Even though winter doesn’t seem willing to loosen its grip on much of the country—April 15 saw snowstorms as far south as Charleston, W.Va.—the threat of density altitude lurks just around the corner. (And of course in some places it’s already arrived: A week before these latest snow showers, temperatures in Miami hit 90 degrees Fahrenheit.)
Compared to, say, ground effect, density altitude isn’t the most difficult aerodynamic phenomenon to explain, but it seems to be one of those classroom lessons that many aviators never internalize well. While it’s not directly implicated in a lot of accidents—typically about a dozen a year between airplanes and helicopters—attributing that to consistently rigorous departure planning on the part of GA pilots might be a little optimistic. The margin for error engineered into the aircraft, the relative infrequency of warm-weather operations at high weights, and a healthy dose of luck probably all make their own contributions.
Certainly the more spectacular of those accidents suggest that somewhere along the way the pilots involved lost track of the whole notion of departure planning and the associated performance calculations. The Piper Lance that crashed into an Arizona high school in June 2010 was loaded within its weight and balance limits, but the remaining 200 pounds of payload capacity wasn’t enough to compensate for a density altitude perhaps 1,500 feet above the maximum addressed in its pilot’s operating handbook performance charts. Its pilot had no good reason to assume the airplane could get off the ground at all, and crosswinds gusting 10 knots above its demonstrated component probably didn’t help. As it was, the Lance never succeeded in climbing out of ground effect before it crashed and burned. First responders were held back when several boxes of ammunition on board began going off in the fire.
Other times the devil was in the details. The Cessna P210 that crashed during an attempted takeoff in Idaho in August 2011 was loaded just short of maximum gross weight. Density altitude was a little over 7,000 feet, certainly manageable for a big-bore turbocharged engine—but the pilot chose to take off downwind, and then aggravated that error by attempting a low-altitude turn. The airplane began descending immediately, hit a wingtip, and cartwheeled, killing a family of four. An upwind takeoff with a careful wings-level climb to gain altitude and airspeed might have worked out.
Density altitude accidents are insidious. High elevations make them more likely (as well as adding a separate risk that a pilot from the flatlands will forget to lean the fuel mixture), but the only real criterion is warm weather. Yes, they’ve occurred at Angel Fire, N.M., base elevation 8,379 feet msl, but in recent years fatal accidents have also taken place at Stockbridge, Ga. (elevation 770 feet msl), Leonardtown, Md. (142 feet msl), and Wasilla, Alaska (342 feet msl). The only common factor was higher-than-usual temperatures (which in Wasilla was 57 degrees Fahrenheit).
The fact that most pilots who don’t fly for a living rarely get close to the operating limitations of their machines makes the need for departure planning (and arrival planning) especially difficult to teach. Schools that see a marketing advantage in imposing requirements more stringent than the FAA minima might consider requiring students to calculate density altitude for every solo flight and every cross-country in an effort to instill it as habit. If the result comes close to the aircraft’s performance limits, that’s a good reason to have those students complete the full suite of takeoff and landing distance assessments before beginning any lesson. While your instructors have plenty of motivation to look out for themselves, students who complete their training and move out into the real world of personal flight may not find it quite as natural to keep these factors in mind.