AVIM 103D Landing Gear Notes Workbook

Transcription

1 AVIM 103D Landing Gear Notes Workbook 8/26/2010 Course Outline Landing gear Types Configurations Alignment Suspension systems Fixed gear Retractable Course Outline Retraction systems Steering systems Brakes Dependent systems Independent systems Anti-skid control Wheel assemblies Tires Safety Shock strut servicing Gear retraction and extension Shimmy damper service Tire servicing and dismounting Eye and skin protection Safety Caustic fluids Burns skin Damages surfaces Flammable fluids Fluid contamination Leave containers closed Read labels, use proper handling equip. Safety Retraction can crush you if you are in the path of the gear Retraction without proper support can destroy an aircraft as well Landing Gear Purposes Supports the aircraft on the ground Absorbs landing shock (some) Absorbs taxi shock (some) Attachment point for: Brakes Steering Wheels and tires Conventional Gear Defn: Wheel Pants The tapered tail end of the pant provides the major part of aerodynamic drag reduction Defn: Cowlings & Fairings A shielded section that provides aerodynamic smoothness to some area or part of the aircraft Defn: Wheel Base Jodel d140c C150 Tail Dragger Conversion Conventional (Tail Wheel) Arrangement Older design C.G. aft of main gear Steering: Rudder pedal cable connection to tail wheel Brake application and castering tail wheel Differential braking to assist steering Tail wheel as far aft as possible to extend wheelbase and increase stability. Conventional (Tail Wheel) Arrangement Advantages Prop clearance for low powered engines Sturdy design for unimproved runways Less drag in flight Greater ground maneuverability Tail wheel failure = minimal aircraft damage Conventional (Tail Wheel) Arrangement Disadvantages Ground loop and nose-over potential Crosswind control problems Restricted visibility during taxi Tricycle Gear Tricycle (Nose Wheel) Arrangement Nose gear as far forward as possible Longer wheelbase more stable Lighter gear assembly due to longer lever arm Castering types use differential braking to steer Tricycle (Nose Wheel) Arrangement Main gear aft of C.G. Advantages Difficult to nose over or ground loop More familiar ground maneuverability Better visibility during taxi Less vulnerable to cross wind landing Steering: Direct linkage with nose wheel bungee Hydraulic nose wheel steering Differential braking Tricycle (Nose Wheel) Arrangement Disadvantages Nose gear damage = major airframe damage Generally not suited for unimproved runways More expensive than conventional gear Much heavier aircraft Nose Wheel Ski Skis Ski systems are usually pivot mounted to the aircraft wheel axle incorporate travel limit straps or cables (front and rear) usually have a bungee or spring to keep the nose up, preventing pearling during landing

2 May be retractable (skis retract higher then bottom of wheel assemblies) Skis Auxiliary gear, nose or tail, may or may not have a ski Are subject to corrosion damage and hard landing damage Floats Floatplane Configurations Floats Amphibious floats wheels and floats Hull floats bottom of aircraft = boat Outrigger pontoons Hang from wing tips or struts Fold down from wing tips Float/Hull/Pontoons Most common are dual float assemblies Usually are uniform shape May have retractable, and or steerable rudder assembly May require a vertical vane installed on lower side of fuselage below vertical stabilizer Float/Hull/Pontoons Almost all water aircraft use a float shape that includes a chined V hull They usually have a stepped section that assists the aircraft in planing across the water (reduces water drag) Flying CG and floating CG may not be the same some hull planes have self flushing ballast sections / wheel well sections Float/Hull/Pontoons basic shape Tandem Wheel Arrangement Aircraft with narrow fuselage Gear positioned directly beneath fuselage Tandem Wheel Arrangement Gliders U-2 AV-8 Harrier Usually has one main set of gears in center, one steerable nose gear, and outrigger gears on the wings Can be fixed or retractable Tandem Wheel Arrangement Gear Types Fixed Gear Popular on older and low speed aircraft Speed and fuel efficiency increase with pants Fixed Gear Are not able to retract into some cavity or aerodynamic shielding within the aircraft May be fully rigid or able to absorb landing / taxi loads Fixed Gear Are usually lighter and less complex than retractable gear aircraft Have overall lower purchase and operating costs than retractable gear The benefits from lighter weight can exceed the benefits of reduced drag from retractable gear Are subject to corrosion damage and hard landing damage Retractable Gear Streamlines aircraft reducing drag More complex and heavier than fixed gear Retraction methods: Mechanical Electrical Hydraulic Anatov AN 225 Trailing Link landing Gear B747-8 Landing Gear Ship Set Skid Landing Gear Used on helicopters that do not ground taxi High skids and pop-out floats available May or may not have shock absorbing devices May or may not have skid pads (stellite faced) Left skid / nose low wear pattern Loose skids may cause Vibration Ground resonance (fully articulated rotor) Skid Landing Gear May have detachable wheel assemblies for ground handling Are also found on early aircraft in place of the tail wheel assembly Wooden skid with brass or steel plate for hard surface or leather plate for grass Pop Out Floats Spring Steel Gear - Cessna Type Load transfer only Minimal rebound protection Generally not field repairable Serialized Cessna component Check Cessna maintenance manual table of limits for alignment data Tubular Steel Nose Gear Grumman TR2 Load transfer only Minimal rebound protection Sometimes field repairable by welding Some have bungee shock cord Wheel Alignment This is much more critical for tail draggers. The aircraft should be level and the wheels should be on some form of grease plates to eliminate gear binding. The aircraft should be located inside where it is not subject to winds.

