8/26/2010
AVIM 103D Landing Gear – Notes Workbook
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
• 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.
• 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
Aircraft Suspension Systems
Suspension Systems
• 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 Systems
• 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 Systems
• 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
Suspension Systems
• 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
Suspension Systems
• 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
Suspension Systems
• 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.
Suspension Systems
• 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
Suspension Systems
• 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
Suspension Systems
• 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)
Suspension Systems
• 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
Suspension Systems
• 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 28889 most common
– AN 6287
– AN 812 older models
MS 28889 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 28889 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 28889
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 75 - 100 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 150 - 1000 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
• 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
• 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
– 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 Steering Systems
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
Steering Systems
• Two basic types
• Open - found on conventional geared aircraft
• Closed - most common, pedals, third gear and
rudder are looped in the system
Steering Systems
• Open loop system
Steering Systems
• Closed Loop System
Steering Systems
• 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
Steering Systems
• Tail wheels can be fully castoring, or steerable
and castering
• Castering pivot must be vertical
or gear can get stuck
Steering Systems
• 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
Steering Systems
• 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
Steering Systems
• 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
Steering Systems
• 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
Steering Systems
• 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
Steering Systems
• Dual Piston Steering Damper
• 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)
Steering Systems
• 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
Steering Systems
• 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
Steering Systems
• The steering input is the same as the steering
action
• Again the steering input unbalances the
compensating device and the steering action
rebalances it.
Steering Systems
• 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
–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 0.0001/99.9999 (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
Brake Assemblies
–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
Brake Assemblies
–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
Brake Assemblies
–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
Brake Assemblies
–The three sections of any brake system
include:
– The brake assembly: friction device
– The control or actuating system
– The linkage, plumbing, power boost system
Brake Assemblies
–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
Brake Assemblies
Mechanical Brakes
Brake Assemblies
–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
Brake Assemblies
Hydraulic Drum Brakes
Landing Gear
Brake Assemblies
–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
Brake Assemblies
Floating Hydraulic Drum Brakes
Brake Assemblies
–Drum Brakes
–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
Brake Assemblies
–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
Brake Assemblies
–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
Brake Assemblies
–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
Brake Assemblies
–Hydraulic Disc Brakes
Brake Assemblies
–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
Brake Assemblies
–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
Brake Assemblies
–Parts include:
– Pads, pucks, or shoes
– Calipers, or wheel cylinders
– Discs, or drums
– Backing plate
– Landing gear axle assembly
–Wheel and tire assembly
Brake Assemblies
–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
Brake Assemblies
–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
Brake Assemblies
Brake Assemblies
–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
Brake Assemblies
–3 Piston Floating Disc Caliper Assembly
Brake Assemblies
–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
Brake Assemblies
–Auto adjusting piston
Goodyear Brakes
Goodyear Brake Linings
Linings, Rivets and Pins
Lining Limits
Linings
Cleveland Brake Linings
Brake Assemblies
–Pad thickness
–Always refer to manufacturer's specifications
–Pad material may come with back plate or is
riveted to old back plate
Brake Assemblies
–Pad or puck replacement
–Usually done with aircraft wheel removed
–Reservoir vent opened, fluid level lowered as
needed
–Disassemble brake assembly as needed to
remove pad
–If non-riveted type then replace pad and
reassemble
Brake Assemblies
–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
Brake Assemblies
–Pad/puck thickness measuring
Matco Wheel and Brake
T6 STC Brake Conversion
Brake Assemblies
–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)
Brake Assemblies
–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
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
Brake Assemblies
–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
Brake Assemblies
–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
Brake Assemblies
–Multi disc assemblies
Brake Assemblies
Multi disc
assemblies
Brake Assemblies
Multi disc
assemblies
Brake 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
Power 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
pedal operates right brake
–They operate by diverting a controlled amount
of hydraulic fluid from the engine driven
pump to the brake assemblies
Power Boosted Brakes
–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
Power Boosted Brakes
–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
Power Boosted Brakes
–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
Power Boosted 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
Power Boosted Brakes
–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
Power Boosted Brakes
–Pressure Ball-Check Brake Control Valve
Power Boosted Brakes
–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
Power Boosted Brakes
–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
Power Boosted Brakes
–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
Power Boosted Brakes
–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
Power Boosted Brakes
–Air / oil transfer tube
Power Boosted Brakes
–Air / oil transfer tube
Power Boosted Brakes
–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
–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
3600.
