Landing Gear Systems Flashcards
List the steps in servicing shock struts
- position aircraft so that the shock struts are in
the normal ground operating position - remove the cap from the air servicing valve
- check the swivel nut for tightness
- if the servicing valve has a valve core, depress
it to release any air pressure that may be
trapped - loosen the swivel nut
a. for a valve with a valve core - rotate swivel
nut one turn - depress valve core to release
all air in the strut
b. for valve without a valve core - rotate the
swivel nut enough to allow air to escape - aircraft on jacks may need to have the lower
strut jacked with an exerciser jack to achieve
the full compression of the strut - Remove the valve core of an AN6287 valve
with a valve core removal tool - then remove
the entire service valve - fill the strut with hydraulic fluid
- reinstall the air service valve assembly using a
new O ring packing
a. If an AN2687-1 valve, install a new valve
core
10 Inflate the strut according to manufacturers
instructions - inflate slowly to avoid excess
heating and over inflation - tighten the swivel nut and torque as
specified - remove the fill hose fitting and finger tighten
the valve cap of the valve
List the steps to bleeding the shock strut
- construct and attach bleed hose containing a
fitting suitable for making an airtight
connection at shock strut service valve port - jack the aircraft until the shock struts are fully
extended - release any air pressure in the shock strut
- remove the air service valve assembly
- fill the strut to the level of the service port
with approved hydraulic fluid - attach the bleed hose to the service port and
insert the free end of the hose into a
container of clean hydraulic fluid - place an exerciser jack under the shock strut
jacking point. compress and extend the strut
fully by raising/lowering the jack. continue
until all air bubbles cease to form - remove the exerciser jack. lower aircraft and
remove all other jacks - remove bleed hose assembly and fitting from
service port of the strut - install the air service valve, torque, and inflate
the shock strut to manufacturers specs
If a skid is not corrected quickly, what can happen?
- tire blowout
- possible damage to aircraft
- loss of control of the aircraft
What aircraft require anti skid systems?
large aircraft with power brakes
What is the purpose of the anti skid system?
- detects wheel skid
- detects when wheel skid is imminent
How does the anti skid system work?
It automatically relieves pressure to the brake pistons of the wheel in question by momentarily connecting the pressurized brake fluid area to the hydraulic system return line
Most anti skid systems contain these 3 main types of components
- wheel speed sensors
- anti skid control valves
- control unit
Describe wheel speed sensors
- wheel speed sensors are transducers
- may be alternating current (AC) or direct
current (DC) - includes a stator, rotor, and lobes
- the frequency produced is directly
proportional to the speed of wheel rotation
What is a control unit in an anti skid system?
- the brain of the anti skid system
- receives signals from the wheel sensors
- comparative circuits are used to determine if
a skid is imminent - common for it to be located in the avionics
bay of the aircraft
Describe anti skid control valves
- fast acting, electrically controlled hydraulic
valves that respond to the input from the anti
skid control unit - there is one control valve for each brake
assembly - typically located in the main wheel for close
access to hydraulic pressure, return
manifolds, and brake assemblies - they are positioned downstream of the power
brake control valves and upstream of the
debooster cylinders, if equipped
What should be checked when completing an On Aircraft Wheel inspection?
- all wheel tie bolts and nuts should be in place
- a missing bolt is grounds for removal
- thorough inspection of wheel halves
- wheel hub dust cap and anti skid sensor
should be secure - inboard wheel half should interface with the
brake rotor with no signs of chafing or
excessive movement - all brake keys on wheel must be present and
secure - examine wheels for cracks, flaked paint, and
any evidence of overheating - inspect thermal plugs - no sign of fusible alloy
being melted - make sure wheel not abnormally tilted
- flanges should not be missing any pieces
- check wheel for signs of impact damage
Why is Axle Nut Torque extremely important on an aircraft wheel installation?
- if the nut is too loose, the bearing and wheel
assembly may have excessive movement
a. the bearing cup(s) could loosen and spin
damaging the wheel
b. could be impact damage from bearing
rollers leading to bearing failure
** all aircraft axle nuts must be installed and
torqued in accordance with the airframe
manufacturer’s maintenance procedures**
When is an Off Aircraft Wheel inspection required?
