The landing gear system on the orbiter is a conventional aircraft
tricycle configuration consisting of a nose landing gear and a
left and right main landing gear. Each landing gear includes a
shock strut with two wheel and tire assemblies. Each main landing
gear wheel is equipped with a brake assembly with anti-skid protection.
The nose landing gear is steerable. The nose landing gear is located
in the lower forward fuselage, and the main landing gear are located
in the lower left and right wing area adjacent to the midfuselage.
The nose landing gear is retracted forward and up into the lower
forward fuselage and is enclosed by two doors. The main landing
gear are also retracted forward and up into the left and right
lower wing area, and each is enclosed with a single door. The
nose and main landing gear can be retracted only during ground
For retraction, each gear is hydraulically rotated forward and
up during ground operations until it engages an uplock hook for
each gear in its respective wheel well. The uplock hook locks
onto a roller on each strut. Mechanical linkage driven by each
landing gear mechanically closes the respective landing gear doors.
All three landing gear doors have high-temperature reusable surface
insulation thermal protection system tiles bonded to their outer
surface with thermal barriers to protect and prevent the landing
gear and wheel well from the high-temperature thermal loads encountered
during the shuttle's entry into the atmosphere.
For deployment of the landing gear, the uplock hook for each
gear is activated by the flight crew initiating a gear-down command.
The uplock hook is hydraulically unlocked by hydraulic system
1 pressure applied to release it from the roller on the strut
to allow the gear, assisted by springs and hydraulic actuators,
to rotate down and aft. Mechanical linkage released by each gear
actuates the respective doors to the open position. The landing
gear reach the full-down and extended position within 10 seconds
and are locked in the down position by spring-loaded downlock
bungees. If hydraulic system 1 pressure is not available to release
the uplock hook, a pyrotechnic initiator at each landing gear
uplock hook automatically releases the uplock hook on each gear
one second after the flight crew has commanded gear down.
The landing gears are deployed only after the spacecraft has
an indicated airspeed of less than 300 knots (345 mph) and an
altitude of approximately 250 feet.
The shock strut of each landing gear is the primary source of
shock attenuation at landing. The struts have air/oil shock absorbers
to control the rate of compression extension and prevent damage
to the vehicle by controlling load application rates and peak
Each main landing gear wheel contains an electrohydraulic disc
brake assembly with anti-skid control. The main landing gear brakes
are controlled by the commander or pilot applying toe pressure
to the rudder pedals; electrical signals produced by rudder pedal
toe pressure control hydraulic servovalves at each wheel and allow
hydraulic system pressure to perform braking. Main landing gear
brakes cannot be applied until weight on the main gear has been
sensed. The anti-skid system monitors wheel velocity and controls
brake torque to prevent wheel lock and tire skidding. The braking/anti-skid
system is redundant in that it utilizes system 1 and 2 hydraulic
pressure as the active system with system 3 as standby and also
utilizes all three main dc electrical systems.
The nose landing gear contains a hydraulic steering actuator
that is electrohydraulically steerable through the use of the
onboard general-purpose computers, the commander's or pilot's
rudder pedals in conjunction with the orbiter flight control system
in the control stick steering mode, or through the use of the
commander's or pilot's rudder pedals in the direct mode. If hydraulic
system 1 is inoperative, nose wheel steering changes to caster
mode, and the commander or pilot would then apply toe pressure
to the brake pedals to apply hydraulic pressure to the left and
right main gear brakes as required for directional control using
Each landing gear shock strut assembly is constructed of high-strength,
stress- and corrosion-resistant steel alloys, aluminum alloys,
stainless steel and aluminum bronze. Cadmium and chromium plating
and urethane paint are applied to the strut surfaces for space
flight protection. The shock strut is a pneudraulic shock absorber
containing gaseous nitrogen and hydraulic fluid. Because the shock
strut is subjected to zero-g conditions during space flight, a
floating piston separates the gaseous nitrogen from the hydraulic
fluid to maintain absorption integrity.
Landing gear wheels are made in two halves from forged aluminum
and are primed and painted with two coats of urethane paint.
The LG hyd isol vlv 1, 2 and 3 switches on panel R4 control the
corresponding landing gear isolation valve in hydraulic systems
1, 2 and 3. When the LG hyd isol vlv 1 switch on panel R4 is positioned
to close , hydraulic system 1 is isolated from the nose and main
landing gear deployment uplock hook actuators and strut actuators,
nose wheel steering actuator and main landing gear brake control
valves. A talkback indicator next to the switch would indicate
cl. The landing gear isolation valves will not close or open unless
the pressure in that system is at least 100 psi. When the LG hyd
isol vlv 1 switch is positioned to open , it allows hydraulic
system 1 source pressure to the main landing brake control valves
and to the normally closed extend valve. The normally closed extend
valve is not energized until a gear-down command is initiated
by the commander or pilot on panel F6 or panel F8. The talkback
indicator would indicate op . The LG hyd isol vlv 1 switch is
left in the close position during the mission to prevent inadvertent
The LG hyd isol vlv 2 and 3 switches on panel R4 positioned to
close isolate the corresponding hydraulic system from only the
main landing gear brake control valves. The adjacent talkback
indicator would indicate cl . When switches 2 and 3 are positioned
to open, the corresponding hydraulic system source pressure is
available to the main landing gear brake control valves. The corresponding
talkback indicator would indicate op.
