18 / 20

Which of the following ambient conditions may result in the highest fuel consumption during a hover taxi?

  • A

    High density altitude, High temperature, High humidity.

  • B

    Low density altitude, High temperature, Low humidity.

  • C

    Low density altitude, Low temperature, High humidity.

  • D

    High density altitude, Low temperature, Low humidity.

Helicopter's performance depends on air density, which varies with:

  • Temperature
  • Altitude
  • Humidity

1. Density Altitude is the pressure altitude corrected for non-standard temperature, the ISA deviation. In other words, it is the altitude in ISA, which gives the same air density as the prevailing non-ISA combination of temperature - pressure altitude and is used to establish helicopter's performance, as it is a figure that expresses where your engine "thinks" it is, as opposed to where it actually is. As density altitude increases, the air density reduces. This causes the thrust and therefore the output power or performance to decrease at all phases of flight.

2. Water vapour is important because it is invisible, and affects the humidity ratio of the air. Adding water vapour reduces air density because its molecular weight is lower. So, as humidity increases, the air density reduces. This causes the thrust and therefore the power output or performance to decrease at all phases of flight.

3. As ambient temperature increases, the air density reduces. This causes the thrust and therefore the output power or performance to decrease at all phases of flight.

Thus, the more the density of the air decreases, the higher the aircraft "thinks" it is and the less efficiently the engine and rotors will perform. As a result, the aircraft will consume more fuel, since more power will be required.

Your Notes (not visible to others)



This question has appeared on the real examination, you can find the related countries below.