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Illustrated technical information covering Vol 2 Over 800 multi-choice systems questions Close up photos of internal and external components Illustrated history and description of all variants of 737 Databases and reports of all the major 737 accidents & incidents History and Development of the Boeing 737 - MAX General flightdeck views of each generation of 737's Technical presentations of 737 systems by Chris Brady Detailed tech specs of every series of 737 A collection of my favourite photographs that I have taken of or from the 737 Press reports of orders and deliveries Details about 737 production methods A compilation of links to other sites with useful 737 content Study notes and technical information A compilation of links to major 737 news stories with a downloadable archive A quick concise overview of the pages on this site



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*** Updated 14 Nov 2021 ***

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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:

  1. Overheat detected
  2. Fire detected
  3. During an OVHT/FIRE test
  4. Pressing manual override buttons

Pulling a fire switch will do the following:

  1. Arm firing circuits
  2. Allow fire switch to be rotated for discharge
  3. Close engine fuel shut-off valve.
  4. Trip the associated GCR (i.e. switches off the generator)
  5. Close hydraulic supply to EDP & disarms its LP light (Not if APU)
  6. Close engine bleed air valve (If APU will also close air inlet door)
  7. 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)

The cargo holds have many smoke detectors. They use photoelectric cells to detect smoke. The smoke detector will trigger for smoke or if the air temperature exceeds 230F (110C). Each detector is dual-loop and powered by DC bus 1 & 2. If one loop loses power or fails, the DETECTOR FAULT light will illuminate and the system automatically converts to single-loop detection.

737 Cargo Hold Smoke Detector

Cargo Hold Smoke Detector

Normally both loops must detect smoke/heat to give a warning unless one loop has failed. You can select a single loop on the panel if desired. This is normally only done to despatch IAW the MEL.

Cargo Fire Panel

Cargo Fire Panel - Alternative version

The cargo fire bottle(s) are located in the air con distribution bay, between the back of the forward hold and the wheel well. There are options for single, dual or even triple cargo bottles but the panels and way we use them are identical. They give 75 to 195 minutes of fire suppression time. The second (and third for BBJs) bottle fire automatically 15 minutes (MAX) or 60 minutes (NG) after the first bottle, unless the aircraft has landed. The cargo bottle(s) can be discharged into either the forward or aft hold (similar to the engine fire bottles). When the bottle pressure is less than 250 psi, the pressure switch sends a signal to illuminate the amber DISCH light. The squibs on the fire bottle are connected to the EXT lights on the cargo fire control panel.


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.


See also NG engineering notes by M Ferreira

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