Density AltitudeÖ don't be dense

George Gershwin (1898 - 1937) could have had aviation in mind composing 'Summer time'. The phrase in the lyrics 'Then you'll spread your wings and take to the sky. But till that morning there's nothing can harm you,' seems ominous. What can harm us in summer? Here's a clue. Joe Soap returns from a site visit in a six-seat twin engine aircraft. On final approach the same indicated airspeed is maintained as always, but conditions are bumpier and sweat is running down his greying temples. He is maintaining the three degrees glide-slope. Over the runway threshold he neatly reduces from approach to idle power. Yet, he 'inexplicably' somehow finds himself farther along the runway than usual. To avoid missing the standard 'first turn-off', or perhaps overrunning the opposite threshold, he checks forward on the control column to keep the aircraft on the groundÖ and promptly steps on the brakes.

If Joe and his entourage are lucky enough, his brakes will need replacing a little sooner and the discs need skimming. Often the aircraft will tend to 'porpoise' and one or both props hit the surface. Joe will be battling with lateral control and one brake - usually on the turn-off side - will be locked. The side-forces may result in the gear collapsing and a 'double prop-strike' as one or both trunnions gives way.

The key to the 'riddle' lies in the higher true airspeed (TAS) and hence ground speed corresponding with the same indicated airspeed (IAS) in high ambient temperatures. Overall the aircraft will stall at the same indicated airspeed at the same weight and wing loading and hence the approach reference speed or 'Vref' of 1.3 times the stalling speed in the landing configuration, will be the same. The airspeed indicator is not concerned with all this. The lifting surfaces will need the same amount of air particles passing over and under to generate the same amount of lifting force. In heated air the particles are further apart and hence the aircraft will need to run farther through the air - comprehendo?

As private and weekend flyers we sometimes do not pay sufficient attention to the effects of outside air temperatures. Looking at prevailing weather forecasts one might readily expect to depart at typically 15 degrees Centigrade in the morning to return at a searing 35. Most flyers know that one degree Centigrade translates to an extra 120 feet. At first glance the change seems insignificant. But in airlines the magnitude of payload left on the ground in terms of 'reduced take-off' weights in the name of safety, severely affect profit margins. In the commercial and private environment the amount of forfeited take-off power is even more incredible.

A take-off power graph (4-14) for King Air 200 with PT6A-41 engines reveals at 4000 pressure altitude at 2000 r.p.m. 92 percent engine torque is available at 15 degrees Centigrade. At 25 degrees only 87 percent can be obtained and at 35 degrees a mere 79 percent. The crunch comes when considering single engine climb performance reduction. Graph 4-29 at 3966' pressure altitude and 12, 150 lb take-off weight shows the climb gradient reducing from three percent at 15 degrees, to a mere 1.1 percent at 35 degrees. That can wipe out all aboard! As temperatures soar, performance and safety margins deteriorate. Don't be dense when it comes to Density Altitudes. Fly safely!

Johan Lottering - Focused Flying
Aviation Safety

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