737 Flight Crew Operations Manual
Fire Protection Chapter 8
System Description Section 20
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D6-27370-866-EGP 8.20.1
20 System DescriptionSystem DescriptionFile Highlight
Introduction
There are fire detection and extinguishing systems for:
The engines also have overheat detection systems.
The main gear wheel well has a fire detection system, but no fire extinguishing
system.
Engine Fire Protection
Engine fire protection consists of these systems:
• engine overheat and fire detection powered by the battery bus
• engine fire extinguishing powered by the hot battery bus.
Engine Overheat and Fire Detection
Each engine contains two overheat/fire detector loops. Each loop provides both
fire and overheat detection. As the temperature of a detector increases to a
predetermined limit, the detector senses an overheat condition. At higher
temperatures, the detector senses a fire condition. Normally, both detector loops
must sense a fire or overheat condition to cause an engine overheat or fire alert.
The ENG OVERHEAT light or engine fire switch remains illuminated until the
temperature drops below the onset temperature.
An OVHT DET switch for each engine, labeled A, B, and NORMAL, permits
selection of either loop A or B, or both A and B, as the active detecting loops.
The system contains a fault monitoring circuit. If one loop fails with the OVHT
DET switch in NORMAL, that loop is automatically deselected and the remaining
loop functions as a single loop detector. There is no flight deck indication of single
loop failure. If both loops fail on an engine, the FAULT light illuminates and the
system is inoperative.
If the OVHT DET switch is positioned to A or B, the system operates as a single
loop system. The non–selected loop is not monitored. If the selected loop fails, the
FAULT light illuminates and the system is inoperative.
The indications of an engine overheat are:
• both MASTER CAUTION lights illuminate
• the OVHT/DET system annunciator light illuminates
• the related ENG OVERHEAT light illuminates.
• engines
•APU
• lavatories
• cargo compartments.
September 18, 2008
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