AMT 111 Aircraft Landing Gear Systems. Chapter 13

AMT 111 Aircraft Landing Gear Systems Chapter 13

Complex

Hard Working

Big

Small

Hard Core

Landing Gear Arrangement Tail Wheel/Tail Dragger/Conventional Tricycle Tandem v Gliders or Military Floats v Amphibious Skis v Retractable Amphibious seaplane/flying boat Fixed or retractable

Tail Dragger/Conventional

Amphibious Tail Dragger

Tricycle Gear

Tandem

Floats (Amphibious)

Skis

Skids

Skids (Wrong Kind)

Landing Gear Diagram

Shock Absorbing Leaf-Type Spring v Gear flexes Rigid Steel, aluminum or composites v Usually helicopters Bungee cords v Older tube/fabric type aircraft Solid, donut-type rubber Shock struts v Called oleo struts v Combination of hydraulic and gas shock absorbers

Leaf-Type Spring

Bungee Cord

Rubber Donut

Shock Struts

Shock Trailing Arm

Shock Strut

Metering Tube Strut

Strut

Servicing Shock Strut Check normal operation v Check height v Check rebound action Look for leaks Jack-up aircraft Inspect for physical damage and leaks Release pressure v If possible, remove valve core after releasing pressure v Valves marked with a H are high pressure valves, be careful

Servicing Shock Strut If the strut must be rebuilt, take care not scratch Fill/replace hydraulic fluid per instructions Install new valve core (they are inexpensive) and slowly pressurize v Nitrogen is preferable v Pressurization level may be defined by PSI or extension of strut

Alignment Toe-in/Toe-out is a measurement along the longitudinal axis (center line) v Toe-in front of tire points toward fuselage v Toe-out front of tire points away from fuselage v Corrected by inserting, removing, or changing the location of washers or spacers at the center pivotal point of the scissor torque links. Camber is the alignment along the vertical axis (up and down)

Alignment

Video 1 Video 2 Video 3 Video 4 Retract Testing

Retraction Systems Small aircraft v Mechanical v All electric v Combined electric and hydraulic Large aircraft v Power pack combination of electric motor and hydraulic pump v Usually hydraulic Usually engine driven v Sequence valve to control order of opening and closing v Orifice check valve will help slow down lowering of landing gear

Emergency Extension If the retraction fails or losses power, the system should fail in the extended position Gravity/free fall v Unlock the gear and it falls/locks into place Hand crank or pump Backup hydraulic system v Redundant pump v Backup electric pump v Ram air driven pump Compressed air

Landing Gear Components Position selector switch up is up and down is down Landing gear indicator lights v Green is down & locked v Controlled by position switches Warn system will alert the pilot when the throttle is retarted and the gear is still up Squat switch detects when pressure is on gear v Used to prevent retraction on the ground v Controls automatic brake application and engine reversers v Controls airframe timers

Landing Gear Maintenance Gear should be inspected and tested at each annual (100 hour inspection) v Clean v Check for wear and smooth action v Check light and indicators v Lubricate v Check doors, latches and braces v Check locks v Perform retract tests v Check emergency extension

Nose Gear Steering

Nose Gear Steering Small Aircraft v Nose wheel driven by the rudder pedals Push rods Spring often used to provide give v Tail wheel driven by the rudder pedals Cable connected to tail wheel Tail wheel connected to rudder Springs often used to provide give v Free swiveling nose wheel Differential braking used to steer aircraft

Nose Gear Steering Large Aircraft v Hydraulically powered v Driven by rudder or separate wheel/tiller

Shimmy Dampers

Shimmy Dampers Reduce nose wheel vibration (shimmy) Before rebuilding shimmy damper check for other causes of shimmy v Tire/wheel balance v Tire wear v Wheel bearings v Linkages Steering Dampers

Wheels

Two piece Wheels Relieve pressure before splitting wheels v Precaution in case wheel bolts have been damaged Often tubeless Aluminum or magnesium v Inspect for damage such as cracks and corrosion Can be refinished/painted v High stressed wheel have regular scheduled NDT NDT Nondestructive Testing

Wheels High stress wheels use thermal plug v Melts when wheel gets to hot Prevents wheel from exploding v Prevent overheating with short ground rolls, slow taxi speeds, minimum braking, and proper tire inflation. v Wheel must be completing inspected after a thermal plug failure Tire should be replaced Wheel have white marks applied to both the wheel and tire v The marks are used to detect tire slippage v Service wheel/tire if slippage is detected

