14 CFR § 27.1141 - Powerplant controls: general.
(b) Each flexible powerplant control must be approved.
(c) Each control must be able to maintain any set position without -
(1) Constant attention; or
(2) Tendency to creep due to control loads or vibration.
(d) Controls of powerplant valves required for safety must have -
(1) For manual valves, positive stops or in the case of fuel valves suitable index provisions, in the open and closed position; and
(2) For power-assisted valves, a means to indicate to the flight crew when the valve -
(i) Is in the fully open or fully closed position; or
(ii) Is moving between the fully open and fully closed position.
(e) For turbine engine powered rotorcraft, no single failure or malfunction, or probable combination thereof, in any powerplant control system may cause the failure of any powerplant function necessary for safety.