3 Adequate measuring equipment should be available. Toe in / out Toe = the distance between the front of the tires and the back of the tires. The best means to measure this is to project lines out to a distance and calculate to the specifications. Toe-in is front of tires in, Toe-out is front of tires out Camber (- +) Camber = the distance between the top of the tires and the bottom of the tires. This can be seen using a large square. Positive is top of tires out. Negative is top of tires in. Castor Castor = only really applies to a wheel assembly that turns or steers. It is the measure of the angle that the pivoting axis tilts front or back. This is similar to the concept of rake used on single strut assemblies such as nose gears or motorcycles. Inclination and Offset Steering inclination = is similar to castor but it is the measure of the angle between the pivot axis and the vertical axis of the wheel with no camber. Trail or offset = The amount of distance between the wheel axis and the steering axis. Wheel Alignment Adjustment Some may be adjustable by shimming the stub axle at the mounting flange Wheel Alignment Adjustment Some may be adjustable by shimming the torque links at the center pivot Wheel Alignment The aircraft must be located on a flat smooth surface, resting on grease plates, leveled as per manufacturer's procedure First determine the landing gear are properly mounted and not damaged or distorted Damage and conformity inspection, symmetry checks, etc Wheel Alignment Several methods for checking toe: Straight edge and a large square Scribe and a measuring tape or bar Line of sight projection to a reference Straight edge and a large square Scribe and a measuring tape or bar Line of sight projection to a reference Camber Is checked using a ruler and a level Scissor Link Disconnected END SECTION ONE

4 Aircraft Provide controlled flexibility to the landing gear systems while maintaining their structural integrity Up to a point they will eliminate the unusual loads incurred during landing and takeoff operations They can also reduce or eliminate ground operation vibrations from uneven or rough taxi surfaces Suspension vs. Absorption Suspension systems are devices that allow flexibility or bounce to occur between the ground and a vehicle This can include low pressure tires, springs (torsion, flex, coiled)(rubber, metal, plastic), telescoping struts Suspension vs. Absorption Absorption is a suppression or restriction to flexibility or bounce The most common form are air / oil filled telescoping struts Less commonly are stiffeners such as plastic or wood straps attached to flexing type gear Very early aircraft had rigidly mounted gear As technology progressed two main forms of suspension came into being Rubber bungee mechanical lever systems Flexible metal tapered bars or shafts The main advantage of these two systems are: they are light easy to maintain relatively inexpensive fairly aerodynamically clean The main disadvantage is they provide no permanent shock absorption Air-Oleo struts were then designed to: suspend, or provide bounce and to truly absorb the shock energy, or prevent spring-back. Note: FAA test questions handle this badly Springs and bungees only delay the shock energy, but eventually spring back. Bungee System Elastic Shock Ring Shock Ring Notes: Remove boot for thorough inspection Beware safety hazard Oil stained cotton cover damaged Necked diameter worn, broken elastic Replacement considered preventive maintenance FAR 43 Appendix A Bungee Cord Bungee Installation / Removal Tools Spring Systems Flexible tapered bars and shafts Spring Systems Flexible tapered bars and shafts Spring Systems Less commonly there are numerous versions of coiled spring, rubber disc, torsion bar, plastic flexible bar, etc. assemblies that all provide some form of flexibility to the landing gear Flexible Gear Servicing Includes checking all fittings for security, tightness, and appropriate free play Inspect main gear for signs of corrosion, fatigue, hard landing damage or taxi damage Inspect auxiliary gear and steering connections for damage and corrosion Repair all worn or failed parts Flexible Gear Servicing Some tubular structures may be repairable by welding All spring type structures are not repairable by welding Spring type may have serial numbers and may be matched pairs which means they are replaced in pairs Struts Oleo Telescoping Strut Over all aircraft it is the most commonly used suspension system Range from 1" in shaft diameter to 10", 12", etc. Can be used as main gear, or as auxiliary gear Can be steerable, fixed or free castoring Oleo Strut - Basic principle of operation A telescoping strut that contains compressed gases and fluid, usually a light oil The compressed gas causes the strut to extend thereby sustaining the changing weight of the aircraft (suspension) For the strut to change length the oil must pass through a restricted orifice Due to the nature of hydrostatic lock this restriction of oil flow "meters" the rate at which the strut can change length (shock absorption) The tapered metering pin determines the rate of compression

