Landing Gear
Landing Gear
Landing Gear
Introduction
Landing gears have two main functions:
Supporting the weight of the stationary aircraft on the ground
Absorbing the loads during touchdown, the landing run and taxiing.
They are divided into two main categories, fixed (non-retractable) or fully
retractable.
Early aircraft had fixed landing gear, which unfortunately produced a large
amount of parasitic drag in flight. Since drag increases at the square of forward
speed, as aircraft began to fly faster, the resulting amount of drag became too
prohibitive.
In the short term, this problem was resolved by simply installing streamlined
fairings over the wheels. However it soon became clear that this drag could be
almost completely eliminated, if the landing gear were retracted after take off and
stowed out of the air-stream.
General
Early landing gear designs consisted of two main legs set just in front of the
centre of gravity (C of G) of the aircraft and a small tailwheel at the rear end of
the fuselage. Putting the C of G just aft of the main gear, ensured the aircraft very
quickly attained flying attitude during take off.
All aircraft at that time, were propeller-driven types and the inclined fuselage gave
ample clearance between the propeller and the ground during taxiing, take-off
and landing.
However the main disadvantage of this configuration was the risk that the aircraft
was likely to nose over when heavy braking was applied and poor vision for the
crew during taxiing and the initial part of the take off run.
This problem was overcome by the development of the Tricycle configuration,
which is now used almost exclusively. This places the main landing gear aft of the
C of G and a supporting nose gear at the forward end of the fuselage. As aircraft
became larger and heavier, landing gear design included multi-leg and multiwheel configurations.
Construction
All landing gears have to be attached to strong points on either the fuselage or
the wing structure, so that the landing loads can be absorbed and transferred
safely to the aircraft structure.
Smaller light aircraft use a steel leaf or tubular steel spring to act as an
undercarriage (figure 3). One end is attached to a strong point on the airframe
while located on the other end is the wheel and axle. The deflection of the spring
tube on landing absorbs the landing loads and transmit them to the airframe. A
properly conducted landing will not cause any undercarriage rebound.
BUNGEE SHOCK
CORD
Bungee Cord Type Landing Gear
Figure 4
Larger more modern aircraft, require more complex and heavier retractable
systems (Figure 5). The larger the aircraft the larger the system. The components
remain similar just the size and quantities change (Figure 6). Each landing gear
unit is basically a wheeled shock absorber (oleo). A forged cylinder body is
attached to the airframe on trunnions to allow it to pivot when lowered and raised.
Articulated side stays are located between the cylinder body and airframe strong
points to give the landing gear strength and rigidity and allow the landing gearleg
to fold. Drag or bracing struts may also be fitted. These absorb the high
acceleration loads during take off and deceleration loads during braking.
MAIN SUPPORT FRAMES
TRUNNION
MAIN ACTUATOR
DOWNLOCK ACTUATOR
BRACING STRUT
DOWNLOCK LINKAGE
(TOGGLE LEVERS)
SIDE STAY
MAIN OLEO
PISTON
MAIN ACTUATOR
DOWNLOCK
ACTUATOR
CYLINDER
BRACING STRUT
PISTON
SCISSOR
(TORQUE) LINK
WHEEL
SINGLE
D OU BLE
TAN DEM
BOGIE
They provide greater safety. As the loads are spread over several wheels a
burst tyre is not so critical as the remaining wheels accept the extra loads.
Due to the large footprint the turning circle is increased to prevent the tyres
from crabbing and increasing wear.
Shock Absorbing
In order to absorb and dissipate the tremendous shock loads of landing, the
kinetic energy of the impact must be converted into other forms of energy. This is
achieved on most landing gear legs by using self-contained hydraulic shock
absorbing struts.
There are three main types of strut commonly used in commercial aircraft:
The strut uses a compressed gas (normally nitrogen) combined with a specific
quantity of hydraulic oil to absorb and dissipate the shock loads. It is essentially
an outer cylinder into which an inner hollow piston is inserted.
