Landing Gear Landing Gear Configurations
Two main types: Conventional, and Tricycle
Tricycle Has nose wheel, which may be steerable Main gear, on either side Example: Cessna Keeps aircraft level during take-off and landing The most important advantage is its ease of ground handling.
Conventional Two main wheels One tail dragger wheel Reduced drag in the air Reduced landing gear weight Requires more skill in ground taxiing The most important advantage is the ability to operate the aircraft over rough terrain.
Classification of Landing Gear
Main landing gear Cushions landing impact
Heavily stressed area Main Landing Gear consists of the main weight-bearing structure Auxiliary landing gear includes tail wheels, skids, nose wheels, etc.
Nonabsorbing Landing Gear
Includes Rigid landing gear, Shock-cord landing gear, Spring landing gear Rigid: helicopters, sailplanes. No flexing other than the structure. Shock cord system: uses “Bungee” cords Spring type uses spring steel (some Cessna‟s)
Shock-Absorbing Landing Gear
Dissipates landing energies by forcing fluid through a restriction This fluid generates heat, dissipated into the atmosphere Two types: Spring Oleo, and Air-Oil Oleo Spring Oleo is history by now Air Oleos are all very similar: a needle valve restricts fluid flow Air in the oleo holds the weight of the a/c on the ground Air Oleos present in both retractable and fixed gears
Non retractable, usually bolted on to the structure Often uses fairings or wheel pants
Cessna 152 Advantages: Lighter weight Less compex Least costly
Designed to eliminate drag (the greatest advantage) Can be either fully or partially retractable Direction of retraction depends on airframe model Methods of retraction: hydraulic, electric, mechanical, pneumatic Critical area of aircraft maintenance for safety reasons
Hulls and Floats
Can be single float, or multiple Definition may include floating hulls (ex. “Lake” aircraft) Floating hulls may only require wing tip floats Skis used for snow and ice (wood, metal, composites) Skis may use shock cord to assist angle of ski attack Skis are mounted on the same strut as tires
Landing Gear Components
Exact definitions of some components will vary
The Oleo strut is the widely used form of shock absorption on aircraft landing gear.
Portion of the landing which attaches to the airframe Supported at the ends by bearings Landing gear traditionally extends from the center
Vertical member, contains the shock absorbing mechanism Top of the strut mounts onto the trunnion Strut forms the cylinder for the oleo (“outer” cylinder) Piston is the moving portion (aka piston rod, tube or inner cylinder) Oil is forced from the lower portion of the strut to the upper Oil flow is restricted or varied according to a metering pin Final weight of a/c rests on air in the top of the strut Snubbers are used to prevent a sudden dropping of gear on takeoff Metering pin controls the flow of fluid between the chambers. The shock of landing is absorbed by the fluid being forced through a metered orifice. The metering pin gradually reduces the size of the orifice as the shock strut extends, which avoids a rapid extension after the initial shock of landing and related bounce. Chevron seals are used in shock struts to prevent the oil from escaping On nose wheel struts, a cam is built into the strut for the purpose of straightening the nose wheel before retraction. Filling a shock strut: “exercise” the strut in order to seat the seals, and remove air bubbles from the fluid.
Most shock strut oil levels are checked by releasing the air, bottoming the strut, and checking to see if the oil is at the level of the filler plug. Information about shock struts: see: Manufacturer‟s maintenance manual Information decal located on the strut Mfr‟s overhaul manual
Also called scissors assembly Two A-frame members Connects and aligns upper and lower cylinders Connects the strut cylinder to the piston Restricts extension of piston during retraction Correctly aligns axle to the strut
Located at the bottom of the strut piston Axles are mounted on the truck Trucks can tilt fore or aft to allow for a/c attitude changes
Stabilizes landing gear longitudiannly May be hinged to allow retraction Also called a drag strut
Side Brace Links
Stabilize gear laterally May be hinged to allow retraction Can be called a side strut
Overcenter Links (aka downlock mechanism)
Use to apply pressure to the center pivot joint in a drag or side brace link Overcenter link is hydraulically retracted to allow gear retraction Also called a downlock, and/or a jury strut
Flexible joint with internal passages Route hydraulic fluid to the wheel brakes Used where space limitation eliminate flex hoses
Hydraulic snubbing unit Reduces tendency of nose wheels to oscillate
Piston type dampers
Piston and rod filled with hydraulic fluid Piston has an orifice restricting speed of travel
Slow movement has no restriction Large shimmy dampers incorporate temperature compensation
Vane type dampers
Employ stationary vanes and rotating vanes Small passages restrict fluid movement Central shaft rotation is restricted from moving quickly
Check for leakage & effectiveness of operation Check mounting bolts and hardware Most dampers are fairly reliable
Some a/c have free castering nose wheels; most have steerable.
