Module 3 Atmospheric Pressure

Today, we're going to be talking about pressure, and uh, I think you'll you'll use a lot of the information that uh is in this lecture uh going forward. And uh, it's pretty darn informative when it comes to uh utilizing an understanding of pressure uh in terms of how you fly in certain areas, such as high-altitude airports. Um, this is a a big deal uh to understand.

Um, if you're already a pilot, this is going to be a refresher on some of the stuff at the end of the lecture uh regarding density altitude. But uh, the importance of this is is pretty high. I'm currently writing a paper on density altitude accidents, and and they're they still happen. So it shouldn't, but they still do happen um because people aren't planning for it ahead of time. So we'll kind of get into that at the end, uh but before we get there, we need to understand a little bit more about pressure and uh uh how pilots use it, which is kind of neat. So we go, so we'll start off with the report card, right? So instead of having to go through all the work, we'll just go straight to the chase and see how everybody's doing.

So this report card that I'm going to show you here in a second, but this was published in NAA magazine in 2018. So it's still relatively new information when it comes to weather, of course, uh and how pilots understand it or perceive it. It's uh it's pretty important to kind of judge uh where everybody's at. So uh this was an Embry-Riddle study uh which was I would say totally comprehensive, but uh it was very informative, if anything at all. Uh, it included about uh it was 204 pilots, and 20% of those were student pilots. And uh that's kind of important because when you start to like dig into all the results and stuff uh you might find inconsistencies because some student pilots, it just depends on like where they're at in training, right? Because if there's three different phases that you have to go through as a pilot before you take your check ride, uh who knows where they're at. So uh but I'm sure they did the best they can to answer the questions that they had, and uh it was uh multiple choice, which is uh improves most people's odds. So hopefully you can eliminate some answers and then guess the right ones uh if you have to. And uh the but some of the results that you can see is the average number of questions that they were unable to answer was 42%, which is a huge number. Now that can't accomp, you know, that that can't be, you know, just blamed on the student pilots. I mean, 42% is a big number. And uh what you're going to see here in a second is uh some of the areas in which they didn't do so well.

But of the 204 pilots, you know, I mentioned 20%, that was 41 students uh out of the 204 were student pilots; 72 of them were private pilots. I have these little notes down here: 50 were private pilots with instrument ratings, and then 41 of them were commercial pilots with instrument ratings. So that 42% probably shouldn't be that high uh based on who was taking the survey uh and this was not such a good one either. So the mean score for the entire test was only 58%, and we all know what a failing percentage is, and so you know that's well below that, so which is really kind of sad considering when we fly an aircraft, we always fly in the weather, regardless of how beautiful the day is; it's still weather. And uh we need to know how it works to keep ourselves safe and uh hopefully enjoy the ride a little bit better, maybe plan around certain things so you don't get maybe tossed around as much, that kind of stuff.

Um, but anyway, to look at the actual data uh that was in this report card uh and what they covered, I'm just going to look at a few things. I mean, there's a bunch of uh different uh pieces of information, which most of it will uh be covered uh in this in this semester uh on weather. But one of the bigger ones: the convective sigmet. There were 11 different items; convective sigmet, of course, uh as you'll learn a little bit later, had to deal with thunderstorms. So they asked quite a few questions about thunderstorms. Um, I do know three of the questions that I'm going to be listing in my thunderstorms lecture is the different stages of thunderstorms, which is important for not only for your personal understanding of thunderstorms, but it's usually a test question, not only with the FAA but with future employers because they want to make sure that you truly understand uh how thunderstorms work uh in order to essentially avoid them uh properly. So uh convective sigmet, so only 61% uh was the mean score on that, which uh that isn't too good.

Next one is called a G airmet Sierra. So a G, the G stands for graphical. In other words, you can just take a look at it like a picture, and it kind of gives you the information that you're looking for or uh without having to decipher information like you had to do back in the day where they give you not necessarily longitudes, but they could uh but they'd give you distances from like VFRs and stuff like that, and you had to kind of piece everything together; it was a real pain in the butt. Um, now it's graphical, which is cool. So you can just go, oh yeah, there it is. So and it's great because my my course is either going to go through it or around it or whatever, so you can it's easier to plan. Sierra stands for uh I have, and when we get into uh airmets, you'll you'll be able to understand what all those different types of airs are. Um, so two questions or two items were on that; there was only a 40% 46% uh pass rate on that. I don't know what the questions were, but the majority of those pilots were instrument rated, so uh it's it's kind of iffy.

The next one that I focused on is one that all pilots need to be very familiar with, hand down, which is the METAR. You'll use that each and every time that you fly. You get METARs of, you know, at least your takeoff and your landing uh airports that you're going to be going to, even if it's the same one, uh and then of course you'll probably want some examples as you fly along besides your terminal area forecast, but you want your METAR along the way just to get an idea of, you know, how how the weather's going. So I had eight questions on that; 44% uh was the mean correct, so that wasn't so good. You guys will be masters hopefully of the METAR after the end of the semester in terms of as long as you put the practice in. So my goal, of course, is to show you the subject, uh show you how to interpret the subject, but it's, you know, it's on you to practice it. And that's the only way you can get better at a lot of stuff in aviation is just to practice; it's pretty simple. Uh, you're going to going to use it every day that you fly anyway, whether you're flying as a private pilot or commercially as an airline pilot. So your modes will just get used to practicing it until it becomes kind of second nature uh when it comes to reading this stuff.

The last one is the station plot. You don't use this one as often, but it's super important for you to know it and recognize it when you see it on charts, which we'll do in the second half of the semester. Um, and there actually is a specific quiz just on the station plot, so you should be uh hands down like a master at all this at the end. But that only got a 27% rate, and part of that is because you don't use it as often, but it's really important when you do see it, understanding what that information is telling you because you'd be surprised at some of the good stuff that's uh in the station plot. As you look at some of these charts, you could pick stuff out like really quickly, which is pretty awesome. So we're going to go over all of this stuff throughout the semester, so just kind of hang on. And it's a lot of stuff you have to learn uh just as a private pilot; there's a lot to learn. And uh of course, as as you continue on getting your instrument, your commercial, and ATP and that kind of stuff, uh you'll continue to learn and practice and use more and more of this information as you go on.

But today we're going to be starting off with uh pressure and how it works with weather. Uh, we'll always be touching on pressure throughout this semester because it's a it's a big component to the atmosphere and what type of weather we can uh uh uh anticipate just by looking at it. So in our overview, we're going to be talking about how it's measured, right? How do we know what the pressure is, and how did they figure that out? Differences between high and low pressure systems; think of like when you're you're looking at the weather uh on TV or whatever, you see the big pressure systems uh the highs and the that kind of stuff. Uh wind, wind is caused by pressure, and we'll show you how that works, which is pretty interesting. And then, of course, air density, which we'll touch on today uh at the end when it comes to high density altitude calculations that you need to do as a pilot to make sure that your plane continues to fly after you rotate and start climbing out. And yeah, there's uh one video just off the top of my head, uh hopefully I remind myself to do it, uh I think I gave myself a reminder earlier. I'm going to post in the modules a video uh on someone who obviously did not do uh what was necessary to uh you know make sure that his flight was going to take off without a hitch. And then, of course, when it comes to judgment, you'll see in the video how he decided not to go ahead and land, irregardless of where it was that he was landing, uh but decided to continue on, and you'll see what happens in the end. So it's a learning, apparently everybody uh let's just say survived this one, but uh what we'll go ahead and get started with is a video I've been using uh I think for almost five years I'll be I've been teaching this. This is kind of a cool video; it's pretty simple. Uh he explains a little bit more about the atmosphere, how pressure works, uh not only in the atmosphere but with the body. Uh there is a slight technical part, uh but if you like math and want to understand a little bit more of that, he mentions that in the video, which is kind of nice. Uh it's kind of interesting to learn as you start to see and and see how pressure affects our atmosphere and that the air that surrounds us uh it's it's pretty pretty interesting. Some of the experiments he does in his video, it's not too long, but uh we'll go ahead and get started with this and uh put us on the right [Music] foot.