When the aircraft is airborne, the landing gear is no longer supporting the aircraft
weight, consequently the piston fully extends under the influence of the nitrogen
pressure. The nitrogen gas being lighter than oil, will settle in the upper portion of
the cylinder with the heavier oil at the bottom. Since in this particular type of strut
there is no separator between the oil and gas, there will be some aeration (froth)
as the oil and gas mix together at the demarcation line.
On landing, the inner piston is forced up into the outer cylinder, reducing the
internal volume. A tapered metering pin and snubber knob which are an integral
part of the piston, are forced into a snubber tube carried by the outer cylinder.
(See Figure 8).
OIL BLEED VALVE
NITROGEN/OIL
CHARGING VALVE
FLAPPER
VALVE
(CLOSED)
INNER
CYLINDER
SNUBBER KNOB
CYLINDER
SNUBBER
TUBE
METERING PIN
SNUBBER
KNOB
PISTON
(Strut Compressed)
Figure 8
METERING
PIN
PISTON
(Strut Extended)
Figure 9
Oil is forced into the upper chamber through a series of holes in the snubber tube
and through the open flapper valve. The tapered shape of the metering pin
steadily reduces the available orifice area as it compresses.
The landing energy is therefore absorbed by the oil, as it is forced through the
ever-decreasing sized orifice and by the compression of the nitrogen gas, as the
oil is forced into the reduced volume of the upper chamber.
The problem now is to absorb the recoil, to prevent the aircraft from bouncing
back up from the runway.
As the piston starts to extend, the oil is now forced downwards into the hollow
piston. The rate at which this transfer takes place is greatly restricted by the
flapper valve slamming shut, leaving only a reduced number of holes in the
snubber tube to permit transfer the oil. This restriction in flow and the associated
increase in internal volume, prevents rapid strut extension and thus dampens the
recoil energy. (See Figure 9).
In this design, the principle is exactly the same as the oleo-pneumatic without
separator type previously described. The main difference is the inclusion of a
floating piston, to separate the oil chamber from the nitrogen chamber and
therefore prevent oil and gas mixing together. It also means that the nitrogen
chamber does not have to be positioned at the top of the leg, or indeed be limited
to one chamber. This makes shock absorbing more efficient, less severe jolting
during taxiing and will simplify servicing (see later).
Liquid Spring
This type does not have a gas compartment. Instead, it relies on the fact that if a
piston is forced into a cylinder completely filled with oil under a static pressure,
energy absorption will take place due to oil compression.
Oil is generally considered to be incompressible, however it is a fluid and will
obey the same rules as for a gas. At normal hydraulic system pressures (typically
3000 psi), the amount of compression is negligible. However, in liquid spring
shock absorbers, pressures in excess of 60,000 psi will often be generated and in
this case the oil will be compressed.
During touchdown, the inner piston is forced up into the upper cylinder as before,
compressing the oil as the volume progressively reduces by what is known as,
jack ram displacement. A restrictor valve inserted as before, will absorb the
recoil in a similar manner to the previous two types.
Servicing Filling and Charging
To guarantee the correct operation of the shock absorber, the strut must be
serviced in order to fill the leg with the proper quantity of oil. Additionally, the oil
must be completely free of air. The nitrogen chamber must also be charged to the
correct value in order to maintain the correct oil/gas ratio.
When correctly filled and charged, the strut will adopt the correct extension when
supporting the aircraft on the ground and the risk of the inner piston coming into
contact with the outer cylinder (bottoming) during touchdown will be eliminated.
Filling and charging procedures will vary between aircraft type, will be detailed in
the Aircraft Maintenance Manual (AMM) and must be strictly adhered to. A
general sequence of events to fill and charge a typical oleo-pneumatic without
separator type of strut (conforming to relevant health and safety regulations), is
detailed as follows:
Normally the aircraft will be positioned on jacks with the wheels clear of the
ground.
Using an approved adapter, completely release the nitrogen pressure via the
charging valve and ensure the valve remains open after all pressure has been
dissipated.
Place a bottle jack under the strut and carefully compress the leg, pushing the
inner piston into the outer cylinder until it bottoms and the leg is fully
compressed.
Open the hydraulic bleed valve and pump oil into the oil filling connection until
fresh clean oil, completely free of air bubbles, emerges from the bleed valve.
