to provide a stabilized source of air for the compressor. (See figure 8-43.)
The aircraft utility hydraulic system provides power to operate the hydraulic-motor-driven air compressor. The air compressor hydraulic actuating system consists of a solenoid-operated selector valve, flow regulator, hydraulic motor, and motor bypass (case drain) line check valve. When energized, the selector valve allows the system to be pressurized to run the hydraulic motor; when deenergized the valve blocks off utility system pressure, stopping the motor. The flow regulator, compensating for the varying hydraulic system flow and pressures, meters the flow of fuid to the hydraulic motor to prevent excessive speed variation and/or overspeeding of the compressor. A check valve in the motor bypass line prevents system return line pressures from entering the motor and stalling it.
The air compressor is the pneumatic system's pressurizing air source. The compressor is activated or deactivated by the manifold pressure-sensing switch, which is an integral part of the moisture separator assembly..
The moisture separator assembly is the pneumatic system's pressure-sensing regulator and relief valve.
The manifold (system) pressure switch governs the operation of the air compressor. When the manifold pressure drops below 2,750 p.s.i., the pressure-sensing switch closes, energizing the separator's moisture dump valve and the hydraulic selector valve which activates the air compressor. When the manifold pressure builds up to 3,150 p.s.i., the pressuresensing switch opens, de-energizing the hydraulic selector valve to deactivate the air compressor and dump valve, thus venting overboard any moisture accumulated in the separator.
The safety fitting, installed at the inlet port of the moisture separator, protects the separator from internal explosions caused by hot carbon particles or fames that may be emitted from the air compressor.
A chemical drier further reduces the moisture content of the air emerging from the moisture separator.
A pressure transmitter senses and electrically transmits a signal to the pneumatic pressure indicator located in the cockpit. The indicating system is an "autosyn" type that functions exactly like the hydraulic indicating systems.
An air-charge valve provides the entire pneumatic system with a single external ground servicing point. An air pressure gage, located near the air-charge valve, is used in servicing the pneumatic system. This gage indicates the manitold pressure.
An air filter (10-micron element) in the ground air-charge line prevents the entry of particle impurities into the system from the ground servicing source.
The high-pressure air exiting from the fourth stage of the air compressor is directed through a bleed valve (controlled by an oil pressure tap on the pressure side of the oil pump) to the highpressure air outlet. The oil pressure applied to the piston of the bleed valve maintains the valve piston in the "closed" position. When the oil pressure drops (due either to restriction of oil flow or to stopping of the compressor), the spring within the bleed valve re-positions the bleed valve piston, thereby connecting the inlet port and the drain port of the valve. This action unloads the pressure from the compressor and purges the line of moisture.
The air filter through which the ground-charge air passes is located immediately upstream of the air-charge valve. Its purpose is to prevent the entry of particle impurities into the system from the ground servicing source. The filter assembly is made up off three basic components--body, element, and bowl.
The pneumatic system air compressor inlet sir is filtered through a high-temperature filter. Its purpose is to prevent particles of foreign matter from entering the compressor's absolute pressure regulator, thereby causing it to malfunction. The filter is an in-line, full-flow type (with integral relief valve) housed in a cylindrical body.
The moisture separator is the pneumatic power system's pressure-sensing regulator and relief valve, and is capable of removing up to 95% of the moisture from the air compressor discharge line. The automatically operated, condensation dump valve purges the separator's oil/moisture chamber by means of a blast of air (3,000 p.s.i.) each time the compressor shuts down. The separator assembly is made up of several basic components, each of which Components The pressure switch controls system pressurization by sensing the system pressure between the check valve and the relief valve. It electrically energizes the air compressor solenoid-operated selector valve when the system pressure drops below 2,750
p.s.i., and de-energizes the selector valve when the system pressure reaches 3,100 p.s.i.
The condensation dump valve solenoid is energized and de-energized by the pressure switch.
When energized, it prevents the air compressor from dumping air overboard; when de-energized, it completely purges the separator's reservoir and lines up to the air compressor.
The filter protects the dump valve port from becoming clogged and thus ensures proper sealing of the passage between the reservoir and the dump port.
The check valve protects the system against pressure loss during the dumping cycle and prevents backflow through the separator to the air compressor during the relief condition.
The relief valve protects the system against overpressurization (thermal expansion). The relief valve opens when the system pressure reaches 3,750 p.s.1.
and re-seats at 3,250 p.s.i.
