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LANDING GEAR SYSTEMS AND MAINTENANCE INSTRUCTOR: MR . JOHN MARK VELANO
LANDING GEAR TYPES • The first airplanes with wheels used a tricycle landing gear. In a tricycle gear configuration, the main wheels are located behind the center of gravity and an auxiliary wheel (nose wheel) is located at the front of the aircraft.
• When airplanes with aft or "pusher" propellers were replaced with those having the propeller up front, the tailwheel landing gear was used in order to keep the propeller further above the ground.
• This configuration became so popular that it is called "conventional" landing gear, even though it has the disadvantage of being difficult for ground retract the wheels into the wing or the fuselage to decrease drag.
LANDING GEAR ARRANGEMENT • Landing gear arrangement is determined by the manufacturer. The most prevalent arrangement on modern aircraft is the tricycle gear configuration. However, it is important that the maintenance technician be familiar with other arrangements, particularly the tailwheel or conventional gear arrangement.
• Occasionally, a technician will encounter some other form of landing gear such as the tandem arrangement in which the wheels are located down the centerline of the longitudinal axis of the aircraft, such as in the case of some gliders.
Decreased drag can be achieved by the use of streamlined fairings or by retracting the gear.
TAILWHEEL TYPE LANDING GEAR
• The majority of modern aircraft do not utilize conventional landing gear, resulting in a generation of pilots who have never flown an airplane with a tail-wheel arrangement..
• Tailwheel aircraft are configured with the two main wheels located ahead of the aircraft's center of gravity and a much smaller wheel at the tail. Moving the rudder pedals that are linked to the tailwheel steers the aircraft on the ground
The configuration of an airplane having a tailwheel-type landing gear is often called a "conventional" landing gear.
TRICYCLE-TYPE LANDING GEAR
• Nearly all currently produced aircraft use the tricycle landing gear configuration in which the main gear are located behind the airplane's center of gravity and the nose of the airplane is supported by the nose gear.
• Steering the nose wheel through connections to the rudder pedals provides control on the ground for small airplanes, while large airplanes utilize hydraulic steering cylinders to control the direction of the nose gear.
The tricycle landing gear configuration substantially improved the ground handling characteristics of modern airplanes.
FIXED OR RETRACTABLE LANDING GEAR
• All aircraft must contend with two types of aerodynamic drag, parasite drag, which is produced by the friction of the airflow over the structure and induced drag which is caused by the production of the lift.
• Parasite drag increases as the speed increases, while induced drag decreases as the speed increases because of the lower angle of attack required to produce the needed lift.
• Slower aircraft lose little efficiency by using the lighterweight fixed landing gear. Faster aircraft retract the landing gear into the structure and thus gain efficiency even at the cost of slightly more weight.
• Fixed landing gear decreases parasitic drag markedly by enclosing the wheels in streamlined fairings, called wheel pants. Many light airplanes utilize fixed landing gear that consist of spring or tubular steel landing gear legs with small frontal areas that produce minimum drag.
SHOCK ABSORBING AND NON-ABSORBING LANDING GEAR • Some aircraft landing gear absorb landing shock and some do not. Non-absorbing gear include spring steel, composite, rigid, and bungee cord construction. Shock absorbing gear incorporate shock absorbers that converts motion into some other form of energy, usually heat.
SPRING STEEL AND COMPOSITES
• Most aircraft provide for absorbing the landing impact and shocks of taxiing over rough ground. Some aircraft, however, do not actually absorb these shocks but rather accept the energy in some form of elastic medium and return it at a rate and time that the aircraft can accept.
• The most popular form of landing gear that does this is the spring steel gear used on most of the singleengine Cessna aircraft.
• These airplanes use either a flat steel leaf or a tubular spring steel strut that accepts the loads and returns it in such a way that it does not cause the aircraft to rebound.
The thin spring-steel landing gear struts of this airplane do not truly absorb the shocks, but rather accept them and return them to the aircraft at a rate that will not cause the aircraft to bounce.
• Certain older types of aircraft use rigid landing gear that transmit all the loads of landing touchdown directly to the airframe's structure. Some of the shock is absorbed by the elasticity of the tires.
• However, this type of landing gear system is not only hard on the aircraft's occupants, but can cause structural failure during a hard landing. Some aircraft, such as helicopters, that normally land very softly utilize rigid landing gear.
