Why does a rescue helicopter have a difficult time flying at altitudes over 6000 meter?

Answer 1

Helicopters in general do not do well at high altitudes. This is because of the lower air density at higher altitudes. Jet airplanes can fly at these heights only because they are going very fast. Helicopters are designed specifically for slow (or hovering) flight, and so have difficulty at high altitudes.

On first examination, one would think that you could overcome this problem by simply designing a helicopter with a faster-spinning rotor, but as helicopter rotors get to a certain speed, their lifting capability becomes spoiled by their own wake and things tend to get unstable. Many helicopters overcome the altitude limitations by simply having a larger diameter rotor to produce more lift from the thinning air, however the larger size also limits the helicopters usability as a rescue platform.

Additionally, rescue helicopters are usually equipped with a large amount of heavy medical equipment which further reduces their performance, helicopters which are specifically used for mountain rescue will be much lighter equipped. However it is often the case that an air-ambulance or other rescue helicopter has been co-opted into a mountain search and rescue and may find themselves overequipped/overweight for the job.