FAA Private Pilot Helicopter (PRH)Performance and LimitationsMedium
A performance chart shows that a helicopter's maximum torque available decreases as pressure altitude and temperature increase. This chart is illustrating the effect of
- Atranslational lift on rotor efficiency
- Bdensity altitude on engine power output
- Cretreating blade stall on maximum airspeed
- Dgross weight on autorotative descent rate
Show answer & explanationAnswer & explanation
Correct answer: B. density altitude on engine power output
Engine power output depends on the mass of air available for combustion; as pressure altitude and temperature increase (raising density altitude), the air becomes less dense, reducing the mass airflow into the engine and lowering maximum available torque/power.
Why the other options are wrong
- A. Translational lift concerns rotor airflow in forward flight, not engine power output charts.
- C. Retreating blade stall relates to airspeed limits, not torque available versus altitude/temperature.
- D. Gross weight affects autorotation descent rate but is unrelated to torque-available charts.
Density Altitude Effect on Engine Power
As pressure altitude and temperature increase, air density decreases, reducing the mass of air available for combustion and lowering maximum engine power output.
- Higher pressure altitude reduces available oxygen for combustion
- Higher temperature further reduces air density
- Torque-available charts are used to verify sufficient power margin before flight
Memory trick: Thin, hot air starves the engine of the oxygen it needs.