14 CFR §27.45

1. (a)Unless otherwise prescribed, the performance requirements of this subpart must be met for still air and a standard atmosphere.
2. (b)The performance must correspond to the engine power available under the particular ambient atmospheric conditions, the particular flight condition, and the relative humidity specified in paragraphs (d) or (e) of this section, as appropriate.
3. (c)The available power must correspond to engine power, not exceeding the approved power, less—
   1. (1)Installation losses; and
   2. (2)The power absorbed by the accessories and services appropriate to the particular ambient atmospheric conditions and the particular flight condition.
4. (d)For reciprocating engine-powered rotorcraft, the performance, as affected by engine power, must be based on a relative humidity of 80 percent in a standard atmosphere.
5. (e)For turbine engine-powered rotorcraft, the performance, as affected by engine power, must be based on a relative humidity of—
   1. (1)80 percent, at and below standard temperature; and
   2. (2)34 percent, at and above standard temperature plus 50 degrees F. Between these two temperatures, the relative humidity must vary linearly.
6. (f)For turbine-engine-powered rotorcraft, a means must be provided to permit the pilot to determine prior to takeoff that each engine is capable of developing the power necessary to achieve the applicable rotorcraft performance prescribed in this subpart.