Dug this out of a book titled "Helicopter test and evaluation":-
Avoid area testing
The final stage of a power-o
ff assessment programme is to define the avoid curve, avoid
area or height/velocity diagram. An example of the type of presentation commonly
seen in aircrew manuals is shown in Fig. 7.13. An avoid area diagram shows the
operational pilot the combinations of airspeed and height above the surface from
which a total loss of power is unlikely to be survivable. Armed with this information
the pilot can plan the aircraft’s flight path to minimize the time spent at risk inside
the area. A number of assumptions are normally made in the construction of height/
velocity diagrams:
Ω
The aircraft is in level flight or the hover at the moment of power loss.
Ω
There is a total intervention delay time of 2 seconds.
Ω
There is no wind and the surface is suitable for a landing.