5 Torque or scissors links maintain wheel alignment May have a flapper return valve that allows the strut to extend quicker then it compresses Very slight seepage of seals is normal to lubricate the piston Oleo Strut Oleo strut telescoping Oleo strut telescoping Oleo strut telescoping Oleo strut telescoping Oleo Strut Parts Main Strut, outer tube Piston, piston rod, inner cylinder Upper or inner strut seal rings Upper inner bearing Snubber or return valve (sometimes) Lower outer collar, bearing and gland nut Oleo Strut Parts Oil and gas fill plug / valves Neoprene V-ring seals Orifice tube Orifice or snubber plate Tapered metering pin Oil Dry gas Oleo Strut Notes Strut service is preventative maintenance Earlier struts used O-ring seals Newer use stacked V-ring seals Fluid pressure is applied to the inside of the V Or D-rings with round side facing movement Piston is hardened polished and or chromed steel Gland nuts are bronze (may or may not be adjustable) Oleo Strut Notes Deflating struts will protect piston from corrosion Piston may have a spline or cam that aligns the nose gear for retraction Strut extension distance at a given weight is the common method for determining gas charge Seal compatibility determines type of oil Strut should have a data plate attached Oleo Strut Notes Dried Nitrogen is the gas of choice Inert Inexpensive No moisture Three type of filling Valves MS most common AN 6287 AN 812 older models MS Fill Valve Has no valve core Base nut and swivel are 3/4" Has a roll pin to keep swivel valve in place Base nut torque is 110 in/lbs Swivel nut torque is 70 in/lbs Pressure rated to 5000psig MS Fill Valve AN 6287 Fill Valve Has high pressure valve core (stamped H) and a swivel nut valve Base nut is 3/4", Swivel nut is 5/8" Base nut torque is 110 in/lbs Swivel nut torque is 70 in/lbs Pressure rated to 3000 psig Do not interchange with MS AN 6287 Fill Valve AN 812 Fill Valve Has only a valve core Base nut is 5/8" Med. press. valve core short type stamped H Base nut torque is in/lbs Pressure rated to 1500psig Do not use in place of MS28889 or AN6287 AN 812 Fill Valve Fill Valve Warning All the fill valves are interchangeable DO NOT DO INTERCHANGE THEM DO NOT ATTEMPT TO USE AUTOMOTIVE VALVE CORES WITH EITHER THE AN 6287 OR THE AN 812 DO NOT INTERCHANGE VALVE CORES OR CAPS BETWEEN ANY OF THEM Oleo Strut Servicing Servicing data may come from current maintenance manual, or data plates Depressurize, remove from plane Disassemble and clean, inspecting for any damage, corrosion or cracks Replace all rubber seal components, worn bushings, and failed parts Oleo Strut Servicing Reassemble, add oil to level with filler opening, bleed air out, and seal Reinstall and repressurize with nitrogen 100hr / annual must include checking strut fluid and gas levels Typical pressures range from psi You will not be able to service a strut with shop air sources Oleo Strut Servicing Use a nitrogen charged bottle, or a strut pump (12:1) Cycle pressurized strut several times to ensure seal seating and air bubble removal

6 Struts can have slow gas leaks, recheck fill after 24 hours Always rock the aircraft prior to measuring strut extension Strut Servicing Strut Service Strut Service Strut Inflation Strut Inflation Strut Inflation B737 Main Landing Gear END SECTION TWO AVIM 103D Aircraft Retraction Systems Retraction Systems Aircraft gear retraction systems can be found on many aircraft From the small experimental Vari-eze to the ultra-large AN 124 (winged building) In most cases the retraction process is accomplished with hydro-electrical force connected to mechanical linkage Retraction Systems In most cases the retraction process includes the opening, and closing of doors or covers that complete the aircraft's aerodynamic shape In most cases all the gear retract With retractable conventional gear the tail wheel often doesn't retract Retraction Systems In most cases steering gear needs to be repositioned correctly for retraction Gear can retract in any direction, forward, backward, inboard, outboard, or rotating to fit into a special compartment. They can retract into the wing or the fuselage They can change the aircraft CG when retracting or extending Retraction Systems They will contain many adjustable devices that limit travel or notify the pilot of landing gear configurations and conditions They must have some emergency, auxiliary means of extension Any hydraulic, or electrical failure cannot cause the gear to automatically retract Most contain safety systems that limit when the gear retr./ext. may be operated Retraction Parts Trunion = the main pivot point, and attach point Drag or Side braces = provide rigidity when locked down Overcenter lock = similar to a knee joint Ground lock or pin = prevents accidental retraction, should have red flag attached Retraction Parts Weight on wheels switch, squat, ground safety sw. etc = usually attached to gear torque scissor links Limit switches = micro switches that electrically sequence gear retraction Sequencing valve = hydraulic valves the sequence gear retraction Retraction Systems Priority valve = same as sequencing valve but is actuated hydraulically Indicating system = red, amber, green lights, horns, barber poles that indicate gear position Red = unsafe, in transition Green = down and locked, up and stowed Lights may have push to test feature Retraction Systems Retraction systems must be tested fully during 100 hr and annual inspections This includes inspection, lubrication, and an operational test with the gear off the ground Any additional safety and alarm systems must also be tested, such as throttle horns, indicator lights Avoid testing the squat switch the hard way Retraction Operation Down and locked Retraction Operation Inboard gear door open, gear in transition Retraction Operation Gear up, inboard gear door in transition Retraction Operation Gear cycle complete gear up Retraction Systems DC-10 uses Oleo strut gear Mains retract inboard, nose retracts forward Mains use a four wheel truck or bogee 2X2 Incorporates the use of axle beams and beam trim cylinders Every wheel contains a brake assembly Retraction and braking is hydro-mechanical Retraction Systems Retraction sequencing is accomplished with a follow-up hydraulic-mechanical control valve System uses various cables, levers and bell cranks to control the landing gear control valve assembly Main gear doors can be locked closed to use as a work platform Emergency Extension All retractable gear system must have an alternate means to extend the landing gear