This kite surfer is hitching a ride on a moving air mass. Our transparent atmosphere reveals itself when it starts to move. Moving air, of course, is wind. The atmosphere surrounding us is a a deep ocean of air extending for kilometers above us. We know that the atmosphere is essential to life on Earth. The components of air, particularly oxygen and carbon dioxide, play a critical role in the chemistry of life. Another important aspect of air is atmospheric pressure. We live at the bottom of an ocean of air. Gravity pulls on this massive layer of gas creating pressure. The pressure is greatest at the bottom, the Earth's surface, the level where we live. Pressure is a force per unit area. The standard SI unit of pressure is the Pascal. One Pascal is 1 Newton of force per square meter. You will also find PSI or pounds per square inch used in parts of the world, particularly America. Air pressure at sea level is 14.7 PSI or 101.3 kilopascals. That's over 101,000 Newtons of force per square meter. If you have a small kitchen table, 1 m square, the force on that area created by air pressure is approximately 101,000 Newtons, equivalent to the weight of 10 cars sitting on the tabletop. If air pressure can generate such huge forces, why don't we notice it? Why doesn't the table collapse? It turns out that pressure in a fluid like air transmits in all directions, not just down. This means that the 101,000 Newtons of force pushing down on the tabletop is balanced by 101,000 Newtons pushing up. Forces are balanced. Similarly, our bodies are not crushed because the fluids contained within our skin exert a pressure that balances atmospheric pressure. Here is a simple demonstration that reveals the pres pres of atmospheric pressure. Fill a glass with water. Cut a small piece of card large enough to cover the top of the glass. Lay the card on the glass. Hold the card in place and rotate the glass full of water. Surprisingly, the water stays in the glass. Here's the explanation: the card provides a rigid membrane between the water and the air in the room. Air pressure pushes up on the card. Air pressure is not pushing down on the water because there is a vacuum at the top of the water. Air pressure is pushing on the outside of the glass, but this does not affect the water. The only downward force from the water is its weight. The upward force of air pressure easily supports the weight of the water. If we drilled a hole in the top of the glass, air pressure would push on the top of the water; the water would fall from the glass. The forces involved in our above demonstration are impressive. The glass I used in this demonstration has a diameter of 6 cm at the open end. The force acting on that small area is an incredible 286 Newtons or 64 lb, easily enough force to support the water in the glass. To confirm my numbers, calculate the area of the open end of the glass, then multiply by atmospheric pressure. Interestingly, we couldn't live without atmospheric pressure. In the few minutes you have spent watching this video, your body has been quietly using air pressure to move air in and out of your lungs. The diaphragm, a muscular membrane below your lungs, alternately contracts and relaxes, changing the volume of the body cavity, which changes internal pressure. Low pressure allows air at atmospheric pressure to move down the trachea into the lungs. High pressure forces it out. Without atmospheric pressure, we could not breathe. Atmospheric pressure also plays a role in the function of many devices, including water pumps, syringes, and drinking straws. When you use a drinking straw, you're not sucking liquid up the tube; air pressure pushes the liquid up the tube. For more videos about our atmosphere and other science topics, visit higher.com. Follow.

All right, so uh when we uh as we continue on with this lecture, just remember some of the some of the concepts that he had mentioned in this video; we'll be talking about as we continue on. Um, but as you can tell just by uh you know watching this video on pressure, uh it surrounds us; it's always it's always around us, and it's important for us to understand the components that make up uh pressure that involve uh aviation. So to kind of get down to some of the I guess they say smaller basics, right, we'll go ahead and look at a kinetic molecular theory, right? So there's there's six six items that we're going to be talking about. You don't have to worry about understanding theory in per se, but just to understand how it works so you have a little bit better understanding of what pressure is and how molecules are affected by it, which is pretty interesting, uh the more I started looking into this. So um it talks about gases as far as, you know, each individual uh particle or or molecule itself, and it just kind they act like spheres as they're kind of bouncing around constantly and in random motion. But when you think of, let's say a parcel of a like we've talked about uh you know earlier uh in this semester, um we talked about parcels then L um if we were to fill those uh fill a parcel full of molecules, we can see them kind of doing their own thing. But uh understanding that when a molecule does move, it it moves kind of in the same fashion as Newton's first law, where once it starts moving in a direction, it will continue in a straight line until it's affected by something else, such as hitting another molecule or tapping off the side of the parcel that we've just made up, right? So in other words, they move randomly, but when they move, they move in a straight line, so it kind of makes sense. In a second, we start to look at uh you know how many molecules are contained within a parcel, and this is an interesting concept because uh I used to think that whenever you had air, right, such as in a parcel, that the entire parcel itself was just completely packed full of molecules; in other words, there couldn't be any space that didn't have a molecule, but apparently that's incorrect. There is space, empty space between molecules, so it's not completely full of molecules, which is pretty interesting. So as we continue to look at the theory itself, we start to understand a little bit more about how molecules are able to kind of move around where there is space actually in between them. Um, we also see that there's no force that are uh especially in a gas or in this particular case air where there's no attraction of the molecules to each other, so they continue in that random motion as they start to kind of in our parcel kind of whack against the side and create pressure. So collisions, of course, uh this is pretty interesting. So whenever something a molecule hits the side of a parcel or each other, right, and bounces off, and then of course continues in a straight line, they don't lose any of that energy or that kinetic energy that we were uh talking about uh in the last lecture where we looked at, let's say the temperature that was associated with molecules where they speed up or slow down or their kinetic energy got greater or smaller uh based upon the temperature. Well, the same thing here is they also don't lose that kinetic energy when they hit each other or the sides of the parcel, so that allows more of a constant when it comes to pressure. And then, of course, there's the average kinetic energy of the particles. So the biggest uh uh uh factor that changes the kinetic energy of the molecules themselves is temperature. So even though pressure and temperature are we have to think of them separately, they are, like I said, uh interrelated uh in the way they behave. So to understand the kinetic molecular theory a little bit better, I have one more video; this one's a short one, little technical, so hopefully this won't turn you off science-wise uh to help you understanding kinetic molecular theory better. You probably like this one. [Music] [Music] The kinetic molecular theory is the theory that all matter is made up of atoms and molecules that are always moving. These particles hold kinetic energy and move around in random directions. In higher temperatures, the particles have high energy, and in low temperatures, the particles have less energy. The more energy a particle has, the faster it will move. In solids, such as ice, the particles hardly move at all and hold a regular pattern because the energy is low and the particles are tightly packed. In liquids, such as water, the particles have more energy and the freedom to move around. Liquids have the ability to flow and fill the shape of any container they are put in. In gases, like steam, the particles have the most energy. These particles are sparse and move around freely. Gases are usually invisible to the eye, and they can be expanded and compressed in different pressures. All matter is composed of atoms and molecules that are always moving. The higher the temperature, the more energy these particles have, and that is the kinetic molecular theory.

All right, so as you can tell, that was a very highly technical uh video, but uh it's kind of nice because it just it's an easy visual way of understanding the concept a little bit better, um because as we find out as we start to learn more and more about weather, uh this is one of those factors; understanding the energy that's associated with the molecules and the pressure and how temperature relates to it is very important, um and as we get into density altitude, you'll find out it's it's extremely important uh to understand kind of the how and the why things happen the way they do uh instead of just getting like a checkout on density altitude where they say, well, this is why this happens and keep it generic; you'll have a

Little bit better, deeper understanding of how it works and, uh, it will actually, um, benefit you as you continue flying. So, nor like the the planes that we're flying right now, right, are unpressurized. You know, if you guys are flying Cessnas and Pipers and stuff, so when you fly up on high, you know, higher in the atmosphere, of course, you know, the molecules are getting a little bit farther apart from each other because the pressure is decreasing. And, uh, because of that, um, eventually you get to a certain point, of course, where you either have to have supplemental oxygen. And if you look at the F.S., it's around 12,500 feet, uh, for more than 30 minutes. So how do you avoid that? Because in order for jet engines to be really efficient, for our true air speeds to be high enough, we need to go up higher in altitude, right? So in order to get higher up in altitude, we need to find a way to increase or maybe compress all those molecules; in other words, pressurize the cabin so that the altitude inside the cabin is much lower than the altitude outside the cabin, uh, which is, of course, the atmosphere that the aircraft is flying in. So to do that, of course, you pressurize the cabin.

To give you an idea of the differences between the two, we got this also highly technical graphic right here, which shows the plane cruising at 36,000 ft. But because we pressurize the cabin, uh, which is done before you push back the airplane if we're talking about an airliner, you set the altitude you're going to be flying at, in this case 36,000 feet, and of course, the computer does the rest of the work as you continue. As soon as you rotate and start climbing out, the cabin will start pressurizing itself as you continue to fly. Usually somewhere around 1,500 ft or so is when it switches from a let's say a go-around or a comeback, uh, or an abort, uh, to going into that cruise phase, right? So as you continue to climb up in the atmosphere, the the cabin continues to pressurize. So when you get up to 36,000 ft inside the fuselage where you're sitting, uh, the altitude is equivalent to around 7,000. It's usually between 5 and 8,000 ft; it's usually never greater than 8,000, uh, mainly because, you know, it's it's there could be issues if you get above 8,000 ft, but most of them, for the most part, uh, we it's somewhere around 7,000 is a good average.