Mechanical Steering Systems
Uses foot power to steer the aircraft – no assistance Some types will disengage when the gear is retracted Some types have an automatic centering device when weight is off the a/c
Conventional gear use the tail wheel to steer
May be a castering type with no steering capabilities (rudder steers) May be lockable, for parking purposes
Power Steering Systems
Used where large amounts of force are required to steer Controlled by pilots rudder pedals, OR By a steering wheel, OR By a combination of both Most will require a towing bypass valve which allows Ground crews to to the a/c without damaging the system
Purpose: reduce drag, or adapt a/c for landing on different surfaces (consider retractable wheels on float systems)
Crank mechanism, or uses a lever pulled by the pilot This method may use a mechanical latch system to lock wheels “up” No emergency backup available for this system
Electrical Retraction Systems
Uses a central motor and push-pull rods Uses microswitches to detect when gear is down/locked, or up/locked
Hydraulic Retraction Systems
Most common system of retraction for most sizes of a/c Used exclusively where landing gear is too large to be retracted by other methods May use ED pumps, electric pumps, hand or wind-driven pumps
Emergency Landing Gear Systems
4 possible methods of dropping gear when hydraulics are lost: air bottle “blows” the gear down hand crank or ratched separate hydraulic system (may be hand pump) mechanical system which releases UP locks, and gear free-falls
Landing Gear Operation
4 main components: shock strut the wheel the brake assembly the trunnion and side/drag brace scissors (torque links) actuating cylinder down & up locks the bungee system
Using hydraulics, landing gear retraction requires greater energy than lowering Gear rotates on the trunnion pin Extending landing gear requires a release of the UP lock first, then The gear can begin free falling, slowed by the snubber in the orifice check valve Final few degrees of travel may require hydraulic pressure assistance Bungee system is used for emergency operation:
Gear doors must be operated before extension & after retraction
Landing Gear Position Indicator Systems
Positive indication must be provided to the pilot that gear is down & locked Safety system includes squat switches and other microswitches Squat switches tell pilot when weight of a/c is on the wheels Squate switches are electrically “open” when on the ground Some a/c use warning horns: they sound when: If gear is retracted, and throttle retarded to below cruise Landing gear position indicators: show position of Landing gear May use a system of different color indicator lights
Transport Aircraft Landing Gear Systems
Corporate Jets and Dual-Wheeled Transports
Most have retractable tricycle-type gear, 2 wheels on each Nose gear will probably be a dual-wheel steerable type Gear will become completely enclosed when retracted
Helicopter Landing Gear
Basic skid gear is common for small & mediums Wheel gear is used on sikorsky aircraft Retractable or cushioning gear may impart ground resonance Skid tubes are replaceable, and repairable Bending and deforming limits are established, and occasionally liberal Skid protectors are available, as are “bear paws” snow shoes Ground handling wheels are bolt-on towing additions
Inspection and Maintenance of Landing Gear
The following must be carefully inspected: Attachments to fuselage or wings Struts Wheels Brakes Actuating mechanisms for gear Hydraulic systems
Fixed Gear Inspection
Inspect for wear, deterioration, corrosion, alignment Jack up the aircraft to relieve the weight on the gear Shock cord should be inspected for age & fraying (5 years, retire) Shock Cord MIL-C-5651A. See diagram for year and quarter. Check oleos for bottoming: air charge has been lost.
Retractable Landing Gear Inspection
Similar to fixed gear inspection, but add inspections for: Wear or looseness in joints or trunnions Leakage of fluids Smoothness of operation Operational check performed by jacking the airplane & operating gear Check for clearance of new tires in the wheel wells Check for operation of gear doors Check operation and adjustment of microswitches
If a oleo bottoms upon initial landing, but operates normally during taxiing, it is likely an indication of low fluid. Check your fluid levels.
