Landing Gear System
A landing gear usually takes the form of two or more main undercarriage units in the wings or fuselage, and an auxiliary undercarriage unit at the nose or tail which carries only a small proportion of the total load and is used for steering purposes.
The functions of a landing gear are
to support an aircraft during ground manoeuvres, dampen vibration, and absorb landing shocks;
Performs the functions of steering and braking.
These objectives are achieved by many different designs, depending on the type of aircraft to which the landing gear is fitted and the degree of sophistication required.
Types of Landing Gears
The landing gears are generally classified as
Fixed Landing Gear
Retractable Landing Gears
With slow, light aircraft, and some larger aircraft on which simplicity is of prime importance, a fixed (non-retractable) landing gear is used which reduces performance caused by the drag of the landing gear during flight, however the simplicity, reduced maintenance and low initial cost are the advantages.
With higher performance aircraft, drag becomes progressively more important, and the landing gear is retracted into the wings or fuselage during flight. however is has disadvantages too like, penalties of increased weight, greater complication and additional maintenance.
The landing gear of an aircraft receive harsh treatment throughout its installed life, being subject to frequent landing shocks and in regular contact with spray, ice, dirt, and abrasive grit. Regular servicing and lubrication are required, therefore, to guard against corrosion, seizure of mechanical parts and failure of electrical components.
Another way of classification of landing gears is the type of landing gear arrangement. Different types of arrangement are shown in the diagram.
TYPES OF LANDING GEAR ARRANGEMENT
Fixed Landing Gears
There are three main types of fixed landing gear as follows :
those which have a spring steel leg
those which employ rubber cord to absorb shock
those which have an oleo-pneumatic strut to absorb shocks
Exceptions include aircraft with rubber in compression, spring coil, and liquid spring struts.
Spring Steel Legs
Spring steel legs are usually employed at the main undercarriage positions. The leg consists of a tube, or strip of tapered spring steel, the upper end being attached by bolts to the fuselage and the lower end terminating in an axle on which the wheel and brake are assembled.
Spring steel under carriages should be inspected regularly for damage and corrosion. The aircraft should be jacked up periodically, so that all load is taken off the wheels, and the security of each undercarriage checked by attempting to move it against the restraint of its attachments to the airframe structure. If there are signs of looseness, the bolts should be removed for detailed inspection and the bolt holes should be checked for cracks or fretting. Axle fittings should be similarly inspected, and all nuts and bolts should be tightened to the specified torque.
When rubber cord is used as a shock-absorber, the undercarriage is usually in the form of tubular struts, designed and installed so that the landing force is directed against a number of turns of rubber in the form of a grommet or loop.
Rubber cord is colour coded to indicate the date of manufacture and the specification to which it conforms, by replacing some of the fibres in the outer cotton covering with coloured threads wound in a spiral.
The undercarriage should be examined for damage, corrosion, wear or cracks at the pivot points, and bent pivot bolts, and should be lubricated as specified in the approved Maintenance Schedule. The rubber cord should be inspected for Chafing, necking, or other deterioration, and it is advisable to replace if it is more than five years old, regardless of its external condition.
Oleo- Pneumatic Struts
Some fixed main under carriages, and most fixed nose under carriages, are fitted with an oleo-pneumatic shock absorber strut. The design of individual struts varies considerably, and reference should be made to the appropriate Maintenance Manual for a particular type, but operation and maintenance procedures for a typical design are covered in the following paragraphs.
TYPES OF FIXED LANDING GEARS
Fig shows the construction of a simple oleo-pneumatic strut, in this instance a nose undercarriage which also includes a steering mechanism. The outer cylinder is fixed rigidly to the airframe structure by two mounting brackets, and houses an inner cylinder and a piston assembly, the interior space being partially filled with hydraulic fluid and inflated with compressed gas (air or nitrogen). The inner cylinder is free to rotate and move up and down within the outer cylinder, but these movements are limited by the torque links, which connect the inner cylinder to the steering collar. The steering collar arms are connected through spring struts to the rudder pedals, and a shimmy damper is attached to the steering collar.