The thermostatically controlled wraparound-blanket type heating element prevents freezing of the moisture within the reservoir due to low-temperature atmospheric conditions. The thermostat closes at 40° F. and opens at 60° F.
Pneumatic Power System Maintenance Maintenance of the pneumatic power system consists of servicing, troubleshooting, removal and installation of components, and operational testing.
The air compressor's lubricating oil level should be checked daily in accordance with the applicable manufacturer's instructions. The oil level is indicated by means of a sight gage or dipstick. When re-filling the compressor oil tank, the oil (type specified in the applicable instructions manual) is added until the specified level. After the oil is added, ensure that the filler plug is torqued and safety wire is properly installed.
The pneumatic system should be purged periodically to remove the contamination, moisture, or oil from the components and lines. Purging the system is accomplished by pressurizing it and removing the plumbing from various components throughout the system. Removal of the pressurized lines will cause a high rate of airflow through the system causing foreign matter to be exhausted from the system, If an excessive amount of foreign matter, particularly oil, is exhausted from any one system, the lines and components should be removed and cleaned or replaced.
Upon completion of pneumatic system purging and after re-connecting all the system components, the system air bottles should be drained to exhaust any moisture or impurities which may have accumulated there.
After draining the air bottles, service the system with nitrogen or clean, dry compressed air. The system should then be given a thorough operational check and an inspection for leaks and security.
340 --..
CHAPTER 9
LANDING GEAR SYSTEMS
GENERAL The landing gear of a fixed-wing aircraft consists of main and auxiliary units, either of which may or may not be retractable. The main landing gear forms the principal support of the aircraft on land or water and may include any combination of wheels, floats, skis, shock-absorbing equipment, brakes, retracting mechanism with controls and warning devices, cowling, fairing, and structural members necessary to attach any of the foregoing to the primary structure. The auxiliary landing gear consists of tail or nose wheel installations, outboard pontoons, skids, etc., with necessary cowling and reinforcement.
Landing Gear Arrangement Many aircraft are equipped with a tricycle gear arrangement. This is almost universally true of large aircraft, the few exceptions being older model sircraft. Component parts of a tricycle gear arrangement are the nose gear and the main gear.
Nose gear equipped aircraft are protected at the fuselage tail section with a tail skid or bumper. The nose gear arrangement has at least three advantages:
(1) It allows more forceful application of the brakes for higher landing speeds without nosing over.
(2) It permits better visibility for the pilot during landing and taxiing.
(3) It tends to prevent aircraft ground-looping by moving the aircraft c.g. forward of the main wheels. Forces acting on the c.g.
tend to keep the aircraft moving forward on a straight line rather than ground-looping.
The number and location of wheels on the main gear vary. Some main gears have two wheels as shown in figure 9-1. Multiple wheels spread the aircraft's weight over a larger area in addition to providing a safety margin if one tire should fail.
Heavier aircraft may use four or more wheels.
When more than two wheels are attached to one strut. the attaching mechanism is referred to as a "bogie," as shown in figure 9-2. The number of wheels that are included in the bogie is determined by the gross design weight of the aircraft and the surfaces on which the loaded aircraft may be required to land.
The tricycle arrangement of the landing gear is made up of many assemblies and parts. These consist of air/oil shock struts, main gear alignment units, support units, retraction and safety devices, auxiliary gear protective devices, nose wheel steering system, aircraft wheels, tires, tubes, and aircraft brake systems. The airframe mechanic should know all about each of these assemblies, their inspection procedures, and their relationships to the total operation of the landing gear.
Shock Struts Shock struts are self-contained hydraulic units that support an aircraft on the ground and protect the aircraft structure by absorbing and dissipating FIcURE 9-1. Dual main landing gear wheel arrangement.
Pivot trunnion Upper side brace Trunnion beam Drag brace Lower side brace Shock strut Upper outer cylinder Shock strut Torque arms Lower inner cylinder
- Independent Pivot fork and shaft rotating wheels Pivot beam - Positioner Tubular "H" member Axie FWD Equalizer FIGURE 9-2. "Bogie" truck main landing gear assembly.
the tremendous shock loads of landing. Shock struts must be inspected and serviced regularly to function efficiently.
Since there are many different designs of shock struts, only information of a general nature is included in this section. For specific information about a particular installation, consult the applicable manufacturer's instructions.
A typical pneumatic/hydraulic shock strut (figure 9-3) uses compressed air combined with hydraulic fluid to absorb and dissipate shock loads, and is often referred to as the air/oil or oleo strut.
