Aircraft Landing Gear Systems Terms

Valve Core

A check valve within a tire air valve which permits air pressure chucks without undue loss of air pressure. The core should not be considered a valve seal.

Bungee Cord

A cushioning material used with the nonshock absorbing landing gears installed on older aircraft. Bungee cord is made up of many small rubber bands encased in a loose woven cotton braid.

Tandem

As the name implies, this type of landing gear has the main gear and tail gear aligned on the longitudinal axis of the aircraft. Sailplanes commonly use tandem gear, although many only have one actual gear forward on the fuselage with a skid under the tail. A few military bombers, such as the B-47 and the B-52, have tandem gear, as does the U2 spy plane.

Axle Nut

Axle nuts are pieces of hardware that are used to help secure a wheel to an axle.

Overheating

Blue tint- caused by lack of sufficient lubrication results in a bluish tint to the metal surface. The ends of the rollers shown were overheated causing the metal to flow and deform, as well as discolor. The bearing cup raceway is usually discolored as well.

Boosted Brakes

Boosted brake actuating systems augment the force developed by the pilot with hydraulic system pressure when needed.

Transducers

Commonly used transducers change mechanical movement or pressures into electrical signals.

Floating Disc Brake

Disk is keyed to the inside of the wheel. calipers are fixed to the landing gear. self adjusting- pressure out, spring return.

Thermal Plug

During heavy braking, temperatures can become so great that tire temperature and pressure rise to a level resulting in explosion of the wheel and tire assembly. The thermal plug core is filled with a low melting point alloy. Before tire and wheel temperatures reach the point of explosion, the core melts and deflates the tire.

Brake Puck

Each brake assembly has brake linings or pucks. Three are located on the ends of the pistons, which are in the outboard side of the caliper. They are designed to move in and out with the pistons and apply pressure to the outboard side of the disc.

Pontoons

Floats (also called pontoons) are airtight hollow structures, similar to pressure vessels, designed to provide buoyancy in water. Their principal applications are in watercraft hulls, aircraft floats.

MS28889-1 Valve

High Pressure Strut Valve

Over-inflation

It makes the tire more susceptible to bruises, cutting, shock damage, and blowout.

Multiple-Disc Brakes

Large, heavy aircraft require the use of multiple-disc brakes. Multiple-disc brakes are heavy duty brakes designed for use with power brake control valves or power boost master cylinders.

Leaf-Type Spring Gear

Many aircraft utilize flexible spring steel, aluminum, or composite struts that receive the impact of landing and return it to the airframe to dissipate at a rate that is not harmful. The gear flexes initially and forces are transferred as it returns to its original position.

Keys and Key Screws

On most aircraft inner wheel halves, keys are screwed or bolted to the wheel to drive the brake disc(s). The drive keys are subject to extreme forces when the brakes are applied. As mentioned, there should be no movement between the wheel and the keys.

Trunnion

Projections from the cylinder of a retractable landing gear strut about which the strut pivots retract.

Hydroplaning

Skidding on a wet, icy, or dry runway is accompanied by the threat of tire failure due to heat build-up and rapid tire wear damage. Hydroplaning on a wet runway may be overlooked as a damaging condition for a tire.

Single Disc Brakes

Small, light aircraft typically achieve effective braking using a single disc keyed or bolted to each wheel. As the wheel turns, so does the disc. Braking is accomplished by applying friction to both sides of the disc from a non rotating caliper bolted to the landing gear axle flange. Pistons in the caliper housing under hydraulic pressure force wearable brake pads or linings against the disc when the brakes are applied. Hydraulic master cylinders connected to the rudder pedals supply the pressure when the upper halves of the rudder pedals are pressed.

Torque Links

The hinged link between the piston and cylinder of an oleo-type landing gear shock absorber. The torque links allow the piston to move freely in and out of the landing gear cylinder, but prevent it rotating. The torque links can be adjusted to achieve and maintain the correct wheel alignment. Torque links are also called scissors and nutcrackers.

Brake Control Valve

The key element in a power brake system is the brake control valve, sometimes called a brake metering valve. It responds to brake pedal input by directing aircraft system hydraulic fluid to the brakes. As pressure is increased on the brake pedal, more fluid is directed to the brake causing a higher pressure and greater braking action.

Shock Struts

True shock absorption occurs when the shock energy of landing impact is converted into heat energy, as in a shock strut landing gear. This is the most common method of landing shock dissipation in aviation. It is used on aircraft of all sizes. Shock struts are self-contained hydraulic units that support an aircraft while on the ground and protect the structure during landing. They must be inspected and serviced regularly to ensure proper operation.

