Transcription
Hey, guys. John from Fly8MA.com. And today, we're here to look at the inner workings of an altimeter. Basically, what is happening as we twist this little knob around and change the setting on our Kollsman window there? And what makes these needles or these hands spin around and indicate our altitude as the aircraft ascends or descends? (Instrumental, upbeat music.)
So, for starters, let's go ahead and disassemble our instrument here. If we look at the back side, that little hole there is where this sealed instrument, when it was sealed before we cut it all apart, had a static line going to the back of it, which went to our static port on the outside of the aircraft, which measured ambient air pressure, static pressure. So as we take that apart and take the case off here, we get to see the inner workings.
And the first thing I'll show you guys is basically what's happening when we twist this little knob here and change that setting in our Kollsman window, for the local altimeter setting. Well, we know that when we move it about a tenth of an inch, we should see about a 100-foot change in altitude like that. We use about a tenth of an inch for 100 feet, or a general rule of thumb would be one inch for a thousand feet. And so as we're turning this little Kollsman knob here, or changing our setting in our Kollsman window, not only are we moving this little round display there, we're also twisting the entire assembly of the instrument inside the casing. And by twisting that entire assembly of the instrument, what we're doing is moving the hands that are mounted to that instrument. So as we twist that whole assembly backwards, you can see the hand moving backwards. Or we twist the whole assembly forward, the hand moves forward. And of course, as the hundredths-hand moves, it moves the thousandths-hand, and the ten-thousands’ hands, ever so slightly.
So now let's look at what actually makes up the altimeter. It's a pretty simple-looking instrument in here, but very sensitive with some very delicate parts. First off, we're talking about ambient air pressure. So we know that the pressure is less up high. And so as we ascend, there's less pressure. We have these two discs here that are sealed with gas inside them. And that gas is going to expand as the ambient air pressure, as the aircraft ascends, decreases. As the ambient air pressure is decreasing, these discs are expanding and applying pressure through this little armature here, which then moves our hands to indicate a higher altitude. We call these two little discs Aneroid wafers, and you can see how little this armature moves to go from zero all the way up to 15,000 feet when it was barely an eighth of an inch. So it's a very sensitive, delicate instrument. And you can see how the vibration of the engine over time is going to affect this negatively.
And that's why we accept variations in the altimeter indication, say when we have an altimeter setting of two, niner, niner, zero, and we have that set up there and we read, okay, we're at about 450 feet here and field elevation is actually 400. Well, we still accept that as a normal indication because it's within plus or minus 75 feet of field elevation. We know that this thing is really delicate. And to get it to indicate 50 feet within field elevation is still acceptable, pretty good. So these two very delicate Aneroid wafers will expand as the aircraft ascends, push on that little armature there, and give you a higher reading. And you can see as this armature is pushed on what it's really doing, it's turning these gears inside there. You can see that little guy down inside there, with some teeth on him. That spins, those gears down inside there, which are all geared at different ratios to, of course, move your hundredth’s hand faster than your thousands hand, faster than your ten thousandths hand.
And this particular altimeter is really only good up to about 20,000 feet. And without forcing it, I get up to about 15,000 feet there or so. So there's 15,000 as I ease up there. 14,000. 13, 12, 11. In fact, 9, 8, 7, 6. And I’ll just let go of it. So a very delicate instrument. And obviously, we would never treat one like this if we were actually going to reuse it. However, this altimeter has seen the extent of its life. But hopefully, that gives you some good insight as to what's actually happening when you twist this little knob here. A lot of people, even some CFIs, out there, don't know what is happening when you're twisting this little knob. And in fact, this entire assembly is rotating inside there to change the indication of your 1/100’s hand, your 1/1,000’s hand, and your 1/10,000’s hand.
Also, something interesting to note here is that we have this little checker pattern here, that little stripe pattern that will kind of catch your attention to let you know that, yeah, that's zero feet there. So literally zero feet MSL is what that's indicating. And then although it looks very similar, that is 10,000 feet MSL and it looks like zero feet again. But notice the 1/10,000’s hand is over at the ten. And part of that stripe pattern is covered up and altitude is tilted, kind of giving you some clues that, hey, you're not at zero feet, you're at 10,000 now. And that's obvious when you're in the aircraft, not, you know, misread your altitude by 10,000 feet. But sometimes we'll get people on the written exam. So we definitely want to keep an eye out when they depict our pictures of altimeters on your written exam. They could easily give you, say, 12,000 feet. And it may look like 2,000 unless you're catching the fact that the 1/10,000’s hand is over to the right. And this is in fact 12,000 feet indicated.
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