737-300/400
LANDING GEAR
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Slide 1 of 56
Landing Gear Panel
Instrument Panels
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Slide 2 of 56
Landing Gear Panel
LANDING GEAR Lever:
UP –
Retract landing gear.
OFF –
Hydraulic pressure is removed from the landing
gear system.
DN –
Extend landing gear.
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Landing Gear Panel
Red Landing Gear Indicator Lights:
Illuminated –
• Respective landing gear is not down and locked
when either or both forward thrust levers are
retarded to idle.
• Respective landing gear is in disagreement with
LANDING GEAR lever, i.e. landing gear is in
transit or is unsafe.
Extinguished –
Respective landing gear is up and locked with
LANDING GEAR lever UP or OFF.
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Landing Gear Panel
Green Landing Gear Indicator Lights:
Illuminated –
Respective landing gear is down and locked.
Extinguished –
Respective landing gear is not down and locked.
Note: Landing gear configuration warning horn is
deactivated with all landing gear down and
locked.
See picture of
main gear and nose gear
indicator sensors
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Slide 5 of 56
Landing Gear Panel
LANDING GEAR LIMIT Speed Placard:
Indicates maximum speed when operating the
landing gear and after gear extension.
See closeup picture of
LANDING GEAR LIMIT
speed placard
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Slide 6 of 56
Landing Gear Panel
LOCK OVERRIDE (Landing Gear Override) Trigger:
Pull –
Bypasses the landing gear lever locking system.
Note: When the aircraft is on the ground, the
air/ground sensing system engages the landing
gear lever locking system and prevents the lever
from moving to the UP position. When the aircraft
becomes airborne, the air/ground sensing system
disengages the landing gear lever locking system
which allows the lever to be moved from DN to UP.
It is possible, however, for the air/ground sensing
system to remain in the “ground-mode” even
though the aircraft is in flight. In this case, the
locking system will remain engaged after takeoff
and the landing gear lever will not move to the UP
position. Non-normal procedures may recommend
pulling the landing gear override trigger to bypass
the locking system and consequently raise the
gear (reference your company operations manual).
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Slide 7 of 56
Manual Gear Extension
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Slide 8 of 56
Manual Gear Extension
Manual Gear Extension Handles:
If “A” system hydraulics is inoperative, the
landing gear may still be extended via the manual
extension system. The manual gear extension
handles are located in the flight deck floor, aft and
to the left of the First Officer’s seat. When the
extension handles are pulled to their limit, the
respective gear will release from its locked
position. Gravity and air loads will assist the gear
in free-falling to the down and locked position.
The main gear manual extension handles need to
be pulled out approximately 14 to 16 inches (35 to
40 cm) with a force of about 60 pounds. The nose
gear manual extension handle needs to be pulled
out approximately 8 inches (20 cm) with a force of
about 80 pounds. Down and locked verification
can be made using the green landing gear
indicator lights.
Note: LANDING GEAR lever must be in DN or
OFF position to prevent hydraulic lockup during
manual extension.
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Slide 9 of 56
Main Gear Viewer
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Slide 10 of 56
Main Gear Viewer
Plywood cover installed
Plywood cover removed
The main gear viewer is located in the main cabin floor. It is opposite the third window that is aft of the
overwing exit, one foot left of center. After pulling back the carpet that is concealing the viewer, a
plywood cover will be exposed and needs to be removed. The pilot will now position their eyes over the
viewer. Two mirrors inside the viewer are used to ensure that the three red paint stripes on each main
gear are visible and in horizontal alignment. It may be necessary to turn on the wheel well lights.
Note: Depending on the aircraft seat configuration, some viewers may be positioned under an aisle seat.
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Slide 11 of 56
Main Gear Paint Stripes
See picture of main
gear viewer mirrors
The red paint stripes are located on the lower side strut of each main gear. When looking at the
mirrors in the main gear viewer, the three red paint stripes should be visible and in horizontal
alignment. This is an indication that the respective main landing gear is down and locked.
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Slide 12 of 56
Nose Gear Viewer
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Slide 13 of 56
Nose Gear Viewer
Cover plate closed
Cover plate open
The nose gear viewer is located on the flight deck floor, just inside the door. Access to the viewer is
made by opening the cover plate. Under the cover plate is a small circular viewing window. As the
pilot looks into the viewing window, two red arrow heads should be visible. If the two red arrow
heads are facing each other and appear to be in alignment (in contact), the nose gear is down and
locked.
