Par 8-92 Page 8-46 the wood between the propeller and the flange due to loose propeller mounting bolts.
Composition propellers should be inspected in accordance with the propeller manufacturer’s instructions.
a. Fixed-pitch propellers are normally removed from the engine at engine overhaul periods. Whenever the propeller is removed, visually inspect the rear surface for any indication of cracks. When any defects are found, disassemble the metal hub from the propeller. Inspect the hub bolts for wear and cracks at the head and threads and, if cracked or worn, replace with new equivalent bolts. Inspect for elongated bolt holes, enlarged hub bore, and cracks inside the bore or anywhere on the propeller. Repair propellers found with any of these defects. If no defects are found, the propeller may be reinstalled on the engine. Before installation, touch up with varnish all places where the finish is worn thin, scratched, or nicked. Track and balance the propeller and coat the hub bore and bolt holes with some moisture preventive, such as asphalt varnish. In case the hub flange is integral with the crankshaft of the engine, final track the propeller after it is installed on the engine. In all cases where a separate metal hub is used, make a final balance and track with the hub installed on the propeller.
b. On new, fixed-pitch propeller installations, inspect the bolts for proper torque after the first flight and after the first 25 hours of flying. Thereafter, inspect and check the bolts for proper torque at least every 50 hours. No definite time interval can be specified since a bolt’s proper torque is affected by changes in the wood caused by the moisture content of the air where the airplane is flown and stored. During wet weather, some moisture is apt to enter the propeller wood through the holes drilled in the hub. The wood then swells, and because expansion is limited by the bolts extending between the two flanges, some of the wood fibers become crushed. Later, when the propeller dries out during dry weather or due to heat from the engine, a certain amount of propeller hub shrinkage takes place, and the wood no longer completely fills the space between the two hub flanges. Consequently, the hub bolts become loose.
c. In-flight tip failures may be avoided by frequent inspections of the metal cap, leading-edge strip, and surrounding areas. Inspect for such defects as looseness or slipping, separation of soldered joints, loose screws, loose rivets, breaks, cracks, eroded sections, and corrosion. Inspect for separation between the metal leading edge and the cap, which would indicate the cap is moving outward in the direction of centrifugal force. This condition is often accompanied by discoloration and loose rivets. Inspect the tip for cracks by grasping it with the hand, slightly twisting about the longitudinal blade centerline, and slightly bending the tip backward and forward. If the leading edge and the cap have separated, carefully inspect for cracks at this point. Cracks usually start at the leading edge of the blade. A fine line appearing in the fabric or plastic may indicate a crack in the wood. Check the trailing edge of the propeller blades for bonding, separation, or damage.
d. Examine the wood close to the metal sleeve of wood blades for cracks extending outward on the blade. These cracks sometimes occur at the threaded ends of the lag screws and may be an indication of internal cracking of the wood. Check the tightness of the lag screws, which attach the metal sleeve to the wood blade in accordance with the manufacturer’s instructions. Inspect and protect the shank areas of composition blades next to the metal sleeve in the same manner as that used for wood blades.
Par 8-93 Page 8-47
8-93. METAL PROPELLERS AND BLADES. These propellers and blades are generally susceptible to fatigue failure resulting from the concentration of stresses at the bottoms of sharp nicks, cuts, and scratches. It is necessary, therefore, to frequently and carefully inspect them for such injuries. Propeller manufacturers publish SBs and instructions which prescribe the manner in which these inspections are to be accomplished. Additional information is also available in AC 20-37, Aircraft Propeller Maintenance.
a. Steel Blade Inspection. The inspection of steel blades may be accomplished by either visual, fluorescent penetrant (see Chapter 5), or magnetic particle inspection. The visual inspection is easier if the steel blades are covered with engine oil or rust-preventive compound.
The full length of the leading edge, especially near the tip, the full length of the trailing edge, the grooves and shoulders on the shank, and all dents and scars should be examined with a magnifying glass to decide whether defects are scratches or cracks.
b. Aluminum Propellers and Blades. Carefully inspect aluminum propellers and blades for cracks and other flaws. A transverse crack or flaw of any size is cause for rejection. Multiple deep nicks and gouges on the leading edge and face of the blade is cause for rejection. Use dye penetrant or fluorescent dye penetrant to confirm suspected cracks found in the propeller. Refer any unusual condition or appearance revealed by these inspections to the manufacturer.
c. Limitations.
