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Why Is it Harder to Fly a Helicopter at High Altitudes?

On May 14, 2005, Didier Desalle reached the 29,035-foot peak of Mount Everest. Unlike the nearly 700 climbers who have conquered the summit on foot, however, Desalle rocked the aviation world because he got there by helicopter -- the first ever to do so. High altitude flight has always been a limitation of rotary wing craft. While the altitude envelope is continually being adjusted upward, choppers are still cramped under a lower ceiling than fixed wing planes. Three factors influence the performance of helicopters at high altitudes.

How High Is Too High?
- The altitude at which a helicopter can no longer sustain a climb rate of at least 100 feet per minute is defined as its "operational ceiling." When the rate of climb declines to zero, "absolute ceiling" has been reached. Both operational and absolute ceiling of helicopters vary by model. Currently, 25,000 feet is generally the upper limit for advanced, turbo-jet helicopters in translational flight (forward motion) and under the most favorable conditions. Static hovering, which requires more raw lift than transitional flight, is limited to 15,000 to 18,000 feet.
Thinner Air
- Lift produced by the rotating blades of a helicopter is directly related to the density of the air. As altitude increases, air density decreases. Thin air provides the blades less "grab," thus reducing lift. This, in turn, demands more power output from the engine to compensate for the decreased lift.
Decreased Power
- As air density declines at high altitudes, engine power output also depletes due to the decreased oxygen content of thin air. In piston-driven engines, the decline in power occurs at a relatively low altitude. Gas turbo-jet engines are capable of higher altitudes before the combined tipping points of reduced lift and declining engine output impose operational then absolute ceiling.
High Winds
- Extreme winds are also a factor at high altitudes. With lower forward speeds than fixed wing aircraft, rotary wing craft are more vulnerable to powerful wind shifts. This is even more pronounced when attempting to hover. The longstanding difficulties in executing high-altitude helicopter rescues from mountain peaks above 25,000 feet are as much a result of winds at those elevations as low air density.

Aviation