- discrepancies found while inspecting a wheel
mounted on the aircraft may require further
inspection with the wheel removed from the
aircraft - other items, such as bearing condition can
only be performed with the wheel assembly
removed - a complete inspection requires the tire be
removed from the wheel rim
Why should you deflate the tire before removing the wheel assembly?
- wheel assemblies have been known to
explode while removing the axle nut
What are the steps for removing the wheel assembly?
- deflate the tire
- let aircraft cool (min 3-4 hours)
- approach wheel assembly from front or rear
- do not stand in the path of the released air
- make sure to remove only one tire and wheel
assembly from a pair at a time - this leaves a
tire and wheel assembly in place in case
aircraft falls from jack
What are the steps in an Off Aircraft Wheel Inspection?
- loosen tire from the wheel rim
- disassemble the wheel
- clean the wheel assembly
- clean the wheel bearings
- inspect the wheel bearings
- lubricate bearings
- inspect wheel halves
- inspect wheel tie bolt
- inspect key and screws
- inspect fusible plug
- balance weights
List the things to know when loosening the tire from the wheel base
- bead must be broken to remove tire - there
are mechanical and hydraulic presses made
for this purpose - an arbor press can be used, if above not
available - never pry a tire off the rim with a screwdriver
or other device
Disassembly of the wheel - What are some things to know?
- should take place in a clean area on a flat
surface, such as a table - remove wheel bearing first - set aside for
cleaning/inspection - don’t use an impact tool to disassemble the
tie bolts - aluminum and magnesium alloys
too soft for repeated hammering
Cleaning the Wheel Assembly - What are some things to know?
- clean wheel halves with solvent
recommended by wheel manufacturer - use a soft brush
- avoid abrasive techniques, materials, and
tools, such as scrapers - when wheels clean - dry with compressed air
What is used to clean the wheel assembly?
a soft brush and solvent recommended by the wheel manufacturer
What can happen to the wheel if abrasive techniques/materials are used to clean the wheel assembly?
corrosion can quickly form and weaken the wheel if the finish is missing in an area
How do you dry the wheels once clean?
compressed air
Why shouldn’t you use an impact tool when disassembling tie bolts?
aircraft wheels are made of relatively soft aluminum and magnesium alloy - these materials are not designed to receive repeated hammering of an impact tool
Describe the process of cleaning the wheel bearings
- bearings should be removed from the wheel
- clean with manufacturer approved solvent,
such as Varsol, Naptha, or Stoddard solvent - brush clean with a soft bristle brush
- dry with compressed air - never rotate the
bearing while drying - avoid steam cleaning of the bearings
Why should wheel bearings never be steam cleaned?
the surface finish of the metals will be compromised leading to early failure
Why should you never rotate wheel bearings when drying with compressed air?
the high speed metal to metal contact of the bearing rollers with the race causes heat that damages the metal surfaces
List the common conditions of a bearing that are cause for rejection
- Galling
- Spalling
- Overheating
- Brinelling
- False Brinelling
- Staining and Surface Marks
- Etching and Corrosion
- Bruising
What is Galling?
- reason for bearing rejection
- caused by rubbing of mating surfaces
- the metal gets so hot it welds
- the surface metal is destroyed as the motion
pulls the metal apart in the direction of
motion
What is Spalling?
- reason for bearing rejection
- a chipped away portion of the hardened
surfaced of a bearing roller or race
What is Overheating?
- reason for a bearing rejection
- caused by lack of sufficient lubrication
- results in a bluish tint to the metal surface
- bearing cup raceway is usually discolored as
well
What is Brinelling?
- reason for a bearing rejection
- caused by excessive impact
- appears as indentations in the bearing cup
raceways - any static overload or severe impact can
cause true brinelling which leads to vibration
and premature bearing failure
What is False Brinelling?