Now, what that what what does that allow you to do? Of course, is it allows you to function just like you would on the ground, for the most part. Um, everybody is sitting except when you're getting up to go to the bathroom or something. And so because you're not expending a lot of energy, they don't have to worry about you getting, let's say, shorter breath or whatever the case might be. It's because if if anybody's been up to Big Bear, which I use quite a bit in this semester as far as an example airport because it's close, you eventually will probably fly there. And then, of course, if you've ever experienced anything in the mountains, I'll use that as an example. But if you were up in Big Bear, let's say, and you started walking around, uh, you know, it's close to 7,000 feet, uh, around the lake, and you can get tired faster or you feel a little bit shorter of breath doing the same kind of energy or same kind of task that you do down here, like walking or jogging or whatever. And, uh, that's just because the molecules are a little bit further apart. But when you pressurize the cabin, of course, it just makes it easier, uh, just to be able to deal with, you know, flying from point A to point B, uh, in a in a comfortable fashion. So, uh, just a reminder that we use pressure for a lot of different things.

Um, if you want to know where you get the air from to pressurize the cab, and it actually comes from the engines themselves, uh, before it actually goes through that combustion phase; in other words, it comes off of the engine, uh, as just air. As the air goes through the compressor part, which is just compressing it, warming it up, it takes the air off of that section, so it's just air coming into the cabin. And then these little outflow valves at the back of the aircraft is what the computer uses to regulate the amount of pressure that's uh maintained within the aircraft fuselage. And then sometimes parts of the uh the baggage compartment, like if you have dogs and and stuff that you're you're transporting. So anyway, that's a kind of a short, long and short way of explaining how that works and why or how we use pressure to cruise at higher altitudes, which is kind of neat. Um, things that you have to memorize, just like I mentioned before when we talked about temperature, there were two numbers that you needed to memorize. Hopefully you guys are thinking about those numbers right now, but you also need to know what standard pressures are. So we use these two different numbers in different ways. The first one is the one we've set in our Kohlman's window, which you'll see here in a minute, um, and the 2992 is our standard. So just like temperature, pressure has a standard, and the reason why, of course, that we use it is so that we can consistently, you know, uh, design aircraft engines and all the other components that we need uh to fly on aircraft. And then you need to be be able to have these standards in which to compare because, of course, Mother Nature is always different and it's never always where we want it to be in case uh in terms of pressure.

Um, the other number, the 1013.25, is also a very important number for you to memorize because you want to be able to to be able to recognize this number when you look at charts. So when we start to look at charts, they don't use the 2992; they use the 1013.25 because everything is millibars. And what your job to do is as a pilot is when you look at some of these charts is to be able to recognize one: is the pressure are the pressures higher or lower than standard? And then you need to be able to look at whether or not one pressure system system is higher or lower than another one. So a lot of times when it comes to that second number, it's not only to recognize what the standard is, but it's also to be able to recognize when a number is higher or lower in pressure because you'll be able to now distinguish high-pressure systems from low-pressure systems. And from that, you'd be able to determine what type of weather that you might be anticipating. It'll all tie together as we continue on in this lecture or this uh this semester. So this is pretty cool.

Um, so the question is, we're talking about pressure in the atmosphere; is who was the one who determined or figured out how to measure the atmosphere? So we know from the video that they had mentioned that the atmosphere has mass, and the because it has mass, it has weight because it's being pulled down towards the Earth via gravity, right? Well, someone had to figure out, well, how do you how do you weigh it? Because we can weigh ourselves, right? So we have mass, our our our person, right, and the we still are affected by gravity, and if we put a scale on the ground between ourselves and the surface, we will find out there's a number; it's not always the number you want to see, but there's a number that comes out, and just it's based upon gravity, and it tells you how much much you weigh or how much your mass weighs, right? So unfortunately, even though if we go to space and you don't weigh anything, which makes you feel pretty good for a couple of seconds until you realize your mass is well, it's still the same. So if I'm chubby on Earth, I'm gonna be chubby out in in space. So you know, it's just mass doesn't change; it's just how it's measured. So how does the atmosphere get uh to measured here on on Earth? Uh, this was discovered by Evangelista Torricelli. Torricelli. So if there's any Italians out there, I completely destroyed that name. So anyway, it's written right there so you can read it yourself. He's got a cool mustache, and what he did is he decided to I don't know why he decided, but it's kind of neat; he took a huge vat of mercury. Now mercury, of course, is a very heavy metal, and it looks and you know and and it's it looks liquid as you kind of swirl it around, which is kind of wild. It almost looks like if you melted something, uh, but of course, it's not melted; it's just the the how it's how it's composed. And, uh, like when we were kids, you should never do this, by the way, and when we were kids, they used to actually in class they would put a piece of mercury in your hand; you can kind of roll it around in your hand; it would just roll around in circles, which is wild, uh, very bad because it's poisonous, so don't do that at home ever.

But what this guy did was pretty interesting. So he has a big vat of mercury, and then he decided, Well, I'm going to take this big huge tube, then that tube, of course, would be closed at one end so I can fill it up. So he went ahead and filled the tube up all the way up to the very top with mercury, put his finger on the end, flipped it upside down, and stuck it inside the huge vat of mercury. And what that did was the heavy mercury that was inside the tube settled because of gravity, so it pulled it down, created a little gap at the top, which is kind of like a a small vacuum, and it allowed the measurement to be able to be measured, uh, just the atmosphere itself. So they could put little marks on the side of that tube and then eventually figure out what that pressure number was, and then they could start to compare, which is pretty interesting. So, uh, let's see. So to let you know what it looked like, uh, you've probably seen these before, uh, is the object that's on the left-hand side, which the mercurial barometer with, of course, his name down at the bottom, uh, as you can tell just by looking at it, it's hopefully as I described, and you can tell as the atmosphere that surrounds that big uh, you know, vat of mercury pushes down or releases it, it will affect the uh the level that's within the tube. So you can just visually look at it, say, oh, the pressure is higher or the lower just by looking at it, which is pretty simple, um, to get away from the fact that you can't just kind of roll around with mercury in your hands, right? So they decided to come up with the anoid barometer, which is portable, which is great. You can see that this kind of looks like a nautical device, uh, which it is, which you'd find on a boat. I have one around here somewhere; I couldn't find it.

But what this does is kind of neat; it works similar to an altimeter, which we're going to uh show you kind of a breakout piece of an altimeter today, um, where there's a little bellows inside it, and uh, it'll as as it uh contracts or expands, it starts to affect the little dials and and and gears and stuff like that which rotate that needle that's on the face of it, telling us what the pressure is uh outside. The nice thing about that is if you look closely at the aneroid barometer, the very top of it, there's a little black piece that's kind of pointing, a little pointer that's pointing down towards the gauge, and uh, you can set that, of course, manually. And the whole idea, just like when you're looking at pressures on a map, you want to compare where it is now to where it is, let's say, tomorrow or the next day, and you can start to see trends, whether or not the weather is getting better or worse based upon whether or not the pressure is getting higher or lower. Pretty interesting. Um, other things that you can use uh barometers for, uh, other devices that use uh this the same kind of science behind it, uh, this is kind of interesting because uh I have to read this. So the microelectromechanical systems or the MEMS, um, are used in watches, uh, in this case, this is a Garmin watch that or my Garmin watch, uh, and it's kind of neat because you can always take a look at where the barometer is set; you can actually set it. I use it when I call AIS or something, and I'll reset it to make sure it's accurate, um, but you'll also find it not only in watches, uh, I think the the the Apple Watch and there's other watches out there that have barometers associated with it, um, it's also part of inertial navigation. So if you're not sure what inertial navigation is, it's a component, an avionics component where when you fire up the airplane, uh, this system goes through and figures out based upon pressure and GPS kind of where it's at, where it's located, uh, you know, around the world; it actually finds itself, unlike GPS, which uses satellites directly to figure out where it's at, uh, the inertial navigation uses its own separate system to find itself, um, and then it uses a lot of barometric pressure to do that. The neat thing is, of course, now it's become a redundant system, so not only do you have an inertial navigation system because once it's set after you start up the aircraft and it kind of finds itself, um, you now have backups, which is kind of cool for navigation.

