‘Autorotation’ means a rotorcraft flight condition in which the lifting rotor is driven entirely by action of the air when the rotorcraft is in motion. 
 
‘Auxiliary Power Unit (APU)’ means any gas turbine-powered unit delivering rotating shaft power,  compressor air, or both which is not intended for direct propulsion of an aircraft.
 
‘Auxiliary rotor’ means a rotor that principally serves to counteract the effect of the main rotor torque on a rotorcraft and/or to manoeuvre the rotorcraft about one or more of its three principal axes. 
 
‘Back-up Mode’ (Engine related definition) means the Control Mode of the back-up system.
[Amdt. No.:Def/1]
 
‘Back-up System’ means a part of the Engine/Propeller Control System where the operating characteristics or capabilities of the Engine/Propeller control are sufficiently different from the Primary System that the operating characteristics or capabilities of the aircraft, crew workload, or what constitutes appropriate crew procedures may be significantly impacted or changed.
[Amdt. No.:Def/1]
 
‘Brake Horsepower’ means the power delivered at the main output shaft of an aircraft engine.
 
‘Calibrated airspeed’ means indicated airspeed of an aircraft, corrected for position and instrument error. Calibrated airspeed is equal to true airspeed in standard atmosphere at sea level.