- reason for a bearing rejection
- caused by vibration of the bearing while in a
static state - identified by a rusty coloring of the lubricant
- also known as frictional corrosion
Describe Staining and Surface Marks on bearings
- reason for a bearing rejection
- located on the bearing cup
- appears as grayish black streaks with the
same spacing as the rollers - caused by water that has gotten into the
bearing - it is the first stage of deeper corrosion that
follows
Describe Etching and Corrosion
- reason for a bearing rejection
- caused when water and the damage caused
by water penetrates the surface treatment of
the bearing element - appears as a reddish/brownish discoloration
Define Bruising
- reason for a bearing rejection
- caused by fine particle contamination -
possibly from a bad seal or improper
maintenance of bearing cleanliness - leaves less than smooth surface on the
bearing cup
What causing galling on a bearing?
- rubbing of mating materials
- metal gets hot - melds - destroys surface
metal
How does overheating appear on a bearing?
a bluish tint on the metal surface
How does brinelling appear on a bearing?
appears as indentations in the bearing cup raceways
How can false brinelling be identified?
a rusty coloring of the lubricant
How does bruising appear on a bearing?
it leaves a less than smooth surface on the bearing cup
Describe the process of Bearing Handling and Lubrication
- avoid conditions where contamination,
moisture, and vibration can occur - install and torque bearings according to
manufacturers instructions - lubricate with lubricant recommended by
manufacturer - use of a pressure bearing packing tool or
adapter is recommended
Describe the process of inspecting the wheel halves
A thorough visual inspection should be
conducted - use of magnifying glass
recommended
Corrosion -
1. check for corrosion carefully - check locations
where moisture is trapped closely
2. it is possible to dress out some corrosion
according to manufacturers instructions - use
an approved protective surface treatment and
finish
3. corrosion beyond stated limits is cause for
rejection of the wheel
Cracks - cracks in certain areas are common
1. Bead Seat Area - check using Eddy current
3. Wheel Brake Disc - check using dye penetrant
test - no corrosion permitted in this area
How do you check for cracks in the bead seat area?
check using an Eddy current
How do you check for cracks in the wheel brake disc?
check using dye penetrant test
Name two areas where cracks are commonly found during inspection of the wheel assembly
- Bead Seat Area
- Wheel Brake Disc
Wheel Tie Bolt Inspection - things to know
- tie bolts stretch and change dimension at the
threads and under the bolt head - most cracks are found in this area
- magnetic particle inspection can be used
What inspection technique is used to check for cracks in wheel tie bolts?
Magnetic Particle Inspection
Where are cracks in tie bolts usually found?
At the threads and under the bolt head
Key and Key Screw Inspection - things to know
- drive keys are subject to extreme forces when
brakes are applied - bolts should be inspected for security
- keys should be inspected for cracks
- there is a limitation on how worn keys can be -
too much wear allows excessive movement - wheel manufacturers maintenance
instructions should be used to perform the
inspection
How are fusible plugs or thermal plugs inspected?
- fusible plugs or thermal plugs are inspected
visually
Fusible plug inspection - things to know
- inspected visually
- check for deformation due to high
temperatures - if detected, all thermal plugs in the wheel
should be replaced with new plugs
Inspecting Balance Weights - things to know
- when manufactured - each wheel set is
statically balance - weights are added to balance, if needed
- weights are permanent part of the wheel
- balance weights are bolted to the wheel
halves - balance weights can be removed when
cleaning/inspecting the wheel - must be re-fastened in their original position
What does Pascal’s Law state?
states that pressure applied to any part of a confined liquid is transmitted with undiminished intensity to every other part
this means that if a number of passages exist in a system, pressure can be distributed through all of them by means of the liquid
What properties and characteristics must be considered when choosing the best liquid for a particular hydraulic system?
- Viscosity
- Chemical Stability
- Flash Point
- Fire Point
Define Viscosity
- viscosity is internal resistance to flow
- most important properties of any hydraulic
fluid - most hydraulic fluids are low viscosity
What instruments are used to measure viscosity?
- viscometers or viscosimeters
Define chemical stability
- very important property in choosing hydraulic
fluid - it is the liquid’s ability to resist oxidation and
deterioration for long periods
Define flash point
- the temperature at which a liquid gives off
vapor in sufficient quantity to ignite
momentarily or flash when a flame is applied - a high flash point is desirable for hydraulic
liquids - it indicates good resistance to
combustion and low degree of evaporation at
normal temperatures
Define fire point
- the temperature at which a substance gives
off vapor in sufficient quantity and continue to
burn when exposed to a spark or flame. - a high fire point is required of desirable
hydraulic fluid
What are the 3 principal categories of hydraulic fluid?