Alignment of Main Gear Wheels
Check for Camber and Toe-in. Camber is the tilt of the top of the wheel inboard or outboard Toe is the angling of the forward edge of the tire Oleo type gear often use angled shims or washers A common Toe setting is 0º, with a tolerance of ½º Some a/c may require alignment to be set while weight is on the gear. Use grease plate for this
Inspection of Floats and Skis
Consider pop out floats and fixed floats (helicopters) Standard float assemblies for small a/c are sensitive to salt water Check for corrosion thoroughly Use standard sheet metal repair techniques Check for leaks See powerpoint slides for float parts familiarization diagram
Tires and Wheels
Aircraft Tire Operation Characteristics
Tires for aircraft must endure higher loads and higher speeds than automobiles and trucks; the safety issue is much higher as well. Heat generation is higher in aircraft tires Rubber, the major material used, dissipates heat slowly Underinflating or overinflating increases shear forces in between the plies: tension will be higher in outer plies than in inner.
Type I tires: smooth contour Type II tires: high pressure III: low pressure IV: extra low pressure V: not applicable VI: low profile VII: tires are constructed for extra high pressure; jet aircraft. VIII: extra high pressure, low profile, low speed or high speed.
Types I, II, IV and VI are being phsed out.
Aircraft Tire Nomenclature
Tire ply rating refers to the maximum static load and its inflation pressures
Tire markings: manufacturer, country of mfr, design type, load rating, tube or tubeless, tire size, part number, ply rading Number of recaps used to be stamped on the sidewall, but not all models have provision for this. Chafers are used to protect the wheel rim-to-tire bead chafing. The most important part of an aircraft tire is the bead. A tire with smooth tread is used for very light aircraft, grass runways, and locations where braking is only used as an aid to taxiing. Ribbed tread tires are used for directional stability, good tread wear, and to allow water to escape from between the tire tread and runway. A Chine sided tire is used to deflect water or slush away from the intake of the jet engines. Double or single chine wheels exist. Inboard halves of an aircraft wheel are different from the outboard by the provisions made for mounting & securing the disk brake assembly, and the presence of fusible plugs. The fusible plug is used to prevent tire blow-out due to heat build-up. The plug has a low melting point core which melts under high temperatures which may build up during heavy braking.
Steel wire beads form the inner diameter Plies are diagonal layers of rubber coated nylon cord fabric Chafers protect the tire during mounting/demounting. They also provide a good seal between tire and wheel. Breakers are used to increase structural strength Inner liner acts as a built-in tube; prevents air from seeping through the casing plies. Beads anchor the plies and provide mounting surfaces for the wheel Tread is the surface of the tire for runway contact Most treads have groove patterns with 3 – 6 ribs, depending on size and type of service
The light point on a tire is indicated by a red dot on the tire.
Tires should be stored vertical, in racks, in cool dry places. Ensure there are no sources of light or electrical appliances (for the ozone) nearby. Same with chemical fumes.
Made of rubber sections vulcanized together. Air valve is vulcanized to the tube for inflation/deflation Tubes can be checked for leaks by immersing in a water trough with a light inflation pressure applied.
Aircraft Wheel Construction
Aircraft tires are too stiff to stretch over wheel rims. Damage will occur to the bead of the tire Split rims are used, and the two halves are sealed with an O-ring Most wheels are constructed of forged aluminum or magnesium alloys Wheels rotate on two tapered roller bearings; the cups are shrink fitted into the hub. Wheels are constructed with three fusible plugs, equally spaced around the wheel. The bead seat area is strenthened by rolling: it pre-stresses the surface Nose wheels are smaller in diameter and width than mains: rarely have brakes
Demounting and Mounting Tires
Safety procedures: Deflate the tire beforee loosening the axle nut. This must occur in case the rim is cracked or the wheel bolts fail. Before we loosen the wheel half retaining bolts, we must deflate the tire, and remove the valve stem. Break the tire beads away using a bead breaker – do not use sharp tools Then dismantle the wheel halves & inspect the bolts Always use a safety cage when inflating a tire. Bearings are generally a tapered roller bearing in hubs. Grease seals are used to prevent foreign material from entering and contaminating the wheel bearing grease. Stuck wheel bearing? Don‟t beat it to death with a hammer & punch ~ use a bearing puller. The only place to obtain proper inflation pressures is in the A/C M/M. That manual will quote pressures for the aircraft when loaded. (all-up weight) Installing tubes requires a dusting of talc into the tire for lubrication purposes. Tube should be placed so that the yellow strip (the heavy spot) is adjacent to the red dot (the light spot) on the tire. If using a tubeless tire; inspect the new tire to make sure it is a tubeless type. Install bolts and nuts ensuring they are installed in the correct direction. Use an alternating sequence to tighten. Only partially inflate the first time to allow the tire bead to seat to the wheel. A strap around the surface of the tread may be used to prevent the tire from expanding radially. Nitrogen is preferred, but not always necessary. Tires should be allowed to set for 12 to 24 hours before being installed. This allows for “growth” and natural stretch, and the air pressure to settle. Do not use soap/water installation liquids; may cause slippage of tire on the rim during landing.
