Hydraulic Brakes - Heavy Maintenance Checks
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Heavy maintenance checks and landing gear repair.
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a. The landing gear on aircraft may be fixed or retractable.
b. Retractable gear on aircraft is usually operated with hydraulic or electric power, although some models of light general aviation aircraft have manual retract systems operated by a lever in the cockpit.
(1) In addition to the normal operating system, emergency systems are usually provided to ensure that the landing gear can be lowered in case of main-system failure.
(2) Emergency systems consist of backup hydraulic systems, or stored nitrogen aas bottles that can be directed into actuating cylinders, mechanical systems that can be operated manually, or free-fall gravity systems.
Wear points, such as landing gear up-and-downjack-screws, latches, door hinges, cables, pulleys, bellcranks, and pressure-type grease fittings, should be lubricated after every cleaning operation.
To prevent possible failure of a component due to incompatibility or breakdown of the grease, the following should be observed:
1. Use only greases approved for use by the product manufacturer.
2. Never mix different kinds of grease without approval from the product manufacturer.
3. Follow the manufacturer's instructions or FAA approved process for cleaning, purging, and lubricating of the component.
To obtain proper lubrication of the main support bushings, it may be necessary to jack the aircraft.
Those companies with approval for Boeing heavy maintenance are concerned with retractable landing gearand thorough inspection of the landing gear is part and parcel of heavy maintenance checks. This involves the entire structure of the gear, including attachments, struts, wheels, brakes, actuating mechanisms for retractable gears, gear hydraulic system and valves, gear doors, and all associated parts. The manufacturer's inspection procedures should be followed where applicable.
It is recommended that only easily removable neutral solutions be used when cleaning landing gear components. Any advantage, such as speed or effectiveness, gained by using cleaners containing corrosive materials, can be quickly counteracted if these materials become trapped in close-fitting surfaces and crevices.
NOTE: Any time the aircraft is on aircraft jacks, check the landing gear main supportbushings for wear. Consult the aircraft manufacturer's overhaul manual for specific wear tolerances.
During winter operation, excess grease may congeal and cause increased loads on the gear retraction system, electric motors and hydraulic pumps. This condition can lead to component malfunctions; therefore, it is recommended that cleanliness be stressed during and after lubrication.
Fixed landing gear should be examined regularly for wear, deterioration, corrosion, alignment, and other factors that may cause failure or unsatisfactory operation. During a 100-hour or annual inspection of the fixed gear, the aircraft should be jacked up to relieve the aircraft weight. The gear struts and wheels should be checked for abnormal play and corrected.
Narrow bodied aircraftmaintenance
a. Old aircraft landing gear that employs a rubber shock (bungee) cord for shock absorption must be creamed for age, fraying of the braided sheath, narrowing (necking) of the cord, and wear at points of contact with the structure and stretch. If the age of the shock cord is near 5 years or more, it is advisable to replace it with a new cord. A cord that shows other defects should be replaced, regardless of age.
b. The cord is color-coded to indicate when it was manufactured and to determine the life of the shock cord. According to MIL-C-5651A, the color code for the year of manufacture is repeated in cycles of 5 years.
c. The color coding is composed of interwoven threads in the cotton wool that holds the strands of rubber cord together. Two spiralthreads are used for the year coding and one thread is used for the quarter of the year rose, e.g. yellow and blue would indicate that the cord was manufactured in 1994 during April, May, or June.
d. Shock struts of the spring-oleo type should be examined for leakage, smoothness of operation, looseness between the moving parts, and play at the attaching points. The extension of the struts should be checked to make sure that the springs are not worn or broken. The section of the strut piston should be free of nicks, cuts, and rust.
e. Air-oil struts should undergo an inspection similar to that recommended for spring-oleo struts. In addition, the extension of the strut should be checked to see that it ' Tomb conforms to the distance specified by the manufacturer. If an air-oil strut"bottoms", that is, it is collapsed-the gas charge and hydraulic fluid has been lost from the air chamber. This is probably due to a loose or defective air valve or to defective o-ring seals.