• 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.
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
B737-800 Brake Change
Beechcraft Super King Air
END OF SECTION SIX
Aircraft Wheels
–Aircraft Wheels
–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
Aircraft Wheels
–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
Aircraft Wheels
–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 Wheels
–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
Aircraft Wheels
–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
Aircraft Wheels
–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
Aircraft Wheels
–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
Aircraft Wheels
–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
Aircraft Wheels
–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
Aircraft Wheels
–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
–Replace any bearing with rust, or water marks
Aircraft Wheels
–Bearing lubrication
–MIL-G-3545C or MIL-G-81322
–Coloration of grease is due to dyes used by
manufacturer
–Some extra grease in the hub area will assist in
heat dissipation
–Too much grease will push the wheel seals out
Aircraft Wheels
–Pressure packing bearings is the quickest way,
always keep grease systems very clean
–Hand packing is done by working grease into
bearing cage dragging cage lip across a hand
full of grease
–Do not contaminate the brake components with
wheel bearing grease
–Repacking wheel bearings is P.M.
Wheel Types
• Drop Center (Single Piece)
– Tire bead forced over rim (automotive)
• Demountable (Removable) Flange
– Easier tire mount and dismount for stiffer tires
• Split Center (Split Rim)
Wheel Materials and Manufacture
• Aluminum alloy or Magnesium alloy
• Cast or Forged
• O-ring between wheel halves -Tubeless
• Knurled flanges (on some wheels) -Tube
Wheel Classification for Tire Casing
• Type I - Smooth contour
• Type II - High pressure
• Type III - Low pressure
• Type IV - Extra low pressure
• Type VI - Low profile
• Type VII - Extra high pressure
• Type VIII - Extra high pressure – Low Profile
Drop Center Wheel
Split Center (Split Rim)
Inboard Wheel Half
• Steel reinforced keyways or steel keys
• Bearing cup (interference fit)
• Tapered caged roller bearing
• Grease seal, two retainers and snap ring
• Fusible plug(s)
• Over-inflation valve (on very large wheels)
–May also be mounted on outboard wheel half
Outboard Wheel Half
• Bearing cup (interference fit)
• Grease seal, two retainers and snap ring
• Inflation valve (tubeless tires) or hole for innertube valve stem
• Axle cap and retaining ring
• Anti-skid bracket attached to cap
On Aircraft Wheel Inspection
• Light aircraft verify proper tire pressure daily
• Heavy aircraft verify before each flight
– Tire cool, or at least 2 to 3 hours after flight
• Check wheel weight security
• Brake tangs must align with wheel slots
• Axle nut torque
– Too loose, bearing cup could spin
– Too tight, damaged bearing
Off Aircraft Inspection
• Deflate tire first
• Break the bead
• Remove and properly store bearings
• Note wheel weight location
• Remove tie bolts
• Clean wheel assembly
• Clean and inspect bearings
Bearing Inspection
Bearing Cup Replacement
Removal
• Heat wheel
– Boiling water for 1 hour
– Oven for 30 minutes at 2250 F.
