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.

Tail Wheel Type UndercarriageFIGURE 1

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 multi-wheel configurations.

Nose Wheel Type UndercarriageFigure 2

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.

Spring Tube TypeFigure 3

Another simple method was to use elastic bungee cord encased in a loose weave cotton braid (Figure 4). The bungee cord is located on a series of support struts which support the wheel and axle. The bungee cord stretches on landing and transfers the landing forces into the airframe.

Bungee Cord Type Landing GearFigure 4

BUNGEE SHOCK CORD

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.

Landing Gear Leg With Bracing StrutsFigure 5

The wheel and axle assembly (bogey) is attached to the piston end. A hinged torque (scissor) link is located between the axle yoke and the cylinder body. This allows the piston to move freely in and out of the cylinder but prevents the piston and wheel assembly from swivelling.

Two actuators are usually fitted. A main actuator attached to the cylinder body to raise and lower the gear and a downlock actuator located on the bracing strut which operates to cause a mechanical lock when lowered. It also unlocks the gear mechanism before raising.

A Hop Damper is often used with multi wheel units to align the bogie at the correct angle for landing and absorbs minor shock loads during taxiing. It is connected between the main landing gear body and the bogie.

Oleo Type Landing GearFigure 6

Multiple Axles And Wheels

To allow for maximum utilisation of aircraft when operating from different runways multi wheel landing gear is used. Typical configurations are shown in Figure 7.

Wheel Axle ConfigurationsFigure 7

The advantages of using multi-wheel configurations are:

They spread the landing loads over a larger area (footprint).

They are easier to stow as the wheel volume is reduced.

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.

The main disadvantages are:

There are more moving parts so they need more maintenance.

They are expensive to produce

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:

Oleo-pneumatic without separator Oleo-pneumatic with separator Liquid Spring

Oleo-pneumatic without separator

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 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).

FLAPPER VALVE (OPEN)

METERING PIN

SNUBBER KNOB

NITROGEN/OILCHARGING VALVE

OIL BLEED VALVE

CYLINDER

PISTON

FLAPPERVALVE(CLOSED

SNUBBERTUBE

SNUBBERKNOB

METERINGPIN

PISTON

CYLINDER

INNER CYLINDER

(Strut Compressed)Figure 8

(Strut Extended)Figure 9

Oleo - Pneumatic without separator

Oleo-pneumatic with separator

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

SEPARATOR

CHARGING VALVE

OIL BLEED

OIL

SEPARATOR

GAS

OLEO - PNEUMATIC WITHSEPARATOR

Figure 10

LEG EXTENDEDFigure 11

Extension And Retraction Systems

As the speed of the aircraft becomes high enough that the parasite drag of the landing gear is greater than the induced drag caused by the added weight of the retracting system it becomes economically practical to retract the landing gear into the aircraft structure.

Raising or lowering of the undercarriage is carried out either hydraulically or pneumatically via a selector lever in the cockpit which is mechanically or electrically linked to a selector valve. When the selector valve is operated it directs the fluid to one side or the other of the piston.

The landing gear is uplocked and downlocked mechanically or hydraulically through the uplock boxes and the downlock toggle levers. Landing gear positions are sensed by proximity switches or microswitches and transmit these positions to the cockpit instrumentation via a control unit.

In the case of fluid or electrical failure, a mechanical emergency lowering system is available. An emergency handle located in the cockpit is operated and by a system of push-pull cables and gearboxes, the uplocks are released.

The landing gear selector valve or a freefall valve is also operated, which opens all extension and retraction lines to return. The landing gear is allowed to fall under gravity and aerodynamic forces but may be assisted by a spring or gas operated free fall assister.

Smaller light aircraft may use differing methods for operating the landing gear. Electric motors may drive actuators, a winding cable system, a simple operating lever with safety locks or a manual hydraulic jacking system may be used to raise or lower the landing gear.

