I live in low country. My airport is a measly 15 feet above sea level, not low enough to be subject to flooding in heavy storms, but plenty low. You’d think that means that my airplane sees peak performance on takeoff all the time. Well, maybe on a standard atmosphere day (59 degrees F. / 15 degrees C. and 29.92 Hg / 1012 hPa, with 50% humidity). Unfortunately its been a while since we’ve seen those temperatures.
Most summer mornings I awake at 0600 to an outside air temperature of 77 F / 26 C, barometric pressure around 29.80 and the humidity? At 96% you might as well breathe water as air. What does that do to my aircraft’s performance? Skunks it. Seriously, density altitude does a number on my high performance machine—and just about everyone else’s, too. (I say “just about” because a few of you out there fly turbocharged or turbo-normalized aircraft, and they cope better with density altitude, up to a point.)
With temperatures and humidity like we have, my airplane performs more like it is at 1,600 MSL, and that is first thing in the morning. It doesn’t take long for the sun to cook up the air to temperatures approaching 100 F / 38 C, more than doubling the density altitude effect and making it perform as if we were above 3,000 MSL.
This is because hot, wet air behaves precisely like the thin air at altitude: it is tough for the engine to breathe (heck, I find it tough to breathe!). With molecules of air farther apart and separated by H2O, even the flying surfaces are impacted, resulting in longer take off rolls and anemic climb out performance.
How does one cope? First: calculate. Density altitude calculators are built-in to every flight planning software package worth its salt (even NOAA publishes one here). Don’t have one of those handy? Call the AWOS or ATIS at your airport (they have local telephone numbers, you can find them here). If you are already at the airplane well, just tune in and listen. No weather reporting at your airport? Just about every airplane has an outside temperature gauge somewhere (your oil temperature will be at ambient temperature before engine start). As for the humidity? If you are sweating on the ramp and you aren’t in Yuma, Arizona, you can guess it is well above 50%. Just figure it in at 90% and you’ll be safe.
Now, pull out the performance charts from the Pilots Operating Handbook that you keep in the airplane (you’ve got to have one, it’s the law). Run your finger across the chart per the instructions and it should spit out a required takeoff roll distance. Is your runway long enough? The climb chart will predict your performance—then you have to ask yourself: is that climb rate adequate (remember, you’ll be going up into even thinner air, so that initial climb performance is likely to deteriorate)?
In my corner of the country side most significant obstacles can be out climbed in the first 300 feet of altitude. Even at 300 fpm climb rate if I’m patient I can get away from terrafirma on a hot afternoon—that is, if I haven’t packed the back of the airplane with passengers and bags to the ceiling then loaded it up with full fuel. These are all variables I can change. I could also opt out of the flight, rescheduling it for a cooler time of day.
And what if I risk it and try the take off? Look at the NTSB database (www.ntsb.gov). Search density altitude and you’ll find a host of general aviation accidents where high DA is listed as “probable cause.” Many occurred in the summer, often from high elevation airports. Nearly every time the aircraft was overloaded for the conditions and was forced into the air by the pilot. He or she managed to get it flying in the cushion of ground effect, but once the airplane pulled away from that crutch it was stall / spin time. A few pilots managed to resist the overwhelming compulsion to pull harder on the control yoke and did the right thing, which is to PUSH the nose over to a flying airspeed and ride the airplane back into ground effect and onto the ground in a controlled crash. Not pretty, but survivable.
A few years ago I had my first experience in a Redbird simulator. The instructor with me set up the hot/high demo, where I attempted to fly a Cessna 172 off a mountain valley grass strip on a hot afternoon with no wind. Honestly? It was awful. Even with my best short field technique I felt the airplane sinking as I pulled away from ground effect and I instinctively pushed the nose over, pulled the power and jockeyed the airplane to a landing and a ground loop to avoid the trees at the end of the strip. Not pretty at all, but we did manage to keep life and hardware intact.
With practice I learned what that Cessna could and could not do on that little back country strip. It was an education. Want to try it for yourself? Redbird is partnering with the National Association of Flight Instructors and the Experimental Aircraft Association at EAA AirVenture this summer to bring attendees the Pilot Proficiency Center. There you can sign up to brush the rust off or try skills you’ve yet to master in one of 12 Redbird LD simulators (Advanced Aviation Training Devices). The building is air conditioned and the sim instructors are volunteers. And yes, you can log it as flight sim time with an instructor sign off.
Give it your best. You’ll impress yourself.