CONSTRUCTION OF AN OLEO STRUT
Under static conditions the weight of the aircraft is balanced by the strut gas pressure and the inner cylinder takes up a position approximately midway up its stroke.
Under compression (e.g. when landing), the strut shortens and fluid is forced through the gap between the piston orifice and the metering rod, this restriction limiting the speed of upward movement of the inner cylinder.
As the internal volume of the cylinders decreases, the gas pressure rises until it balances the upward force.
As the upward force decreases, the gas pressure acts as a spring and extends the inner cylinder. The speed of extension is limited by the restricted flow of fluid through the orifice. Note: On some struts an additional valve is fitted to the piston or inner cylinder, to further restrict the flow of fluid during extension, and prevent violent extension of the strut if upward force is suddenly released, such as when a bounce occurs.
Normal taxying bumps are cushioned by the gas pressure and dampened by the limited flow of fluid through the orifice.
Movement of the rudder pedals turns the nose wheel to facilitate ground manoeuvres, the spring struts being provided to allow for vertical movement of the nose wheel, and prevent shocks from being transmitted through the rudder control system.
Most of the nose and tail wheels are fitted with shimmy dampers to prevent rapid oscillation during ground manoeuvres.
A simple damper consists of two friction discs, one connected to a fixed part of the undercarriage and the other connected to the oscillating part. The discs are held in contact by spring pressure and resist relative movement between the parts to which they are connected.
A type of damper commonly found on light aircraft consists of the piston rod is connected to the steering collar and the cylinder attached to a fixed part of the strut. The cylinder is completely filled with fluid, and small holes in the piston allow a restricted flow of fluid when force is applied to the piston rod. Movement of the nose undercarriage is therefore slowed down, and oscillations damped.
Retractable Landing Gear
The majority of modern transport aircraft, and an increasing number of light aircraft, are fitted with a retractable landing gear, for the purpose of improving aircraft performance. Retraction is normally effected by a hydraulic system, but pneumatic or electrical systems are also used.
In some instances power is used for retraction only, extension being effected by gravity and slipstream. Retractable landing gear is also provided with mechanical locks to ensure that each undercarriage is locked securely in the retracted and extended positions; devices to indicate to the crew the position of each undercarriage; and means by which the landing gear can be extended in the event of failure of the power source.
In addition, means are provided to prevent retraction with the aircraft on the ground, and to guard against landing with the landing gear retracted. Undercarriage wells are normally sealed by doors for aerodynamic reasons, but one particular aircraft type employs inflatable rubber bags to seal the main undercarriage wells.
Retractable under carriages normally consist of an oleo-pneumatic shock-absorber strut, but supported in a trunnion bearing which is fixed to a spar or strengthened box section in the wings or fuselage; the strut is braced longitudinally by drag struts, and laterally by side stays.
In some designs the drag strut or side stay is in two parts, and hinges about the centre point to provide a means of retraction, while in others the retraction jack operates on an extension of the shock-absorber strut housing.
TYPICAL RECTRACTABLE LANDING GEAR
Hydraulic Retraction system
A hydraulic system for retracting and extending a landing gear normally takes its power from engine driven pumps, alternative systems being available in case of pump failure. On some light aircraft a self-contained ‘power pack’ is used, which houses a reservoir and selector valves for the landing gear and flap systems; an electrically driven pump may also be included, or the system may be powered by engine driven pumps. This type of system normally provides for powered retraction of the landing gear, extension being by ‘free-fall’, with the assistance of spring struts.