Thus, only hydraulic system 1 is used to deploy the nose and
main landing gear and for nose wheel steering. When the nose-
and main-landing-gear-down command is initiated by the commander
or pilot on panel F6 or F8, hydraulic system 1 pressure is directed
to the nose and main landing gear uplock hook actuators and strut
actuators (provided that the LG hyd isol vlv 1 switch is in the
open position) to actuate the mechanical uplock hook for each
landing gear and allow the gear to be deployed and also provide
hydraulic system 1 pressure to the nose wheel steering actuator.
The main landing gear brake control valves receive hydraulic system
1 source pressure when the LG hyd isol vlv 1 switch is positioned
to open. If hydraulic system 1 is unavailable, a pyrotechnic actuator
attached to the nose and main landing gear uplock actuator would
deploy the landing gear automatically one second after the gear-down
command, actuate the mechanical uplock hook for each landing gear
and allow the gear to be deployed. Because powered nose wheel
steering would not be functional, directional control for steering
would be accomplished by differential braking to caster the nose
The GPC position of the LG hyd isol vlv 1, 2 and 3 switches on
panel R2 permits the onboard computer to automatically control
the valves in conjunction with computer control of the corre sponding
hydraulic system circulation pump. The LG hyd isol vlv 2 and 3
switches provide fluid circulation to only the main landing gear
brake system, which dead-ends at the brake control valves. The
LG hyd isol vlv 1 switch is left closed to prevent inadvertent
The normally open hydraulic system 1 redundant shutoff valve
is a backup to the retract/circulation valve to prevent hydraulic
pressure from being directed to the retract side of the nose and
main landing gear uplock hook actuators and strut actuators if
the retract/circulation valve fails to open during nose and main
landing gear deployment.
The normally closed hydraulic system 1 dump valve is energized
open to allow hydraulic system 1 fluid to return from the nose
and main landing gear areas when deployment of the landing gear
is commanded by the flight crew.
The activation/deactivation limits of the hydraulic fluid circulation
systems can be changed during the mission by the flight crew or
the Mission Control Center-Houston. The program also includes
a timer to limit the maximum time a circulation pump will run
and a priority system that automatically monitors hydraulic bootstrap
pressure to allow all three circulation pumps to be on at the
same time. The software timers allow this software to be used
in contingency situations for ''time-controlled'' circulation
pump operations in order to periodically boost an accumulator
that is losing hydraulic fluid through a leaking priority valve
or unloader valve.
During entry, if required, LG hyd isol vlv 1, 2 and 3 are positioned
to GPC . At 19,000 feet per second, the landing gear isolation
valve automatic opening sequence begins under GN&C; software control.
If the landing gear isolation valve is not opened automatically,
the flight crew will be requested by the Mission Control Center
to open the valve by positioning the applicable LG hyd isol vlv
to open. Landing gear isolation valve 2 is automatically opened
six minutes and 37 seconds later, and this is followed by the
automatic opening of landing gear isolation valve 1 when orbiter
velocity is at 800 feet per second or less. Landing gear isolation
valve 3 is automatically opened at ground speed enable. Landing
gear isolation valve 1 is next to last to ensure that an inadvertent
gear deployment would occur as late (low airspeed) as possible.
Note that the hydraulic system 1 retract/circulation valve would
be automatically closed when the landing gear system is armed
The commander and pilot have a landing gear deployment arm and
dn (down) guarded push button switch/light indicators and landing
left, nose and right indicators. The commander's controls and
indicators are on panel F6, and the pilot's controls and indicators
are on panel F8. The dn push button, when depressed, energizes
the hydraulic system 1 normally closed extend valve, permitting
hydraulic system 1 source pressure for gear deployment and nose
The proximity switches on the nose and main landing gear doors
and struts provide electrical signals to control the landing gear
nose, left and right indicators on panels F6 and F8. The output
signals of the landing gear and door uplock switches drive the
landing gear up position indicators and the backup pyrotechnic
release system. The output signals of the landing gear downlock
switches drive the landing gear dn position indicators. The landing
gear indicators are barberpole when the gear is deploying (or
The left and right main landing gear weight-on-wheels switches
produce output signals to the guidance, navigation and control
software to reconfigure the flight control system for landing.