BUNGEE CORD • Some aircraft use rubber to cushion the shock of landing. This may be in the form of rubber doughnuts or as a bungee cord, which is a bundle of small strands of rubber encased in a loosely woven cloth tube. Rubber bungee cords accept both landing impact and taxi shocks.
Fabric enclosed rubber bungee cords in this landing gear accept both landing impact and taxi shocks.
• The most widely used shock absorber for aircraft is the air-oil shock absorber, more commonly known as an oleo strut.
• The cylinder of this strut is attached to the aircraft structure, and a close fitting piston is free to move up and down inside the cylinder. It is kept in alignment and prevented from coming out of the cylinder by torsion links, or scissors.
• The upper link is hinged to the cylinder and the lower link to the piston. The wheel and axle are mounted to the piston portion of the strut.
The oleo strut has become the most widely used form of shock absorber on aircraft landing gear.
Shock Strut Operation •
The cylinder of a shock strut is divided into two compartments by a piston tube. The piston itself fits into the cylinder around the tube. A tapered metering pin, which is a part of the piston, sticks through a hole in the bottom of the piston tube. To fill the strut, the piston is pushed all of the way into the cylinder. The strut is then filled with hydraulic fluid to the level of the charging valve. With the weight of the aircraft on the wheel, enough compressed air or nitrogen is pumped through the charging valve to raise the
Servicing Shock Struts
• The air-oil type oleo strut should be maintained at proper strut tube extensions for the best oleo action. Both the nose and main gear struts will have a specific length of piston tube exposed. These measurements should be taken with the airplane sitting on a level surface under normal fuel loading conditions.
• Whenever servicing any part of the gear, wheels, and tires, the shock strut should be inspected for cleanliness, evidence of damage, and proper amount of extension. Manufacturer's repair manuals should be consulted for proper specifications.
For proper action, the strut tube must be in a position to travel in both directions. The exact resting position for strut exposure can be found in the manufacturer's service manual.
• The wheels used on many early aircraft were designed as one-piece units. The tires were flexible enough that they could be forced over the wheel rim with tire tools in much the same way we force tires on automobile wheels today.
• However, modern aircraft tires are normally so stiff they cannot be forced over the rims, and, as a result, almost all modern aircraft wheels are constructed of two-piece units.
• The development of tubeless tires promoted the development of two-piece wheels that are split in the center and made airtight with an O-ring seal placed between the two halves. Today, this form of wheel is the most popular for all sizes of aircraft, from small trainers up to large jet transports.
• Aircraft wheels must be lightweight and strong. Most wheels are made of either aluminum or magnesium alloys and, depending upon their strength requirements, may be either cast or forged.
• The bead seat area is the most critical part of a wheel. To increase wheel strength against the surface tensile loads applied by the tire, bead seat areas are usually rolled to pre-stress their surface with a compressive stress.
INBOARD WHEEL HALF
• The inboard wheel half is the half of a two-piece wheel that houses the brake. Rotating brake disks are driven by tangs on the disk which ride in steel-reinforced keyways, or by steel keys bolted inside the wheel that mate with slots in the periphery of the disk.
• One or more fusible plugs are installed in the inboard half of the main wheels of jet aircraft to release the air from the tire in the event of an extreme overheat condition, such as heavy braking that is required during an aborted takeoff.
OUTBOARD WHEEL HALF
• The outboard half of the wheel bolts to the inboard half and holds a shrunk-in bearing cup in which a tapered bearing cone rides. A seal protects the roller and bearing surfaces from water and dirt and retains the lubricant in the bearing.
• Whenever the maintenance technician comes in contact with the wheels, they should be inspected. When the wheels are on the aircraft, inspect for general condition and proper installation, which includes checking for proper axle torque.
• When the wheel is off the aircraft, more extensive checks can be and are performed. These on and off aircraft checks include the following checks and procedures.
CHECK FOR PROPER INSTALLATION
• It is possible with some types of wheel and brake assemblies that the wheel can be installed with the disk drive tangs between the drive slots, rather than mating with the slots.
• When inspecting the wheel the technician must make certain that the brake is correctly installed and everything is in its proper place.