Tire and Wheel Inspection
Inspect outer tread while mounted on the a/c. Certain types of landing gear wear the tread unevenly (spring gear) Check for over or under-inflation, damage to sidewalls, cracks, weather checking, flat spots, chafing, thinning, valve stem movement (indicator slip marks) Dye Penetrant is not always the best answer: in some cases a crack may not show up because of the extreme pressures imposed by inflation and a/c weight. When the tire is deflated & inspected, some cracks will close up, and not show with Dye Penetrant. The most corrosion prone area of a wheel is any area which is exposed to the direct entry of moisture. This includes split lines, cavities, milled areas. Any fuse plugs showing deformation must be replaced. All of „em. The most critical areas on the wheel bolts is where the shank joins the head, or where the shank joins the threads. Pressures of a tire/wheel that is NOT installed on the a/c should read 4% below the recommended pressure. Any single burned patches on a tire tread would likely indicate hydroplaning.
If the Mfr‟s wheel balance weights have to be removed for any reason, you must mark their position, and replace the same weight in the same place.
Aircraft Brake Assemblies
Light a/c can use simple shoe brakes or single discs; low weight & speed
Internal Expanding Shoe Brakes
Older a/c and home-builts One-way (single servo type), or Two way (dual servo) Can be similar to automotive types
Expander Tube Brakes
Four parts: brake frame, expander tube, return springs, and brake blocks Older aircraft Hydraulically operated Each brake block is independent; no tendency to grap
Single Disk Brakes
One of the most popular types Disk held in the wheel by teeth or keys Linings on either side of the disk; compressions forms braking action One lining is attached to axle structure, the other moves according to hydraulic pressure May have multiple pistons (& therefore multiple linings) Cleveland is one type of popular manufacturer Removal of air requires a brake bleeder valve
Multiple Disk Brakes
Used where a substantial amounf of braking force is required Typically will have multiple rotors and pistons, stator plates, a pressure plate and a torque tube (see page 9-22 in Jeppesen) Wear indicators are often included in this design
Segmented Rotor Disk Brakes
Heavy duty brakes design for use with high pressure systems using power brake control valves, or power boost master cylinders Uses stationary high-friction linings with rotating rotor segments
Carbon Composite Brakes
Weigh 40% less than conventional steel segmented rotor brakes. Natural gas is used in forming the brake discs to add carbon Strength does not decrease at elevated temps Carbon against carbon performs excellent as a friction material Carbon brakes can exceed 3000 degrees F
Braking Heat Energy
Huge amounts of heat energy on braking Pilots figure out gross weight of aircraft to establish a ground cooling time for brakes Somewhat reduced by using thrust reversers
Pilot reports excessive brake pedal travel. Check the brake fluid level. Which way should the chevron seals face? The inside of the chevron should face the pressure. An aircraft is reported as having excessive brake travel, but the brakes are still hard and effective. The probable cause is worn brake linings.
Aircraft Brake Systems
Modern aircraft brakes are classified as single or multiple disk Mechanically operated, or hydraulically operated, or pneumatically operated Mechanical is the older smaller systems, using pulleys, cables, bell cranks. Many a/c use hydraulic system pressures to actuate their brakes. Some have an entirely independant system. Pneumatic brake systems use air only; some hydraulics use air as a backup pressure supply.
Independent Brake Systems
Basic systems require a reservoir, a master cylinder actuated by pedal or handle, a brake assembly at the wheel, and all related hosing/tubing. Master cylinder is the energizing unit, usually one for each main gear wheel. Parking brakes are often a simple ratchet affair for holding the pedal or handle in place, which continues to supply pressure to the brakes. Various models of master cylinders; some mount on top of the rudder pedals. Function remains the same. Heel operated brakes or hand-brakes. Parking brake mechanism may be interrelated with the main brakes, but setting of parking brakes when hot may be an issue.
Power Boost Systems
Power boost systems are used on a/c with high landing speeds. Power boost is halfway between manual brakes and power brakes. Power boost uses hydraulic pressure from the main system to the brakes via a check valve. May use a shuttle valve to route emergency air pressure. Larger aircraft require more braking power than can be applied through a master cylinder. Extra pressure can be exerted on the brake system by allowing hydraulic system pressures to act through a spool valve. A Brake Debooster serves the purpose of decreasing system pressure to a useable level in the brake system. It also has the effect of increasing the volume of hydraulic fluid flowing.