Tube-Type Tires

Tube-type tires do not contain this inner liner since the tube holds the air from leaking out of the tire.

Type I Tires

Type I tires are manufactured, but their design is no longer active. They are used on fixed gear aircraft and are designated only by their nominal overall diameter in inches. These are smooth profile tires that are obsolete for use in the modern aviation fleet. They may be found on older aircraft

Inboard Wheel Half

Wheel halves are not identical. The primary reason for this is that the inboard wheel half must have a means for accepting and driving the rotor(s) of the aircraft brakes that are mounted on both main wheels. Tangs on the rotor are fitted into steel reinforced keyways on many wheels.

Balance Weight

When manufactured, each wheel set is statically balanced. Weights are added to accomplish this if needed. They are a permanent part of the wheel assembly and must be installed to use the wheel. The balance weights are bolted to the wheel halves and can be removed when cleaning and inspecting the wheel.

Bogie Truck

When more than two wheels are attached to a landing gear strut, the attaching mechanism is known as a bogie.

Steering Tiller

While taxiing, an airplane is steered with a tiller'. (usually) to the side of the pilot.

Spalling

a chipped away portion of the hardened surface of a bearing roller or race.

Slave Cylinder

a component of the brake system, slows down or stops the vehicle's wheels by pressing the brake pads against the brake. It is otherwise known as a brake caliper cylinder. While pressing the brake pedal, the brake master cylinder induces pressure on the hydraulic fluid

Wear Pin

a device attached to the brake pressure plate and extends through a hole in the bushing. used as a visual indicator for brake wear.

Downlock

a device in an airplane that locks the landing gear in the down position after it has been lowered

Ply Rating

a ply rating is used to convey the relative strength of an aircraft tire. A tire with a high ply rating is a tire with high strength able to carry heavy loads regardless of the actual number of plies used in its construction.

Drag Brace

is used to restrain against the pivot action built into the trunnion attachment. The upper end of the two-piece drag brace is attached to the aircraft structure and the lower end to the strut. A hinge near the middle of the brace allows the brace to fold and permits the gear to retract.

Staining

located on the bearing cup as grayish black streaks with the same spacing as the rollers and caused by water that has gotten into the bearing. It is the first stage of deeper corrosion that follows.

Under-inflation

not servicing the tyre to maintenance manual requirements. not enough air.

AN6287-1 Valve

on the shock strut has a valve core assembly and is rated to 3,000 pounds per square inch (psi). However, the core itself is only rated to 2,000 psi. The swivel nut on the AN6287-1 valve is smaller than the valve body hex.

Outboard Wheel Half

provides a convenient location of the valve stem used to inflate and deflate tubeless tires.

Debooster

reduces all pressure from the control valve to within the working range of the brake assembly.

Safety Switch

safety switch or squat switch is a landing gear safety circuit with solenoid that locks the control handle and selector valve from being able to move into the gear up position when the aircraft is on the ground.

Skis

skis on aircraft that operate on both slippery, frozen surfaces and dry runways. Typically, the skis are retractable to allow use of the wheels when needed.

Nose Gear

the front landing gear in a tricycle type landing gear. often serves the purpose of steering.

Master Cylinder

the hydraulic pump of an automotive braking system that contains a cylinder and one or two pistons, is actuated by the brake pedal, and supplies hydraulic fluid under pressure to the brakes at each wheel.

Main Gear

the landing gear part that supports the weight of an airplane.

Radial Tires

the most modern design of aircraft tyres. It is a design that is similar to that of car tyres. Radial aircraft tyres differ to bias aircraft tyres in that the plies all run radially from bead to bead at approximately 90° to the centre-line of the tyre.

Wheel Tie Bolt

they hold the two halves of the wheel together and must withstand the stress of landing.

Chine

tire used on the nose gear of aircraft with fuselage mounted jet engines to deflect runway water away from the engine intake(s).

Wheel Speed Sensor

typically detect the time varying position or speed of the aircraft wheel and produce an analog or digital output signal that indicates the detected information.

Centering Cam

used to center the nose gear while landing also centers the nose gear before going to the wheel well.

Side Brace

used to lock a landing gear in the down position involves a collapsible side brace that is extended and held in an over-center position through the use of a locking link.

Type II Tires

high pressure tires used in the mid 40s

Drag Strut

holds the trunnion and gear firm for landing and ground operation. It folds at the hinge to allow the gear to retract.

Tire Talc

is commonly used when installing tube-type tires to ensure easy mounting and free movement between the tube and tire as they inflate.

Hydraulic Power Pack

A small lightweight hydraulic power pack contains several components required in a hydraulic system. These include the reservoir, a reversible electric motor-driven hydraulic pump, a filter, high-and-low pressure control valves, a thermal relief valve, and a shuttle valve. Some power packs incorporate an emergency hand pump.

