It’s a mistake to think that challenging density altitude requires, well, altitude. Higher base elevations provide a head start, but the essential ingredient is heat. Don’t believe it? The Furnace Creek Airport in Death Valley National Park, California, lies 210 feet below mean sea level—but on a typical 125-degree summer day, the density altitude can hit 4,400 feet. That’s 100 feet higher than on a winter day in Denver.
Pilots based at fields whose density altitudes rarely exceed 3,000 feet may not feel much need to worry about it. That’s easier to get away with when the runways are long and the departures unobstructed. Put an aircraft loaded close to maximum gross weight on a short strip with close-in obstacles, on the other hand, and a couple of thousand feet of density altitude can be the difference between a takeoff that’s uncomfortably tense and one that’s flat-out impossible. Even when it’s hypothetically feasible, attaining the necessary climb performance requires correct aircraft configuration and exceptionally fine airspeed control.
Case in point: The Cessna 206 floatplane whose pilot tried to make a water takeoff in Florida with four passengers in the middle of an August afternoon. Actual elevation was essentially zero but the density altitude was 2,300 feet, and the airplane might have been slightly overweight (investigators estimated 55 pounds). He could only coax it up to 400 feet before it could no longer maintain altitude, at which point he was also too far away to return for a water landing. The Cessna descended into treetops instead. The engine produced full power afterward, and its performance charts suggested it should have been able to climb at 600 feet per minute—at 65 knots in a clean configuration.
Overweight departures are sometimes also attempted a little too casually, but density altitude eats up whatever margin for error is built into the authorized maximum gross weight. Ocean Springs, Mississippi, also lies at sea level, and the density altitude one March afternoon was “only” 1,135 feet. Nevertheless, a 4,400-hour ATP found out the hard way that his Cherokee 140 wouldn’t climb with three passengers on board. Both wings were torn off in a forced landing on a residential street a half-mile from the airport. Turns out the Cherokee was “only” 100 pounds overweight. That was enough.
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