According to CS 29.87 Height-Velocity Envelope:

(a) If there is any combination of height and forward velocity (including hover) under which a safe landing cannot be made after failure of the critical engine and with the remaining engines (where applicable) operating within approved limits, a height-velocity envelope must be established for:

(1) All combinations of pressure altitude and ambient temperature for which take-off and landing are approved; and

(2) Weight, from the maximum weight (at sea-level) to the highest weight approved for take-off and landing at each altitude. For helicopters, this weight need not exceed the highest weight allowing hovering out of ground effect at each altitude.

(b) For single engine or multi-engine rotorcraft that do not meet the Category A engine isolation requirements, the height-velocity envelope for complete power failure must be established.

• For a multi-engine helicopter that in the event of a single engine failure during take-off, landing or other operations near the surface, a combination of airspeed and height above the ground from which a safe single engine landing on a smooth, level surface cannot be assured.
• For a single engine helicopter it defines a combination of airspeed and height above the ground from which the helicopter will probably not be able to enter autorotation and a safe landing will not be assured.