14 CFR § 29.1517
Limiting height-speed envelope
July 13, 2020
For Category A rotorcraft, if a range of heights exists at any speed, including zero, within which it is not possible to make a safe landing following power failure, the range of heights and its variation with forward speed must be established, together with any other pertinent information, such as the kind of landing surface.
[Amdt. 29-21, 48 FR 4391, Jan. 31, 1983]