CHECK FOR AXLE NUT TORQUE
• If too little torque is used on the axle nut, it is possible for the bearing cup to become loose and spin, enlarging its hole and requiring a rather expensive repair to the wheel. If the torque is too high, the bearing can be damaged because the lubricant will be forced out from between the mating surfaces.
• The amount of torque required varies with the installation. Follow procedures used for installing and securing the axle nut that are recommended by the airframe manufacturer.
LOOSENING THE TIRE FROM THE RIM
• Before a wheel can be inspected, the tire must be removed. • Before loosening the wheel half retaining bolts, BE SURE THE TIRE IS COMPLETELY DEFLATED. It is also advisable to deflate the tire before removing the wheel/tire assembly from the axle. In the event that the bolts holding the wheel halves have failed, the only thing holding the assembly together is the axle nut.
• After the tire has been deflated, the bead of the tire should be broken from the wheel by applying an even pressure to the tire as close to the wheel as possible. Screwdrivers or any type of tire tool should never be used to pry the bead away from the rim, because it is easy to nick or damage the soft wheel in the critical bead area. Any damage here will cause a stress concentration that can
The bead of the tire should be broken away from the bead seat of the wheel with a steady pressure as near the rim of the wheel as possible.
DISASSEMBLY OF THE WHEEL
• The wheel should be placed on a clean, flat surface and the bearing seals and cones removed from both wheel halves. The nuts from the wheel bolts are removed to separate the wheel halves. Impact wrenches are not used on aircraft wheels.
• Even though it is common practice with automotive wheels to use impact wrenches for speed, the uneven torque produced by these wrenches creates stresses these lightweight wheels are not designed to take.
CLEANING THE WHEEL ASSEMBLY
• Stoddard solvent or similar cleaning fluid should be used to remove any grease or dirt from the wheel. A soft bristle brush will aid in removing stubborn deposits.
• Do not use scrapers that will remove any of the protective finish from the wheel. After all of the parts have been cleaned, they should be dried with compressed air.
CLEANING THE BEARINGS • Clean solvent should be used to wash the wheel bearings. Soak them to soften the grease and any hardened deposits in the bearings, then brush them with a soft bristle brush to remove all of the residue. Dry the bearings by blowing them out with low-pressure dry compressed air.
• DO NOT SPIN THE BEARINGS AS YOU DRY THEM. Rotating dry metal against dry metal will damage both the rollers and the races. Bearings should never be cleaned with steam, because the heat and excess oxygen will cause a premature breakdown of the bearing surface.
BEARING INSPECTION • If a bearing is difficult to remove from the axle shaft, it should be removed with a special puller. It should never be driven from the shaft with any form of drift. Bearings that have been difficult to remove from the shaft often have indications of galling on their inner bore, which is cause for rejection of the bearing cone.
• Water stains on a bearing may not look bad, but they are an indication of intergranular corrosion in the surface of the rollers or the races. Any bearing showing signs of water marks should be rejected.
Water stains on a bearing are evidence of intergranular corrosion.
Spalling, or failure of the bearing surface, is reason to reject the bearing.
These bearings have been overheated.
• Bearings should be packed using the grease specified by the aircraft manufacturer. MIL-G81322D is the most recently developed type of lubrication and has superior qualities to previously developed lubrications.
• Not all brands of MIL specification grease are the same color, but those having the same specification number are compatible and interchangeable, regardless of their color.
INSPECTING THE WHEEL HALVES
• The most difficult area of an aircraft wheel to inspect is the bead seat region. This area, which is highly stressed by the inflated tire, can be distorted or cracked by a hard landing or a seriously overinflated tire.
• When all of the forces are removed and the tire dismounted, these cracks may close up so tightly, especially on forged wheels, that penetrant cannot enter the crack. This makes any form of penetrant inspection useless for examination of the bead seat area.
The bead seat area of a wheel assembly is the most difficult area of a wheel to inspect. Eddy current inspection is a type of inspection that can reliably find cracks in this area
Cracks in the disk drive area of the wheel can be inspected by penetrant type inspection because cracks in this area have no tendency to close.
Corrosion is likely to occur anywhere that water can be trapped against the surface of the wheel.
WHEEL BOLT INSPECTION
• The wheel bolts should be inspected by magnetic particle inspection. Pay particular attention to the junction of the head and shank, and to the end of the threaded area. Because the cross-sectional area of the shank changes at these two locations, these are the most likely locations for cracks to form.