Other things: accelerometers, which are really important not only to be able to tell you how many steps you've walked, how many stairs you've climbed, uh, but it's also used in drones. So if you want to figure out how drones work, where, you know, how do they hover all by themselves, and say people say, do gyros, it's like, yeah, but the gyros are affected by uh pressure. So when those, you know, you think about all those different uh, you know, rotors that are hanging off of, let's say, a helicopter-style, um, uh drone, right? When it notices it's going up or down, right, pressures are changing, but it's the computer's fast enough that it allows it just to hover in one space because it's that good at deterring the differences in pressure as it starts to move. So uh this kind of uh the pressure subject is is pretty interesting as we start to look at different ways it's used in aviation. So kind of moving forward, when we start to talk about pressure changes with altitudes, uh, in order to understand that you need to H you need to understand how pressures change um as you go, let's say, from sea level to something like Denver. So to measure between stations on a standard day, now you'll see the word stations; stations means could be anything, but usually it means an airport, uh, for the most part. There are other places where you can get some of this information that's not an airport, uh, but the whole idea, of course, is just to be able to get the data from wherever that is uh that you're getting it from. But for most of us as pilots, we look at stations as uh as as airports. And as we measure the differences between the two, we know that sea level pressure standard, which we just memorized, of course, a 2992, uh, if we were to take that pressure and we were to go all the way up to Denver at 5,000 feet, well, does the pressure change? Well, yes, of course, it does. So as you start to go up in altitude, every 1,000 feet you will lose 1 inch per 1,000 ft of pressure. So if you were at sea level and it was a standard day throughout between here in Colorado, right, just imagine it being a standard day, and you were to go ahead and look at 2992 at sea level, and then what would be the pressure if you lose one inch per 1,000 feet and it's at 5,000 ft? Let's do some simple math here: the pressure on a standard day at Denver would be 24.92 in.

In an airplane, when we start to adjust that Kohlman's window, which you'll hear me say quite a bit because I just want to kind of ram that home so you remember to do it every single time you hear a pressure, um, but in this particular case, uh, our our altimeters, which we'll learn about here in a second, are calibrated for sea level pressure, so you don't actually have to turn your Kohlman's window every time you go up 1,000 feet to adjust it because as as long as the pressure is consistently standard uh throughout that whole area that you're flying in, you don't have to change anything, right? The altimeter does all the work for you, so you don't have to worry about it. Of course, going the opposite direction, of course, the calculation goes uh a different way, right? So remember when it talks about the the one inch per 1,000 feet, that's the atmosphere itself, not the altimeter, but the atmosphere, so you have to kind of separate what an altimeter gives you versus what the the actual atmosphere, the raw data that you have to be able to calculate, all right? And then, of course, station pressure, I just remind you down at the bottom, it's the pressure that's measured at usually an airport. So if you're a visual learner, of course, here's a nice picture for you to be able to see it. Cool thing about this picture is you just take a look at the tube that's in the middle of the the mercury vat, and you can see if it's higher or lower; it's like that's simple, right? So the higher the pressure, of course, it's on top of the mercury; it's going to push the mercury up the tube; the less pressure uh kind of releases it, and of course, it allows uh the tube, the mercury in the tube to drop, so you can just see it; it's pretty simple pressure variations. All right. So uh there are pressure variations that are associated with temperature, just like we had mentioned before; they're separate, but they're also interrelated, right? So as you travel up, the difference between pressure and uh and and temperature, in this case, pressure will consistently drop uh one inch per 1,000 feet. The only catch with temperature is that might not always be the case, right? Because we get some interesting stuff here in Southern California; we get inversions uh quite often, and when that happens, of course, it's an inversion, so therefore it's not consistently going down, but because of the inversion, it goes down to a certain point and then it starts to go back up because of the type of weather we get here in Southern California. So we'll talk about that later, but for as far as pressure variation, there isn't any; it's very consistent; it does change in the higher atmosphere, uh, but we're not flying there, so we'll just learn it this way for now. And if you want to get more into the atmosphere, of course, you'll start to learn that it does change; that number does change as you get up a little higher. All right. Um, so air temperature, this is very important, understanding that when air expands, right, or as as air gets warmer, it expands, and as air gets cooler, it contracts as it cools, right? That's really huge, uh, as we start, especially when we start to look at how it affects an altimeter as we're flying along into different like air masses and stuff, all right. Um, let's see. So there are different altitudes that we have to understand when it comes to pressure; these are all related, uh, when we start to look at uh the pressures not only in the atmosphere but also the gauges that we're looking at when we're flying our airplanes, whether it's glass or the regular panel display, um, so you need to understand what these four different altitudes are. We'll start off with true altitudes because that's usually the easiest one for most people to understand because it's kind of what we think of uh when we think of altitude; in other words, you go to Big Bear, it's 6,752 feet or wherever, right? It's based upon that elevation compared to sea level, right? So most people are used to whatever the elevation they're at, like if you look at GPS or something like that, and it's based upon sea level of what the the number it gives you; that's your true altitude, right? And MSL stands for mean sea level, and that's the number that you're going to use very consistently when you look at a chart, and the chart will give you, let's say, an example of the size of a tower; it will give you the size of that tower in true altitude, but it also gives you another, which we'll talk about in a sec. Pressure altitude, of course, is

The standard altitude is 29.92, and what it is is this: you have to remember that they use pressure altitude when they design and create these planes. We use that number to compare it to whatever it is today in the environment that surrounds us, to be able to determine the performance we're going to get out of the plane compared to the standard in which that plane was built.

So indicated altitude is when you frankly just look at the instrument itself. If I look at my altimeter, whatever it says, that's what it's at. If I'm looking at my glass panel, same thing; it actually just tells you what the number is. So indicated is whatever your instrument gives you when you read it off the face; it's pretty simple.

Then there's absolute. Sometimes—I mean, I remember this was just because it is worded differently—but an absolute altitude is the altitude that is above the surface that you're flying over. So, in other words, it's not just sea level, right? That would be true altitude, but now we're starting to look at, well, how high are we relative to the surface? Like if we're flying over mountains, if we're flying over Orange County, right? Orange County is not at sea level, right? So LA, Mammoth, whatever—we need to know where our aircraft is located relative to the ground. So AGL, so above ground level. As a pilot, you really need to understand the differences between above ground and mean sea level, right? Because while you're flying in an airplane, right, when you look at your altimeter, that altimeter is based on your true altitude if it's set properly, which is your height above sea level. So every time you're flying, everything is relative to MSL; it's all related to MSL when you're on the ground or if you're looking at a chart. Okay? There's also AGL, or above ground level, so you need to be able to understand what the absolute altitude is of, let's say, as an example, the ceilings. If you went out to John Wayne Airport this morning, the ceilings were around 2,000 ft. I don't know what they're at right now; it's probably burning off. Um, but that's an absolute altitude or above ground level. So, above John Wayne Airport, 2,000 ft above whatever altitude John Wayne Airport's at is the ceiling that you had this morning at John Wayne. So really big to understand the difference between MSL and AGL, uh, when it comes to just understanding the concept but also when you start to look at charting and that kind of stuff. All right.

The altimeter itself, of course, is sensitive. It's sensitive, uh, as you'll see in this next video, uh, he puts it in a little different way, but it's something that you can adjust with the Kohlman's window, which is either a knob—well, it's like if you're using a G1000, it's also still a knob that you rotate in order to adjust the altimeter to whatever the current pressure is at the station that you're at. If you're at John Wayne Airport, you set it to whatever the pressure is at John Wayne Airport so that your altimeter reads properly.

So in order to explain this a little bit better, I found a great video. Uh, it does include a couple of other instruments, but they're all pressure-related, and it's really good that you guys understand all of these instruments, but really focus on the altimeter, which is what we're talking about, uh, so you get a better idea of not only how it works but what, like, what does it look like on the inside and how do you adjust it to uh, the current pressure. So I think you're going to like this.