The leg is now completely filled with oil to the correct quantity.
Close and tighten the oil charging valve and oil bleed valve.
Remove the bottle jack, connect a nitrogen rig to the nitrogen charging valve.
Slowly and carefully inflate the leg with nitrogen until the leg is fully extended
and the inflation adapter gauge shows the correct gas pressure obtained from
the AMM.
Close and tighten the nitrogen charging valve and remove the charging rig.
Repeat if required on the other main leg.
Lower the aircraft off jacks.
The legs are now properly filled and charged.
OIL BLEED POINT
OIL CHARGING
VALVE
OIL
OIL BLEED
SEPARATOR
SEPARATOR
CHARGING
VALVE
GAS
OLEO - PNEUMATIC WITH
SEPARATOR
Figure 10
LEG EXTENDED
Figure 11
The landing gear selector valve operates, and the down lines to the actuators and
the return lines to the reservoir are opened. The fluid pressure flows through the
selector valve to the actuators and extends the actuators. Once the main
actuators are fully extended and the undercarriage legs have mechanically
locked, excess pressure is bled back through the low pressure control valve to
the reservoir.
When all 3 wheels are down and locked, proximity switches send signals to a
control unit which turns the hydraulic pump off, closes the selector valve lines and
sends signals to the instrument panel indicating that the undercarriage is locked
down, (green triangles).
Retraction System
The retraction procedure is basically the opposite of the extension procedure.
When the selector lever is selected GEAR UP a micro-switch on the lever is
made which powers up the hydraulic pump, the hydraulic pressure is then fed to
the downlock actuators to unlock the mechanical locks on the bracing struts. Its is
also fed to the selector valve and opens the uplines to the main actuators and the
return lines to the reservoir.
Movement of the undercarriage legs breaks the downlock proximity switches
which send signals to the control unit which indicates on the instrumentation
panel that the landing gears are in transit, (red triangles) and that the
undercarriage is unlocked.
The fluid pressure flows through the selector valve to the main actuators and
retracts the landing gear. The undercarriage legs on full retraction mechanically
lock the uplocks. Once the main actuators are fully retracted and the
undercarriage legs are locked up, excess pressure is bled back through the low
pressure control valve to the reservoir. When all 3 wheels are up and locked,
uplock limit switches send signals to a control unit which turns the hydraulic pump
off, closes the selector valve lines and change the red triangles to black on the
indicating panel.
If a red triangle remains on when the undercarriage is fully extended or retracted
there is a fault in the system. A squat switch system and an electro-mechanical
stop on the selector lever, will prevent the landing gear from being retracted when
the aircraft is on the ground. The landing gear will not be able to be retracted until
certain parameters are met. This is normally when all landing gear legs have fully
extended after take off. This is sensed by proximity switches on each leg.
Uplock mechanism
On large modern aircraft when the landing gear is being retracted the uplocks will
operate mechanically. A roller on the landing gear leg will locate and engage into
the uplock hook. Limit switches will sense when the landing gear leg has
engaged in the lock hook and will turn off the hydraulic pressure. The gear will
then be held retracted in place purely mechanically. (Figure 14)
LIMIT SWITCH
Locked Uplock
Figure 14
Normal release of the uplock is by a hydraulically actuated valve. The supplied
hydraulic pressure pushes a plunger against the lock lever which rotates about its
pivot. This action allows the uplock hook to disengage under its own spring
tension. The landing gear will then be extended hydraulically by the main
actuator. (Figure 15)
LOCK LEVER ASSEMBLY
LIMIT SWITCH
PLUNGER
UNLOCK ACTUATOR VALVE
UPLOCK HOOK
Unlocked Uplock
Figure 15
Downlock mechanism
The downlock actuator can have either a single or double direction operation
depending on the aircraft. A single direction operation would unlock the downlock
mechanism (upper and lower toggles) prior to retraction, the leg relying on its own
extension to provide the over centre lock. The double direction actuator will lock
the downlock mechanism on extension and unlock it prior to retraction.