FUSIBLE PLUG INSPECTION • Carefully examine the condition of the fusible plugs in the wheel to be sure that none of them show any sign of the core melting. Even if only one of the plugs indicates any deformation, all of the plugs must be replaced.
Fusible plugs should be inspected for signs of softening due to excessive heat.
• Almost all wheels having a diameter of more than ten inches are statically balanced when they are manufactured. If the weights, installed by the manufacturer, have been removed for any reason, they must be put back in their original position.
• The final balancing of the wheel is done after the tire is mounted. The weights for this final balancing are usually installed around the outside of the rim of the wheel or at the wheel bolt circle.
Wheel balancing is done statically at manufacture and the weights must be returned to their original positions if removed for any reason.
NOSE WHEEL STEERING SYSTEMS
• Nose wheel steering is found on most tricycle gear aircraft. On small aircraft the nose wheel is usually controlled by a direct connection between the rudder pedals and the nose gear. Large aircraft steering is usually activated by a hydraulic actuator that is controlled by the rudder pedals or by a separate steering
SMALL AIRCRAFT • Almost all airplanes with tricycle landing gear utilize some type of nosewheel steering on the ground by controlling the nose wheel. Some of the smallest airplanes, however, have a castering nose wheel. In these cases, differential braking does the steering. Other small airplanes link the nose wheel to the rudder pedals directly.
LARGE AIRCRAFT • Large aircraft are steered on the ground by directing hydraulic pressure into the cylinders of dual shimmy dampers. A control wheel operated by the pilot directs fluid under pressure into one or the other of the steering cylinders. The actual control of the fluid can be transmitted from the pilot's control to the hydraulic control unit mechanically, electrically or
The nose wheel steering system for a large aircraft consists of actuators, control valves and other associated components typically found in most hydraulic systems.
SHIMMY DAMPERS • The shimmy damper is a small hydraulic shock absorber that is installed between the nose-wheel fork and the nose-wheel cylinder. • Shimmy dampers are normally small piston-type hydraulic cylinders that control the bleed of fluid between the two sides of the piston. The restricted flow prevents rapid movement of the piston, but has no effect on normal steering.
A shimmy damper reduces the rapid oscillations of the nose wheel, yet it allows the wheel to be turned by the steering system.
STEERING DAMPERS • In many cases, the steering actuators serve as the steering dampers because they are constantly charged with hydraulic fluid under pressure. As the nose wheel attempts to vibrate or shimmy, these cylinders prevent movement of the nose gear. This type of system is used on large aircraft while a piston type shimmy damper is usually used on small aircraft.
LANDING GEAR ALIGNMENT, SUPPORT AND RETRACTION
• In order for the wheels to do their part in supporting the aircraft, there must be a structure that connects the wheels to the aircraft. This structure is the landing gear. The landing gear must be accurately aligned, provide adequate support for the aircraft at any design gross weight, and allow the wheels to retract if necessary.
WHEEL ALIGNMENT • Alignment of the main gear wheels is very important in that misalignment adversely affects landing and takeoff, roll characteristics, tire wear, and steering during ground operations. Severe misalignment can cause malfunction and failure of some of the major components of the landing gear system.
• Alignment consists of checking and adjusting the toe-in or toe-out configuration and the camber of the gear. The aircraft maintenance manual normally specifies the amount of toe-in and camber the landing gear should have. The torque links are also very important in the alignment of the landing gear.
The torque links, sometimes called scissors, limit the extension of the oleo strut and keep the wheel in alignment.
• As an aircraft moves forward in a toe-in arrangement, the wheels try and move closer together. A toe-out configuration causes the wheels to try and move apart.
• In order to measure toe-in, a carpenter's square is held against a straightedge placed across the front of the main wheels. The straightedge should be perpendicular to the longitudinal axis of the aircraft. If this is correct, then the distance between the blade of the carpenter's square and the front and rear flanges of the wheel will indicate toe-in or toe-out.
One method of checking the toe-in or toe-out of an airplane landing gear utilizes a straightedge and a carpenter's square.
This illustration shows the shims used to align the main landing gear on a spring steel landing gear strut. Toe-in is adjusted on spring steel landing gear by placing shims between the axle and the gear leg.