CAUTION: Before an air-oil strut is removed or disassembled, the air valve should be opened to make sure that all air pressure is removed. Severe injury and/or damage can occur as the result of disassembling a strut when even a small amount of air pressure is still in the air chamber.
f. The method for checking the fluid level of an air-oil strut is given in the manufacturer's maintenance manual. An alternate means of servicing an oil strut is to jack up the aircraft, remove the strut's valve cap, release the air charge in the strut by depressing the valve core, remove the strut's valvecore, attach a clean two-foot rubber or plastic hose to the threaded portion that houses the valve core, and secure with a hose clamp. Put the other end of the hose into a clean two quart container filled with the correct hydraulic fluid for the strut. Cover the container with a clean rag to prevent spillage. Now, slowly raise the gear/strut assembly either manually or with another jack under the strut. This will drive the remaining air out of the strut into the container of hydraulic fluid. Once the gear is fully retracted, slowly lower the gear. The hydraulic fluid in the can will be sucked into the strut. Repeat this procedure until you cannot hear any more air bubbles in the container when the wheel strut is fully retracted. With the strut fully retracted, remove the hose, insert thevalve core, lower the gear, and service the strut with nitrogen to get the proper strut extension.
g. The entire structure of the landing gear should be closely examined for cracks, nicks, cuts, corrosion damage, or any other condition that can cause stress concentrations and eventual failure. The exposed lower end of the air-oil piston is especially susceptible to damage and corrosion, which can lead to seal damage, because the strut is compressed and the piston moves past the lower strut seal, causing the seal to leak fluid and air. Small nicks or cuts can be filed and burnished to a smooth contour, eliminating the point of stress concentration. If a crack is found in a landing gear member, the part must be replaced.
h. All bolts and fittings should be checked for securityand condition. Bolts to the torque links and shimmy damper tend to wear and become loose due to the operational loads placed on them. The nose-wheel shimmy damper should be checked for proper operation and any evidence of leaking. All required servicing should be performed in accordance with the aircraft service manual.
Retractable landing gear Inspection of
Inspection of the retractable landing gear should include all applicable items mentioned in the inspection for the fixed gear. In addition, the actuating mechanisms must be creamed for wear in any joint looseness, trunnion, or bearings; leakage of fluid from any hydraulic lines or units; and, smoothness of operation. The operational check is performed by jacking the aircraft according to the manufacturer'sinstructions and then operating the gear retracting and extending system.
Landing-gear doors should be checked. Improper adjustment of sequence valves may cause doors to rub against structures or gear wheels. The manufacturer's checklist should be followed to ensure that critical items are checked. While the aircraft is still on jacks, the gear can be tested for looseness of mounting points, play in torque links, condition of the inner cylinder strut in wheel bearings, play, and play in actuating linkages. Emergency blow down gear bottles should be creamed for damage and corrosion and weighed to see if the bottle is still retaining the charge.
Mechanics should be aware that retread tyres can be dimensionally bigger than a "new" tyre. While this does not pose a problem onfixed landing gear aircraft, it may present a serious problem when installed on retractable landing gear aircraft. It is strongly recommended that if a retread tyre is installed on a retractable landing gear retraction test aircraft, to be performed. With the gear on the up-and-lock position, the mechanic should determine that if the tyre expands due to high ambient temperatures, heat generated from taxi and take-offs, landings, or repeated heavy braking, the tyre will not expand to the point that it becomes wedged in the wheel well.
The proper operation of the antiretraction system should be checked in accordance with the manufacturer's instructions. Where safety switches are actuated by the torque links, the actual time of closing or opening switch can be checked by removing all airfrom the strut and then collapsing the strut. In every case, the adjustment should be such that the gear control cannot be placed in the UP position or that the system cannot operate until the shock strut is at the full extended position.
During the operational tests, the smoothness of operation, effectiveness of up-and-down operation of locks, the warning horn, indicating operation of systems, and clearance of tyres in wheel wells should all be checked.
Emergency landing gear systems.
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