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Nose Gear Arrow Heads
See picture of
viewing window
in wheel well
One nose gear arrow head is located on the nose gear lock brace and the other arrow head is located
on the nose gear lock link. When looking into the nose gear viewing window the two red arrows
should be visible. If the two red arrow heads are facing each other and appear to be in alignment
(in contact), the nose gear is down and locked. It may be necessary to turn on the wheel well lights.
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Slide 15 of 56
Autobrake Panel
Instrument Panels
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Slide 16 of 56
Autobrake Panel
AUTO BRAKE DISARM Light:
Illuminated –
• RTO is selected on the ground.
 When RTO is selected on the ground, the
AUTO BRAKE DISARM light will illuminate
while the braking system performs a self-test.
One to two seconds later, the light will
extinguish if the self-test was successful.
• Manual brakes applied during RTO or landing.
• A fault exists in the automatic braking system.
• Antiskid has been selected off.
• Thrust levers are advanced during an RTO or
landing.
• Landing made with RTO selected.
• SPEED BRAKE lever moved to DOWN detent
during an RTO or landing.
Extinguished –
• Autobrakes armed.
• AUTO BRAKE select switch is positioned to OFF.
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Slide 17 of 56
Autobrake Panel
AUTO BRAKE Select Switch:
OFF –
Automatic braking system deactivated.
1, 2, 3, or MAX –
Selects desired deceleration rate for landing.
Note: Switch must be pulled-out to select MAX
deceleration.
RTO (Rejected Takeoff) –
At or above 90 knots during takeoff,
automatically applies maximum brake
pressure (3,000 psi) when thrust levers are
retarded to idle.
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Slide 18 of 56
Antiskid Panel
Instrument Panels
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Slide 19 of 56
Antiskid Panel
ANTI SKID INOP Light:
Illuminated –
• The antiskid fault monitoring system has
detected a system fault.
• Antiskid switch is OFF.
Extinguished –
Antiskid system is operating normally.
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Slide 20 of 56
Antiskid Panel
ANTISKID Control Switch:
ON (guarded position) –
• Provides power to the antiskid unit for both
inboard and outboard antiskid protection.
OFF –
• Disables the antiskid system.
• Illuminates the ANTI SKID INOP light.
• Illuminates the AUTO BRAKE DISARM light if
the autobrake system is armed.
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Hydraulic Brake Pressure Indicator
Instrument Panels
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Slide 22 of 56
Hydraulic Brake Pressure Indicator
HYD BRAKE PRESS Indicator:
Indicates brake accumulator pressure.
• Normal accumulator pressure – 3,000 psi
• Maximum accumulator pressure – 3,500 psi
• Normal accumulator precharge pressure – 1,000 psi
Note: The brake accumulator is attached to the aft
wall in the main gear wheel well, aircraft right. The
accumulator precharge is either compressed air or
nitrogen.
See picture of
brake accumulator
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Slide 23 of 56
Rudder/Brake Pedals
Rudder/Brake Pedals:
Push full pedal –
Turns nose wheel up to 7° left or right of center.
Push top of pedal only –
Activates wheel brakes.
Note: See the Flight Controls presentation for a
description of the rudder.
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Slide 24 of 56
Parking Brake
Control Stand
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Slide 25 of 56
Parking Brake
PARKING BRAKE PULL Lever:
Positioned forward –
Parking brake is released.
Pulled aft –
Sets the parking brake when either Captain’s
or First Officer’s brake pedals are fully
depressed.
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Slide 26 of 56
Parking Brake
PARK BRAKE LIGHT:
Illuminated –
Parking brake is set.
Extinguished –
Parking brake is released.
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Slide 27 of 56
Nose Wheel Steering Switch
Instrument Panels
1
1 AS INSTALLED
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Slide 28 of 56
Nose Wheel Steering Switch
NOSE WHEEL STEERING Switch:
NORM (guarded position) –
Hydraulic system “A” is providing power for
the nose wheel steering system.
ALT –
Hydraulic system “B” is providing power for
the nose wheel steering system.
1
1 AS INSTALLED
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Slide 29 of 56
Nose Wheel Steering Wheel
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Slide 30 of 56
Nose Wheel Steering Wheel
Nose Wheel Steering Wheel:
Rotate –
• Turns nose wheel up to 78° left or right of center.
• Overrides rudder pedal steering.
Note: The nose wheel steering wheel is attached
to the side wall, left of the Captain’s control
column. The steering wheel is also known as the
tiller.