Power Brake Systems
Used where manual and boosted brakes are not adequate. Uses a power brake control valve to direct hydraulic system pressure to the brakes. Power brake control valve is also called a brake metering valve; generally one for each main landing gear brake. Typical system has four lines to each valve; pressure, return, brakes, and automatic braking. Automatic braking is used to stop wheel rotation during retraction on take-off. Sources pressure from the landing gear UP position in hydraulic system.
Used to reduce the system hydraulic pressure to a lesser pressure in the braking system. Generally exchanges High Pressure/Low Volume into Low Pressure/High Volume.
Multiple Power Brake-Actuating Systems
Brake actuation systems get very complex at this level. Requires a lot of research and careful work on the AME‟s part to learn these individual systems
Brakes are operated by 2 independent systems. (#1, and #2) Each systems consists of daul-brake-control valves, pressure accumulators, brake-pressure transmitters and indicators, brake quantity-limiter valves, a skid-control manifold for each gear, and a parking-brake valve. All contribute the actuation of the independent cylinders in the eight main wheel brakes. Either system is capable of stopping the airplane on a maximum gross-weight landing.
Construction of Brakes Sintered brake linings are another term for metallic linings. Segmented rotor discs produce three benefits: Eliminate heat buildup in the disc Produce more efficient braking Allow for longer braking action. Floating calipers are used to adjust for brake lining wear. Those types of aircraft which use a large amount of fluid to operate the brakes will incorporate a power brake control valve. Carbon disk brake lining material is usedfor light weight, better wear resistance, and better heat resistance. Automatic adjusters are installed in modern systems to maintain a set clearance between the disc and brake lining. Heavy steel plates called Pressure Plates are used to act as a backing against which the linings are forced by the pistons.
Certain a/c have brake temperature readouts in the cockpit Temp ranges are relative to a scale of 0 to 9 Brake temperatures can increase even after brakes have been applied and released due to heat soaking Temp values above a “5” illuminates a BRAKE TEMP light
Some types of bonded linings are not fully cured at the time they are installed. The curing process requires they be installed and then used in a moderate-to-heavy application of the brakes. BCIT students will learn more about curing processes in the a/c composites section of level 3. For routine maintenance, check indicator pins for brake pad wear. Check lugs or keys holding rotor disks Check fusible plugs in the wheels for yeilding or cracks Examine fittings for leakage Ensure you are servicing the brakes with the appropriate fluids. Inspect hoses fro swelling, leakage, sponginess Check for reports of dragging brakes, fading brakes, excessive pedal travel, pedal creep or non responsive braking. Dragging brakes? Check for air in the system, sticking valves, and weak or worn return springs Grabbing brakes? Check for oil or FOD on linings. Fading brakes? Check for overheated linings and glazing Excessive travel? Check for lining wear limits, lack of system fluid, air in the system, or maladjusted brakes. Pedal creep? Inspect for leaks in a master or slave cylinder
Brake Bleeding Purpose is to remove any air from the braking fluids system and all related valves and cylinders Air will cause sponginess or dragging
Gravity bleeding uses a clear plastic tube, attached at one end to the bleed fitting at the brakes, and the other end is immersed in a container of fluid. Apply pressure to brakes, and open the bleed fitting. Trapped air bubbles will be removed with the fluid, and can be seen in the container. Maintain fluid levels in the reservoir. Pressure bleeding uses special tooling for the specific aircraft. Most types use a pressurized reservoir attached to the brake bleed fitting. Fluid is forced through the system back to the reservoir.
Several reaons apply why anti-skid systems aare in use on many modern aircraft: They prevent wheel lockup They prevent skidding They reduce the chance of hydroplaning They help reduce excessive heat build up
A successful anti-skid system will have two main features: A form of wheel sensor that can detect a change in the rate of deceleration A valve system that can rapidly apply and release the brakes, which will prevent a skid
The Three main Components of an anti-skid system: Wheel speed sensor(s) Control unit (computer) Control valves
Two types of wheel speed sensors are: The AC sensor, which creates a variable frequency AC current A DC unit, (basically a DC generator)
AntiSkid system operation
Antiskid systems are generally armed by a switch in the cockpit. System will utilize the squat switch to prevent current from flowing to the system during flight. System allows full pilot control over braking at speeds below 20 mph. System will perform its function when the wheel deceleration indicates an impending skid.