7 In smaller systems a mechanical, or hydraulic release allows the gear to free fall into place There may be an emergency hand pump, accumulator or auxiliary pump There may be a pneumatic extension system Emergency Extension There may be a pneumatic extension system Air flash blow down bottle There may be a mechanical hand cranking system Hydraulic/pneumatic may use a detented shuttle valve to separate the normal system Once extended via emergency system the normal system should be defeated Emergency Extension Some use a freefall system Release control for the main gears may be separate from the release control for the nose gear Main gear may need to be extended first Politically correct terminology for emergency extension is Alternate Extension System Mechanical gear retraction system will prevent gear retraction with weight on wheel beams Retraction Nomenclature Ground Lock Landing Gear Safety Switch Limit Switches (Up and Down) Down Lock Up Lock Indication and Warning Green indicator light(s) or wheels symbol In-transit indicator Red warning light Warning horn Cessna 310 Gear Indication and Warning Beechcraft King Air PIPER POWERPACK PIPER POWERPACK Beechcraft Retraction MIL-H-5606 fluid, system capacity 10 quarts No mechanical up locks Powerpack 28 vdc electric motor turns a variable displacement hydraulic pump Regulated bleed air (18 PSI) for reservoir pressurization Two solenoid selector valves direct pump discharge for gear extend and gear retract Beechcraft Retraction 4-second time delay reservoir fluid level sensor System accumulator nitrogen pre-charged to 800 PSI serviced with aircraft on jacks Fill tank for replenishment of reservoir Service valve permits gear retraction with aircraft on jacks; service valve micro switch disables landing gear relay Beechcraft Retraction Main gear actuators- external down locks Nose gear actuator internal down lock 200 to 300 PSI required to unlock down locks Three port actuators Pressure check valve opens at 750 PSI to provide fluid return path during extension Hand pump dump valve opens under hand pump pressure to provide fluid return path during emergency extension Beechcraft Retraction Controls, Switches, Lights and Circuit Breakers Handing gear handle Illuminates red for gear unsafe Manual down lock release Two squat switches (one on each strut) prevents retract relay operation down hook spring loaded over landing gear control handle Beechcraft Retraction Controls, Switches, Lights and Circuit Breakers Three green down and locked indicator lights Two ampere circuit breaker protects l.g. relay Normal retraction 6-8 seconds; 14-second time delay relay opens landing gear relay circuit Beechcraft Retraction Pressure switch terminates retraction at 2775 PSI Accumulator holds gear retracted Powerpack may cycle every 30 minutes in flight Down lock limit switches terminate power pack operation during extension Powerpack duty cycle One minute cooling cycle; five minutes after five cycles Troubleshooting Powerpack runs more than 10 seconds On retraction or extension check reservoir fluid level On retraction - check stowage of alternate hand pump On extension faulty down lock limit switches Troubleshooting Powerpack motor cycles frequently in flight Accumulator gas precharge low Gear will not extend Defective service valve micro switch Defective power pack solenoid valve

8 Defective down lock switch Gear will not retract Defective squat switch Hand pump handle not stowed SHUTTLE VALVES SEQUENCE VALVE SIMPLIFIED LANDING GEAR SCHEMATIC RETRACTION SCHEMATIC PRIORITY VALVE RETRACTION ILLUSTRATION: RETRACTABLE NOSE GEAR: DOWN LOCK MECHANISM: UP LOCK MECHANISM: F4S Gear Indication Gear Swing B727 Gear Selector Cessna Citation L.G. Safety Switch King Air L.G. Safety Switch Grob G120 Military Aircraft Up Lock Cessna Citation Cessna Gear Retraction Cessna 182R Panel Airbus Piper Twin Comanche Bungee Roller Comanche Over-Center Down lock END SECTION THREE Aircraft Key Steering Needs Pedals actuate steering gear and rudder Large A/C may also have separate steering wheel Extended steering gear needs to be straight ahead for touch down and gear stowage. Needs to steer when weight on wheels (WOT) Needs to allow rudder action when locked straight ahead or stowed Two basic types Open - found on conventional geared aircraft Closed - most common, pedals, third gear and rudder are looped in the system Open loop system Closed Loop System In open loop cable systems there are pedal return springs to maintain cable tension Tail wheels are usually attached to rudder post assembly via bell cranks and springs Tail wheels can be fully castoring, or steerable and castering Castering pivot must be vertical or gear can get stuck Smaller nose wheel systems use a Whiffle tree and mechanical linkage to close the "loop" Larger aircraft use hydraulic power steering systems In most nose wheel aircraft there is a shimmy damper that eliminates nose wheel shimmy Nose wheel shimmy is similar to control surface flutter, it can tear a nose gear off in less than a second Two basic types of steering dampers are Piston Vane Both types operate by creating chambers on either side of a moveable plate Due to hydraulic lock the plate cannot move unless a small metering hole is introduced Cessna 152 Nose Gear Nose wheel De Havilland DH.82 Tiger Moth Tail Wheel STC SA2359NM XP Modification XP Modifications Inc XPM Tail Wheel, features a 500x5 tire mounted on a specially designed assembly that keeps bearings and key wheel parts up and out of soft sand and mud. Advantages provided by the large tail wheel: Smooth operations Less drag on soft ground Better taxi visibility Shorter take-off rolls Improved ground handling Improved maneuverability Turn Limits Larger aircraft must use some form of power assist, or full power steering system Hydraulic power is used almost universally There can be either a separate nose wheel steering wheel a rudder pedal nose wheel steering system a mix of both Any time a hydraulic power/boost/assist system is used there must be some form of a follow-up differential control system This functions by disengaging the hydraulic actuator after the nose wheel has pivoted the desired amount Dual Piston Steering Damper

