55. Flight crew communication

An AOC holder must ensure that every person assigned duty as a flight crew member on a relevant aircraft, and who is required to be on flight deck duty for the aircraft, communicates through a boom or throat microphone —

if the aircraft is a small aeroplane, whenever the aeroplane is flying at an altitude that is below the higher of —

15,000 ft; or

the transition level or altitude; or

if the aircraft is a helicopter, at all times.