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The Russian Mil Mi-17 helicopters have a dust protection device fitted at engine intake (see the rounded part in front of the intake): Mi-17 with dust protection highlighted

Its checklist gives an altitude limitation from sea level to 3000m AMSL. Why is the Mi-17 dust protection device not permitted to be used above 3000m?

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Anything that blocks or restricts flow to the engines will hamper performance.

At low altitude, the aircraft has sufficient performance margin that this is acceptable. However, engine performance drops as altitude increases.

They faced the decision:

  1. Have no dust protection system -- the aircraft will have the best performance at all altitudes, but will not be able to operate in dusty environments without damaging the engines.

  2. Always have the dust protection system -- operate in dusty environment, but suffer the performance penalty and limit the aircraft's ceiling and altitude performance.

  3. Make the dust protection system removable and placard the aircraft that it shall not be operated at high altitude when the dust protection is installed.

To maximize operational flexibility, they went with 3).

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