9 Oleo actuated shut off valve prevents steering when strut extended Self centering device insures that nose gear is centered for retraction Control cable moves bevel gears in differential control (Follow-up) Orifice check valves are installed for shimmy damper action Compensator valve maintains small positive pressure for two reasons: Prevents cavitation if wheel is moved suddenly Controls thermal expansion Solenoid shut off valve allows inter-connection for towing, and failure Differential Follow-up Steering Control The steering input is opposite the steering action therefore a gear set must be used to reverse the direction of the input or the output The steering input unbalances the compensating device and the steering action rebalances it. Differential Follow-up Steering Control The steering input is the same as the steering action Again the steering input unbalances the compensating device and the steering action rebalances it. In most cases the large aircraft dual system steering will allow for limited steering from the rudder pedals while allowing for more range from the cockpit steering assembly There may be a steering wheel lock out above certain speeds They may combine the differential steering control with the steering damper Shimmy Damper Piper Steering (PA28R) Roller alignment guide is disconnected from track while a/c is on the ground Steering rods cause bell crank to pivot at center Bushings on steering arm serve as a bearing surface for turning the steering arm Torque is fed down through the center of strut to turning collar Cessna Bungee Steering Rudder pedal extensions attached to steering bell crank complete rudder "circuit" since it is impossible to put cables under compression Always inspect rubber boots for CO leakage Rudder pedals interconnect with rudder, nose wheel steering and rudder trim Rigging order: Rudder, nose wheel steering, rudder trim Cessna Bungee Steering Functioning: On Ground: Initial Movement of Pedal Turning force is applied to steering bell crank (whiffletree) Rudder moves by cable actuation Spring bungee is compressed at this time and nose gear does not turn much until rolling Cessna Bungee Steering Torque is fed down through the center of strut to turning collar Cessna Bungee Steering In Flight: Initial movement of Pedal Rudder moves because action of cables through spring bungee Nose wheel is locked out of system by centering cam Cessna Bungee Steering Continued Movement of Pedal: Nose wheel remains locked out of system and bungee moves Rudder Trim Interconnect: Rudder trim prepositions rudder by means of threaded shaft which compresses spring within bungee and displaces rudder and pedal only. Since spring is compressed within the bungee, the nose wheel does not turn. END SECTION FOUR Aircraft Brakes The basic principle behind any braking operation is to create a controlled friction process that increases the rate of deceleration Acceleration converts heat energy into motion Deceleration converts motion into heat energy Aircraft Brakes Two main methods of increasing aircraft friction or drag in a controlled manner Increase aircraft to surrounding air drag Airbrakes, spoilers, flaps, reverse thrusters, drag chutes, etc.. Increase aircraft to ground drag Anchors, skids, mechanical brakes, hydraulic brakes, pneumatic brakes Aircraft Brakes One main method of increasing aircraft friction or drag in an uncontrolled manner Aircraft Vs Automotive Some of you may be familiar with the power assist systems used in automotive

10 This type of system power assists the mechanical application of a hydraulic brake system. The hydraulic brake system is independent from the power assist system (Pneu. or Hyd.) This system is rarely used on aircraft Aircraft Vs Automotive Aircraft and automotive braking needs are very different Aircraft braking speeds far exceed automotive Aircraft braking weights far exceed auto Auto braking duration far exceeds aircraft Automotive ratio of braking/nonbraking much closer to 20/80, aircraft / (est.) Aircraft Brakes In any case the braking system for any vehicle must be able to meet or exceed the coefficient of friction between the tire and the braking surface Anti-skid systems (covered later) are an attempt at splitting the line between meeting and exceeding the tire's skidding ability Brake Maintenance You must be at least Airframe rated to perform and return to service any brake work Brake systems may be rebuilt, resealed, rehosed, new brake material installed, new fluid installed, new or serviceable parts installed, etc. Remember to always be extremely clean and thorough with any brake work. Aircraft Brakes Braking systems fall into three basic categories Mechanical brakes - independent Hydraulic brakes - both Pneumatic brakes - dependent (depends on external pressure source) Independent Brakes Do not use an external power source other than the operator's mechanical application Usually consist of one complete system for the left main gear, and one for the right main gear (nose gear use brakes on some large aircraft) In some cases they will use the same reservoir for both sides (Piper) Commonly the reservoir is a part of each M/C Independent Brakes Common manufacturers: Bodell/Firestone Cleveland Goodrich Goodyear Matco Warner They all function by forcing a moving surface to rub or drag against a stationary surface The two surfaces usually differ greatly in composition and hardness In most cases this rubbing motion is a rotating motion and is associated with wheel rotation If the rotation rate of the wheel is slowed down then the linear speed of the aircraft will be slowed down providing the wheel does not slide Extreme amounts of heat will be generated at any point where sliding friction occurs Some Vehicle Gross Weights are established by the ability to brake, not the ability to carry a load The three sections of any brake system include: The brake assembly: friction device The control or actuating system The linkage, plumbing, power boost system Mechanical Brakes Tend to be very weak Heavy Need constant adjustment Often subject to binding and failure Used only on small early or experimental aircraft Mechanical Brakes Hydraulic Drum Brakes Much stronger Lighter systems overall Are usually self adjusting Rarely subject to binding and failure Used only on small early or experimental aircraft Hydraulic Drum Brakes Landing Gear Floating Hydraulic Drum Brakes Even stronger The piston actuates the primary shoe The primary shoe begins to drag actuating the secondary shoe The secondary shoe does most of the braking action Floating Hydraulic Drum Brakes Drum Brakes

