Transcription
[Music] Density altitude: what causes it and how does it affect your airplane? Let's jump right in.
Heat, humidity, and altitude all affect air density and therefore increase density altitude. Heat causes air molecules to have a lot of energy and move farther apart. Water molecules, as in humidity, displace air molecules, and a lower atmospheric pressure (found at altitude) causes air molecules to spread farther apart. All of this has a major impact on how your aircraft performs, as the wings, engine, and propeller struggle to grip the thinner air.
Dealing with high-density altitude is commonplace for pilots who often fly in hot weather, high humidity, or higher elevations, but for many of us, it can be easily overlooked with profound implications. So pay attention to density altitude weather reported during flight planning via the ATIS or by ATC, and use your POH and an E6B to calculate density altitude and aircraft performance.
Now, what can you do about it? Hmm. Here are a few tips:
Fly light to avoid max gross weight and high-density altitude ramifications. When dealing with an airplane already performing differently than you're used to, determine proper fuel/air mixture. If your checklist suggests SFIL rich mixture at sea level, don't assume the same setting at higher density altitudes. Engines differ, so refer to your airplane's POH or the engine's digital monitor or fuel flow gauge manual for optimum power. For a quick and easy method, advance the throttle to takeoff power during ground run-up and begin reducing or leaning the mixture until the RPM increases and peaks—that's your optimum setting. If you lean beyond this point, the RPM begins to decrease, and eventually, the engine will sputter as it is starved for fuel. Similarly, before joining the traffic pattern, determine the best mixture for optimum power during landing or go-around by applying full power, then leaning until RPM peaks.
Fly indicated airspeeds. If rotation speed is 55 knots, don't try to rotate at 40 knots because you're eating up more runway than usual. Likewise, on landing, the ground and near objects will pass by more quickly—that's correct; your ground speed is faster for the equivalent airspeed at higher density altitudes. When the airplane needs more space to get the required air molecules to fly, use long runways. But remember the abort point rule: reach at least 70% of your rotation speed at the runway's midpoint or abort. For example, a Cessna 172 with a rotation speed of 55 knots departs a 3000-foot runway; if the airplane's not at 38 knots by 1500 feet, abort.
Watch airspeed and trends and maneuver carefully on climb-out and during cruise, as your airplane isn't going to accelerate rapidly nor climb effectively. On takeoff, get airborne and then lower the pitch angle to accelerate and use ground effect for the remaining available runway. As you approach the runway end, begin a gradual climb and pitch to climb speed. Chair fly this a few times so you're comfortable with the procedure. Be aware that pitch or bank changes will result in a quicker deceleration than at sea level, as airspeed bleeds off faster at higher density altitudes. Aircraft responds to flap retraction; especially early flap retraction will be more exaggerated than at lower density altitudes. Since the engine is less powerful, you can't regain speed and lift quickly. So monitor POH speeds and positive climb indications carefully for the same reasons. Be extremely attentive to flying into canyons and close to mountains or obstacles and do not rely on visual cues you'd normally depend on to clear terrain because your climb path at higher density altitude is much shallower than expected.
Fly early in the day when temperatures are cooler, so performance will be better. We're all well aware that aviation is terribly unforgiving of carelessness or neglect, so let's pay attention to density altitude and its effect on airframe, engine, and propeller performance. Think through your density altitude procedures and never just hope you'll be fine.
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