• Tap cup out with fiber drift
Replacement
• Reheat wheel
• Chill cup with dry ice
• Coat cup exterior with zinc chromate primer
• Drift cup in with fiber drift
Inspect Wheel Halves
• Bead seat
– Eddy current inspection
• Keys or Key slots
– Dye penetrant, Magnetic particle, Dimensional
– Check key attachment screw stake
• Internal and external surfaces
– Dye penetrant, Dimensional
• Bolts and other hardware
–Magnetic particle
Inspect Wheel Halves
• Fusible plug(s)
– Visual, replace all if any distorted
• Corrosion
– Check bead seat for trapped water
– Remove corrosion per manufacturers’
instructions
– Treat aluminum surfaces with Alodine
– Treat magnesium surfaces with Dow 19
– Finish with two coats zinc chromate primer
(except mating surfaces and bolt bosses – one
coat only)
Fusible Plugs
Reassemble Wheel (Tubeless Tire)
• Clean bead seat area – isopropyl alcohol
• Usually inboard wheel half first
– Inspect and lubricate wheel O-ring (tubeless)
• Install tire on inboard wheel half
• Index outboard wheel half so that red dot on
tire is adjacent to inflation valve
• Lubtork bolts, washers and nuts if specified
• Torque per manufacturers’ recommendations
• Inflate tire in cage to ½ static inflation pressure
• Final tire inflation or adjustment on aircraft
Reassemble Wheel (Tube Tire)
• Clean bead seat area – isopropyl alcohol
• Prepare and position inner tube
• Prepare and position tire
• Position brake disk (Cleveland brakes)
• Lubtork bolts, washers and nuts if specified
• Torque per manufacturers’ recommendations
• Inflate tire in cage to ½ static inflation pressure
• Adjust axle nut torque
• Final tire inflation or adjustment on aircraft
2006 Mechanic Killed
B737 Nose wheel Tire
166 PSI required – exposed to 3000 PSI from
unregulated nitrogen cylinder
Cleveland 40-76A
P38 Main Gear Wheel
T38 Nose
END OF SECTION SEVEN
Aircraft Tires
–Aircraft Vs automotive/truck
– Auto/truck need a medium speed tire
– Long duration
– Low bounce protection
– High traction needs
– High water displacement needs
–Weight / size not very critical
Aircraft Tires
–Aircraft tire needs
– High speed
– Short duration
– Very high bounce protection
– Low traction
– Low water displacement
–Weight and size very critical
Aircraft Tires
–The major difference is the aircraft tire sustains
much higher side wall deflection
–Tires:
–Type I Smooth contour type, from smooth
profile
– Non retractable landing gear Size: = 0D
Obsolete
–Type II High Pressure Tire
– Retractable landing Gear Made obsolete by
Type VII
– Size: OD X Section Width
Aircraft Tires
–Type III: Most popular on GA aircraft
– Section width usually wider than bead
diameter.
– Lower pressures possible, as bead seat traps
tire
– Size: Section Width X Wheel Diameter
– TSO C62
–Type IV: Extra Low pressure
– Rough and Unimproved runways (donut type)
– Obsolete
– Size: OD X Section Width X Rim Diameter
Aircraft Tires
–Type V: Streamlined tires
–Type VI: Low profile tire
–Main wheel space saver tire
– Limited height decreases the amount tire will
drop when flat
– Size: OD X Section Width X Rim Diameter
–Type VII: Extra High Pressure
– Standard for turbine aircraft
– High load carrying ability
– Size: OD X Section Width
Aircraft Tires
Aircraft Tires
–Tire Data:
–Tubeless tires are marked tubeless
–Tire deflection 32%-35% twice what is found in
automobile tires
–Nylon Stretch: New tires stretch in the initial 24
hr period after mount
–May result in a 5 to 10 percent drop in pressure
Aircraft Tires
–Suggested to let tire stand for 12 hours and
inflate
–Tubeless Air Diffusion: Maximum diffusion is
5% for any 24 hr. period. Allow tire to stand 12
hours before check
–Dual Tires: Difference of more than 5 PSI, note
it in the log book
Aircraft Tires
–Source of Pressure Data: Aircraft maintenance
manual
–Air pressure in a tire will drop by 1 PSI for
every 4 degree F change
–Inflation Pressure As specified in the
maintenance manual
–Always use safety cages and safety gear when
filling aircraft tires
–Tire size
–Tire type
–Date of Man
–Slippage mark
–Balance mark
–Ply rating
–Band material
–Retreading Co.