The pitot-static system is connected to the airplane's altimeter, airspeed indicator, and the vertical speed indicator. These instruments tell the pilot how high they are, how fast they are going, and how fast they climb or descend. These are determined by measuring the pressure of the atmosphere. Before we go any further, let us become familiar with the atmosphere. If we take a column of air starting at the ground and traveling up all the way into space, you notice that the molecules of air at the Earth's surface are much more compacted together than the molecules up in space. The reason for this is gravity. Since air is matter, it has weight, so all the air molecules at the top of the column are pushing down on the molecules below them, which compresses them. This compression results in a higher pressure at sea level compared to the pressure on top of a mountain. How much of a pressure change are we talking about? Well, if we say the atmosphere goes up to about 375 mi above the surface of the Earth, 50% of those air molecules would be found in the first 18,000 ft of the surface. So, because the pressure of the atmosphere decreases the higher up we go, our pitot-static instruments are able to measure that pressure and calculate our altitude and speed. The pitot-static system gathers its pressure information from two sources: the pitot tube and the static ports, hence the name pitot-static. The pitot tube is designed to measure the pressure of the air as the airplane flies through it. In most smaller aircraft, this tube is located under the wing so it can measure the flow of air without any interference. Sometimes aircraft manufacturers will use a pitot mast instead of a tube shape, but it still functions exactly the same way. On the back side of the tube, there is a drain hole which allows any rain or water that is collected while flying to drain out and not go into the system. Finally, the pitot tube can also be heated. This is used to prevent ice from forming on the tube, which could potentially block the hole and prevent the system from functioning correctly. The counterpart of the pitot tube is the static port. The location of this port will vary with different aircraft designs but should be in a location where it can measure the static pressure of the air unaffected by the dynamic air flow around the airplane. On the Cessna 172, the static port is located on the left side of the forward fuselage. The pitot tube and static port openings are connected to tubes that join into the pitot-static instruments. The pressure inside the instruments matches the pressure of the outside atmosphere. All three pitot-static instruments connect to the static port, but only the airspeed indicator connects to the pitot tube. Sounds simple, right? Let's discuss how each pitot-static instrument works.

Altimeter. Perhaps the most basic of all pitot-static instruments is the altimeter, which displays the airplane's altitude. The instrument contains a set of anoid wafers which expand and contract based on the pressure. The air inside the wafers is trapped, but the air in the rest of the case is able to change to match the pressure from the static port. As we increase altitude, the static pressure goes down. This means that the air inside the case will escape out the back and result in less air pressure in the case compared to the wafers. Because of this, the wafers will expand until both pressures are equal. Getting the wafers to result in an altitude readout is done through a series of gears, pinions, arms, and levers, also known as the mechanical linkages. These linkages will rotate the hands on the face of the instrument and show the airplane's altitude. Now, when the airplane descends, the opposite happens. Descending to a lower altitude results in higher static pressure. Air from the static port will now enter the case of the instrument and squeeze the anoid wafers until both of the case pressure and the wafer pressures are equalized. The mechanical linkages will then rotate the hands on the face to show a lower altitude. The face of the altimeter contains three hands: the 10,000 ft, 1,000 ft, and 100 ft hands. These hands move clockwise and counterclockwise to display the appropriate altitude. Most altimeters in smaller aircraft will only work up to around 20,000 ft, but those airplanes usually can't get that high anyway. Here's some examples of altitudes: 3,000 ft, 8,400 ft, 12,000 ft, 5,280 ft. This altimeter is actually called a sensitive altimeter, not because you have to hug it every once in a while, but because it can be adjusted for the current atmospheric pressure. Because the pressure at any given point on the earth never stays the same, altimeters would always read incorrectly. Fortunately, pilots can correct this issue. Once the pilot knows the current atmospheric pressure, also known as the altimeter setting, all they have to do is rotate the dial on the lower left side of the instrument until the current pressure is selected in a little window on the face called the Kohlman window. This then realigns the gears inside, and the instrument reads accurately. The realignment is accomplished by rotating the entire inside mechanics of the instrument.

Vertical Speed Indicator. Another instrument that uses only the information from the static port is the vertical speed indicator, more commonly called the VSI. The VSI measures the vertical speed of the aircraft in terms of feet per minute. This is accomplished by comparing the current pressure of the air with the pressure of the air from a few seconds ago. Inside the VSI is a diaphragm connected to some mechanical linkages that move the needles on the face of the instrument. The diaphragm has a direct connection to the static port, meaning that the pressure inside of it matches the current atmospheric pressure from the outside. The case of the instrument is also filled with static pressure, but the connection between the case and the static port is constricted by what is called a calibrated leak. This calibrated leak is nothing more than a tiny hole which limits the rate at which the pressure of the case can change. When a plane climbs or descends, the diaphragm pressure will change instantly, but the case pressure changes slowly. This results in two different pressures. The difference in pressure allows the instrument to display the vertical speed. Be aware, however, that it takes a few seconds to read accurately, so anticipate a slight amount of lag. Here's an example of how it works: say you have a Cessna flying at 3,000 ft. The air pressure in the diaphragm and the case are the same, so the VSI is reading 0 ft per minute. If the Cessna starts to climb, the air pressure in the diaphragm is decreasing, but the case pressure is decreasing more slowly. This results in the case having a higher air pressure than in the diaphragm. This higher air pressure will squeeze the diaphragm and make the VSI show a climb. Once the aircraft levels off again, the case pressure will finally equalize with the diaphragm, and then the VSI will show zero again.

Air Speed Indicator. The airspeed indicator is the only pitot-static instrument that uses both input from the static port and the pitot tube. The pitot tube is used to measure what's called ram pressure. The faster the airplane travels, the greater the ram pressure is. The ram pressure entering the pitot tube gets sent to the airspeed indicator and, similar to before, goes into a diaphragm. The greater the pressure, the more the diaphragm expands. So the diaphragm will expand as the airspeed increases. Then, through mechanical linkages, the appropriate airspeed will display on the front of the instrument. But what about the static port, you ask? Well, remember how the pressure of the atmosphere changes with altitude? Well, if this wasn't taken into account, then the airspeed would indicate different speeds at different altitudes, even if the plane was actually going the same speed. To fix this, the static port connects and fills the case surrounding the diaphragm with static air. This will subtract out the static air pressure that the pitot tube captured and only allow for the dynamic pressure to be read on the instrument. This keeps the airspeed indicator reading the correct values no matter what the altitude of the airplane is. The face of the instrument displays color-coded speed ranges that the pilot should be aware of while they fly to avoid exceeding any limitations of the aircraft. The green arc is for normal operations; the white arc is when you're allowed to extend the flaps; the yellow arc is limited to flight in smooth air only; and the red line indicates the maximum allowed speed.

So hopefully you enjoyed that little short video, uh, on the three, uh, pressure-sensitive, uh, instruments that we use when we fly. Uh, the same is in what if you're flying glass, uh, except, of course, instead of a round dial, that'll be a tape, uh, but you'll get the same—let's say—color-coordinated lines on the tape itself giving you the same information. Um, but as far as the pitot-static system is concerned, we're also going to be touching on, or you'll probably ask questions about, uh, the pitot tube or the airspeed when we talk about taking off and landing at high-density airports or airports that are, uh, a little bit higher in, uh, in altitudes, such as Big Bear, Mammoth, Tahoe, Truckee, that kind of stuff. So keep that in mind as we start to cruise through, um, this—as we start to talk about altimeter errors.

So we just learned about how the altimeter works, uh, with the bellows along with these static pressures in the back of it to be able to compare, and what we're going to look at now is what errors could we potentially get, uh, from the altimeter if it's not set properly, right? So this is a biggie. So whether you're flying VFR or IFR, uh, this one is a big one only because—excuse me—only because in this particular case, if it's not set properly and your altimeter is reading incorrectly, you might catch the attention of air traffic control that might ask you, "Cesna 234, can you say your current altitude?" That's usually how they inquire, and you usually give them the number, and it's probably wrong. So if you hear those words from ATC, it's probably because they're going to tell you you're at the wrong altitude. And let me explain how that happens.

So you'll see this picture in the book and, uh, the FAA book, and it's—this one's all over—the nice thing is is very consistent. So, uh, to explain the chart real briefly: on the left-hand side, of course, is our true altitudes of what the airplane's actually flying at; that's on the left. And then down at the very bottom, if you look at the numbers, uh, it actually gives you pressure numbers, and each one of these pressure numbers for the three different columns are different because the aircraft is flying through different pressure systems. All right? So it's flying from a high-pressure system to a low-pressure system as it goes from the left to the right. So, of course, the middle one is set properly. You'll see the three altimeters down at the bottom; they're all indicating 3,500 ft. They all have the exact same number in the Kohlman's window, that's where you set your altimeter pressure, and therefore, if everything is set properly, but you're flying through different pressure systems, something's going to have to change, and in this case, it's your true altitude that actually changes.