A shock strut is made up essentially of two telescoping cylinders or tubes, with externally closed ends (figure 9-3). The two cylinders, known as cylinder and piston, when assembled, form an upper and lower chamber for movement of the fluid. The lower chamber is always filled with fluid, while the 342
Air valve - Air valve Outer cylinder -Metering Pin Centering cam Orifice plate Inner cylinder.
- Metering tube Orifice ' Torque Piston arms Inner cylinder (piston) Wheel axle Towing eye FIcURE 9-3. Landing gear shock strut of the metering pin type.
upper chamber contains compressed air. An orifice is placed between the two chambers and provides a passage for the fluid into the upper chamber during compression and return during extension of the strut.
Most shock struts employ a metering pin similar to that shown in figure 9-3 for controlling the rate of fluid flow from the lower chamber into the upper chamber. During the compression stroke, the rate of fluid flow is not constant, but is controlled automatically by the variable shape of the metering pin as it passes through the orifice.
On some types of shock struts, a metering tube replaces the metering pin, but shock strut operation is the same (figure 9-4).
Some shock struts are equipped with a damping or snubbing device consisting of a recoil valve on FIGURE 9-4. Landing gear shock strut of the metering tube type.
the piston or recoil tube to reduce the rebound during the extension stroke and to prevent too rapid extension of the shock strut. This could result in a sharp impact at the end of the stroke and possible damage to the aircraft and the landing gear.
The majority of shock struts are equipped with an axle attached to the lower cylinder to provide for installation of the wheels. Shock struts not equipped with axles have provisions on the end of the lower cylinder for easy installation of the axle assembly. Suitable connections are also provided on all shock struts to permit attachment to the aircraft.
A fitting consisting of a fluid filler inlet and air valve assembly is located near the upper end of each shock strut to provide a means of filling the strut with hydraulic fluid and inflating it with air.
A packing gland designed to seal the sliding joint between the upper and lower telescoping cylinders 343
is installed in the open end of the outer cylinder. A packing gland wiper ring is also installed in a groove in the lower bearing or gland nut on most shock struts to keep the sliding surface of the piston or inner cylinder free from dirt, mud, ice, and snow. Entry of foreign matter into the packing gland would result in leaks.
The majority of shock struts are equipped with torque arms attached to the upper and lower cylinders to maintain correct alignment of the wheel.
Shock struts without torque arms have splined pis- 18721 Cylinder Upper locating cam Lower locating cam Piston Torque arm Fork Axle FICURE 9-5. Nose gear shock strut.
ton heads and cylinders which maintain correct wheel alignment.
Nose gear shock struts are provided with an upper locating cam attached to the upper cylinder and a mating lower locating cam attached to the lower cylinder (figure 9-5). These cams line up the wheel and axle assembly in the straight-ahead position when the shock strut is fully extended. This prevents the nosewheel from being cocked to one side when the nose gear is retracted, thus preventing possible structural damage to the aircraft. The mating cams also keep the nosewheel in a straightahead position prior to landing when the strut is fully extended. Some nose gear shock struts have attachments for installing an external shimmy damper.
Generally, nose gear struts are equipped with a locking (or disconnect) pin to enable quick turning of the aircraft when it is standing idle on the ground or in the hangar. Disengagement of this pin will allow the wheel fork spindle to rotate 360°, thus enabling the aircraft to be turned in a very small space, as in a crowded hangar.
Nose and main gear shock struts are usually provided with jacking points and towing lugs. Jacks should always be placed under the prescribed points; and when towing lugs are provided, the towing bar should be attached only to these lugs.
All shock struts are provided with an instruction plate which gives brief instructions for filling the strut with fluid and for inflating the strut. The instruction plate attached near the filler inlet and air valve assembly also specifies the correct type of hydraulic fluid to use in the strut. It is of utmost importance to become familiar with these instruction prior to filling a shock strut with hydraulic fluid or inflating it with sir.
Figure 9-6 shows the inner construction of a shock strut and illustrates the movement of the fluid during compression and extension of the strut.
The compression stroke of the shock strut begins as the aircraft wheels touch the ground; the center of mass of the aircraft continues to move downward, compressing the strut and sliding the inner cylinder into the outer cylinder. The metering pin is forced through the orifice and, by its variable shape, controls the rate of fluid flow at all points of the compression stroke. In this manner the greatest possible amount of heat is dissipated through the walls of the shock strut. At the end of the downward stroke, the compressed air is further compressed, limiting the compression stroke of the strut.
