ATA 26: Boeing 737 (Technical Notes)

The Boeing 737 is equipped with fire detection and extinguishing systems for the following:

• Engines: Includes fire detection and extinguishing systems, as well as an overheat detection system.
• APU: Fire detection and extinguishing systems are provided.
• Main gear wheel well: Fire detection system only, with no fire extinguishing capability.
• Cargo compartment: Equipped with smoke detection and fire suppression systems.
• Lavatories: Feature smoke detection and fire extinguishing systems.

ENGINE

• Engine and APU fire detection module – Fire detection control unit (FDCU).
• Engine overheat/fire detector – Dual Loop: loop A and loop B.
• Fire extinguisher bottles – 2 : top left corner of the main wheel well.
  • Halon
  • Pressurized with nitrogen at – 800 psi at a temperature of 21°C.
  • Send either one or both bottles to the same engine.
  • Color coding in hoses.
  • If the bottle temperature increases to 130°C, the safety relief port ruptures. This releases halon into the wheel well.

Each engine has 14 detectors. The detectors monitor 7 sections of the engine. In each section, two detectors attach to a support tube and make an assembly. An assembly has one detector from loop A and one from loop B.

• Two on the upper fan case section – Overheat at 149°C & Fire at 277°C.
• Two on the lower fan case section – Overheat at 149°C & Fire at 277°C.
• Two on the right side fan case section – Overheat at 149°C & Fire at 277°C.
• Two on the left side mid fan case section – Overheat at 149°C & Fire at 277°C.
• Two on the left core section – Overheat at 343°C & Fire at 454°C.
• Two on the right core section – Overheat at 343°C & Fire at 454°C.
• Two on the combustor burn-thru section – Overheat at 343°C & Fire at 454°C.

During the OVHT/FIRE test, the APU fire and wheel well fire circuits are also tested.

During the FAULT/INOP test, the APU fire detector fault circuit is also tested.

There is no indication in the flight compartment of single loop operation until you do the OVHT/FIRE test.

If the FDCU changes to single loop operation it will be identified during the OVHT/FIRE test as a fault. To configure the system for dispatch, move the OVHT DET switch to the operating loop position (A or B). This enables a good OVHT/FIRE test.

The fuel control switch light on the control stand comes on during a fire condition to show the pilot the correct engine to shutdown.

The OVHT DET switches on the engine and APU fire control panel allow selection of the mode of operation. Each switch has these positions:

• NORMAL – makes loop A and loop B agree before the alarm condition operates
• A – only takes fire/overheat signals from loop A
• B – only takes fire/overheat signals from loop B.

The FAULT/INOP test does a check of the FDCU fault detection circuitry and related flight compartment indications. These are the indications of a good FAULT/INOP test:

• MASTER CAUTION and OVHT/DET annunicator lights come on
• Amber FAULT light comes on
• Amber APU DET INOP light comes on.

NOTE: YOU CAN ALSO DO THE FAULT/INOP TEST AT THE FDCU. THE INDICATIONS IN THE FLIGHT COMPARTMENT WILL BE THE SAME.

The OVHT/FIRE test does a check of the engine fire detection circuit and flight compartment indications. These are the indications of a good OVHT/FIRE test:

• Eng 1, eng 2, and APU fire handles unlock
• Amber eng 1 and eng 2 OVERHEAT lights come on
• Amber MASTER CAUTION and OVHT/DET lights on glareshield P7 come on
• Red eng 1, eng 2, and APU fire handle lights come on
• Red engine start lever lights come on
• Two red FIRE WARN lights on glareshield P7 come on
• Bell sound from aural warning unit
• Remote APU control panel horn and red light come on
• Red WHEEL WELL light comes on (115v ac power is necessary).

Note: If the OVHT DET switch is in the NORMAL position, both loops on one engine must have a fault condition before the FAULT light comes on. If loop A has a fault, the FAULT light will come on only when the OVHT DET switch is in the A position. If loop B has a fault, the FAULT light will come on only when the OVHT DET switch is in the B position.

Extinguisher (EXT) TEST Switch – (spring–loaded to center)

• 1 or 2 – tests bottle discharge circuit continuity for all three extinguisher bottles (Both ENG & APU).