11 43.13 indicates drums can sustain 1 inch cracks as long as they don't reach an edge Overall these brakes are limited in the amount of friction surface area that can be compacted into a small space Single Servo Shoe Brakes Single Servo Brake Assembly Duo Servo Brake Assembly Bendix Duo-Servo One version of the drum type brake is the expander tube brake used from the 30s - 50s This uses a flat hydraulic inner tube that expands when pressurized causing the surrounding braking pucks to rub against the outer drum These tended to swell and leak causing dragging and occasional brake fires Expander tube brakes Can have more than one row of pucks Tend to take a set when extremely cold P47 Expander Tube Expander Tube Brake Expander Tube Brake Expander Tube Brake Hydraulic Disc Brakes Strongest type of brake system available Lightest system overall Are always self adjusting Rarely subject to binding and failure Used on most aircraft Hydraulic Disc Brakes The discs are steel, and rotate with the wheel The shoes, or pads/pucks are mixtures of asbestos, organic compounds such as nut shells, and soft metal chips such as brass, lead, aluminum, or carbon These are installed in a hydraulic clamping device that is attached to the landing gear As the aircraft gets bigger multiple disks and pads can be stacked into each assembly In some cases the metal discs rotate and the braking discs are stationary In other cases the braking discs rotate and the metal discs are stationary Parts include: Pads, pucks, or shoes Calipers, or wheel cylinders Discs, or drums Backing plate Landing gear axle assembly Wheel and tire assembly Pneumatic brakes are not very common on aircraft They can be found used as a back up system Large non aircraft vehicles use pneumatic systems (Trains, trucking, etc..) They can be pressure applied, or pressure deapplied - spring applied Single piston brake assembly Used on small general aviation aircraft One piston with a floating caliper Fixed disc (to the wheel assembly) As the pressure increases the piston forces the pressure plate lining into the disc, and the floating caliper forces the backplate lining into the other side of the disc These assemblies can have more then one piston They can have more then one caliper assembly The caliper assembly can be fixed and the disc is floating 3 Piston Floating Disc Caliper Assembly Wear Indicator Caliper Has a pin sticking out the visible side that indicates pad or puck wear Pin also functions as a part of the piston retraction mechanism Refer to manufacturer's specifications for proper pin depths Auto adjusting piston Goodyear Brakes Goodyear Brake Linings Linings, Rivets and Pins Lining Limits Linings Cleveland Brake Linings Pad thickness Always refer to manufacturer's specifications Pad material may come with back plate or is riveted to old back plate Pad or puck replacement Usually done with aircraft wheel removed Reservoir vent opened, fluid level lowered as needed

12 Disassemble brake assembly as needed to remove pad If non-riveted type then replace pad and reassemble If riveted type then remove rivets and old puck, by drilling and punching out old rivet Clean & inspect backing plate Install new pucks with new rivets installed in the same direction as old materials Rivets are commonly copper, can be squeezed with small hammer and drift, or an arbor press Pad/puck thickness measuring Matco Wheel and Brake T6 STC Brake Conversion Disc coneing and warpage They can cone in either direction They can warp like a potato chip They can wear to uneven thickness radially They can wear to uneven thickness in circumference They can crack in many different ways (heat) Disc coneing Shoe Brake Brake Cooling Main brake cooling system Ducted manifold system from air inlet scoop Feeds ram air into wheel well Directs cold air onto brake assemblies when gear is retracted Probably doesn t do much since brakes get hottest on landings, more than takeoffs Brake Maintenance Some brake pucks come with a back plate bonded to the lining Some must have the lining riveted to a mounting plate Some linings are just inserted into a retainer and held in place by the assembly Always use the manufacturer's brake pucks and retainer parts Brake Maintenance To install puck linings on the puck backing plate, use the appropriate manufacturer's rivets, and the proper rivet set Can be set by hammer, or by an arbor press Setting too tight will shatter the puck Setting too loose will cause the puck to move and wallow out the rivet hole The rivet shop end is usually on the puck side Brake Maintenance New brake pucks must be seated into the discs New brake pucks must be cured with heat from initial applications Too much heat will burn the bonding resins Too little heat will wear the cured pad portion away without curing the new surface material Brake Maintenance To properly condition brake pucks apply brakes medium amounts five to six times at 25 to 30 MPH Allow partial cooling between applications Unusual brake puck wear, brake shimmy, brake pull can be due to improperly tempered brake linings Actuating Systems It is very common for the brake pedals to be the upper part of the rudder pedals These are called toe brakes In some installations the whole pedal pushes for rudder / steering action, and rocks or pivots for braking action Actuating Systems The most common type of brake actuating system used on aircraft is the hydraulic system Two basic types Independent: Not dependent on engine driven hydraulic system Dependent: Dependent on engine driven hydraulic system Independent Brakes A typical master cylinder will consist of a: Piston Cylinder Piston connecting rod Reservoir or inlet port Pressure or outlet port Pressure return or compensating valve Independent Brakes In the relaxed position the compensating valve is open, the piston is retracted The first section of travel the return valve closes, no brake actuation occurs The next section of travel the piston moves down creating pressure, which in turn actuates the brake assembly When the brake returns to relaxed, the compensating valve is opened, releasing all pressure Independent Brakes Fluid return, and brake release is caused by Return springs in the brake assembly Slight flexing of the caliper piston seals The disc rotor just pushes the piston back Independent Brakes Typical Master Cylinder Independent Brakes Typical Master Cylinder