The extension stroke occurs at the end of the compression stroke as the energy stored in the compressed air causes the aircraft to start moving upward in relation to the ground and wheels. At this instant, the compressed air acts as a spring to return the strut to normal. It is at this point that a snubbing or damping effect is produced by forcing the fluid to return through the restrictions of the snubbing device. If this extension were not snubbed, the aircraft would rebound rapidly and tend to oscillate up and down, due to the action of the compressed air. A sleeve, spacer, or bumper ring incorporated in the strut limits the extension stroke.
For efficient operation of shock struts, the proper fuid level and air pressure must be maintained. To check the fluid level, the shock strut must be deflated and in the fully compressed position. Defating a shock strut can be a dangerous operation unless servicing personnel are thoroughly familiar with high-pressure air valves. Observe all the neces.
sary safety precautions. Refer to manufacturer's instructions for proper deflating technique.
Two of the various types of high-pressure air valves currently in use on shock struts are illustrated in figure 9-7. Although the two air valves 5/8-inch hex nut Care 3/4-inch hex body - Valve core housing - - Yellow Compression stroke - Packing -O-ring
• Air orifice Valve assembly (A) Yellow valve cap - 3/4-inch hex nut - Air 3/4 inch hex body - Stem Extension stroke Hydraulic fluid Air orifice FICURE 9-6. Shock strut operation.
• Back-up-ring O-ring If there was an insufficient amount of fluid and/or air in the strut, the compression stroke would not be limited and the strut would "bottom." FIGURE 9-7. High-pressure air valves.
are interchangeable, there are some important dif- jacks to service the shock struts.)
ferences in their construction. One valve (figure 9-7A) contains a valve core and has a 5/8-in. 9-8A).
swivel hex nut. The other air valve (figure 9-7B) has no valve core and has a 3/4-in. swivel hex nut.
Servicing Shock Struts ose hyruts osiorkthe on (F) FICURE 9-8. Steps in servicing shock struts.
(6) Release the air pressure in the strut by slowly turning the swivel nut counterclockwise (figure 9-8E).
(7) Ensure that the shock strut compresses as the air pressure is released. In some cases, it may be necessary to rock the aircraft after deflating to ensure compression of the strut.
(8) When the strut is fully compressed, the air valve assembly may be removed (figure 9-8F).
(9) Fill the strut to the level of the air valve opening with an approved type of hydraulic fluid.
(10) Re-install the air valve assembly, using a new O-ring packing. Torque the air valve assembly to the values recommended in the applicable manufacturer's instructions.
(11) Install the sir valve core.
(12) Inflate the strut, using a high-pressure source of dry air or nitrogen. Bottled gas should not be used to inflate shock struts.
On some shock struts the correct amount of inflation is determined by using a high-pressure air gage. On others it is determined by measuring the amount of extension (in inches) between two given points on the strut. The proper procedure can usually be found on the instruction plate attached to the shock strut, Shock struts should always be inflated slowly to avoid excessive heating and over inflation.
(13) Tighten the swivel hex nut, using the torque values specified in the applicable manufacturer's instructions.
(14) Remove the high-pressure air line chuck and install the valve cap. Tighten the valve cap fingertight.
Bleeding Shock Struts If the fluid level of a shock strut has become extremely low, or if for any other reason air is trapped in the strut cylinder, it may be necessary to bleed the strut during the servicing operation.
Bleeding is usually performed with the aircraft placed on jacks. In this position the shock struts can be extended and compressed during the filling operation, thus expelling all the entrapped air. The following is a typical bleeding procedure:
(1) Construct a bleed hose containing a fitting suitable for making an airtight connection to the shock strut filler opening. The base should be long enough to reach from the shock strut filler opening to the ground when the aircraft is on jacks.
(2) Jack the aircraft until all shock struts are fully extended.
(3) Release the air pressure in the strut to be bled.
(4) Remove the air valve assembly.
(5) Fill the strut to the level of the filler port with an approved type hydraulic fluid.
(6) Attach the bleed hose to the filler port and insert the free end of the hose into a container of clean hydraulic fluid, making sure that this end of the hose is below the surface of the hydraulic fluid.
(7) Place an exerciser jack (figure 9-9) or other suitable single-base jack under the shock strut jacking point. Compress and extend the strut fully by raising and lowering the jack until the flow of air bubbles from the strut has completely stopped.