So a great example of this is if you're flying from, let's say, John Wayne Airport to—you're doing a cross-country to Palm Springs, right? That's kind of a common cross-country; get all sorts of stuff accomplished, like turbulence and landing in the desert and all sorts of good stuff. But anyway, so let's say you start off and you're that center aircraft, okay? So the initial setting for the altimeter is 30.00. So you make sure that's set in your altimeter, and when you're flying along, in this case 3,500 ft, because you have your—your altimeter set properly, your true altitude is also 3,500, so your indicated and true match, which is what you want. Now, as you start to fly towards Palm Springs, there is a higher pressure in at Palm Springs typically. So as you start to fly towards Palm Springs, if you did nothing to your altimeter, you just continued flying towards Palm Springs without changing anything, something mysterious will start to happen, and you won't even realize it's happening. Your plane will start to climb, but you really won't notice it because it's really insidious. So, uh, you'll—you'll notice that the needle starts to move, and as it starts to move, you being the super pilot that you are will, of course, compensate for that to make sure that you maintain 3,500, but the reason why it's not just turbulence that's moving the needle, you're actually from a lower pressure area to a higher pressure area, and as you continue to do that, your true—not true airspeed—your true altitude will continue to climb while your altimeter stays the same, and that's because the pressure is changing. So with, uh, in—in this case, we're showing a higher pressure which gives you a higher altitude on your altimeter, um, and then, of course, just the opposite happens if you went from, uh, let's say, say John Wayne Airport and you flew through an area that has a lower pressure system, your true altitude in your aircraft would actually decrease.

Why is that a problem? Well, the reason why that's a problem is if you're working, let's say, on your instrument rating and you're flying an instrument approach, and when you fly an instrument approach, usually you have a plate that tells you how to fly to get to the airport, what avionics that you're using to get there, right? And it also tells you the lowest altitude that you can fly to as you descend towards that airport. The whole idea, of course, is to make sure that you see the airport before you land, right, as you go through the clouds, because technically you shouldn't be able to see anything until you break out, right? So let's say you're that middle aircraft in this picture; everything's set properly, but when you fly from that current position towards the airport that you're going to land at, the pressure in the environment decreases as you get closer to the airport. Unfortunately, you didn't change your Kohlman's window when the air traffic controller told you what the landing airport's pressure was, and you can tell at the bottom it's at 29.50 versus 30.00. If your altitude was, let's say, at, uh, the lowest altitude that you could fly on the approach was 500 ft, right? When you reach 500 ft and you went into the lower pressure area like in the case on the right and you've lost 500 ft in true altitude, what would happen when your altimeter reads 500 ft? Your true altitude will be on the airport itself. So, in other words, you're probably going to crash into the runway if the fog goes all the way to the ground. So that's why it's important to make sure that your altimeter is always set properly, and anytime you hear the, uh, air traffic controller give you an altimeter setting that you change it immediately, uh, on the panel. It's super important. I know in cruise some people kind of blow this kind of stuff off, but you'll find out when—especially when you start to fly in instrument conditions—how very, very important it is. And if you don't have that habit ingrained and it's just not second nature, like when you drive your car and you're not thinking about what you're doing, you're just driving it, right? Same thing here: always check your—your—your pressure setting, your Kohlman window. An easy thing to remember on both—not only pressure but temperature, which we'll talk about in a sec—is, uh, from high to low, look out below. Some of you have probably heard that before. There is another one, going towards higher pressures, but that makes you higher in altitudes—your front—I'm really more concerned about from high pressure to low pressure, look out below, or from high temperature to low temperature, also look out below. So keep both those in mind; they work with both scenarios. Especially when you're taking a test, you're trying to figure out, you know, what happens to my airplane's true altitude if the pressure or the temperature changes. From high to low, look out below, and you know that your airplane will descend if it's going from a high pressure to a low pressure. Um, now we'll talk about temperature. I was going to ask you that if you guys had any questions, but I'll wait till after this one. So when we look at temperature, you'll notice that the—the plane isn't changing that much in this because it's a similar picture, right? Because they have the true altitudes on the left-hand side, you have the pressures down at the bottom. In this case, of course, they're all 30.00, uh, so everything—the altimeters, the Kohlman window, everything is exactly the same. The only difference is is we're flying from different temperatures as we cruise along, right? So in warmer air, you can kind of look at as that—just that parcel of air below the airplane starts to expand, your airplane will climb. As it cools off, the parcel gets smaller, your airplane starts to descend, uh, in the temperature. So that's mainly the difference between the two, except with temperature you'll notice there is no adjustment for it, right? So you can't change the temperature in your altimeter. That's not such a bad thing because when you're flying in an area, let's say Big Bear or Mammoth or or even John Wayne Airport, um…

When you fly into an area with other aircraft, as long as they have their pressure set properly, you will all be higher or lower based upon the temperature. None of you will see it, but the whole idea here, of course, between myself and the FAA, is to make sure you understand what happens or what's the potential that could happen to your aircraft based upon how temperature affects the atmosphere. So I will go ahead and continue on with density altitude. Uh, usually get a few questions when we start to talk about this, but this one's like super important. It's because I want to make sure none of you guys whack into some granite or take a dirt nap somewhere or something. So this is really important because, surprisingly, why it happens, it's still happening today, uh, where there's accidents associated with this phenomena. So, um, anyway, and we'll talk about both the air speed indicator and the altimeter when we start to talk about this. So here's your example.

So if you've never seen Big Bear before, that's what the airport looks like. Uh, a lot of fun to fly to, either while you're doing your flight training or when you do a transition after you become a private and you get a checkout, both for uh, Big Bear airport and Catalina. Both a lot of fun places to fly; both have great restaurants uh, that you can get some food at or whatever, so give you a reason to go fly there in the first place. Big Bear, of course, has uh, ski resorts and stuff that you can also do, uh, which is kind of neat because if you think about it, if there's good snow, uh, you can just imagine what the traffic is like, regardless of which Highway you use to get to Big Bear; it's probably going to stink. So cool thing is, if you're a pilot, who cares? I'll just fly up there, no worries, because I'll get there in like 45 minutes or less, no traffic. I can just take—I'll be all fresh and ready to go. So that's kind of cool.

Um, so here's some data on Big Bear itself. I mentioned the altimeter or the altimeter, the altitude before pattern altitude, typically in most air forch is about a thousand feet higher, round stuff like that, off when you're flying. If you tried to fly that number on your altimeter, uh, good luck, especially in the mountains, because it usually is monkey. Uh, the runway is 5,850 ft, so it's uh, just about—it's a little bit longer than the runway at John Wayne, that big Runway at John Wayne Airport, just to give you an idea of size. So it's a big Runway, which is great, but there's a reason for it, so we'll talk about that in a second of why it's a longer Runway up in the mountain. Since it's relatively a small airport, but the runway's big, it's also surrounded by Peaks. So it's very important for you to get the checkout to understand when you get down towards the airport; you can look to your left and right as you landed, have you landed, and you can see that there are Hills; you're in a valley. Uh, so if you take off at Big Bear and you can't see where you're going, well, you better be doing a departure procedure, because uh, it's—you could whack into something. So and that's—that doesn't end well either. So that's Big Bear. So now we're going to talk about density altitude in relation to Big Bear or any mountainous airport. All right.

So we're first going to start off with what the definition is. So sometimes the the the name of it kind of confuses uh, some of the newer Pilots when you start to learn this stuff. So uh, it's pressure altitude adjusted for non-standard temperature. So that's what density altit—that's what you're doing when you do the calculation; right, you're you're finding out how has temperature changed the performance that I'm going to get out of my airplane, which is really important, not only how much power it develops but how—how long is my ground roll? Is—is the thing going to be able to climb out? Does it have that capability because I have more or less power and that kind of stuff. All right. Remember that warmer temperatures, right, they create—their density—it starts to warm up, right? Our molecules are getting farther apart, right, and when they get farther apart, just like our bodies in that original video that we showed at the beginning of the the lecture, you know, our bodies are used to that 14.7 PSI. Right, as we go up higher in the atmosphere, we know that gets lower; our bodies don't change right away; we have to acclimate, which usually takes a couple couple weeks and increases our red blood cells so we can actually hold more oxygen; that's the acclimation process. But when we're going up during the day just to go skiing or whatever ever, well, our bodies don't change, and because we're taking the plane up there, of course, the plane doesn't acclimate at all, so it only deals with whatever the atmosphere is, whatever it's flying through, right, whether it's down at sea level or up at up up in the mountains. But warmer temperatures can create a less dense atmosphere, or it creates density altitudes that are higher. So and this is how it relates, so you understand why they use the term high. So remember that your airplane thinks it's higher when you start to calculate this, and therefore it's going to perform—just remember it's going to perform crappier at at these altitudes, when especially as it starts to get warmer, right? Your performance will be less. All right.