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Slide 31 of 56
Nose Wheel Steering Wheel
Nose Wheel Steering Indicator:
LEFT –
Indicates nose wheel steering displacement left of
center position.
CENTER –
Normal straight ahead position.
RIGHT –
Indicates nose wheel steering displacement right
of center position.
This concludes the review of the Landing Gear controls and indicators.
The next section will discuss the system in greater detail.
Click Next to continue.
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Slide 32 of 56
Landing Gear Operation
The aircraft has two main landing gear and single nose gear. Each gear is equipped
with two tire and wheel assemblies. Each main gear wheel is fitted with disc-type
hydraulic brakes which are modulated by the antiskid system. To absorb the impact on
landing, air-oil type shock struts are installed on all three gear. “A” system hydraulics
supplies power for normal retraction, extension, and nose wheel steering.
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Slide 33 of 56
737-300/400 Hydraulic
Schematic (specific to landing gear)
“A”
system
reservoir
Return
Hydraulic shutoff
Engine
driven
pump
To reservoir
No. 1
fuel
tank
Hydraulic
heat
exchanger
Motor
Electric
pump
“A” system
hydraulics
supplies power
for normal
retraction,
extension, and
nose wheel
steering.
Supply
“A”
system pressure
“B”
system pressure
Standby
system pressure
Nose wheel
steering
Landing gear
Case drain
return
Landing gear
transfer unit
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Slide 34 of 56
737-300/400 Hydraulic
Schematic (specific to landing gear
transfer unit)
“B”
system
reservoir
“A”
system
reservoir
Return
Return
Hydraulic shutoff
Engine
driven
pump
To reservoir
No. 1
fuel
tank
Hydraulic shutoff
Hydraulic
heat
exchanger
Hydraulic
heat
exchanger
Motor
No. 2
fuel
tank
Engine
driven
pump
Motor
Electric
pump
Electric
pump
A landing gear transfer unit is installed in the main gear wheel well. When all of the
following conditions exist, “B” system hydraulics will retract the landing gear via the
landing gear transfer unit:
• Airborne
• No. 1 engine drops below 56% N2
• LANDING GEAR handle in the UP position
• Landing gear not up
When this becomes especially useful is during a loss of the No. 1 engine immediately
after takeoff. With the No. 1 engine inoperative, the “A” system engine driven pump is
also inoperative. Even though the “A” system electric pump is operating normally,
the volume output of the pump is not great enough to raise the gear at a rate which is
required during single engine operations. As the No. 1 engine N2 gauge drops below
56%, the landing gear transfer unit will connect “B” system hydraulics to the landing
gear. “B” system will now raise the gear at a normal retraction rate.
Nose wheel
steering
To reservoir
Landing gear
Supply
“A”
system pressure
“B”
system pressure
Standby
system pressure
Case drain
return
Landing gear
transfer unit
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Slide 35 of 56
Landing Gear Operation
See picture of
uplock hook and
uplock roller
The main gear is locked in the “down” position by a folding lock strut, and held in the
“up” position by an uplock hook and uplock roller. When the LANDING GEAR lever is
moved to the UP position, the main gear retracts inboard into the main gear wheel well.
During retraction, main gear wheel rotation is stopped by “A” system hydraulics via
the alternate brake metering valves. When the main gear is fully retracted, the doors
are faired with the fuselage. Rubber blade-type seals and oversized hubcaps complete
the fairing of the outboard wheels.
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Slide 36 of 56
Landing Gear Operation
See picture of
snubbers
The nose gear is locked in the “up” and “down” position by a folding lock strut. When the
LANDING GEAR lever is moved to the UP position, the nose gear retracts forward into the
nose gear wheel well. As the gear retracts, nose gear wheel rotation is stopped by tire
contact with the snubbers. The snubbers are installed on the ceiling of the nose gear
wheel well. The nose gear doors are mechanically linked to the nose gear and will fair
with the fuselage when the gear is fully retracted.
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Slide 37 of 56
Brake System
See picture of
one brake
wear indicator pin.
The brake system includes a normal brake system, an alternate brake system, one brake
accumulator, an autobrake system, antiskid protection, and a parking brake. Each main gear
wheel has a multi-disc hydraulic powered brake. Four brake wear indicator pins are provided
on each main gear to indicate when the brakes need to be replaced.
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Slide 38 of 56
Brake System
Each brake is provided with pistons which actuate the brake when hydraulic pressure is
applied. The brake system is manually controlled by the the Captain’s or First Officer’s brake
pedals thru linkage and cables to the brake metering valve for each main gear. The intensity of
pressure established in the brakes varies directly with the amount of pedal force maintained.