Compress the strut slowly and allow it to extend by its own weight.
(8) Remove the exerciser jack, and then lower and remove all other jacks.
(9) Remove the bleed hose from the shock strut.
(10) Install the air valve and inflate the strut.
-Hvdraulic fluid bleed hose Exerciser jack FIGURE, 9-9. Bleeding a shock strut using an exerciser jack.
Shock struts should be inspected regularly for Aircraft structural leakage of fluid and for proper extension. Exposed member portions of the strut pistons should be wiped clean daily and inspected closely for scoring or corrosion.
MAIN LANDING GEAR ALIGNMENT, SUPPORT, RETRACTION Trunnion The main landing gear consists of several compo- support nents that enable it to function. Typical of these are the torque links, trunnion and bracket arrange- brackets ments, drag strut linkages, electrical and hydraulic gear-retraction devices, and gear indicators.
Alignment Torque links (figure 9-10) keep the landing gear pointed in a straight-ahead direction; one torque link connects to the shock strut cylinder, while the Shock LTrunnion other connects to the piston. The links are hinged at strut the center so that the piston can move up or down in the strut. cylinder FICURE 9-11. Trunnion and bracket arrangement.
The upper end of the drag strut (figure 9-12) connects to the aircraft structure, while the lower end connects to the shock strut. The drag strut is hinged so that the landing gear can be retracted.
Trunnion bearing surface Drag strut FICURE 9-10. Torque links.
Support To anchor the main gear to the aircraft structure, a trunnion and bracket arrangement (figure 9-11) is usually employed. This arrangement is constructed to enable the strut to pivot or swing forward or backward as necessary when the aircraft is being steered or the gear is being retracted. To restrain this action during ground movement of the Retracting mechanism aircraft, various types of linkages are used, one being the drag strut. FIcURE 9-12. Drag strut linkage.
Manual control torque tube Landing gear motor Manual control gearbox • Drag strut Gearbox Trunnion support Retracting mechanism Shock strut
• Drag strut Universal joints FIGURE 9-13. Electrical retraction system.
Electrical Landing Gear Retraction System An electrical landing gear retraction system, such as that shown in figure 9-13, has the following features:
(1) A motor for converting electrical energy into rotary motion.
(2) A gear reduction system for decreasing the speed and increasing the force of rotation.
(3) Other gears for changing rotary motion (at a reduced speed) into push-pull move-
(4) Linkage for connecting the push-pull movement to the landing gear shock Basically, the system is an electrically driven jack for raising or lowering the gear. When a switch in the cockpit is moved to the "up" position, the electric motor operates. Through a system of shafts, gears, adapters, an actuator screw, and a torque tube, a force is transmitted to the drag strut linkages. Thus, the gear retracts and locks. If the switch is moved to the "down" position, the motor reverses and the gear moves down and locks. The sequence of operation of doors and gears is similar to that of a hydraulically operated landing gear system.
Hydraulic Landing Gear Retraction Systems Devices used in a typical hydraulically operated landing gear retraction system include actuating cylinders, selector valves, uplocks, downlocks, sequence valves, tubing, and other conventional hydraulic components. These units are so interconnected that they permit properly sequenced retraction and extension of the landing gear and the landing gear doors.
The operation of a hydraulic landing gear retraction system is of such importance that is must be covered in some detail, First, consider what happens when the landing gear is retracted. As the selector valve (figure 9-14) is moved to the "up" position, pressurized fluid is directed into the gear up line. The fluid flows to each of eight units; to sequence valves C and D, to the three gear downlocks, to the nose gear cylinder, and to the two main actuating cylinders.
Notice what happens to the fluid flowing to sequence valves C and D in figure 9-14. Since the sequence valves are closed, pressurized fluid cannot flow to the door cylinders at this time. Thus, the doors cannot close. But the fluid entering the three downlock cylinders is not delayed; therefore, the gear is unlocked. At the same time, fluid also enters the up side of each gear-actuating cylinder and the gears begin to retract. The nose gear completes 349
Selector valve From main hydraulic Down system pressure manifold
• To return manifold check valve Orifice Sequence valves Orifice check valve Gear cylinder Gear cylinder Up Up Up-lock Up-lock Close Down-lock Down-lock Door cylinde Door cylinder Sequence valves D Up-lock Nose gear Down-lock cylinder Code Pressure manifold Return manifold Up gear up, door close line Up Gear down, door open line FIGURE 9-14. Hydraulic landing gear retraction system schematic.