- minerals
- polyalphaolefins
- phosphate esters
Where do you find the correct hydraulic fluid to use in a system?
aircraft manufacturers maintenance manual or the instruction plate affixed to the reservoir of unit being serviced
Describe Mineral Based Fluids
- oldest hydraulic fluid from the 1940’s
- used in many systems where the fire hazard is
low - suppliers generally ship components with MIL-
H-6803 - processed from petroleum
- odor similar to penetrating oil and is dyed red
- some are dyed purple or green
What is Phosphate Ester-Based Fluid?
- used in most commercial transport category
aircraft - Skydrol & Hyjet are examples
- Today types IV and V fluids are used
Name 2 general contaminants of hydraulic fluid
- abrasives, including core sand, weld spatter,
machining chips, and rust - nonabrasives, including those resulting from
oil oxidation and soft particles worn or
shredded from seals and other organic
components.
Checking the Hydraulic System for contamination - things to know
- to determine which component is defective,
liquid samples should be taken from the
reservoir and various other locations in the
system - samples are taken according to
manufacturers instructions - some systems are equipped with permanently
installed bleed valves for taking liquid
samples - in other systems the lines must be
disconnected
What is the Hydraulic Sampling Schedule
- Routine Sampling - each system should be
sampled
a) at least once a year
b) every 3000 flight hours or
c) whenever airframe manufacturer suggests - Unscheduled Maintenance - when
malfunctions may have a fluid related cause - Suspicion of Contamination - fluids should be
drained and replaced, with samples taken
before and after the maintenance procedure
Describe the hydraulic fluid sampling procedure
- Pressurize and operate hydraulic system for
10-15 minutes - operate various flight controls
to activate valves - Shut down and depressurize the system
- Wipe off sampling port or tube with a lint free
cloth - Place a waste container under the reservoir
drain valve and open valve so a steady stream
flows - Allow 1 pint of fluid to drain
- Insert precleaned sample bottle under the
fluid stream and fill - leave an air space at top - Close drain valve
- Fill out sample id label
- Service system reservoirs to replace the fluid
that was removed - submit samples for analysis to laboratory
What information is included on a hydraulic fluid sample id label?
- customer name
- aircraft type
- aircraft tail number
- hydraulic system sampled
- date sampled
- indicate if a routine sample or taken due to a
suspected problem
What procedures should be followed to control contamination?
- Keep tools and work area clean and dirt free
- a suitable container should always be used to
receive the drained hydraulic fluid - clean affected area with dry cleaning solvent
before disconnecting hydraulic lines or fittings - all hydraulic lines and fittings should be
capped or plugged immediately after
disconnecting - wash all parts with a dry cleaning solvent
before assembly of any hydraulic components - dry parts thoroughly and lubricate them with
recommended preservative or hydraulic fluid - all seals and gaskets should be replaced
during the reassembly procedure - connect all parts carefully to avoid stripping
metal slivers from threaded areas / all fittings
and lines installed and torqued - all hydraulic servicing equipment should be
kept clean and in good operating condition
Hydraulic system filters - things to know
- a screening or straining device used to clean
the hydraulic fluid, preventing foreign
particles and contaminating substances from
entering - filter rating given in microns, which is the
smallest particle size that is filtered out - replacement interval of filters is established
by the manufacturer and included in the
maintenance manual - can be located in reservoir, pressure line, etc
What is the recommended service life of filter elements?
- Pressure filters - 3000 hours
- Return filters - 1500 hours
- Case drain filters - 600 hours
How often should pressure filters be replaced?
every 3000 hours
How often should return filters be replaced?
every 1500 hours
How often should case drain filters be replaced?
every 600 hours
Describe the procedure for flushing the hydraulic system?
- Connect a ground hydraulic test stand to the
inlet and outlet test ports of the system.