Once the landing gear has been fully extended and is sensed by a limit switch
hydraulic pressure is directed to the downlock actuator which extends the
actuator piston. The piston acts against a toggle lever which move both toggle
levers to an over centre position. This over centreing of the toggle levers forms a
mechanical lock which prevents the landing gear leg from collapsing. (Figure 16)
MAIN LEG
DOWNLOCK ACTUATOR
PROXIMITY SWITCH
SIDE BRACE
UPPER TOGGLE
LEVER
LOWER TOGGLE
LEVER
PROXIMITY SWITCH
CENTRE LINE
Linkage Downlocked
Figure 16
Once the aircraft has landed and parked up, a red flagged safety pin is inserted
through alignment holes in the toggle levers to prevent inadvertent collapse or
retraction of the landing gear on the ground. This safety pin is removed before
flight.
On selecting the landing gear up, the hydraulic pressure is directed initially to the
downlock actuator and retracts the piston. As the piston retracts it moves the
lower toggle overcoming the mechanical lock, moving both toggle levers from the
over centre position to an under centre position, so that the landing gear can now
fold. (Figure 17)
MAIN LEG
DOWNLOCK ACTUATOR
PROXIMITY SWITCH
SIDE STAY
UPPER TOGGLE
LEVER
UNDER CENTRE POSIITION
LOWER TOGGLE
LEVER
PROXIMITY SWITCH
CENTRE LINE
Linkage Unlocked
Figure 17
Emergency Landing Gear Operation
The uplocks can be released manually if the actuator or hydraulic system fails. An
emergency landing gear lever, operated from the cockpit will act on and rotate the
hook locks, releasing the landing gear legs from the uplock hooks. The
emergency mechanism lever will also operate a lever on the landing gear selector
valve which will open all hydraulic lines to return. This allows the hydraulic fluid to
free flow through the system, to allow the landing gear to extend.
Once the uplocks are released the landing gear legs will extend under gravity and
aerodynamic forces. Spring or gas operated free fall assistors may be used to
help the gear extend. The proximity and limit switches will operate as normal
giving a cockpit indication of the gear in transit and down locked.
LIMIT SWITCH
UPLOCK HOOK
UNLOCK ACTUATOR VALVE
CABLE
EMERGENCY
OPERATING HANDLE
To further reduce the drag some doors will close when the landing gear has been
extended. The landing gear doors may have a manual unlocking mechanism to
allow the door to be opened on the ground for access in carrying out
maintenance tasks and inspections.
Anything that jeopardises the sequence can cause considerable damage to the
aircraft structure and could lead to an unsafe landing condition. Door sequencing
relies on the movement of valves operated by the doors and the movement of the
legs. The sequencing valve can be therefore be either door operated or gear
operated.
RED TRANSIT
GREEN
LOCKED
DOWN
ADDITIONAL NOSE
LOCKED DOWN GREEN
GREEN
LOCKED
DOWN
LIGHTS
AMBER FAULT
LIGHTS
Micro switches or proximity sensors are fitted to each leg to relay information the
flight deck indicators. A change the output voltage whenever the uplock or
downlock mechanisms are made or broken during the retraction or lowering
sequences, determine indicator output.
Other methods can be mechanical indicators outside the aircraft, visible from the
cockpit. There may be painted indicator lines on the landing gear legs toggle
levers which align when the gear is down and locked. (Figure 24)
UNLOCKED
LOCKED
Some aircraft have pop up indicators which stand proud on the upper wing
surface when the gear is down and locked (Figure 25). These are plunger
operated through a cable linkage attached to the toggle levers. When the landing
gear extends and is locked down a plate attached to the toggle lever operates a
spring loaded plunger which by cable connection moves the indicator from its
housing, proud of the airframe skin. The indicator returns under spring pressure
into its housing when the landing gear is retracted
POP UP INDICATOR
AIRFRAME SKIN
UNLOCK ACTUATOR
PLUNGER
TOGGLE
LEVERS
SIDE STRUT
POP UP INDICATOR
AIRFRAME SKIN
UNLOCK ACTUATOR
PLUNGER
TOGGLE
LEVERS
SIDE STRUT