Singapore legislation
Regulation 56
of Air Navigation (121 — Commercial Air Transport by Large Aeroplanes) Regulations 2018
Regulation 56
Flight crew communication
An AOC holder must ensure that every person assigned duty as a flight crew member of a large aeroplane, and who is required to be on flight deck duty on the aeroplane, communicates through boom or throat microphones whenever the aeroplane is flying at an altitude below the higher of —
(a)
15,000 ft; or
(b)
the transition level or altitude.