13 Independent Brakes Typical Master Cylinder Independent Brake System Independent Brakes Independent Brake Troubleshooting Dragging brake Broken master cylinder return spring Dirty, corroded piston/caliper Restricted master cylinder compensating port (contaminated or binding pedal assembly) Spongy brake Air Deteriorated brake hose Brake grabs Fluid leak on brake lining Brake fade or parking brake creeps Off Internal master cylinder leak Independent Brakes Pedal Pulsing Uneven wear on rotor Warped rotor Wheel shimmy with brakes applied Uneven wear on rotor Warped rotor Scraping noise with brakes applied Linings worn out Puddles on ground Failed o-rings or hoses Independent Brakes Flushing Done to clear system free from contaminates Water, air, dirt, oil, debris System can be flushed from low to high using a pressure pot System can be flushed from high to low using a hose and a bottle of fluid Most common fluid used is H-5606 Parking Brakes Is usually a racheting master cylinder that feeds both independent brakes Not wise to leave aircraft locked with this brake on heat can rupture a system Aircraft cannot be moved by ground support Brake Bleeding END OF SECTION FIVE Multi disc assemblies Commonly use carbon braking disc Still use steel wearing discs These systems are designed to withstand very extreme temperature, and weather operating conditions The various discs can be solid, segmented, slotted, internal or external tangs or notches In every case they will index alternately to the inside or the outside, with one side being attached to the gear and the other a part of the wheel These will have an even distribution of pistons in the complete circumference of the brake disc assembly Multi disc assemblies Multi disc assemblies Multi disc assemblies Multi disc assemblies Mig 21 Tire, Wheel, Brake Off-Aircraft Inspection/Servicing AN MS and Special bolts and other hardware Visual, dimensional and magnetic particle inspection Inlet and bleeder adapter Torque tube and pressure plate Visual, dimensional and magnetic particle inspection Piston Housing Visual, dimensional and fluorescent penetrant inspection Pistons, seals, backup rings and insulators Off-Aircraft Inspection/Servicing Stationary and rotating discs Thickness, wear, cracks at relief slots Tangs and slots Loose rivets and pads that are curled Glazed pads Self-adjusters Visual and magnetic particle inspection Semi-Boosted Brakes Boost assisted brakes hydraulic systems are not independent of each other The mechanical action of the operator does some of the work Engine driven hydraulics do the rest of the work Semi-Boosted Brakes Similar to semi-boosted in theory, the operator's actuating force is not part of the brake actuating force They are similar to the independent brakes in that left pedal operates left brake, and right

14 pedal operates right brake They operate by diverting a controlled amount of hydraulic fluid from the engine driven pump to the brake assemblies In some large aircraft systems the nose gear will also have braking capabilities If both pedals are being applied equally the nose brake will assist braking In theory of operation they are also similar to the differential follow-up steering devices They are dependent on the aircraft hydraulic system for operating power The braking function calls for the operator to apply a fixed amount of pedal travel to get a fixed amount of braking As long as the pedal remains in the same position you should get the same amount of braking Although hydraulic valves can regulate they still either let fluid flow or don't let it flow, based upon a fixed amount of travel By modifying the valves to be self adjusting using balancing springs, and pressure differential changes across the spool valve, we create a valve system that will allow a fixed amount of fluid flow for a fixed amount of pedal travel No Boost Brakes By modifying the valves to be self adjusting using balancing springs pressure differential changes across the spool valve we create a valve system that will allow a fixed amount of fluid flow for a fixed amount of pedal travel Pressure Ball-Check Brake Control Valve Very similar to PBCV Instead of a spool for valving it uses a piston and a check-ball Instead of two coiled balanced coil springs it uses one coil spring and a flexing lever The application of hydraulic pressure on the piston springs closes the check-ball Pressure Ball-Check Brake Control Valve Hydraulic fluid source, High pressure Power brake control valves Pedal assemblies and linkage Control valves Emergency Pneumatics Anti skid Air/oil transfer tube Deboosters Emergency valve Shuttle valves Pressure cylinder Debooster Assemblies Much like an electronic transformer, trading pressure for volume instead of voltage for current As the debooster reaches the maximum range of its travel a pin opens a through flow check valve allowing full pressure to reach brakes: used for emergency situations such as a leak Lockout Debooster Assemblies Much the same as a normal debooster except it can be locked to a closed through flow state when the debooster piston reaches full extention It must be manually set to open via pin handle This allows for a complete lock out of each brake in the event of t major leak Shuttle valve Keeps the normal brake hydraulic system separated from the emergency system during normal operation Will allow brake system to swap to an alternate pressure source during emergency braking Air / oil transfer tube This is a tank full of oil that will be fed into the hydraulic system during emergency brake operations The oil is forced into the system by gas pressure from an emergency discharge bottle In principle it is very similar in operation to a pressure accumulator Air / oil transfer tube Air / oil transfer tube Air / oil transfer tube Anti Skid Brakes The main purpose of aircraft anti-skid is to maximize braking effectiveness during all braking conditions The basic operation is to monitor all wheel rotation speeds When a difference begins to occur the offending brake is automatically deactivated slightly, until it comes back up to speed Anti Skid Brakes Will prevent the aircraft from touching down with the brakes on Will reduce the possibility of tire hydro planeing Generally does not operate under 20 mph