Power Brakes

Aircraft brakes that use the main hydraulic system to supply fluid for the brake actuation. Aircraft that require a large amount of fluid for their brake actuation normally use power brakes, and the volume of fluid sent to the brakes is increased by the use of deboosters.

Auto Brakes

Aircraft equipped with auto brakes typically bypass the brake control valves or brake metering valves and use a separate auto brake control valve to provide this function. In addition to the redundancy provided, auto brakes rely on the anti-skid system to adjust pressure to the brakes if required due to an impending skid.

Antiskid Control Valve

An antiskid control valve uses a torque motor controlled flapper in the first stage of the valve to adjust pressure on a spool in the second stage of the valve to build or relieve pressure to the brake.

Brake Stator

Stators are flat plates notched on the internal circumference to be held stationary by the torque tube spines. They have wearable brake lining material riveted or adhered to each side to make contact with adjacent rotors. The liner is typically constructed of numerous isolated blocks.

Tail Wheel

Tail wheel-type landing gear is also known as conventional gear because many early aircraft use this type of arrangement. The main gear are located forward of the center of gravity, causing the tail to require support from a third wheel assembly.

Brake Rotor

The Rotor is the brake disk that attaches to the wheel

Camber

The amount the wheels of an aircraft are tilted, or inclined, from the vertical. If the top of the wheel tilts outward, the camber is positive. If the top of the wheel tilts inward, the camber is negative.

Caster

The angle created by the steering pivot point from the front to the back. Caster is positive if the line is angled forward, and negative if backward.

Parking Brake

The brakes are applied with the rudder pedals and a ratcheting system holds them in place when the parking brake lever on the flight deck is pulled.

Goodyear Brakes

The cylinder is always filled with air-free, contaminant-free hydraulic fluid as is the reservoir and the line that connects the two together. When the top of the rudder pedal is depressed, the piston arm is mechanically moved forward into the master cylinder. It pushes the piston against the fluid, which is forced through the line to the brake. When pedal pressure is released, the return springs in the brake assembly retract the brake pistons back into the brake housing. The hydraulic fluid behind the pistons is displaced and must return to the master cylinder. As it does, a return spring in the master cylinder move the piston, piston rod and rudder pedal back to the origina position

Conventional Gear

The main gear are located forward of the center of gravity, causing the tail to require support from a third wheel assembly. A few early aircraft designs use a skid rather than a tail wheel. This helps slow the aircraft upon landing and provides directional stability. The resulting angle of the aircraft fuselage, when fitted with conventional gear, allows the use of a long propeller that compensates for older, underpowered engine design. The increased clearance of the forward fuselage offered by tail wheel-type landing gear is also advantageous when operating in and out of non-paved runways.

Tricycle

The most commonly used landing gear. It is comprised of main gear and nose gear. Tricycle-type landing gear is used on large and small aircraft with the following benefits: 1. Allows more forceful application of the brakes without nosing over when braking, which enables higher landing speeds. 2. Provides better visibility from the flight deck, especially during landing and ground maneuvering. 3. Prevents ground-looping of the aircraft.

Cleveland Brakes

The popular Cleveland brake uniquely features the ability to change the brake linings without jacking the aircraft or removing the wheel. On these assemblies, the torque plate is bolted to the strut while the remainder of the brake is assembled on the anchor bolts. The disc rides between the pressure plate and back plate. Linings are riveted to both plates. By unbolting the cylinder housing from the backplate, the backplate is freed to drop away from the torque plate. The remainder of the assembly is pulled away, and the pressure plate slides off of the torque bolts.

Sidewall

The sidewall of an aircraft tire is a layer of rubber designed to protect the carcass plies. It may contain compounds designed to resist the negative effects of ozone on the tire. It also is the area where information about the tire is contained. The tire sidewall imparts little strength to the cord body. Its main function is protection.

Bias Ply Tires

The term "bias ply" refers to the internal construction of the tire. In a bias ply tire, the cords (plies) run at a 45-degree angle from bead to bead.

Tire Bead

The tire bead is an important part of an aircraft tire. It anchors the tire carcass and provides a dimensioned, firm mounting surface for the tire on the wheel rim. Tire beads are strong. They are typically made from high strength carbon steel wire bundles encased in rubber.

Tread

The tread is the crown area of the tire designed to come in contact with the ground. It is a rubber compound formulated to resist wear, abrasion, cutting, and cracking. It also is made to resist heat build-up. Most modern aircraft tire tread is formed with circumferential grooves that create tire ribs. The grooves provide cooling and help channel water from under the tire in wet conditions to increase adhesion to the ground surface.

