H21741
Control Amplifier Installation 527
26-16-21 Figure 401 Nov 1/74
Page 402
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
WHEEL WELL OVERHEAT DETECTION SYSTEM -DESCRIPTION AND OPERATION
1. General
A. The wheel well overheat detection system provides overheat detection in the main wheel well area. Fire detection is provided by a sensing element along the ceiling of the wheel well (Fig. 1). The detection system is an electrical, heat sensing system that responds to an overheat condition by activating fire warning lights and an alarm bell.
B. The wheel well overheat detection system consists of the fire protection system module, compartment overheat accessory unit (control module) M237, alarm bell, master fire warning lights and sensing elements. The sensing element is connected to the compartment overheat accessory unit. The accessory unit contains the overheat control unit test relays and alarm control for the wheel well, as well as circuits for the wing and body overheat systems. The overheat control unit monitors the electrical resistance of the sensing element. When a temperature rise causes the element resistance to drop to the value corresponding to the alarm temperature, the control unit will provide a signal to the alarm cutout control card, wheel well warning light and a ground to the master fire warning lights. When the control card receives the signal, it actuates the fire alarm bell in the aural warning devices box.
C. The overheat detection system can be tested on the ground or in flight by operation of the overheat fire test switch on the fire protection module. The fire test switch tests continuity through the wheel well system by simulating an overheat condition and energizing the alarm bell and warning lights.
Fire Protection System Module Components
A. The fire protection system module components described in the following paragraphs will be only those associated with the wheel well overheat detection system. The fire protection system module is located on the aft electronic panel (P8).
500
Feb 20/90 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 26-17-0 Page 1
L20739
Wheel Well Overheat Detection System Equipment Location 541
26-17-0 Figure 1 May 20/82
Page 2
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
EFFECTIVITY
AIRPLANES WITH BOEING M237 COMPARTMENT OVERHEAT ACCESSOR UNIT
G43205
539 Wheel Well Overheat Detection System Schematic
Feb 20/90 Figure 2 26-17-0
Page 3
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
B. Wheel Well Warning Light
(1) A red warning light indicates an overheat condition in the wheel well area. An overheat condition in the wheel well area causes the steady illumination of the warning light and the master fire warning light.
C. Test Switch
(1) A overheat fire test switch is provided for testing the wheel well overheat detection system. The test switch may be used while the airplane is in flight or on the ground (Fig. 2). Holding the switch to the fire position operates control circuits in the accessory unit, which simulate an overheat condition in the wheel well. The wheel well overheat warning components are then energized.
D. Bell Cutout Switch
(1) The switch can be pressed to cut off the fire alarm bell ringing or extinguish the master fire warning lights (Fig. 2). The fire alarm bell and master fire warning lights can also be cut off by depressing the master fire warning lights.
3. Compartment Overheat Accessory Unit Components (Airplanes with Boeing M237 compartment overheat accessory unit)
A. The compartment overheat detection control module is located on electronic equipment rack E3 in the electronic compartment.
B. Overheat Control Unit
(1) The overheat control unit plugs into the accessory unit and is held in place by screws. Each control unit contains the overheat detection circuitry necessary to continuously monitor the sensing element resistance and provide an alarm signal when the element resistance drops to the predetermined trip level (Fig 2). When the overheat condition has passed, the system returns to its standby condition.
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