Alright, fellow aviators, we’re about to unravel the secrets of density altitude.
Longer Takeoff Rolls and Climb Rate Woes
You probably know that density altitude is the enemy when it comes to airplanes. When it creeps up, your takeoff roll turns into a marathon, and your climb rate is doing its best impression of a sloth on a Monday morning.
But wait, there’s a lot more to it. You may also know that you should still rotate and climb at the same value on your trusty airspeed indicator, but what you might not realize is that you might notice the ground whizzing by faster than a caffeinated squirrel. Your lizard brain’s screaming, “Lift off, buddy!”
Indicated Airspeed vs. True Airspeed
Picture this: You’re cruising at sea level, sunglasses on, wind in your hair. Your trusty airspeed indicator says, “Hey, buddy, we’re doing 80 knots!” Cool, right? But guess what? When it reads 80 knots at 6000 feet and 85 degrees Fahrenheit, you’re actually cruising along at 88 knots true.
So, when you’re itching to rotate, remember: The ground’s in a hurry, but your wings are all about the air molecules. Indicated airspeed is what matters. For landing, you will also be tempted to pull back more and more when you see the ground whizzing by. Indicated airspeed is what matters.
Vy, the Sneaky Chameleon
Vy is our climb speed sweetheart. If you want to get as high as possible as fast as possible (and hey, who doesn’t?), that’s the way to go. But here’s the plot twist: At higher density altitude, two things happen. First of all, Vy changes. That number you memorized is actually only good for a specific weight and specific altitude. Fortunately it doesn’t drop by all that much, so you can maybe get away with pretending it’s just that single magic number.
But one other thing also happens at higher density altitudes. You’ve probably learned to pitch for airspeed, at least I hope you have! Well, the same pitch attitude that you might have etched in your brain for a typical Vy climb won’t produce the same airspeed when you use it at higher density altitude. Your lizard brain may be fighting with you again, but you need to pay attention to that airspeed indicator and recalibrate your brain to the new pitch attitude that will give you this magic speed at the higher density altitudes.
Lean Engines and Surprise Go-Arounds
One last item to consider. If you’re not used to leaning the engine for takeoff, you might be in for an even worse takeoff — or possibly not be able to take off at all! The engine simply won’t produce as much power at higher density altitudes if you leave it fully rich. Yes, it can be significant. Fire up your favorite simulator and try it out.
Just as importantly, if you’re a “full rich” pre-landing enthusiast, brace yourself. Go-arounds will get your attention, and not in a good way. Lean the mixture for landing as you did for takeoff, despite what your mental checklist tells you.
So there you have it. Density altitude: Next time you’re up there, wink at the air molecules, adjust that pitch, and lean like a pro. Happy flying!
Rob Montgomery