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Given the following information, which statement is correct regarding a helicopter filming operation of a bicycle race in three different locations?

1. On day 1, the Pilot notices a hover power required of 90% (with maximum available 100%) at a field with Pressure altitude 1 000 ft AMSL and OAT +10°C
2. On day 2, the helicopter is located at a field with Pressure altitude 1 000 ft and OAT +20°C
3. On day 3, the helicopter is located at a field with Pressure altitude 5 000 ft and OAT +20°C

  • A

    The hover power required is highest on Day 1.

  • B

    If hover power is available on Day 2, it will certainly be available on Day 3, as well.

  • C

    The hover power required is lowest on Day 2.

  • D

    The hover power available might be insufficient on Day 3.

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.

  • Density altitude = Pressure altitude + (120 ft x ISA deviation)

1. On Day 1:

  • ISA deviation = OAT - ISA temperature at 1 000 ft = +10°C - (15°C - 2°C x 1 000 ft/1 000) = -3°C.
  • Density altitude = 1 000 ft + 120 ft x (-3) = 640 ft.

2. On Day 2:

  • ISA deviation = OAT - ISA temperature at 1 000 ft = +20°C - (15°C - 2°C x 1 000 ft/1 000) = +7°C.
  • Density altitude = 1 000 ft + 120 ft x (+7) = 1 840 ft.

3. On Day 3:

  • ISA deviation = OAT - ISA temperature at 5 000 ft = +20°C - (15°C - 2°C x 5 000 ft/1 000) = +15°C.
  • Density altitude = 1 000 ft + 120 ft x (+15) = 2 800 ft.

Therefore, on Day 3 the helicopter will have the most degraded performance.

The performance of a helicopter will lie in the relationship between the power available and the power required. The greater the difference between them the greater the margin of power. A surplus of power available over the power required exists over the greater part of the speed range.

For a helicopter, the power available is considered to be the power which is available to the rotor and not that which is available from the rotor. For any given altitude, this power will remain more or less constant and it, therefore, appears on the power graph as a straight line.

The power available depends on the air density. More specifically, is affected by the:

  • Pressure Altitude
  • Temperature
  • Humidity

Flying at a high pressure altitude combined with a high temperature, then the power availble will decrease and the power required will increase (decreased power margin) and for the forementioned conditions the hover power available might be insufficient on Day 3.

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