Serious maintenance checks

Serious maintenance checks

Serious inspections for maintenance and repair of the landing gear.

a. Aircraft landing gear can be fixed or retractable.

b. Retracting equipment on airplanes is usually operated by hydraulic or electrical power, although some models of light general aviation aircraft have manual retraction systems operated by a lever in the cockpit.

(1) In addition to the normal operating system, emergency systems are normally provided to ensure that the landing gear can be lowered in the event of a failure of the main system.

(2) Emergency systems consist of backup hydraulic systems or stored nitrogen cylinders such as cylinders that can be directed to actuating cylinders, mechanical systems that can be operated manually, or gravity free fall systems.

Wear points such as landing gear lift and lower locks, jack screws, door hinges, pulleys, cables, bells and all pressure grease fittings should be lubricated after each cleaning operation.

To prevent possible component damage due to incompatibility or grease failure, the following must be observed:

1. Use only greases approved for use by the product manufacturer.

2. Never mix different types of grease without the approval of the product manufacturer.

3. Follow the manufacturer’s instructions or the FAA-approved process for cleaning, flushing, and lubricating the component.

To obtain proper lubrication of the main bearing bushings, it may be necessary to raise the aircraft.

These Boeing heavy maintenance approved companies deal with retractable landing gear and a thorough inspection of the landing gear is an integral part of the heavy maintenance checks. This includes the entire gear structure including fixtures, struts, wheels, brakes, retracting gear actuators, hydraulic system and gear valves, gear doors and all associated parts. Manufacturer’s inspection procedures should be followed when applicable.

It is recommended to use only easily removable neutral solutions when cleaning landing gear components. Any advantage, such as speed or efficiency, gained by using cleaners containing corrosive materials can be quickly counteracted if these materials enter tight surfaces and crevices.

NOTE: Whenever the airplane is on airplane jacks, inspect the main landing gear support bushings for wear. Consult the aircraft manufacturer’s overhaul manual for specific wear tolerances.

During winter operation, excess grease can solidify and cause increased stress on the gear retract system, electric motors and hydraulic pumps. This condition can cause components to malfunction; therefore it is recommended to emphasize cleanliness during and after lubrication.

Fixed landing gear should be inspected regularly for wear, deterioration, corrosion, misalignment, and other factors that could cause damage or unsatisfactory performance. During a 100-hour or annual fixed equipment inspection, the airplane must be lifted to reduce the weight of the airplane. The struts and pinion wheels should be checked for abnormal clearance and adjusted.

Maintenance of narrow-body aircraft

a. Older aircraft undercarriages that use a rubber shock absorbing (bungee) cord should be inspected for age, wear of the braided wrap, constriction (kink) of the cord, and wear at the points of contact with the structure and stretch. If the age of the shock cord is close to 5 years or more, it is recommended to replace it with a new cord. A cable that shows other defects should be replaced, regardless of age.

b. The cable is color coded to show when it was manufactured and to determine the life of the shock cable. According to MIL-C-5651A, the color code for the year of manufacture repeats in 5-year cycles.

° C. The color coding consists of threads woven into the cotton sheath that holds the threads of the rubber cord together. Two spiral threads are used to encode the year and one thread is used for the quarter of the year wrap, for example yellow and blue will mean the cord was manufactured in 1994 in April, May or June.

e. Spring-oleo shock absorbers should be checked for leaks, smoothness of operation, looseness between moving parts and play at attachment points. The extension of the struts should be checked to ensure that the springs are not worn or broken. The strut piston section must be free of nicks, cuts and rust.

e. Air-oil struts should be subjected to an inspection similar to that recommended for spring-oil struts. In addition, the strut extension should be checked to see if it matches the distance specified by the manufacturer. If the air-oil strut is “down”, that is, it is bent – the gas charge and hydraulic fluid are lost from the air chamber. This is probably due to a loose or defective air valve or defective o-rings.