The nose gear wheels do not incorporate any type of braking system.
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Slide 39 of 56
737-300/400 Hydraulic
Schematic (specific to the normal
brake system)
“B”
system
reservoir
Return
Hydraulic shutoff
Hydraulic
heat
exchanger
To reservoir
No. 2
fuel
tank
Engine
driven
pump
Motor
Electric
pump
“B” system
hydraulics
supplies power to
the normal
braking system
for each main
gear wheel.
Supply
“A”
system pressure
“B”
system pressure
Normal
brakes
Standby
system pressure
Case drain
return
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Slide 40 of 56
737-300/400 Hydraulic
Schematic (specific to the alternate
brake system)
“B”
system
reservoir
“A”
system
reservoir
Return
Return
Hydraulic shutoff
Engine
driven
pump
To reservoir
No. 1
fuel
tank
Hydraulic shutoff
Hydraulic
heat
exchanger
Hydraulic
heat
exchanger
Motor
To reservoir
No. 2
fuel
tank
Engine
driven
pump
Motor
Electric
pump
If “B” hydraulic
system loses
pressure, separate
hydraulic lines are
utilized to
automatically
supply “A”
hydraulic system
pressure to each
main gear wheel.
This is known as
alternate brakes
Electric
pump
Supply
“A”
system pressure
Alternate Brakes Selector Valve
Alternate
brakes
Normal
brakes
Accumulator
isolation valve
“B”
system pressure
Standby
system pressure
Case drain
return
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Slide 41 of 56
737-300/400 Hydraulic
Schematic (specific to the
“B”
system
reservoir
accumulator braking)
“A”
system
reservoir
Return
Return
Hydraulic shutoff
Engine
driven
pump
To reservoir
No. 1
fuel
tank
Hydraulic shutoff
Hydraulic
heat
exchanger
Hydraulic
heat
exchanger
Motor
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No. 2
fuel
tank
Engine
driven
pump
Motor
Electric
pump
If “A” and “B”
hydraulic system
pressure is lost,
the brake
accumulator will
supply trapped
hydraulic fluid to
each main gear
wheel. Enough
pressure exists in
the accumulator
for approximately
six brake
applications.
Note:
Antiskid protection
is available during
accumulator
braking.
To reservoir
Electric
pump
Supply
“A”
system pressure
“B”
system pressure
Alternate Brakes Selector Valve
Alternate
brakes
Normal
brakes
Hydraulic brake
pressure accumulator
P
Accumulator
isolation valve
See picture of
brake accumulator
Standby
system pressure
Case drain
return
Slide 42 of 56
Autobrake System
The autobrake system provides automatic braking upon landing when a pre-selected
deceleration rate (1, 2, 3 or MAX) is chosen. It also provides automatic braking in the
case of a rejected takeoff (RTO) after a wheel speed of 90 knots is attained. The
autobrake system operates utilizing “B” system hydraulics only. If the “B” hydraulic
system is inoperative, the autobrake system is also inoperative. Antiskid protection is
available during autobrake operation.
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Slide 43 of 56
Autobrake System
If automatic braking is desired upon landing, a deceleration rate must be selected while still
airborne. When a rate is selected, a self-test is initiated and the AUTOBRAKE DISARM light
will not illuminate unless the system has detected a fault. After landing, autobrake
application begins when both forward thrust levers are retarded to idle and wheel speed of
at least one wheel on each main gear is greater than 60 knots. Autobrake pressure will
gradually increase during the first three seconds after touchdown. The system will then
maintain a constant deceleration rate and, if allowed, will bring the aircraft to a complete
stop unless braking is terminated by the pilot or a system fault is detected.
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Slide 44 of 56
Autobrake System
To maintain the selected deceleration rate, pressure from the autobrake system is reduced
as additional drag (thrust reversers, spoilers) contributes to the total deceleration.
The following deceleration rates are provided:
“1” – 4 feet/second2
“2” – 5 feet/second2
“3” – 7.2 feet/second2
“MAX” – 14 feet/second2
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Slide 45 of 56
Autobrake System
If automatic braking is desired during a rejected takeoff, the AUTO BRAKE select
switch must be positioned to RTO. After selecting RTO, the AUTO BRAKE DISARM
light will illuminate while the braking system performs a self-test. One to two seconds
later, the light will extinguish if the self-test was successful.
