14 CFR § 27.1505

The never-exceed speed, VNE, must be established so that it is—

Not less than 40 knots (CAS); and

Not more than the lesser of—

0.9 times the maximum forward speeds established under § 27.309;

0.9 times the maximum speed shown under §§ 27.251 and 27.629; or

0.9 times the maximum speed substantiated for advancing blade tip mach number effects.

VNE may vary with altitude, r.p.m., temperature, and weight, if—

No more than two of these variables (or no more than two instruments integrating more than one of these variables) are used at one time; and

The ranges of these variables (or of the indications on instruments integrating more than one of these variables) are large enough to allow an operationally practical and safe variation of VNE.

For helicopters, a stabilized power-off VNE denoted as VNE (power-off) may be established at a speed less than VNE established pursuant to paragraph (a) of this section, if the following conditions are met:

VNE (power-off) is not less than a speed midway between the power-on VNE and the speed used in meeting the requirements of—

§ 27.65(b) for single engine helicopters; and

§ 27.67 for multiengine helicopters.

VNE (power-off) is—

A constant airspeed;

A constant amount less than power-on VNE; or

A constant airspeed for a portion of the altitude range for which certification is requested, and a constant amount less than power-on VNE for the remainder of the altitude range.