CAUTION: Before an air-oil support is removed or disassembled, the air valve must be opened to ensure that all air pressure is removed. Serious injury and/or damage can result from strut disassembly when there is still even a small amount of air pressure in the air chamber.

f. The method for checking the air-oil shock fluid level is given in the manufacturer’s maintenance manual. An alternative way to service an oil prop is to jack up the airplane, remove the prop valve cap, release the air charge in the prop by depressing the valve core, remove the prop valve core, attach a clean two-foot rubber or plastic hose to the threaded part where the valve core is located and secure with a hose clamp. Place the other end of the hose in a clean two liter container filled with the correct hydraulic fluid for the prop. Cover the container with a clean cloth to prevent spillage. Now slowly lift the gear/strut assembly by hand or with another jack under the strut. This will force any remaining air from the prop into the hydraulic fluid container. Once the gear is fully retracted, slowly lower it. The hydraulic fluid in the box will be sucked into the strut. Repeat this procedure until you hear no more air bubbles in the container when the wheel support is fully retracted. With the prop fully retracted, remove the hose, install the valve core, lower the gear and service the prop with nitrogen to get the proper prop extension.

g. The entire landing gear structure should be carefully inspected for cracks, gouges, cuts, corrosion damage, or any other condition that could cause stress concentration and possible failure. The exposed lower end of the air-oil piston is particularly susceptible to damage and corrosion, which can lead to seal failure as the bearing is compressed and the piston moves past the lower bearing seal, causing fluid and air to escape from the seal . Small nicks or cuts can be filed and polished to a smooth contour, eliminating the stress concentration point. If a crack is found in a landing gear element, the part must be replaced.

h. All bolts and fittings should be checked for security and condition. The bolts in the torque and shimmy damper linkages tend to wear and loosen due to the operating loads on them. The nose wheel shim damper should be checked for proper operation and any evidence of leakage. All necessary service must be performed in accordance with the aircraft service manual.

Inspection of retractable landing gear

The inspection of the retractable landing gear should include all applicable items mentioned in the inspection of the fixed landing gear. In addition, actuators should be checked for loose wear in any linkage, journal, or bearings; leakage of fluid from any hydraulic lines or assemblies; and smoothness of operation. The operational check is performed by raising the aircraft according to the manufacturer’s instructions and then operating the gear retract and extend system.

The landing gear doors should be checked. Improper adjustment of the sequential valves can cause the doors to rub against gears or wheels. The manufacturer’s checklist should be followed to ensure critical items are checked. While the airplane is still on jacks, the gear can be tested for looseness at the mounting points, play in the torque links, condition of the inner strut cylinder, play in the wheel bearings, and play in the actuating links. Emergency purge cylinders should be checked for damage and corrosion and weighed to see if the cylinder still holds a charge.

Mechanics should be aware that retreaded tires can be sized larger than a “new” tire. Although this is not a problem for fixed landing gear aircraft, it can be a serious problem when fitted to retractable landing gear aircraft. It is strongly recommended that if a retreaded tire is installed on an aircraft with retractable landing gear, a retraction test be performed. With the gear in the up and locked position, the mechanic must determine that if the tire expands due to high ambient temperature, heat generated from taxiing and takeoff, repeated landings, or hard braking, the tire will not expand to the point of jams into the wheel socket.

Correct operation of the anti-retract system should be checked in accordance with the manufacturer’s instructions. When safety switches are actuated by rotary links, the actual closing or opening time of the switch can be checked by removing all air from the strut and then collapsing the strut. In any case, the adjustment must be such that the gear control cannot be placed in the up position or that the system cannot operate until the shock is in the fully extended position.

During the operational test, the smoothness of operation, the effectiveness of the up and down locks, the operation of the warning horn, the operation of the indicating systems and the clearance of the tires in the wheel wells should be checked.

Emergency landing gear systems.

Exercise emergency landing gear systems periodically to ensure proper operation and to prevent inactivity, dirt, and corrosion from rendering the system inoperative when needed. Most emergency systems use either mechanical capabilities, or pressure cylinder extension capabilities, or free fall capabilities. Check that the actuation mechanisms are properly secured and that the necessary plates and necessary accessories such as cranks, levers, handles, etc. are present. Emergency purge cylinders should be inspected for corrosion damage and then weighed to see if the cylinder is still retaining a charge.

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