737 Operations Manual

Introduction
The airplane has two main landing gear and a single nose gear. Each main gear is a conventional two–wheel landing gear. The nose gear is a conventional steerable two–wheel unit.
Hydraulic power for retraction, extension, and nose wheel steering is normally supplied by hydraulic system A. A manual landing gear extension system is installed, and--on some airplanes--an alternate source of hydraulic power for nose wheel steering is also provided.
The normal brake system is powered by hydraulic system B. The alternate brake system is powered by hydraulic system A. Antiskid protection is provided on both brake systems, but the autobrake system is available only with the normal brake system.
Landing Gear Operation
The landing gear are normally controlled by the LANDING GEAR lever. On the ground, a landing gear lever lock prevents the LANDING GEAR lever from moving to the up position. An override trigger in the lever may be used to bypass the landing gear lever lock. In flight, the air/ground system energizes a solenoid which opens the lever lock.
Landing Gear Retraction
When the LANDING GEAR lever is moved to UP, the landing gear begins to retract. During retraction, the brakes automatically stop rotation of the main gear wheels. After retraction, the main gear are held in place by mechanical uplocks. Rubber seals and oversized hubcaps complete the fairing of the outboard wheels.
Note:  Manual brake application during flight may cause brake pedal vibration until pedal force is released.
The nose wheels retract forward into the wheel well and nose wheel rotation is stopped by snubbers. The nose gear is held in place by an overcenter lock and enclosed by doors which are mechanically linked to the nose gear
Hydraulic system B pressure is available for raising the landing gear through the landing gear transfer valve. Hydraulic system B supplies the volume of hydraulic fluid required to raise the landing gear at the normal rate when all of the following conditions exist:
.
airborne

.
No. 1 engine RPM drops below a limit value

Landing Gear -System Description 737 Operations Manual

.
LANDING GEAR lever is positioned UP

.
either main landing gear is not up and locked.

Hydraulic pressure is removed from the landing gear system with the LANDING GEAR lever in the OFF position.
Landing Gear Extension
When the LANDING GEAR lever is moved to DN, hydraulic system A pressure is used to release the uplocks. The landing gear extends by hydraulic pressure, gravity and air loads. Overcenter mechanical and hydraulic locks hold the gear at full extension. The nose wheel doors stay open when the gear is down.
Landing Gear Manual Extension
If hydraulic system A pressure is lost, the manual extension system provides another means of landing gear extension. Manual gear releases on the flight deck are used to release uplocks that allow the gear to free–fall to the down and locked position. The forces that pull the gear down are gravity and air loads.
Copyright . The Boeing Company. See title page for details.
14.20.2  June 8, 2001

Landing Gear -System Description
Nose Wheel Steering

NOSE WHEEL STEERING
The airplane is equipped with nose wheel steering which is powered by hydraulic system A when the NOSE WHEEL STEERING switch is in the NORM position and when the airplane is on the ground. When the NOSE WHEEL STEERING switch (as installed) is in the ALT position, nose wheel steering is powered by hydraulic system B. In the event of a hydraulic leak downstream of the Landing Gear Transfer Unit, resulting in a loss of hydraulic system B fluid in the reservoir, a sensor closes the Landing Gear Transfer Valve and alternate steering will be lost.
Primary steering is controlled through the nose wheel steering wheel. Limited steering control is available through the rudder pedals. A pointer on the nose steering wheel assembly shows nose wheel steering position relative to the neutral setting. Rudder pedal steering is deactivated as the nose gear strut extends.
Landing Gear -System Description 737 Operations Manual

A lockout pin may be installed in the towing lever to depressurize nose wheel steering. This allows airplane pushback or towing without depressurizing the hydraulic systems.
Brake System
Each main gear wheel has a multi–disc hydraulic powered brake. The brake pedals provide independent control of the left and right brakes. The nose wheels have no brakes. The brake system includes:
.
normal brake system . antiskid protection

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alternate brake system . autobrake system

.
brake accumulator . parking brake

Normal Brake System
　
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本文鏈接地址：737-300機組操作手冊Flight Crew Operations Manual FCOM 3(102)