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

**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) =
- 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) =
- 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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