The two weight-on-nose-gear signals run to the main landing gear
brake/skid control boxes to prevent the main landing gear brakes
from being applied until the nose gear is in contact with the
runway and also to the GN&C; software, which computes a nose wheel
steering enable signal. This enable signal is then sent to the
NWS control box to prevent NWS until the nose gear is in contact
with the runway.
The six group 1 switches are signal conditioned by the landing
gear proximity sensor electronics box 1, located in avionics bay
1. The six group 2 switches are signal conditioned by the landing
gear proximity sensor electronics box 2, located in avionics bay
Landing gear deployment is initiated when the commander or pilot
depresses the guarded arm push button switch/light indicator and
then the guarded dn push button switch/light indicator at least
15 seconds before predicted touchdown and at a speed no greater
than 300 knots (345 mph).
Depressing the arm push button switch/light indicator energizes
latching relays that close the hydraulic system 1 landing gear
retract/circulation valve and the normally open redundant shutoff
valve to the retract/circulation valve. It also arms the nose
and main landing gear pyrotechnic initiator controllers and illuminates
the yellow light in the arm push button switch/light indicator.
The dn push button switch/light indicator is then depressed.
This energizes latching relays that open the hydraulic system
1 landing gear normally closed extend control valve, permitting
the fluid in hydraulic system 1 to flow to the landing gear uplock
and strut actuators and nose wheel steering. The relays also open
the normally closed dump valve, allowing the landing gear retract
line fluid to flow in to the hydraulic system 1 return line. The
green light in the dn push button switch/light indicator is illuminated.
Hydraulic system 1 source pressure is routed to the nose and
main landing gear uplock actuators, which releases the nose and
main landing gear and door uplock hooks. As the uplock hooks are
released, the gear begins its deployment and mechanical linkage
attached to the doors and fuselage is powered by landing gear
strut camming action, during gear extension, which opens the landing
gear doors. There are two landing gear doors for the nose gear
and one for each main gear. The landing gear free falls into the
extended position, assisted by the strut actuators and airstream
in the deployment. The hydraulic strut actuator incorporates a
hydraulic fluid flow through orifice (snubber) to control the
rate of landing gear extension and thereby prevent damage to the
gear's downlock linkages.
If hydraulic system 1 fails to release the landing gear within
one second after the dn push button is depressed, the nose and
left and right main landing gear uplock sensors (proximity switches)
will provide inputs to the pyro initiator controllers for initiation
of the redundant NASA standard detonators (nose, left and right
main landing gear pyrotechnic backup release system). They release
the same uplock hooks as the hydraulic system. The nose landing
gear, in addition, has a PIC and redundant NSDs that initiate
a pyrotechnic power thruster two seconds after the dn push button
is depressed to assist gear deployment.
The landing gear drag brace overcenter lock and spring-loaded
bungee lock the nose and main landing gear in the down position.
The ldg gr/arm/dn reset switch positioned to reset on panel A12
unlatches the relays that were latched during landing gear deployment
by the landing gear arm and dn push button light/switch indicators.
This is primarily a ground function, which will be performed only
during landing gear deactivation.
The reset position also will extinguish the yellow light in the
arm push button switch/light indicator and the green light in
the dn push button switch/light indicator. In addition, the hydraulic
system 1 landing gear dump valve is closed, the extend control
valve is closed, the retract/circulation valve is opened only
if the switch is in the open position, and its redundant shutoff
valve is opened (de-energized) and de-energizes the landing gear
The nose landing gear tires are 32 by 8.8 inches and will withstand
a burst pressure of not less than 3.2 times the normal inflation
pressure of 300 psi. The inflation agent is gaseous nitrogen.
The maximum allowable load per nose landing gear tire is approximately
45,000 pounds and rated at 224 knots (258 mph) landing speed.
The nose landing gear shock strut has a 22-inch stroke. The maximum
allowable derotation rate is approximately 9.4 degrees per second
or 11 feet per second, vertical sink rate.
The main landing gear tires are 44.5 by 16 and 21 inches. The
normal inflation pressure is 315 psi, and the inflation agent
is gaseous nitrogen. The maximum allowable load per main landing
gear tire is 123,000 pounds. If the orbiter touches down with
a 60/40 percent load distribution on a strut's two tires, with
one tire supporting the maximum load, then the other tire can
support a load of only 82,410 pounds. Therefore, the maximum tire
load on a strut is 205,410 pounds with a 60/40 percent tire load
distribution. The tires are rated at 225 knots (258 mph).
The main landing gear shock strut stroke is 16 inches. The allowable
main gear sink rate for a 212,000-pound orbiter is 9.6 feet per
second; for a 240,000-pound orbiter, it is 6 feet per second.
With a 20-knot (23-mph) crosswind, the maximum allowable gear
sink rate for a 212,000-pound orbiter is 6 feet per second; for
a 240,000-pound orbiter, it is approximately 5 feet per second.
The landing gear tires have a life of one landing.