Water vapor and density. So this is a question I like to ask the the students: Is if you have two Parcels of Air, One parcel is completely filled with air, just air; there's nothing else in it; it's just like nitrogen, oxygen, carbon dioxide, argon, all that kind of stuff, right? So it's just air, and the other one, some of the like the air molecules, right, have been, you know, changed out for water vapor; in other words, it's more humid; is probably an easy way, simple way to remember which one has more density, just the air or the air with the water vapor in it? Which one has more mass? Air has more mass than the water vapor that replaces the air. Pretty wild, huh? So a water vapor molecule weighs less than an air molecule, so when you have this displacement, you'll find out that in like uh, when you go to, let's say, down south, you go to Texas, Louisiana, Georgia, Florida, some of these places that have high humidities, occasionally on the East Coast you get high humidities, um, when that happens, the air becomes less dense; the performance on your aircraft becomes less because of the humidity associated with it. So in other words, think about your carburetor intake manifold or wherever that air is going into the engine, right? If the water vapor is replacing some of the air, and the engine needs air in order to get that fuel air mixture properly, well, there's less air coming into the carburetor or intake manifold because there's water vapor that took its place. So in other words, it becomes less dense, so there's less performance. So I like to point that one out uh, because uh, it's good for sports. So if you're a human environment, right, your ball's going to travel far farther if you golf or play basketball or or baseball or soccer, whatever. So keep that in mind, so you want to go to play in places like that, and then of course uh, like Denver, right, when they're hitting baseballs, right? It's less dense at 5,280 feet, and of course the ball travels farther, so the—that's the difference between water vapor and density. It's very important for you to understand that, and and it's also important for you to understand the two different ways you can calculate density altitude to find out whether or not you're going to have a good day or you're going to have to work for your money, right? So we'll go ahead and start off with a chart.

Now there's a couple different, you know, as you start to go through these pilot operating handbooks, there's different ways that they want to you to calculate density altitudes; some of them have this chart, uh, some of them have something you can calculate. I will show you both of them, um, but but before you do any of them, uh, you need to get some basic information. So uh, let's just use a scenario: You're in Big Bear; you're walking out to the airplane, and there's two things you can either get on your phone and dial up the AWOS, and it'll give you the current weather; it's called minute weather because it changes usually every minute uh, because a computer does it automatically. Uh, so you listen for that, and what you want to get from that is the temperature. So we already know if we're walking across the tarmac to our airplane that we're at 6,752 ft, right? So when we hop inside our airplane and we have the current pressure number, we'll keep that in mind, but the first thing we're going to do when we get inside our airplane is we're going to go ahead and grab that Colman's window or that knob on your G1000, and we're going to spin it so it does this standard pressure number, which is 29.92, right? So we'll turn that C1 299 or 2, and we will read the indicated altitude that's on our our our aircraft, and it will give us a number. The number that it gives you on the altimeter because you put in the standard pressure is now the pressure altitude, so it's the altitude that you know based on the altimeter setting, right? It—the airplane thinks it's at, but we haven't calculated density altitude, so it's actually incorrect, but that's the starting point. You need to have your pressure altitude uh, before you can do any of these calculations. So if you're looking at this chart on the on the the right hand side, you'll see that the airport pressure altitude is listed on the right, so we need to find that. So in our case, it's a standard day 299 or 2, so we don't have to worry about that because all we're really focused on right now is temperature and how it affects our density altitude. So we'll go ahead and pick our our altitude, put it on, and we'll—oh, sorry. So we picked our our altitude, and then the other number that we need is the temperature, which we got uh, when we listened to the AWOS or ASOS or wherever you got your weather information, right? So we're going to draw a line from the altitude over to the temperature number. Fulla, there you go; there's your line. I used 80 as—it's 25° C in my example, uh, as you can tell down at the bottom, yeah, it's showing at the bottom for you, uh, it's it's it's roughly around 80. So on this case, uh, as we draw a line through it, what we really want is the data that's in the middle, right? So we want to know how much long—how much more Runway do we need before we rotate and start climbing out, and then once we start climbing out, I want to know how well is this airplane going to perform? How well is it going to climb out as we start to get out and come back, let's say, to John Wayne Airport, right? So as you look at this line, if you can—I just ballparked it—and you look at the numbers on the left hand side, is the increase in your takeoff distance, right? And in that case, it's 220%, so you have to increase your takeoff distance 220%. You can already see that's a big number, and then on the right hand side it gives you your rate of climb, so we look at that; it's around 75, so 75% off of our climb. So we look up how well this aircraft normally climbs; we take 75% of that number off of it, so you can see density altitude—have a big—just by the thinking in this perspective, it has a big impact on the performance of your aircraft.

Other ways uh, that can get it—you can actually calculate your uh, your pressure altitude uh, by taking the standard minus the current pressure, and then I'll show you all that stuff here in a minute. Um, charts are way easier if you can get them, and uh, and then sometimes like if you use a—I use a a CX3 um, myself or an E6B, you can do the same thing uh, just with the whiz wheel uh, to be able to get the same number. All right. Um, that's if you're—let's say if you're at at home or or in a cabin or or just finished skiing or whatever and you wanted to be able to figure it out and you didn't have the access to the plane to find out what your pressure altitude is, you can calculate it using those devices or get an app on your phone. All right. So uh, pressure altitude, of course, these are the little things that you have to make sure that when you read the chart, you use the chart properly, so it's not the altitude that the airplane's at, like the elevation, right? It's the airport pressure altitude; it just so happens in our example is the same because our pressure was right—the standard pressure. The other thing that's listed on this chart, and it's very important to see this stuff—it's all those little notes that'll eventually potentially kill you—is the way I kind of push it home, so hopefully you'll remember to read all the little notes and stuff because they're very important. This says you need to take whatever that number is at 220% and at 75% and apply them to the sea level values on your charts. So what does that look like? Oh, look, we have charts. So if you open up your POH, you'll find these charts. We're going to look at two different ones; we're going to look at a takeoff distance, and we're going to also look at a climb. So in order to read these charts, they're fairly straightforward. First, of course, they list like max weight and short field, that kind of stuff; those are important notes, but also those three notes below it—make sure you pay attention to those and read those—but we have temperatures all at the very top, and on the left hand side it shows your weight, takeoff speed, right, and altitudes, but the important one of course for you to see is the pressure altitude in feet. Remember that number we just got; that's what we look at. All right. But what we want to be able to do is first find the temperature, and in this case it's 25 degrees Celsius was our example, and in the way I look at it is I want to be more conservative. If you're in Stanley's class, right, you're going to go ahead and interpolate between 20 degrees and 30 degrees will be 25. So in other words, you interpolate those two to get the proper number that will give you the correct answer, his test, in the real world, which is what I told you—the way I teach this course is—what would you do if you're rolling out there after skiing or snowboarding? I'm a skier; sorry, that's why I always say skiing. Um, I'd look at the chart and go, well, I'll just go 30, because if I can do it at 30, I could definitely do it at 25, right? So I'm being more conservative, so I pick 30. Uh, to make life a little bit easier, I go ahead and select the sea level, which was—it told me to use the sea level numbers, right? So that line goes across, and I can see my ground roll 1070, and total to clear a 50-foot obstacle—the obstacle in our case would be a tree, right—what, you know, how far would I have to fly? In this case, it would be 1880 ft before I hit 50 feet, but that's at sea level on a standard day. Well, we're not at sea level, so now we have to figure that part out. So in order to figure that out, we go ahead and add those numbers that we we got from the chart to the numbers that are on this chart. So our ground roll shows on the chart of 170; well, how much would the ground roll be? We go ahead and add 220% to the 170, and we come up with—I did the numbers already—2354 ft. So you can already tell it's, you know, significantly—it's it's easily double or more of what you're used to seeing as a pilot when you take off, and this is the reason why you get a checkout because at John Wayne you're used to going down a certain amount of the runway, right, and and rotating and climbing out. When you go to a Mountain Airport, and you're in this case, you will notice you're going to take a lot more of that Runway before you get to the air speed that you're looking for as you start to climb out. It's a big difference, and it's very noticeable when you first do it, which is why you should get a checkout with the CFI, so just to be able to—it's so it's okay to feel uncomfortable because the CFI is with you, and you can more experience what's happening as you're doing it, so they'll keep you safe while you're gone. Whoa, this is wild.