APU

• APU fire detectors – single loop
• Engine and APU fire detection module – Fire detection control unit (FDCU)
• APU electronic control unit
• APU remote control panel P28 is in the right main wheel well, on the aft bulkhead.
• One fire extinguisher bottle – horizontal stabilizer accessory compartment.
  • Halon
  • Pressurized with nitrogen at – 800 psi at a temperature of 21°C.
  • If the bottle temperature increases to 266°F (130°C) or bottle pressure to approximately 1800 psi, the safety relief port breaks open. This blows out the disc and discharges the extinguishing agent into the accessory compartment.
  • Only one electrical connector and discharge port is used. The second electrical connector is capped and the discharge port is also capped.

The APU has three detectors. The detectors monitor three sections of the APU.

• Upper APU compartment – 232°C
• Lower APU compartment (door) – 232°C
• Tailpipe – 135°C

CARGO

• Smoke detectors (4 fwd & 6 aft) – Pans in the ceiling
• Cargo electronic units (1 fwd & 1 aft) – LH ceiling

The smoke detectors are in a dual loop configuration – total 10.

• FWD – two in loop A and two in loop B.
• AFT – three in loop A and three in loop B.

Each smoke detector monitors the cargo compartment for smoke and heat.

The smoke detectors use photoelectric cells to detect smoke. The smoke detector gives an alarm signal if it senses smoke or if it senses air temperature more than 110°C.

You can test the smoke detection system with the cargo fire control panel or the cargo electronic units.

Extinguisher Bottle – halon 1301

• High Rate Discharge (HRD) fire extinguisher bottle – used for fire extinguishing.
  • 630 in³
• Low Rate Discharge (LRD) fire extinguisher bottle – used for fire suppression.
  • 378 in³ – standard bottle (basic configuration)
  • 800 in³ – big bottle for EDTO

Discharge nozzle –  5

• 2 – ceiling of the forward cargo
• 3 – ceiling of the aft cargo

The fire extinguisher bottles are in the air conditioning distribution bay. The HRD bottle is on the left side and the LRD bottle is on the right side of the air conditioning distribution bay.

The filter/dryers are in the discharge lines of the LRD fire extinguisher bottle.

The safety relief and fill port is a fitting on each fire extinguisher bottle.

The filter/dryer removes moisture, filter debris, and meter the flow of halon during release from cargo LRD fire extinguishing bottle. Each LRD fire extinguisher bottle discharge line contains one filter/dryer. The filter/dryer meters the halon gas so that the halon level in the compartment stays above 3% for 75 minutes (195 minutes if 800 cubic inches bottle)

WHEEL WELL

• Main wheel well fire (overheat) sensing element (loop A and loop B)
• Compartment overheat detection controller – Compartment overheat detection module (CODM)

The wheel well fire detection system uses overheat sensing elements in the main wheel well. It monitors the wheel well for fire conditions.

• The sensing element has a single strand of nickel wire embedded in insulation.
• The sensing elements are in the ceiling of the left and right main wheel wells.
• The sensing element is a thermistor type with resistance inversely proportional to temperature. As the element temperature increases, the resistance decreases. If any portion of the sensing element is above the alarm temperature of 205°C, the element resistance decreases sharply.

The minimum bend radius for the sensing elements is one inch (2.5 cm). If possible, do not bend the element in a radius less than three inches (8 cm).

Sensing elements are in clamps with color-coded bushings. The element end brackets have collars that prevent the element from twisting when the electrical connectors are put on.

During the wheel well fire test, the engine overheat, the engine fire, and the APU fire systems also do a test. The test does a check of the continuity of the sensing element.

WING AND BODY

The wing and body overheat detection system uses sensing elements adjacent to the pneumatic ducts. It monitors the pneumatic distribution system ducts for overheat conditions.