–Number of Ret.
–Name of Manuf.
Aircraft Tires
–Ply rating
–This is a means of rating tires based upon the
original cotton plys found in early tires
–Today's tires usually have less plys then the
stamped rating value
Aircraft Tires
–Bead: Anchors carcass and provides mounting
of tire to bead.
–Bundles of wire
–Apex strip: Streamlines bead
–Flippers: Insulate carcass plys from bead
–Carcass: rubber coated nylon cord fabric cut on
a bias for balance and strength
Aircraft Tires
–New classification system
–(prefix) (nominal outside dia.) X (nominal
section width) - (bead dia.)
–Prefix determines width ratio and bead ledge
angle
–Width ration = section width / rim width
–Bead ledge = angle at the base of the bead
Size Designations
Aircraft Tires
–Tire storage
–Store in a cool, dry, dark area
–Do not stack, store vertically
–Avoid any petroleum product exposure
–Avoid any electrical equipment the generate
ozone in the storage area
Tire Care
• Guard against heat build-up
– Short ground rolls, slow taxi, minimum braking,
proper tire inflation
• Maintain tire pressure in accordance with
aircraft maintenance manual
• Visual inspection
– Tread condition and depth, sidewalls
Tire Maintenance
• Avoid exposure to gasoline, oil, grease, electric
motors (ozone)
• Store racked in a cool, dark, dry place
• Check pressure weekly or sooner;
recommended before each flight
• Check pressure tire pressure when cool (two
hours after flight; three hours on a hot day)
Tire Maintenance
• New mountings: check pressure for several
days
• Allow for nylon stretch: 5% to 10% drop in air
pressure in 24 hour period
• Tubeless Diffusion: After 12 hour inflation,
maximum 5% loss in any 24 hour period.
• Dual inflation pressure: More than 5 PSI
difference, log book entry noted and condition
corrected.
Aircraft Tires
–Always look for damage or debris inside the
tire, wrinkles in the tube, damaged fill valves
–Any ribbed section cut more than 1/2 way
across should be retreaded or scrapped
–Any plies showing is cause for replacement
–Any sidewall damage is cause for scrapping
the tire
Aircraft Tires
–Small weather checking is not cause for
concern
–Move aircraft once in a while to prevent
permanent tire set
–Always keep tire at proper inflation for the type
of landing being conducted
–Harder for paved strips, softer for gravel or
grass strips
Aircraft Tires
–Corner wear is usually a camber problem
–Rapid wear and shifting weight at higher lighter
speeds is a toe in problem
Tire Maintenance
• Nylon flat spotting: roll aircraft
• Leaks: check with water
• Replace all valve caps
• Tread injury: follow aircraft manufacturers’
instructions
• Cuts exposing or penetrating cord body:
remove, repair, recap or scrap.
• Sidewall or tread bulges: remove, mark area
before deflation
Schrader Valve
Tire Deflection Markers
Aircraft Jacking
• Jack per manufacturer’s directions
• Use correct tools and jacks
• Leave nothing under the aircraft in case it
drops
• Jack evenly
• Locate aircraft out of high wind areas
• Use caution for CG shift when jacking
–May need tail/nose weight
• May need several persons
Tailweight
Mounting and Demounting
• Dust inside of tire and outside of tube with talc
• Index tube valve stem
• Assemble wheel correctly; use tire cage for
inflation
• Inflate, deflate and re-inflate tube-type tires
Tire Cage
B747 Tire Rupture
Tube Inspection
• Size “should” match tire
• Inflate no more than to “round-out” tube
• Valve stem
• Wrinkles: if excessive scrap
• Chafing or thinning: scrap
Aircraft Tires
–Tire balancing
–Most common to see it not done, or done
statically on small G/A
–Any tire should be balanced dynamically with a
computerized spin balancer
–Allow for tire to stretch initially prior to
balancing (24 hours)
Static Balance Stand
Selecting Balance Weight
Balancing Weights
Screw Balance Weight
END OF SECTION EIGHT