All right, the second thing, of course, is the climb out to clear a 50-foot obstacle. 50 feet is is a good number that they use because at 50 feet you're clear of um, ground effect. I don't know why I had a brain fart there, uh, but clear of ground effect, which is about half the distance of your wing itself, right? So figure half the distance of your wing, let's say 20-ish feet, right? So 50 feet is way past that. So in other words, this will actually tell you whether or not you'll be able to climb after you get out of ground effect, which is why that number is important. So in this case, uh, the 50-foot obstacle is 1880, and uh, we'll go ahead and also add that 220% to it, and you notice that it's at 4,136 ft. That's a huge number. So 4,136 feet—that Airport's only 5,800 ft—so you're like 3/4 of the way down that Runway before you get 50 feet high. I mean, you know, when you first do it, it's kind of puckering; it's like, oh my God, you know, it's crazy because it doesn't feel like you're climbing that much, and that's also the reason why you get a checkout because it—it'll freak you out a little bit um, when you first do it, but it's kind of neat because if you do all the numbers properly, you can enjoy the experience because you know you're doing it safely. So the next thing you want to be able to do, of course, is figure out how well I'm going to climb, which is related to that 50-foot obstacle. So our maximum rate of climb, which is that other piece of information we got from the chart, uh, we'll go ahead and do those calculations. So we'll go ahead and look at the temperatures that we see at the top, you know, on the left hand side we see weight, pressure altitude, right? It's really important for you to recognize pressure altitude, and then of course climb speeds and stuff like that, but so looking all the way over to the right, we see that I've highlighted the due temperatures. In this case, 25 degrees Celsius isn't like a number you could easily interpolate, right? So what do I do? Well, let's—is I'm still going to interpolate because neither one of them's close, so I'm just going to go straight in the middle, which gives me 30, which is still more conservative than 25. So if it's good at 30, it's good at 25. So here's our sea level numbers, uh, as we kind of highlight that sea level which we're supposed to use from the chart. I interpolated 30° C from this, and I come up with 633. So if the normal climb at sea level at 30° C is 633 feet per minute, I'm going to have to now remove 75% of that 633 to tell me what my airplane's really going to climb like, and it comes out to about 362 ft per minute. You think, well, how does that compare to a normal plane? Well, normally you climb between 500 to 800 feet per minute, depending on how new your engine is, so it's just depends on, but typically, you know, 500 to 700 feet per minute is kind of typical of like a Cessna or Piper, it's in, you know, good running order. 362 feet is—if you ever have done that before or do it with a CFI, you'll find out it's not very fast. So as you start to climb, you'll go, I'm not climbing fast enough out of the airport. So uh, this is why it's important for you to make sure that uh, you get this stuff. All right. So if you're a numbers person and you enjoy math—I'm a little math disabled, not really, but you know, in this—I prefer to do it the other ways because it's faster, um, but I have the example.

Here, so you can actually calculate it if you'd like. So I put the same kind of information on this slide, which you'll get, of course, later. Uh, I show you here how to go ahead and compensate. So, uh, first, I remind you that we lose one inch per 1,000 feet. Right? And so in this case, I show one inch per 1,000 feet. We start with standard 29.92. We decrease one inch per 1,000 ft. And, uh, what do we get? Our pressure at Big Bear, of course, is the actual pressure; the raw data number is 23.22. All right. We don't actually use that in aviation, but just so you guys are reminded that that's what actually happens. So, uh, the temperature, of course, this is an actual weather report; these are real numbers, was 17 degrees Celsius that day. Uh, our altimeter setting is 30.26, and our pressure altitude was calculated at 6,412.

So the first thing we're going to look at real briefly is, of course, the temperature. Now, you guys all remember what our standard temperature is at sea level, right? And that number is 15 degrees Celsius. Right? So as we go up each 1,000 feet, you guys already mentioned this, you drop 2 degrees C per 1,000 ft. We're going to round Big Bear to 7,000 because, well, numbers are easy; 7,000 or 7 * 2 gives us 14°. So we take the 15, we minus the 14, and then we get 1. Why do I mention that? Well, it's important because if the standard temperature in Big Bear should be 1 degree Celsius, what is the actual temperature? Well, it's 17; that's massive. So the difference in temperature from what it should be in a standard to 17, 16, I mean, it's huge, right? So you already know that the higher the temperature, the less dense the air, the crappier the performance of your plane, just by looking at it. So see how you did that quick math, and we look at the temperature, and you're like, okay, I truly need to do this calculation.

So, uh, the pressure altitude itself, uh, the reason why the number is lower, like when you set the C's window to 29.92, you can tell that the pressure 30.26 is higher than 29.92. So therefore, there's more pressure, right, on the atmosphere, and more pressure on the atmosphere, the pressure altitude comes out to 6,412. So it'll be lower because the pressure is higher. All right. So, uh, how do we calculate? So we'll start off with, there we go, calculating density altitude. We will start off with, where is it? There it goes, our pressure altitude. So this is how you'd actually calculate that number instead of turning the C's window. You go ahead and calculate it this way. This is where you take your standard altitude, right, 29.92, and then you minus it by whatever number you got on the weather, which is 30.26. The difference between the two, of course, shows you at -0.34. That, of course, gets multiplied by 1,000 in order to give you feet. So how many feet difference is there, and do we have to adjust for? So it's 340, it's a -340 ft. Then we minus that from the altitude, which comes out to 6,412. So that's how we got that number. Right. We know that the ISA, which we've just talked about, 15 - 14 gives us 1° C, and we know that's a bad number, and we go ahead and put it into this formula. So here's the formula, if you like formulas; here's the formula for density altitude, and all you do is plug in the numbers, and you get your answer. The 120 is a constant, so it's always going to be 120; all the other numbers will be adjusted. So we'll go ahead and start off with our pressure altitude that we calculated, then we'll add that constant times the temperature. We'll go ahead and make that adjustment from the standard, right? As we continue down, we made that adjustment, so it's the 16. So we'll do the multiplication, and then we'll go ahead and add these together, which gives us our 8,332 feet. I actually looked at the reported density altitude; it was 8,001. Ours is more conservative, so we'd go with that number. And so what, in the end, what does that mean? What that means is that number 8,332 feet that we calculated is what the airplane thinks you're at when it tries to fly. So if you were to look at the charts and how does this plane perform at 8,332 ft, right, it's going to be a lot worse, right, than sea level. So you have to keep that in mind.

And why does it perform worse? The atmosphere is less dense; less dense means less molecules; less molecules means less lift over the airfoils; less air going into the, I'm sorry, the engine to create the power that you need in order to turn the propeller. But the propeller can only create as much thrust as per the density of the air. So if it's less dense, you get less thrust. And as we kind of talked a little bit about earlier, in the case of an engine, you also need to adjust something else. Does anybody know, that's a pilot in this class, can tell me what you need to adjust to make sure you get maximum power out of the airplane at a at a mountain airport? Mixture. So you want to adjust the mixture of the plane. Why? Why? Because at sea level, everything's fine, right? Because that airplane was designed based upon sea level. But when we're up in the mountains and we don't have any other devices to to compensate for that, like turbochargers and stuff, we're just using a regular old airplane, right? Well, if you keep the mixture full rich, right, where you kind of normally put it when you take off from a sea level airport, um, a certain amount of fuel goes into the the intake or the carburetor to give you the power out of the plane, which is great at sea level. But when you're up in the atmosphere or up higher, like Big Bear, it's less dense. If it's less dense up there, there's less air getting in; less molecules is a better way to put it, getting into the intake manifold. And if there's less air coming in, right, we need to compensate by reducing the amount of fuel because if you keep pouring in the same amount of fuel that you have at sea level, it's too much fuel, and that perfect harmony that you get between the right air fuel mixture doesn't exist. So you have to reduce the amount of fuel that's going in so it matches the air, and the fuel matches where it's supposed to be. And the CFI will show you that uh that technique; it's really simple to be able to do it; full power, you just adjust it, and won't get into the details, but you need to make that air fuel mixture match so you can get as much power out of the aircraft as you can. But remember when you did that, when you start to lean out the engine, reducing the amount of fuel going in to get it perfect, you're also reducing, because you're reducing the fuel, you're reducing the ability of the engine to create power. Since you can't compensate for the air, you must compensate for the amount of fuel going in, and so you're just going to have to accept the fact that your aircraft can now not produce the same amount of power that it does when it's at sea level. So you can already expect you're not going to get as much power; it's going to take a lot longer for your airplane to start going down the runway before enough molecules go over uh the top of the wing to generate lift and there's enough air going over the top of the propeller to create that thrust that you need. So you can see all three of these things together is they're going to impede the the the performance of your aircraft. That's why you calculate this stuff. Here's a nice picture of avoiding a thunderstorm; this is how you do it, airline style; you just go around the thing, right? And a nice beautiful day like this is super easy to do. Anyway, that's it for the luxury.