14 CFR 23.1203 - Fire detector system.
(a) There must be means that ensure the prompt detection of a fire in -
(1) An engine compartment of -
(i) Multiengine turbine powered airplanes;
(ii) Multiengine reciprocating engine powered airplanes incorporating turbochargers;
(iii) Airplanes with engine(s) located where they are not readily visible from the cockpit; and
(iv) All commuter category airplanes.
(2) The auxiliary power unit compartment of any airplane incorporating an auxiliary power unit.
(b) Each fire detector must be constructed and installed to withstand the vibration, inertia, and other loads to which it may be subjected in operation.
(c) No fire detector may be affected by any oil, water, other fluids, or fumes that might be present.
(d) There must be means to allow the crew to check, in flight, the functioning of each fire detector electric circuit.