TYPICAL LANDING GEAR SYSTEM
Operation of the system is as follows :
When the landing gear selector is moved to the ‘up’ position, fluid under pressure is directed to the ‘up’ line and fluid from the ‘down’ line is directed back to the hydraulic reservoir. Fluid flows to the sequence valves, retraction jacks, main undercarriage down-lock jacks , and nose undercarriage down-lock; it cannot pass the sequence valves, which are closed, but operates the retraction jacks and down locks. The locks operate first, releasing the landing gear and allowing the retraction jacks to raise each undercarriage, the nose undercarriage engaging its spring-loaded up-lock first, because of the jack’s smaller size. At the end of upward travel of the main undercarriage units, a striker on each leg contacts the plunger of its associated sequence valve, and opens the valve, allowing fluid to flow to the door jacks. The main undercarriage engages the up-locks and the doors close, engaging locks. Fluid in the ‘down’ lines returns to the reservoir, flowing unrestricted through the restrictor valves and overcoming the small restriction of the spring loading of the sequence valves.
Note : The nose undercarriage doors are operated mechanically by linkage to the nose shock-absorber housing.
When the landing gear selector is moved to the ‘down’ position, fluid under pressure is directed to the ‘down’ line, and fluid from the ‘up’ line is directed back to the reservoir. Fluid flows to the sequence valves , door jacks, door locks, nose undercarriage retraction jack and the nose undercarriage up-lock .
The sequence valves are closed, so fluid pressure releases all the door locks and the nose undercarriage up-lock, and the doors and nose undercarriage extend, the nose undercarriage engaging its down-lock at the end of its travel.
When the doors are fully open, the door jacks strike the plungers of their associated sequence valves and open the valves, allowing fluid to flow through the restrictor valves to the main undercarriage up-locks and retraction jacks. These locks are released, and the retraction jacks lower the main undercarriage fully, the spring-loaded lock-jacks imposing a geometric lock on the side stays. Main undercarriage doors are held open by fluid pressure.
Note: Restrictor valves are normally fitted to limit the speed of lowering of the main undercarriage units, which are influenced in this direction by gravity. The nose undercarriage often lowers against the slipstream and does not need the protection of a restrictor valve.
Pneumatic Retraction System
Operation of a pneumatic retraction system is similar to that of a hydraulic system, except that pressure in the return lines is exhausted to atmosphere through the selector valve. Pressure is built up in a main storage cylinder by engine driven air pumps, and passes through a pressure reducing valve to the landing gear selector valve.
Operation of the selector valve to the ‘up’ position directs pneumatic pressure through the ‘up’ lines to the retraction rams, and opens the down line to atmosphere. Operation of the selector valve to the ‘down’ position directs pneumatic pressure through a second pressure reducing valve and the down lines, to the up-lock rams and retraction rams.
Note: A low pressure is used for landing gear extension, for the same reason that restrictor valves are used in hydraulic systems, which is to prevent damage occurring through too-rapid extension of the undercarriage units.
Retraction rams are usually damped to prevent violent movement. The hollow piston rod is filled with oil or grease,
which is forced through the angular space between the inner surface of the piston rod and a stationary damper piston whenever the ram extends or retracts, thus slowing movement.
Up-locks and down-locks are similar to those used with hydraulic systems, the geometric down-locks being imposed by over-centering of the drag strut at the end of retraction ram stroke, and the up-locks by spring-ram operated locks.
Down-locks are released by initial movement of the retraction rams during retraction, and up-locks are released by
pneumatic pressure in the spring-rams during extension.
Undercarriage doors are operated mechanically, by linkage on the shock-absorber housing.
Electrical Retraction System
In a number of the smaller types of aircraft having a retractable landing gear system, the extension and retraction of the main wheels and nose wheel, is accomplished by means of electrical power.
The motor is of the series-wound split-field type which is mechanically coupled to the three "leg" units, usually by a gearbox, torque shafts, cables, and screw jacks, The 28 volts d.c. supply to the motor is controlled by a selector switch, relay, and switches in the "down-lock" and ''up-lock" circuits.
A safety switch is also included in the circuit to prevent accidental retraction of the gear while the aircraft is on the ground. The switch is fitted to the shock-strut of one of the main wheel gear units, such that the compression of the strut keeps the switch contacts in the open position as shown in the diagram.
