Density altitude isn’t the most difficult aerodynamic phenomenon to explain (we might nominate ground effect), but it seems to be one of those concepts that not everyone internalizes well. True, it’s not directly implicated in a lot of accidents—typically about a dozen a year in airplanes and helicopters combined—but attributing that to consistently rigorous departure planning might be optimistic. Certainly, the most spectacular of those accidents suggest that the whole notion of departure planning and performance calculations got lost along the way.
The Piper Lance that crashed into an Arizona high school, for example, was loaded within its weight and balance limits, but the remaining 200 pounds of payload capacity didn’t compensate for density altitude 1,500 feet above the maximum addressed in its performance charts.
There was no good reason to assume the airplane could get off the ground at all, and crosswinds 10 knots above its demonstrated component probably didn’t help. As it was, the Lance never climbed out of ground effect before it crashed and burned—after which several boxes of ammunition on board began going off in the fire.
Other times the devil was in the details. The Cessna P210 that went down in Idaho in August 2011 was loaded just short of maximum gross. Density altitude was a little over 7,000 feet, manageable for a turbocharged big-bore engine—but the pilot chose to take off downwind, then attempted a low-altitude turn. The airplane began descending immediately, hit a wing tip, and cartwheeled, killing the family of four. An upwind takeoff with a careful wings-level climb to gain altitude and airspeed might just have worked.
Density altitude accidents aren’t restricted to heavy weights or high elevations (though the latter do add the additional risk that flatland pilots will forget to lean the mixture). The only real requirement is warm weather. Yes, they’ve occurred at Angel Fire, New Mexico, base elevation 8,379 msl, but recent years have also seen fatal accidents at Stockbridge, Georgia (770 feet msl) and Leonardtown, Maryland (142 feet msl). The common factor was higher-than-usual temperatures.
Many of us who don’t fly for a living rarely come close to the operating limitations of our machines, making the need for departure (and arrival) planning easy to forget. The time to remember is not while mushing along in ground effect, watching the trees rush toward you.
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