Verify the ground unit fluid is clean and
contains the same fluid as the aircraft - Change the system filters
- Pump clean, filtered fluid through the system
and operate all subsystems until no obvious
signs of contamination are found. Dispose of
contaminated fluid and filter - Disconnect the test stand and cap the ports
- Ensure the reservoir is filled to the full line or
proper service level
How are hydraulic filters rated?
in microns
What are the 3 main classes of seals?
- packings
- gaskets
- wipers
Describe V Ring packings(AN6225)
- one way seals
- always installed with open end of V facing the
pressure - must have a male and female adapter to hold
them in place - torque seal retainer to manufacturer
specifications
Describe U-Ring packings
- U ring packings and U cup packings are used
in brake assemblies and brake master
cylinders - lip of packings must face toward the pressure
- U ring packings are primarily low pressure
packings to be used with pressures less than
1000psi
O Ring Color Coding - what information is provided by color coding?
- system fluid or vapor compatibility
- not a reliable source of identification
Describe O Rings
- most packings and gaskets used in aircraft are
in the form of O rings - the O ring packing seals most effectively in
both directions - the sealing is done by distortion of its elastic
compound
What is a Backup Ring?
- made of Teflon
- don’t deteriorate with age
- unaffected by any system fluid or vapor
- can tolerate extreme temperatures
- their dash numbers indicate their size and the
number of the O ring it is suited for - installed downstream of the O ring
What is a Gasket?
- used as static (stationary) seals between two
flat surfaces - commonly made from asbestos, copper, cork
and rubber - asbestos sheeting is used for exhaust system
gaskets
What are seals made of?
- most seals are made from synthetic materials
that are compatible with hydraulic fluid
Seal Compatibility - what you should know
- Seals used for MIL-H-5606 hydraulic fluid are
not compatible with phosphate ester-based
fluids - Seals used for MIL-H-5606 are made of
neoprene or Buna-N - Seals for phosphate ester-based fluids are
made from butyl rubber or ethylene-
propylene elastomer
What is a pressure control valve?
- some are an escape for excess pressure
- some only reduce the pressure to a lower
pressure system - some keep the pressure in a system within
the required range
What is a pressure relief valve?
- used to limit the amount of pressure being
exerted on a confined liquid - necessary to prevent the failure of
components or rupture hydraulic lines under
excessive pressure
What are the most common types of pressure relieve valves?
- Ball type
- Sleeve type
- Poppet type
Pressure relief valves may be used as what?
- System relief valve
- Thermal relief valve
How is a pressure relief valve used as a thermal relief valve?
used to relieve excessive pressures that may exist due to thermal expansion of the fluid
What is a Pressure Regulator
- a device used in hydraulic systems that are
pressurized by constant delivery type pump
What are the 2 purposes of the pressure regulator?
- to manage the output of the pump to
maintain system operating pressure within a
predetermined range - to permit the pump to turn without resistance
(unloading the pump) at times when pressure
in the system is within normal operating range
What is a Pressure Reducer?
Pressure reducing valves are used in hydraulic systems where is is necessary to lower the normal system operating pressure by a specified amount
What does a pressure reducer do?
- provide a steady pressure into a system that
operates at a lower pressure than the supply
system - can be set for any desire downstream
pressure
What is a Shuttle Valve?
the main purpose of the shuttle valve is to isolate the normal system from an alternate or emergency system
The housing of the shuttle valve contains what 3 ports?
- normal system inlet
- alternate or emergency system
- outlet
Name the four types of shuttles
- sliding plunge
- spring-loaded piston
- spring-loaded ball
- spring-loaded poppet
What is a Shutoff Valve?
- used to shut off the flow of fluid to a
particular system or component - generally electrically powered
- also used to create a priority in a hydraulic
system and controlled by pressure switches
What is an Accumulator?
- a steel sphere divided into 2 chambers by a
synthetic rubber diaphragm
What is the function of an Accumulator?
- dampen pressure surges
- aid or supplement the power pump when
several units operating at once - Store power for the limited operation of a
hydraulic unit when the pump isn’t operating - supply fluid under to pressure to compensate
for small internal or external leaks
What are the 2 types of accumulators normally used in aircraft hydraulic systems?
- Spherical
- Cylindrical