ELECTRICAL RETRACTION SYSTEM
After take-off, the weight of the aircraft comes off the landing gear shock-struts, and because they have
a limited amount of telescopic movement, the strut controlling the safety switch causes it to close the switch contacts. Thus, when the pilot selects "gear up", a circuit is completed via the selector switch, and closed contacts of the up lock switch, to the coil of the relay which then completes the supply circuit to the "up" winding of the motor, When the landing gear units commence retracting, the down-lock switch Is automatically actuated such that its contacts will also close, and will remain so up to and in the fully retracted position.
As soon as this position is reached the up lock switch is also actuated so as to open its contacts, thereby Interrupting the supply to the motor. and the ''down" winding circuit of the motor is held in readiness for extending the landing gear. As and when the appropriate selection is made, and the landing gear units commence extending, the up-lock switch contacts now close and when the landing gear is down and locked, and the aircraft has landed.
To prevent over-run of the motor, and hence overtravel of the landing gear units, some form of braking is necessary. This is accomplished in some cases, by incorporating a dynamic brake relay in the circuit. The relay operates in such a manner that during over-run, the motor is caused to function as a generator, the resulting electrical load on the armature stopping the motor and gear instantly.
In retractable landing gear systems, it is, necessary to provide some indication that the main and nose landing gear units are locked in their retracted positions during flight, and in their extended positions safe for landing. The indication method most widely adopted is based on a system of indicating lights which are connected to microswitches actuated by the uplock and downlock mechanisms of each landing gear unit.
To guard against landing with the landing gear retracted or unlocked, a warning horn is also incorporated in the indication system. The horn circuit is activated by a microswitch the contacts of which are made or broken by the engine throttle.
CIRCIUT DIAGRAM FOR LANDING GEAR INDICATOR
The system operates from a 28 volts d.c. power supply which is connected to lamps with in the indicator case, and also to the up lock and down lock micro switches of the main and nose landing gear units. Three of the lamps are positioned behind red screens, and three behind green screens; thus, when illuminated they indicate respectively, ''gear up and locked" and ''gear down and locked".
In the ''gear up and locked'' position all lights are extinguished. In the event of failure of a green lamp filament , provision is made for switching-in a standby set of lamps.
The circuit as drawn, represents the conditions when the aircraft is on the ground in a completely static condition.
As soon as power goes onto the bus, bar, the three green lamps will illuminate because their circuits are completed to ground via the left-hand set of contacts of the corresponding down-lock micro switches, the engine throttle is closed, and although its microswitch is also closed, the warning horn circuit is isolated since there is no path to ground for current from the busbar.
Assume now that the aircraft has taken off and the pilot has selected "landing gear up"; the down-lock mechanisms of the gear units are disengaged and they cause their micro switches to change contact positions, thus interrupting the circuits to the green lamps. At the same time, the red lamps are illuminated to indicate that the gear units are unlocked, the power supply for the circuit passing to ground via the up-lock switches, and the right-hand contacts of the down-lock switches.
When the landing gear units each their retracted positions, the up-lock mechanisms are engaged and cause their microswitches to interrupt the circuits to the red lamps; thus, all lamps are extinguished. When the pilot selects “landing gear down”, the up-lock mechanisms now disengage and the micro switches again complete the circuit to the red lamps to indicate an unlocked condition.
As soon as the gear units reach the fully extended position, the down-lock mechanisms engage and their microswitches revert to the original position i.e., red lamps extinguished, and green lamps illuminated to indicate "down and locked".
A warning horns included in the system, the making and breaking of the horn circuit being controlled by a throttle operated micro switch.
In the static condition, the throttle microswitch is closed, but the warning horn will not sound since the circuit is interrupted by all three down lock microswitches. Similarly, the circuit will be interrupted by the throttle microswitch which is opened when the throttle is set for take-off and normal cruise power.