Thermal Plug

Wheel Bearings Bearing Cup/Race Bearing

Bearing Consist of the bearing and bearing cup v Bearing cup is also called the bearing race Bearing and cup are matched v Don t mix Bearing and cup have hardened metal surfaces v Any damage, replace v Replace both bearing and cup v Cup uses an interference fit Heat wheel to remove Heat wheel and refrigerate cup to insert

Bearing Bearing are packed with grease or packing v Lubricates bearing and keeps water/dirt out v Follow manufactures recommendation for grease used v Clean bearing and cup before packing Use mineral spirits and soft brush Dry before packing Don t spin bearing when dry Inspect regularly RTFM v Manufacturer often recommends a time interval Also consider calendar time v Every annual is recommended

Bearing Wheel held on by axle nut v Follow torque recommendation v Some procedures call for two step torquing v Insure axle nut is locked v Insure wheel turns freely

Floating Disk Brake

Fixed Disc

Fixed Disc Cleveland wheel showing the brake disk that bolts to the inner wheel half.

Fixed Disc

Multiple Disc

Expander Tube Brake

Brakes Floating single disc v Goodyear v Disc floats in wheel/caliper is fixed Fixed single disc v Cleveland v Disc is fixed to wheel/caliper floats Multiple disc v Large aircraft Expander tube obsolete v Must be hydraulic/can t be mechanical

Brakes Actuating System All modern brake systems are hydraulic Independent system all human powered Booster system use the aircraft s hydraulic system intermittently Power system completely powered by the aircraft s hydraulic system

Independent Master Cylinder

Remote Reservoir

Hydraulically Link

Independent Master Cylinder Pilot pushes on the top of the rudder pedal, which pushes on the master cylinder v Left and right wheels are independent Master cylinder creates pressure that is sent to the calibers which squeezes the brake pads against the brake disc Reservoirs v Stores extra hydraulic fluid v Built-in part of the master cylinder v Remote Container separate from the master cylinder

Independent Master Cylinder Compensating port permits the fluid to flow toward or away from the reservoir as temperature changes v Fluid comes from/goes to reservoir v Helps compensate for fluid leaks

Independent Master Cylinder

Large Aircraft Brakes Boosted brakes Part of the braking force comes from the pilot, part of the force comes from the hydraulic system

Boosted

Large Aircraft Brakes Power Brakes v All the braking power comes from the hydraulic system v Used because the aircraft is too large, too fast or the system requires too much hydraulic fluid for human power to supply v A brake control valve is used to convert the pilot s input to hydraulic pressure A special hydraulic valve that gradually controls pressure Also called a brake metering valve

Large Aircraft Brakes Power Brakes v Brake debooster valves/systems reduce the hydraulic pressure to a level more suited to braking systems Like a gear system, trades pressure for volume v Reduces pressure, increases volume Lower pressure helps increase release brakes more rapidly Lockout deboosters prevent main hydraulic and brake hydraulic fluids from mixing

Power Brake System

Brake Systems Emergency brake systems backup system to help in the event of a failure in the primary system v Shuttle valves are used to isolate emergency system from main system Parking brake are used to hold the aircraft in a stationary position v Physical lock brake pedal or use a valve to lock pressure in brakes v Don t apply when brakes are hot warps disks v Don t leave on for long periods of time

Brake Systems Anti-skid system prevents the tire from skidding while braking v Once a tire starts to skid, it loses traction v Skids can also damage the tires and cause blowouts v If the onset of a skid is detected, the brake pressure will be released Pressure release maybe accomplish by pulsing v Wheel speed sensors are used to detect wheel speed v A control unit determines if the wheel is slowing down too fast and releases brakes

Brake Systems Touchdown protection will prevent the application of brakes at the moment of touchdown v Wheels should be rolling before applying brakes v Landing gear squat switches are used to detect that the aircrafts weight is on the tires Auto brakes some large aircrafts will automatically apply brakes to reduce rolling distance v The level of braking can be selected by the pilot

Brake System Maintenance Lining measure minimum thickness v Rivet new linings to back plate Caliper maintenance v Check for cracks, corrosion, and leaks v Can be rebuilt Insure pistons and cylinders surfaces are clean and not scratched

Brake System Maintenance Master cylinder maintenance v Check for cracks, corrosion, and leaks v Can be rebuilt Insure pistons and cylinders surfaces are clean and not scratched v Broken spring can cause dragging of brakes Disk must be checked for physical damage and minimum thickness v Some disks can be turned to repair damage v Warped disks can cause chattering or squealing

