737 Flight Crew Operations Manual
If a main landing gear tire is damaged during takeoff, it is possible that braking of the main gear wheels during retraction may be affected. A spinning tire with a loose tread must be stopped prior to entering the wheel well or it can cause damage to wheel well components. When a spinning tire with loose tread impacts a fitting in the wheel well ring opening, that gear stops retracting and free falls back to the down position. The affected gear cannot be retracted until the fitting is replaced.
Landing Gear Transfer Unit
Hydraulic system B pressure is available for raising the landing gear through the landing gear transfer unit. 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 lever is positioned UP

.  
either main landing gear is not up and locked.

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 remain 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.
With the manual extension access door open:
.  
manual landing gear extension is possible with the LANDING GEAR
lever in any position

.  
normal landing gear extension is possible if hydraulic system A pressure
is available

.  
landing gear retraction is disabled.

Following a manual extension, the landing gear may be retracted normally by accomplishing the following steps:
.  
close the manual extension access door

.  
move the LANDING GEAR lever to DOWN with hydraulic system A
pressure available, and then

.  
position the LANDING GEAR lever to UP.

Copyright . The Boeing Company. See title page for details.
14.20.2   June 1, 2005

Landing Gear -System Description 737 Flight Crew Operations Manual
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. Nose wheel steering is powered by hydraulic system B when the NOSE WHEEL STEERING switch is placed to ALT. Nose wheel steering is powered only when the airplane is on the ground. 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.
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 . autobrake system

.
alternate brake system . parking brake

.
brake accumulator YC411, YC413

.
antiskid protection . brake temperature indication

Normal Brake System
The normal brake system is powered by hydraulic system B.
Alternate Brake System
The alternate brake system is powered by hydraulic system A. If hydraulic system B is low or fails, hydraulic system A automatically supplies pressure to the alternate brake system.
Brake Accumulator
The brake accumulator is pressurized by hydraulic system B. If both normal and alternate brake system pressure is lost, trapped hydraulic pressure in the brake accumulator can still provide several braking applications or parking brake application.
Copyright . The Boeing Company. See title page for details.
Landing Gear -System Description 737 Flight Crew Operations Manual
　
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