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:
- Overheat detected
- Fire detected
- During an OVHT/FIRE test
- Pressing manual override buttons
Pulling a fire switch will do the following:
- Arm firing circuits
- Allow fire switch to be rotated for discharge
- Close engine fuel shut-off valve.
- Trip the associated GCR (i.e. switches off the generator)
- Close hydraulic supply to EDP & disarms its LP light (Not if APU)
- Close engine bleed air valve (If APU will also close air inlet door)
- Close thrust reverser isolation valve (Not if APU)
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 Compartment (Optional)
Cargo Fire Panel
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.)
Lavatory Smoke Detection (Optional)
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.
Passenger Compartment (Optional)
The cargo 737's had a pressurisation feature which allowed the crew to pressurise or unpressurise the passenger compartment for smoke clearance.