Brake System Maintenance Fluid maintenance v Follow replacement schedule v Replace if contaminated v Bleeding the brakes must follow any opening of the hydraulic system Remove air from system System will feel spongy if air is in system v Firm brakes are an indication the air has been removed v Follow procedures exactly for anti-skid systems Check condition of brake lines v Old brake lines can flex and cause spongy brakes

Bleeding Brakes

Current Tire Types Type III type small aircraft v Designation A-B A - Tire width B Rim diameter e.g. 9.00-6 is 9 width with a 6 rim diameter v Low pressure Type VII high performance for jets v Designation A X B A Total diameter B Tire width e.g. 26 X 6.6 is 26 tall and 6.6 wide v High pressure

Current Tire Types Type VIII high performance for jets v Designation A X B - C A Total diameter B Tire width - - means bias ply, R means radial C Rim diameter e.g. 30 X 8.8 R 15 is 30 tall, 8.8 wide, radial, 15 rim v Prefixes B the tire has a 60 to 70 percent wheel rim to section width ratio with a bead taper of 15 degrees. H the tire has a 60 to 70 percent wheel rim to section width ratio with a bead taper of 5 degrees. v High pressure

Tires Ply Rating plies are reinforcing layers v Ply rating used to indicate the number of layers v Nowadays, ply rating is not an indication of tire layers Tube-type or tubeless both types are still used Bias ply or radial classification on how the construction plies are wrapped v Bias plies are 30 to 60 in relation to rotation v Radials are 90 v Both types are used Radial are usually used on high speed aircraft

Tire Construction

Tire Construction Bead anchors tire to wheel Carcass plies forms the tire carcass and provides the tire s strength Tread crown to the tire, comes in contact with the ground Sidewalls outside rubber designed to protect carcass v Green or white dots mark seepage release vents on tubeless tires Must be kept clean

Tire Chine

Tire Inflation Proper inflation helps maintain tire life and operation Check inflation with a calibrated gauge Check a cold tire v Should be at ambient temperature v After landing, the tire should be allowed to cool down for 3 hours before checking inflation Pressure changes 1% for every 5 F v If going from hot airport to cold, inflate to maximum v If going from cold airport to hot, inflate to minimum

Tire Inflation Inflation should be check every week Inflation information comes from the aircraft maintenance manual not the tire manual If a multi-tire gear has more than 5 pounds inflation difference, the difference should be corrected and recorded in the logbook v The logbook entries will help track if there is a problem Over inflation can damage the wheel flange

Tire Wear vs. Inflation From AC43.13-1B

Tire Wear vs. Inflation

Tire Condition Tread Depth Replacement v Worn to the bottom of a tread groove along more than 1/8 of circumference v If the first layer of ply is exposed for more than 1/8 of the circumference Uneven Wear v Determine cause and fix Flat spots may involve pilot training v Switching tires from side to side may help with wear For damage limits, consult manufacturer

Tire Removal

Tires Tire repair should be handled by a certified tire shop Tires can be retreaded an unlimited number of time if the carcass is not damaged Tire storage v Stack vertically v Keep away from ozone sources electric equipment v Wrap in dark plastic or paper v Keep away from UV light (sun), fuel, oil, hydraulic fluids and solvents v Keep away from extreme heat and cold

Tire Mounting Assemble wheel in tire v Light side of wheel halves should be opposite each other Light side is marked with an L, S/N or red dot v Lubricate wheel half O ring Use proper lubricate v Tire bead may require lubrication v Align heavy part of wheel with light part of tire Light part of tire has red mark Default heavy part of wheel is valve stem v Follow torque requirements v Video

Tire Mounting Inflate in a safety cage Nitrogen is used for inflation to minimize corrosion from moisture After inflation, leave unloaded for 12 hours v 5-10% pressure loss is normal due to tire stretching

Tube Tire Mounting Lightly inflate tube to give it shape before installing tire v Insure tube is wrinkle free Match heavy spot on tube with light spot on tire v Valve stem is default heavy spot Use tire talc inside tire and outside of tube Completely inflate tire, completely deflate and then inflate again v Helps tube seat properly

Tire Balancing Prevents heavy spots (land on the same spot) and vibrations Most tire balancing is done statically v The tire should not stop at the same place every time it is rotated

FAA Question An electric motor used to raise and lower a landing gear would most likely be a v a. shunt field series-wound motor. v b. split field shunt-wound motor. v c. split field series-wound motor. Answer: C