14 CFR §27.939

1. (a)Turbine engine operating characteristics must be investigated in flight to determine that no adverse characteristics (such as stall, surge, or flameout) are present, to a hazardous degree, during normal and emergency operation within the range of operating limitations of the rotorcraft and of the engine.
2. (b)The turbine engine air inlet system may not, as a result of airflow distortion during normal operation, cause vibration harmful to the engine.
3. (c)For governor-controlled engines, it must be shown that there exists no hazardous torsional instability of the drive system associated with critical combinations of power, rotational speed, and control displacement.