In the case of an approach to land, the engine power is reduced by closing the throttle to a particular approach power setting and this action closes the throttle microswitch. If, in this flight condition, the landing gear has not been selected down in readiness for landing, then the warning horn will sound since the circuit to ground is then completed via the right-hand contacts of the down-look micro switches.
After selecting "down'', the horn continues to sound, but it may be silenced by operating a push switch which, as will be noted from the diagram, energizes a relay lo interrupt the horn circuit. The relay incorporates a hold-in circuit so that it will remain energized until the d.c. power supply is finally switched off. Functional testing of the horn circuit on the ground, and under engine static conditions, may be carried out by closing the throttle and its microswitch, and then operating a test switch.
TYPICAL LANDING GEAR PANEL
Since the correct operation of the landing gear is of the utmost importance, a number of safety features are included in the retraction system to ensure its correct operation under all conditions.
To avoid damage to the airframe structure, the nose wheel must always be aligned in a fore and aft direction during retraction, and a number of methods are used to ensure that this happens automatically. One method utilizes a cam and cam track between the inner and outer cylinders on the shock-absorber. The cam is fixed to the top of the inner cylinder, and the track to the bottom of the outer cylinder. As the strut extends under internal gas pressure after takeoff, the cam engages the track and centres the nose undercarriage before it retracts. A second method is the use of a peg located at the top of the shock-absorber strut, which engages a track fixed to the strut housing or in the wheel bay, and this device centres the undercarriage as it retracts.
To prevent inadvertent retraction of the landing gear when the aircraft is resting on its wheels, a safety device is incorporated which prevents movement of the selector lever; mechanical ground locks are also provided for servicing purposes. The safety lock consists of a spring-loaded plunger which retains the selector in the down position land is released by the operation of a solenoid. Electrical power to the solenoid is controlled by a switch mounted on the shock-absorber strut; when the strut is compressed the switch is open, but as the strut extends after take-off, the switch contacts close and the electrical supply to the solenoid is completed, thus releasing the selector lever lock and allowing the landing gear to be selected up. A means of overriding the lock, such as a separate gated switch to complete the circuit, or a mechanical means of avoiding the locking plunger, is provided for emergency use and for maintenance purposes.
To guard against landing with the landing gear retracted or unlocked, a warning horn is incorporated in the system and connected to a throttle-operated switch. If one or more throttle levers are less than approximately one third open, as would be the case during approach to land, the horn sounds and the red warning lamp illuminates if the landing gear is in any position other than down and locked. A horn isolation switch is often provided to allow certain flight exercises and ground servicing operations to be carried out without hindrance.
A means of extending the landing gear and locking it in the down position is provided to cater for the eventuality of main system failure. On some aircraft the up-locks are released manually or by means of an emergency pneumatic system; the landing gear ‘free-falls’ under its own weight and the down locks are engaged by spring jacks. On other aircraft the landing gear is extended by an emergency pressure system which often uses alternative pipelines to the jacks. Pressure for the emergency system may be supplied by a hydraulic accumulator, a hand pump, a pneumatic storage cylinder, or an electrically powered pump.
Light aircraft generally employ a simple steering system, in which the nose wheel is mechanically linked to the rudder pedals. Larger aircraft require powered steering arrangements, in which the nose wheel is turned by hydraulic, pneumatic, or electrical power. A powered steering system generally includes a cockpit steering wheel or tiller, a control valve, steering cylinders to actuate the nose undercarriage, a follow-up device to hold the nose wheel at the correct angle, and a power source.
The control unit is a hydraulic metering or control valve. It directs hydraulic fluid under pressure to one or two actuators designed with various linkages to rotate the lower strut. An accumulator and relief valve, or similar pressurizing assembly, keeps fluid in the actuators and system under pressure at all times. This permits the steering actuating cylinders to also act as shimmy dampers.
A follow-up mechanism consists of various gears, cables, rods, drums, and/or bell-crank, etc. It returns the metering valve to a neutral position once the steering angle has been reached. Many systems incorporate an input subsystem from the rudder pedals for small degrees of turns made while directing the aircraft at high speed during takeoff and landing. Safety valves are typical in all systems to relieve pressure during hydraulic failure so the nose wheel can swivel.