Tubeless Tires

Tires that are made to be used without a tube inserted inside have an inner liner specifically designed to hold air.

Retractable Gear

To decrease drag in flight undercarriages retract into the wings and/or fuselage with wheels flush with the surrounding surface or concealed behind flush-mounted doors; this is called retractable gear. If the wheels don't retract completely but protrude partially exposed to the airstream, it is called a semi-retractable gear.

Toe-in / Toe-out

Toe-in. A condition of landing gear alignment in which the front of the tires are closer together than the rear. When the aircraft rolls forward, the wheels try to move closer together. Toe-out. A condition of landing gear alignment in which the front of the tires are further apart than the rear. When the aircraft rolls forward, the wheels try to move farther apart.

Type VIII Tires

also known as three-part nomenclature tires.They are inflated to very high-pressure and are used on high-performance jet aircraft. The typical Type VIII tire has relatively low profile and is capable of operating at very high speeds and very high loads. It is the most modern design of all tire types. The three-part nomenclature is a combination of Type III and Type VII nomenclature where the overall tire diameter, section width, and rim diameter are used to identify the tire. The X and "-" symbols are used in the same respective positions in the designator.

Carbon Brakes

are approximately forty percent lighter than conventional brakes. The carbon fiber discs are noticeably thicker than sintered steel rotors but are extremely light. They are able to withstand temperatures fifty percent higher than steel component brakes.

Segmented Rotor-Disc Brakes

are multiple-disc brakes but of more modern design than the type discussed earlier. There are many variations. Most feature numerous elements that aid in the control and dissipation of heat. Segmented rotor disc brakes are heavy-duty brakes especially adapted for use with the high pressure hydraulic systems of power brake systems. Braking is accomplished by means of several sets of stationary, high friction type brake linings that make contact with rotating segments. The rotors are constructed with slots or in sections with space between them, which helps dissipate heat.

Carcass Plies

are used to form the tire. Each ply consists of fabric, usually nylon, sandwiched between two layers of rubber. The plies are applied in layers to give the tire strength and form the carcass body of the tire. The ends of each ply are anchored by wrapping them around the bead on both sides of the tire to form the ply turn ups.

Brinelling

caused by excessive impact. It appears as indentations in the bearing cup raceways. Any static overload or severe impact can cause true brinelling that leads to vibration and premature bearing failure.

Galling

caused by rubbing of mating surfaces. The metal gets so hot it welds, and the surface metal is destroyed as the motion continues and pulls the metal apart in the direction of motion.

Etching

caused when water and the damage caused by water penetrates the surface treatment of the bearing element. It appears as a reddish/brown discoloration.

Type III Tires

common general aviation tires. They are typically used on light aircraft with landing speeds of 160 miles per hour (mph) or less. Type III tires are relatively low-pressure tires that have small rim diameters when compared to the overall width of the tire. They are designed to cushion and provide flotation from a relatively large footprint. Type III tires are designated with a two number system. The first number is the nominal section width of the tire, and the second number is the diameter of the rim the tire is designed to mount upon.

Ground Locks

commonly used on aircraft landing gear as extra insurance that the landing gear will remain down and locked while the aircraft is on the ground. They are external devices that are placed in the retraction mechanism to prevent its movement.

Shimmy Damper

controls nose wheel shimmy through hydraulic damping.

Uplock

designed to lock the landing gear in a retracted position and assist in carrying the weight of the gear during flight

Fixed Gear

down and welded all so known as conventional landing gear.

Bruising

fine particle contamination- caused bad seals, loose dust cup.

Metering Pin

for controlling the rate of fluid flow from the lower chamber into the upper chamber. During the compression stroke, the rate of fluid flow is not constant. It is automatically controlled by the taper of the metering pin in the orifice. When a narrow portion of the pin is in the orifice, more fluid can pass to the upper chamber. As the diameter of the portion of the metering pin in the orifice increases, less fluid passes.

FOD

foreign object damage

False Brinelling

frictional corrosion, oxidation. caused by vibration of the bearing while in a static state. Even with a static overload, lubricant can be forced from between the rollers and the raceway. Submicroscopic particles removed at the points of metal tometal contact oxidize. They work to remove more particles spreading the damage. This is also known as frictional corrosion. It can be identified by a rusty coloring of the lubricant.

Type VII Tires

high performance tires found on jet aircraft. They are inflated to high-pressure and have exceptional high load carrying capability. The section width of Type VII tires is typically narrower than Type III tires. Identification of Type VII aircraft tires involves a two-number system. An X is used between the two numbers. The first number designates the nominal overall diameter of the tire. The second number designates the section width.