15 Usually has several common components found on most vehicles that use anti skid Anti Skid Brakes Used exclusively on aircraft with power brake systems Some form of wheel speed sensor, usually one for each braked wheel Some form of brake servo valve, usually one for each braked wheel Some form of electronic control unit, often internally independent for each wheel Anti Skid Brakes To prevent an inadvertent locked wheel during touchdown the systems leaves the brakes fully released until the WOW switch is moved to ground Two basic types of wheel speed sensors are an A/C sine wave signal generator, and a D/C voltage generator. The A/C type control box has an internal signal converter. Probably a rectifier circuit Anti Skid Brakes The wheel servos operate by releasing brake fluid pressure back to return, until the wheel comes back up to speed They then start reapplying the brake to a lessor degree, attempting to achieve maximum braking action Using a linear elector motor that deflects fluid flow, the valve spool is position by varying degrees of fluid pressure Anti Skid Brakes The computer control unit is able to sense when a wheel is begging to change speed and predicts impending skid By using data from the other wheels, and remembering the what the wheel speed was prior to slippage it can determine when the wheel is back up to proper speed Anti Skid Brakes Since the aircraft is decelerating it is actually looking for a change in the rate of deceleration of any given wheel There are various different activation thresholds for different systems, but it is common for these modern systems to be reacting within several hundredths of a second All systems include operator indication and self test functions Anti-Skid Highlights Electro-hydraulic system Armed by a cockpit switch Electric AC or DC wheel speed sensors Operates just below the skid point at an impending skid Warning lamp illuminates when the system off or during a system failure Skid sensed control valve relieves pressure from brake Touchdown protection through squat switch no signal sent to control box Ground System Test Simulates wheel lock-up, release and restoration of brakes: Cockpit anti-skid switch ON Depress pedals left and right brake lights illuminate With pedals still depressed, press test switch lights remain on; switch released brake lights extinguish and then illuminate Fight System Test Aircraft configured for landing Cockpit anti-skid switch ON Simulates touch down protection feature: Depress pedals left and right brake lights remain off Simulates normal brake function: With pedals still depressed, press test switch lights illuminate as long as switch depressed Tweak Test - Wheel Speed Sensor Simulates skid followed by normal braking: Remove hub cap With brake applied, spin sensor blade Brake will momentarily release, then reapply DC Wheel Speed Sensor Tweak Test Remove wheel hub cap to expose sensor blade. With anti-skid switch ON and brake applied, give blade sharp spin with your finger. In a properly operating system, brakes momentarily release then reapply. If the sensor fails the tweak test, check the resistance using a sensitive ohmmeter. DC Wheel Speed Sensor Resistance Test Remove cable connector and measure resistance of armature while slowly rotating blade Uniformity and amount of resistance through blade travel should be within maintenance manual specifications. DC Wheel Speed Sensor Polarity Test Place meter on lowest DC voltage scale; attach positive lead to pin B and negative lead to pin A. Tweak blade in clockwise direction viewed from drive end. Meter should read upscale.

16 Control Box Check by substitution method Swap cables Problem changes sides control box defective Problem remains on same side wheel speed sensor or control valve defective Control Valve Measure control valve coil resistance using sensitive ohmmeter Resistance within specification, control valve parts are defective B757 HYDRAULIC CONTROL PANEL B757 CONTROL B757 NOSE LANDING GEAR B757 NWS B757 MAIN GEAR B757 PROXIMITY SWITCH B757 BRAKE SYSTEM B757 ANTI-SKID B757 AUTOBRAKES B757 AUTOBRAKES Anti-corrosion Sealant B787 Electric Brake B Brake Change Beechcraft Super King Air END OF SECTION SIX Usually two piece Two opposing conical tapered bearings for each wheel Can be tube type or tubeless Tubeless will have seal rings or sealing compound between halves Wheels are either aluminum alloy or magnesium alloy Are either cast or forged, and therefore can be subject to intergranular corrosion The bead seat area and the bolt hole areas are the most critical inspection areas The inboard half also houses the brake assembly Commonly has fusible plugs that will release pressure if tire exceeds a critical temperature Bearing cups are usually interference fit into each half, or into one half with a flange for the other half Inflation valve, or hole is usually on the outboard half Aircraft tires are generally removed by splitting the wheel in half Must not have any air pressure in tire when loosening bolts, (remove valve core) Can use an arbor press or drill press, turned off, to press bead off of rim, on both sides Wheel inspection is critical for cracks, corrosion, or damaged bead/bolt areas If any fusible plug shows sign of damage, replace all of them Eddy current inspection is the best way to check for subsurface damage Fix a flat tire injection formulas can contain explosive gasses Cracks can also develop in the brake disc mounting areas Bolts may be unidirectional - interference Tighten in a criss cross pattern, in stages Do not use soap on tube type tires, the sudden acceleration of landing will cause them to slip Mount the tire with red dot to the valve stem When reassembling tube types be careful to not pinch the tube or leave any wrinkles Tapered conical wheel bearings Slightly loose is better than slightly too tight Notch in plate washer is used to move washer to test for correct tension Spin wheel when adjusting wheel bearings Always thoroughly clean and regrease bearings and wheels when halves are separated Always replace both the bearing assembly and the bearing cup when replacing a bearing Some axle seals can be reused, but most lip seals should be replaced when removed Always renew cotter pin Make sure cotter pin isn't dragging on dust cap or flange. Builds static charge that can wreck havoc on many things Wheels bearings usually fail due to contamination or being set too tight Heat discoloration, brinelling, spalling, galling, and welding are the stages of wheel bearing failure Bearing cup can wallow loose in wheel half Always replace bearings by part number only It is best to use boiling water and ice to change bearing cups Any damage to metal or plastic bearing cage is cause for rejection of the bearing DO NOT, FOR ANY REASON, AIR SPIN A BEARING RACE OF ANY TYPE