The nose wheel steering wheel connects through a shaft to a steering drum located inside the flight deck control pedestal. The rotation of this drum transmits the steering signal by means of cables and pulleys to the control drum of the differential assembly. Movement of the differential assembly is transmitted by the differential link to the metering valve assembly where it moves the selector valve to the selected position. This provides the hydraulic power for turning the nose gear.
NOSE WHEEL STEERING TILLER
As shown in Figure, pressure from the aircraft hydraulic system is directed through the open safety shutoff valve into a line leading to the metering valve. The metering valve then routes the pressurized fluid out of port A, through the right turn alternating line, and into steering cylinder A. This is a one-port cylinder and pressure forces the piston to begin extension. Since the rod of this piston connects to the nose steering spindle on the nose gear shock strut which pivots at point X, the extension of the piston turns the steering spindle gradually toward the right. As the nose wheel turns, fluid is forced out of steering cylinder B through the left turn alternating line and into port B of the metering valve. The metering valve directs this return fluid into a compensator that routes the fluid into the aircraft hydraulic system return manifold.
NOSE WHEEL STEERING SYSTEM
As hydraulic pressure starts the nose gear turning. However, the gear should not be turned too far. The nose gear steering system contains devices to stop the gear at the selected angle of turn and hold it there. This is accomplished with follow-up linkage. As stated, the nose gear is turned by the steering spindle as the piston of cylinder A extends. The rear of the spindle contains gear teeth that mesh with a gear on the bottom of the orifice rod.
As the nose gear and spindle turn, the orifice rod also turns but in the opposite direction. This rotation is transmitted by the two sections of the orifice rod to the scissor follow-up links located at the top of the nose gear strut. As the follow-up links return, they rotate the connected follow-up drum, which transmits the movement by cables and pulleys to the differential assembly. Operation of the differential assembly causes the differential arm and links to move the metering valve back toward the neutral position.
The compensator unit system keeps fluid in the steering cylinders pressurized at all times. This hydraulic unit consists of a three-port housing that encloses a spring-loaded piston and poppet. The left port is an air vent that prevents trapped air at the rear of the piston from interfering with the movement of the piston. The second port located at the top of the compensator connects through a line to the metering valve return port. The third port is located at the right side of the compensator. This port connects to the hydraulic system return manifold. It routes the steering system return fluid into the manifold when the poppet valve is open.
The compensator poppet opens when pressure acting on the piston becomes high enough to compress the spring. In this system, 100 psi is required. Therefore, fluid in the metering valve return line is contained under that pressure. The 100 psi pressure also exists throughout the metering valve and back through the cylinder return lines. This pressurizes the steering cylinders at all times and permits them to function as shimmy dampers.
Bogie Under Carriages
On heavy aircraft, the need to spread the weight over a large area has resulted in the use of multiple wheel under carriages.
The undercarriage unit normally consists of a shock-absorber strut, at the lower end of which a bogie beam is pivoted, and the axles are attached to each end of the beam. On some aircraft the rear pair of wheels swivels on the bogie beam, and castors when the nose wheel is turned through a large angle; on others, the upper torque link member is replaced by a pair of hydraulic jacks, which, when nose wheel steering is applied, rotates the whole bogie.
Castoring or steering prevents excessive torque on the undercarriage leg and minimizes tyre scrubbing during turns. For normal operation, the swivelling pair of wheels is locked in line with the fixed pair. Brake torque at each wheel is transmitted through compensating rods to the shock-absorber strut, thus preventing excessive loads on the bogie beam.
On retractable landing gear a levelling strut or ‘hop damper’ provides a means of positioning the bogie beam at suitable angles for retraction and landing; this strut is usually connected into the hydraulic system to prevent retraction if the bogie is not at a suitable angle, and combines the functions of hydraulic ram and damper unit.