Density altitude is the reason airplane accidents tend to cluster around mountain resorts during the summer months.

Piloting an aircraft well is a game of numbers: weight and balance, angle and rate of climb, airspeed, power management, heading, rate of descent—each of these items is numerical, absolute, and essential to maintaining safe flight and insuring an uneventful landing. One of the most overlooked elements in aviation however, particularly among general aviation pilots whose experience is often less than that of professionals, is the concept of density altitude or pressure altitude corrected for non-standard temperature.

When an aircraft is designed and flown, its performance figures are always dependent upon the relationship between pressure altitude (a standard datum based on 29.92 inches of mercury at sea level) and air density, which is a figure that changes according to the temperature. Under conditions higher or lower than standard, airplane performance cannot be determined directly from the altimeter, that instrument whose needle seems to unwind at an alarming rate in all movie depictions of an airplane crash.

Every aircraft has an absolute service ceiling, a limit beyond which it will not climb. This number--importantly—is not measured above the ground, or above an obstacle sticking out of the ground, but rather is determined by the density of the air through which the aircraft’s wing passes. As the craft climbs higher and higher, atmospheric pressure becomes lower and lower. Interestingly, half of the earth’s atmosphere is within the first 18,000 feet. Atmospheric pressure at sea level is about 30 inches of mercury. At 18, 000 feet it’s about 15 inches.

Furthermore hot air is less dense than cold air, so a sea level airport on a hot day isn’t at sea level at all, so far as the aircraft’s performance is concerned. On a 100 degree day in summer, the Santa Monica Airport—located slightly higher than sea level in sunny Southern California--has a density altitude of 3000 feet. The sea level performance of an aircraft has deteriorated almost 20 per cent without it's even leaving the ground. A mountain airport, say 5000 feet above sea level, may very well be at 8000 feet, so far as the aircraft’s performance is concerned. Put a 150 horsepower light plane with a service ceiling of 15,000 feet on a 10,000 foot-high mountain runway on a mere 80 degree day and...the plane won't fly. At all. Period.

We’re speaking here in terms of lift—how efficient the wing is working at higher than normal temperatures. The plane’s engine is also adversely affected on "non-standard" days. It too thinks the machine is higher than it really is. The engine runs hotter because cooling air is hotter. Vapor lock can occur, for example, should fuel booster pumps be forgotten on takeoff.

Cold air is a problem too. Though the aircraft flies beautifully, since cold air is denser and the wing has more lift, the plane’s altimeter reads higher than the airplane actually is. The danger is obvious in the mountains in winter.

About the worst feeling an airplane pilot can have is the inability to climb. To see the trees at the end of the runway remain in the same place in your windshield as you approach them is the stuff of nightmares.

Density altitude will kill the pilot who has forgotten his physics.

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