• Wing/body overheat sensing elements – single loop
• Compartment overheat detection controller

EQUIPMENT COOLING

• The supply system and the exhaust system have a smoke detector.
• The two detectors are in the Electronic Equipment (EE) compartment.
  • The supply system detector is adjacent to the fuselage and forward of the supply fan/check valve assemblies. 
  • The exhaust system detector is below the floor board that is forward of the P92 panel and above the exhaust fan/check valve assemblies.
• Operation: Air goes into one port and exits the other port. In the detector, it goes through a light beam. When particles are in the air, the light beam is deflected to a circuit. The circuit energizes a relay. The relay gives a ground to the connector. The ground is the alarm signal.
• If smoke is detected, the detector sends an signal for operation of the automatic reconfiguration for the smoke clearance mode.
• A test panel on the J23 junction box in the EE compartment lets you do a test of the smoke detectors and system operation.

LAVATORY

A smoke detector is in the ceiling of each lavatory.

All photoelectric smoke detectors have a multi-color STATUS INDICATOR LED.

The LED status indicator on the detector will be green or red. When detector power is first applied, the LED is red then green to show normal operation. Normal operation the LED stays green. When the sensor senses smoke or heat, the LED turns red and the internal horn will operate. These will stay on until the horn cancel switch is pushed or the sensor no longer senses smoke.

The HORN CANCEL switch does not have a label. The HORN CANCEL switch is recessed. A small tool or probe is necessary to operate it. When you press and release the horn cancel switch, the horn will stop sounding. The LED will stay red if smoke is still sensed. The red LED will go off when the smoke sensor no longer senses smoke.

If there is a fault when power is first applied, the LED will be constant red. During operation if there is a fault, the LED will flash red at 33Hz. If the smoke detector operates but maintenance is necessary, the LED will flash green at 33Hz.

The SELF TEST switch does not have a label. THe SELF TEST switch is recessed. A small tool or probe is necessary to operate it. Use the SELF TEST switch to do a test of the smoke detector. If the detector is operating normally, the horn will operate and the red LED and external indications will be on.

LAVATORY SMOKE Light (on FWD P5)

• Illuminated (amber) – indicates smoke detected in a lavatory.
• The MASTER CAUTION lights and OVERHEAD annunciator lights illuminate.

LAVATORY FIRE EXTINGUISHING

• Fire extinguisher bottle – extinguishes fires under the wash basin and trash bin area.
• Temperature Indicator Strip – change color (white to black) if overheat or fire.
• The extinguisher bottle has two discharge ports.
• The ports have eutectic plugs. These plugs melt at high temperatures.

PORTABLE FIRE EXTINGUISHERS

There are three types of extinguishers – 

• Water-type: for solid combustible fires.
• Halon: for electrical and flammable liquid fires.
• Clean agent (2-BTP): for electrical and flammable liquid fires.

The water-type extinguishers contain a water and anti-freeze mixture.

CONTROL PANELS

• ENGINE AND APU FIRE CONTROL PANEL
• CARGO FIRE CONTROL PANEL
• REMOTE APU CONTROL PANEL

Fire Warning and Master Caution System

Master Fire Warning (FIRE WARN) Lights

Illuminated (red) – indicates a fire warning (or system test) in engine, APU, main gear wheel well, or cargo compartment.

• fire warning bell sounds
• if on ground, remote APU fire warning horn sounds.

Push – extinguishes both master FIRE WARN lights.

• silences fire warning bell
• silences remote APU fire warning horn
• resets FIRE WARN lights and fire bell for additional warnings.

Note: Pushing the fire warning BELL CUTOUT switch on the ENGINE AND APU FIRE CONTROL PANEL results in the same actions.

MASTER CAUTION Lights

Illuminated (amber) – a system annunciator light has illuminated.

Push – extinguishes both MASTER CAUTION lights.

• system annunciator light(s) extinguish
• resets system for additional master caution conditions.

System Annunciator Panel

Illuminated (amber) – an amber light, relating to illuminated system annunciator, has illuminated on the forward overhead, aft overhead, or overheat/fire protection panel.

Push – either MASTER CAUTION light to extinguish the annunciator light(s). 

Push and Release – either system annunciator panel to recall the illuminated light(s).

• if a master caution condition exists, appropriate system annunciator(s) and MASTER CAUTION lights illuminate.
• a single fault in certain redundant systems, or some simple faults, cause the system annunciator light to illuminate during a recall. The system annunciator light will extinguish when the MASTER CAUTION light is pushed.