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The ''avoid'' areas in a Height-Velocity diagram define the height-speed combinations…
  • A

    In which it is not possible to hover out of ground effect.

  • B
    From which it is not possible to operate because of engine power limitations.
  • C
    In which it is only possible to make a transition into forward flight.
  • D
    From which it is not possible to make a safe autorotative landing.

Refer to figure.
The various undesirable combinations of height and speed are assembled into an "Avoid Area" diagram, sometimes called "H-V curve", and describes the absolute minimum acceptable combinations of height and speed for a safe Engine-Off Landing (a safe aurotative landing in a single-engine helicopter case), otherwise a successful recovery from anywhere near the avoid area will require a high level of skill.

1. At low speeds, a helicopter must accelerate to the minimum rate of descent speed, before a safe Engine-off Landing can be made, entailing a considerable loss of height.

2. If the helicopter is hovering close to the ground, the energy stored in the rotor can be used to cushion the touchdown after an engine failure, without entering autorotation. However, there is a maximum height from which such a landing can safely be made.

3. If flying at high air speed close to the ground, the helicopter will descend after an engine failure during the pilot reaction time. If the helicopter contacts the ground, when the ground speed is above that for a safe run-on, the result is likely to be a crash or at best some significant damage.

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