Most modern light aircraft use a hydraulic power pack. This is a self-contained system and was designed to be lightweight and easy to maintain. The pack contains the fluid reservoir, sight glass, pressure pump, filter, thermal relief valve, pressure relief valve, ground service and replenishment connections.

Extension System

When the selector lever is selected to GEAR DOWN a micro-switch on the lever is made which powers up the hydraulic pump, the hydraulic pressure is then fed to the uplock actuator valves to unlock the uplocks. Once operated, the uplock hooks remain mechanically open under spring pressure. Movement of the undercarriage legs break the uplock limit switches which indicates on the instrumentation panel that the landing gears are in transit.(red triangles) and that the undercarriage is unlocked.

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)

Locked UplockFigure 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)

Unlocked UplockFigure 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)

Linkage DownlockedFigure 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)

Linkage UnlockedFigure 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.

Emergency Release MechanismFigure 18

On aircraft fitted with hydraulically sequenced doors if the hydraulic system fails, the door jack is mechanically unlocked. This will also be carried out by a mechanical linkage connected to the cockpits emergency release mechanism (Figure 18)

Landing Gear Doors Sequencing

To keep the aircraft as streamlined as possible and to reduce drag, the landing gear is normally retracted into bays within the aircraft structure. However some aircrafts landing gear do not fully retract into the structure and some access doors do not fully enclose the landing gear.

The bays have access doors which open and close in relation to the movement of the landing gear. Some doors are mechanically linked to the landing gear, by a system of connecting rods, bellcranks and links, whilst other doors open and close under operation from a hydraulic sequencing valve, signalled by micro-switches or proximity switches via a control unit.

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.

Indications and Warning Indications and Warning

All modern aircraft fitted with retractable landing gear will have a means of indicating on the flight deck whether the legs are locked down, in transit or correctly locked up. Additionally, a separate warning system may be included to show faults, or to indicate that the legs are not in the position selected (‘nips’).

Normally leg position is shown, by a dedicated set of coloured indicators on the front panel, near to the landing gear selector lever. Each leg will have its own set of indicator lights. On some aircraft a ‘nips’ light is included in the selector lever itself.

The actual sequence of indication often varies from aircraft to aircraft, but the modern ‘dark cockpit’ philosophy during flight, usually means that all indicator lights are extinguished (no lights), when the legs are properly locked up. Red lights are often used when the legs are in transit (i.e.: not locked up and not locked down) and green lights illuminate when each leg is down and locked.

On other aircraft, the red transit lights are replaced by the ‘nips’ light in the selector lever, and separate amber warning lights on the front panel will show a fault. (I.e.: if any leg fails to reach its selected position, either locked up or locked down, within a certain time limit.) Also, where for example, visual confirmation from the cabin windows is not possible, usually for nose gear, the locked down indicator may be duplicated, as an additional ‘confidence light’, in case a bulb failure occurs.

RED TRANSIT

GREEN LOCKED DOWN

A RED LIGHT COMES ON WHENEVER:1. THE LEVER IS NOT DOWN AND

GEAR NOT UP.2. THE LEVER IS DOWN AND GEAR

NOT DOWN AND LOCKED.3. ENGINE No. 1 OR 2 THROTTLE IS IN

IDLE RANGE AND ANY GEAR NOT DOWN AND LOCKED.

A GREEN LIGHT COMES ON WHENEVER:1. THE GEAR IS DOWN AND LOCKED.

Gear Position IndicatorFigure 21

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.

GREEN LOCKED DOWN LIGHTS

ADDITIONAL NOSE LOCKED DOWN GREEN

AMBER ‘FAULT LIGHTS’

RED ‘NIPS’ LIGHT INSIDE HANDLE

Landing Gear Selector and IndicationsFigure 22

Gear Downlock SensorsFigure 23

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)

Landing Gear Down Locked Visual IndicatorFIGURE 24

MAIN LANDING GEAR MAIN LANDING GEAR DOWNLOCK SENSOR

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

Landing Gear pop Up Indicator Figure 25