On landing gear using an oleo-type shock absorber, toe-in is adjusted by adding or removing washers from between the torque links.
• The landing gear is generally supported by the aircraft's structure. The wings spars, along with additional structural members, support and attach the main landing gear to the wings on larger aircraft.
• Non-retractable landing gear is generally attached to the aircraft structure by bolting the landing gear struts to the structure directly.
• Retractable landing gear systems must provide for the landing gear to move, so the upper shock strut is attached to the airframe using trunnion fittings, which are extensions or shafts attached to the shock strut that mount into fittings bolted to the airframe.
SMALL AIRCRAFT RETRACTION SYSTEMS
• When the design speed of an aircraft becomes high enough that the parasite drag of fixed landing gear is greater than the induced drag caused by the added weight of the retracting system, retractable landing gear becomes practical. Some smaller aircraft use a simple mechanical retraction system, incorporating a roller chain and sprockets operated by a hand crank. Many aircraft use electric motors to drive the landing gear retracting mechanism and some European-built aircraft use pneumatic systems.
• The simplest hydraulic landing gear system uses a hydraulic power pack containing the reservoir, a reversible electric motor-driven pump, selector valve, and sometimes an emergency hand pump along with other special valves.
This simple landing gear system for small aircraft has an added feature of an airspeed controlled automatic landing gear extension system. At a certain airspeed, the landing gear will automatically extend regardless of gear handle position.
• An additional feature of this particular landing gear system is the automatic extension system that will lower the landing gear when the airspeed slows below a specified value, regardless of the position of the landing gear selector.
• A diaphragm actuated by the difference in pitot, or ram, air pressure and static, or still air, pressure controls the free-fall valve. An airspeed pickup tube on the side of the fuselage brings pitot and static pressure into the automatic extension valve.
LARGE AIRCRAFT RETRACTION SYSTEMS
• The actual system for retracting and extending the landing gear on large aircraft is similar to that just described. However, there are several additional features and components used because of the size and complexity of the system.
• Normally, large aircraft have wheel-well doors that are closed at all times the landing gear is not actually moving up or down. Sequence valves are used in the system to ensure the doors are opened before the landing gear is actuated.
EMERGENCY EXTENSION SYSTEMS
• Retractable landing gear systems must have a means of lowering the landing gear if the primary method of lowering the gear fails. Because there are many methods used to actuate the landing gear, this discussion will be general in nature.
• Emergency extension systems generally use a variety of methods to lower the gear. Some of the methods can include mechanical, alternate hydraulic, compressed air or free-fall techniques to lower the gear. In all cases, the emergency extension system's purpose is to release the up-locks and move
LANDING GEAR SAFETY DEVICES SAFETY SWITCH
• Most aircraft with retractable landing gear are equipped with a means of preventing the retraction of the landing gear while the aircraft is on the ground.
• The landing gear would retract if the aircraft's hydraulic system was powered and the gear handle was moved to the up position. To prevent this from happening, a squat switch with a lever attached prevents the gear control handle from being placed in the up position when there is weight on the aircraft wheels.
GROUND LOCKS • Ground locks are used to secure the landing gear in the down position. These locks are generally removed manually by ground personnel. Ground locks are placed into position after the aircraft lands and are kept engaged until the aircraft is ready for the next flight.
• The locks generally consist of a pin inserted into the retraction mechanism in such a manner to block the retraction of the landing gear.
• Position indicators, generally located close to the landing gear lever, include green gear down and locked lights, a red gear door open light, and red a gear disagreement light, but may use a red gear unsafe/in transit light.
• Smaller airplanes do not use gear door open lights. Generally when the gear is up and locked, all the lights will go out signaling that the gear is up and locked. Switches or proximity probes at each gear position control the lights in the cockpit.
• The nose wheel is equipped with centering cams located in the nose wheel shock strut. These centering cams center the nose wheel when the strut is extended after take-off.
• The nose gear will remain centered until the weight of the aircraft, upon landing, compresses the strut moving the centering cams away from their slots. This allows the wheel to turn as commanded by the steering tiller or the rudder pedals.
LANDING GEAR RETRACTION CHECK
• Landing gear retraction checks are performed as a part of hundred-hour and annual inspections. They are also performed after replacement of landing gear components or after an event that could damage the gear such as a hard landing.