High density altitude’s scary stuff!

Aug 2019 High density altitude’s scary stuff!

High density altitude scares the hell out of me and that’s just the way I like it. When I’m flying outside of my familiar comfort zone and covering new ground (literally) I depend on that strong feeling of potential danger to wake me up and possibly save my butt. 

If you’re from middle America like me, or even further East, density altitude is an often disregarded component to everyday flying. When you learn to fly at an airport with a mean sea level of 386’, and what you’re flying isn’t a total dog, you really never have to concern yourself with density altitude issues. It’s been a very long time since I studied up for the FAA written but admit I’ve occasionally used that knowledge on a few westbound solo cross-countries. For me the reality is “use it or lose it” so before a westbound, high altitude flight, I like a little review. 

So what is density altitude? It’s the air density given as a height above mean sea level. 

I prefer my explanations simple so that translates to me personally in two ways. I always remember that at higher density altitudes my aircraft will have less lift, and reduced engine power. If you understand that, you’ll be where you need to be, ahead of the power curve. 

“As altitude increases, the air’s density decreases. … The thin air at high density altitudes reduces lift, because it exerts less force on your wings; reduces engine power, because there’s less air to mix with the fuel; and reduces thrust because the propeller is less efficient in thin air.” Keep in mind, as the temperature and/or humidity goes up, especially at higher altitudes, the density altitude goes up in addition to the airport’s actual altitude. 

Last week Boyd and I decided to make an impromptu Decathlon flight 1,050 miles to the West. Cody, Wyoming was our flight destination but as luck would have it, Casper was the only location with a rental car available, a car we desperately needed to visit our goal, Yellowstone National Park. Casper was a whopping 5 hour car drive from Yellowstone’s east gate but Casper turned out to be king; a cheap rental car – the ONLY rental car for miles, and it made the decision easy.  I admit we did the one thing I was determined not to do, add additional hours driving time beyond the hundreds of miles we’d surely drive just to see Yellowstone. OK, I’m willing to take a little bad with a lot of good! The more I fly, the less I like road trips. 

Either way, Cody at 5102’ msl and Casper at 5344’ msl were both well outside the density altitude we came from. At our time of arrival the temperature at Casper was 93 degrees with a density altitude of 8404’. Keep in mind that ‘less lift and reduced engine power’ holds true for all high density altitude landings – and take-offs too. 

LANDINGS: When landing at high density altitudes you will have a reduction in engine performance AND a faster ground speed. “As the density of the air decreases a wing needs to fly at a higher true airspeed to maintain the same indicated airspeed. At high density altitude, therefore, a given indicated airspeed equates to a faster ground-speed than it does at sea level (assuming the same wind conditions).” What that means is, don’t land by visual cues, you’ll be too slow. Land using the same indicated airspeeds you would at your home airport, keeping in mind your true airspeed may be as much as 15 mph faster than at sea level. 

DEPARTURE: If a high elevation departure is new to you, do yourself a favor and talk to the locals. When departing at high density altitudes you will likely need to lean your engine for a smooth idle, even during taxi. On take-off, lean the engine for best power. On many aircraft, this is necessary or you will not have enough power to take off. When the airplane comes off the ground, do not climb out of ground effect (one wingspan of height above the ground) until you are at your best-rate-of-climb indicated air speed. Expect a ¼ to ⅓ of what your sea level rate-of-climb would be and a much flatter climb angle. It might take you several miles to even get 500’ higher than your departure point. Pre-plan your obstacle clearances for departure with the knowledge that your climb gradient is going to be very shallow.