*** Updated 20 Nov 2018 ***
Overheat / Fire Protection Panel 3-900 series
Overheat / Fire Protection Panel -200C series. Notice fwd & aft cargo smoke detectors.
Engine & APU fire detection – Battery bus
Engine, APU & Cargo fire extinguishing – Hot battery bus.
There are two fire detection loops in each engine. Failure
of both loops in one engine will illuminate the FAULT light. The individual
loops can be checked by selecting A or B on the OVHT DET switches.
Fire switches will unlock in the following situations:
Pulling a fire switch will do the following:
The engine fire bottles (NG)
There is a wheel-well fire detection system but although the engine fire bottles are located in the wheel-well, there is no extinguishing system for a wheel-well fire. (Suggest extend gear & land ASAP).
The APU only has one bottle. This may be checked externally by looking for the two discharge discs (red for thermal overpressure & yellow for extinguisher discharge) and the pressure sight glass (where fitted) on the aft stbd fuselage.
Cargo Fire Panel
Cargo Fire Panel - Alternative version
The cargo holds have dual-loop smoke detectors powered by DC bus 1 & 2. There is only one cargo fire bottle, it is powered by the hot battery bus and can be discharged into either the fwd or aft hold. On later 737NG's the cargo fire smoke detector sends a signal to the cabin pressure control system. This triggers the cabin pressure to descend at 750 slfpm which helps prevent smoke penetration into the passenger cabin from the lower lobe. (This function is inhibited on the ground.)
Cargo Hold Smoke Detector
Some 737's have a warning light on the flight deck to warn of smoke in the lavatory. If the smoker is in the forward lav you can usually smell it on the flight deck within seconds without a warning light.
The cargo 737's had a pressurisation feature which allowed the crew to pressurise or unpressurise the passenger compartment for smoke clearance.
See also NG engineering notes by M Ferreira