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Slide 46 of 56
Autobrake System
To arm the RTO mode prior to takeoff, the following conditions must exist:
• AUTO BRAKE select switch must be in the RTO position.
• Aircraft must be on the ground.
• Autobrake and antiskid circuits must be operational.
• Both forward thrust levers must be positioned to idle.
• Wheel speed must be less than 60 knots.
After a successful test, the autobrake system is now in an unarmed “waiting-mode.”
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Slide 47 of 56
Autobrake System
When wheel speed is greater than 60 knots, RTO mode is now in a “semi-armed” mode.
However, if the takeoff is rejected with a wheel speed between 60 and 90 knots, automatic
braking is not applied and the AUTO BRAKE DISARM light illuminates.
Once wheel speed is greater than 90 knots, the autobrake system is “fully-armed.” If the
takeoff is rejected and the forward thrust levers are retarded to idle, maximum braking
(3,000 psi) is immediately applied to all four wheels. The system will bring the aircraft to a
complete stop unless braking is terminated by the pilot or a system fault is detected.
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Slide 48 of 56
Autobrake System
If the takeoff was normal and an RTO was not initiated, the RTO mode is automatically
disarmed when the right main gear strut extends (air/ground sensing system in “air-mode”).
When this occurs, the AUTO BRAKE DISARM light will not illuminate and the select switch
must manually be positioned to OFF. However, if a landing is made with the select switch
still in RTO, automatic braking will not occur and the AUTO BRAKE DISARM light will
illuminate two minutes after touchdown.
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Slide 49 of 56
Antiskid Protection
Antiskid protection is provided in the normal and alternate brake systems. The types
of antiskid protection available include normal, touchdown, and locked wheel. When
the pilot depresses the brake pedals, the brake metering valves open and hydraulic
fluid is directed to the brakes via the antiskid valves.
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Slide 50 of 56
Antiskid Protection
Normal brakes (“B” system hydraulics) provide each main gear wheel with individual
antiskid protection. Therefore, each individual wheel has its own normal antiskid
valve. When the normal antiskid system detects a skid, the associated antiskid valve
for the affected wheel(s) reduces brake pressure until the skidding stops. Once the
skidding has stopped, brake pressure is increased via the associated antiskid valve.
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Slide 51 of 56
Antiskid Protection
Alternate brakes (“A” system hydraulics) provide each main gear wheel pairs, instead of
individual wheels, with antiskid protection. Therefore, each wheel pair has its own
alternate antiskid valve. The mated pairs are left and right inboard wheels, and left and
right outboard wheels. When the alternate antiskid system detects a skid, the associated
antiskid valve for the affected pair reduces brake pressure until the skidding stops.
Once the skidding has stopped, brake pressure is increased via the associated antiskid
valve.
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Slide 52 of 56
Antiskid Protection
Normal, touchdown and locked wheel antiskid protection are all available from the
normal antiskid system. However, touchdown and locked wheel protection are not
available from the alternate antiskid system.
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Slide 53 of 56
Air/Ground Sense
Teleflex cable
for main gear
air/ground
sensor
Nose gear
air/ground
sensor
See picture of
teleflex cable for main
gear air/ground sense
See picture of
nose gear
air/ground sensor
Certain systems require electrical control depending upon the condition of the aircraft
(airborne or on the ground). The condition of the aircraft is sensed by the landing gear
air/ground sensors. There are two air/ground sensors installed on the aircraft. The
main gear sensor is located in the main gear wheel well and is connected to the right
main gear via a teleflex cable. The nose gear sensor is located on the steering collar of
the nose gear shock strut. The picture above indicates where the air/ground sensors
are located.
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Slide 54 of 56
Parking Brake
The parking brake is set by depressing either the Captain’s or First Officer’s brake pedals
fully while simultaneously pulling the PARKING BRAKE lever aft. This mechanically
latches the brake pedals in the depressed position and commands the parking brake
valve to close. The PARK BRAKE LIGHT will illuminate indicating the parking brake is
set. The parking brake is released by depressing either the Captain’s or First Officer’s
brake pedals until the PARKING BRAKE lever releases.
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Slide 55 of 56
Nose Wheel Steering
Depressurization Valve
To depressurize the steering system, a lockout pin may be installed in the nose wheel
steering depressurization valve. This is useful if an aircraft is being towed while the “A”
hydraulic system is pressurized. Some airline operators may use the depressurization valve
during push-back from their boarding gate to ensure the safety of the ground personnel.
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