(1) Corrosion may be present on propeller blades in varying amounts. Before performing any inspection process, maintenance personnel must examine the specific type and extent of the corrosion (see Chapter 6 and/or refer to AC 43-4, Corrosion Control for Aircraft).
(2) Corrosion, other than small areas (6 square inches or less) of light surface type corrosion, may require propeller removal and reconditioning by a qualified propeller repair facility. When intergranular corrosion is present, the repair can be properly accomplished only by an appropriately certificated propeller repair facility. Corrosion pitting under propeller blade decals should be removed as described in the propeller manufacturer’s SBs and applicable Airworthiness Directives (AD).
(3) Unauthorized straightening of a blade following a ground strike or other damage can create conditions that lead to immediate blade failure. These unapproved major repairs may sometimes be detected by careful inspection of the leading edges and the flat face portion of the blade. Any deviation of the flat portion, such as bows or kinks, may indicate unauthorized straightening of the blade. Sighting along the leading edge of a propeller blade for any signs of bending can provide evidence of unapproved blade straightening. Blades should be examined for any discoloration that would indicate unauthorized heating. Blades that have been heated for any repair must be rejected since only cold straightening is authorized. All blades showing evidence of unapproved repairs should be rejected. When bent propellers are shipped to an approved repair facility for inspection and repair, the propeller should never be straightened by field service personnel to facilitate shipping because this procedure can conceal damage. Propeller tip damage will sometimes lead maintenance personnel to consider removing damaged material from the blade tips. However, propellers are often manufactured with a particular diameter to minimize vibration. Unless the TCDS and both the engine and propeller manufacturers
Par 8-93 Page 8-48 specifically permit shortening of the blades on a particular propeller, any shortening of the blades would probably create an unairworthy condition. When conditions warrant, inspect the blade tips for evidence of shortening and, if necessary, measure the propeller diameter to determine if it has been changed by an unauthorized repair.
8-94. PROPELLER HUB.
a. Fixed Pitch.
(1) Inspection procedures require removal of the propeller spinner for examination of the prop hub area. Cracks may be present in the hub area between or adjacent to bolt holes and along the hub pilot bore. Cracks in these areas cannot be repaired and require immediate scrapping of the propeller.
(2) Propeller attach bolts should be examined for looseness or an unsafetied or cracked condition. Cracked or broken bolts are usually the result of overtorquing. Correct torquing procedures require all bolt threads to be dry, clean, and free of any lubrication before torquing.
b. Controllable Pitch.
(1) Inspect controllable pitch propellers frequently to determine that all parts are lubricated properly. It is especially recommended that all lubrication be accomplished in accordance with the propeller manufacturer’s instructions.
(2) Complete inspection/servicing requires the removal of the spinner for examination and servicing of the propeller hub and blade clamp area. All inspections and servicing of the pitch-control mechanism should follow the recommendations of the propeller, engine, and airframe manufacturers. Propellers must be in compliance with applicable ADs and the manufacturer’s SBs.
(3) The hub, blade clamps, and pitch-change mechanisms should be inspected for corrosion from all sources, including rain, snow, and bird droppings that may have entered through the spinner openings. Examine the hub area for oil and grease leaks, missing grease-fitting caps, and leaking or missing grease fittings.
(4) Propeller domes should be checked for leaks, both at the seals and on the fill valve (if so equipped). The dome valve may be leak tested by applying soapy water over the fill valve end. Domes should be serviced only with nitrogen or dry air in accordance with the manufacturer’s recommendations. When propeller domes are inspected and found filled with oil, the propeller should be removed and inspected/repaired by an appropriately rated repair facility.
(5) It is especially recommended that all lubrication be accomplished at the periods and in the manner specified by the propeller manufacturer. On makes and models with a grease fitting on the hub, before greasing the hub, remove the grease fitting opposite the one to which you are going to add grease. This will allow the excess grease and pressure to exit through the grease-fitting hole rather than the hub seal.
Par 8-94 Page 8-49
(6) Fiber-block, pitch-change mechanisms should be inspected for deterioration, fit, and the security of the pitch-clamp forks.
(7) Certain models of full-feathering propellers use spring-loaded pins to retain the feathered blade position. Spring and pin units should be cleaned, inspected, and relubricated per the manufacturer’s recommendations and applicable ADs.
(8) Pitch-change counterweights on blade clamps should be inspected for security, safety, and to ensure that adequate counterweight clearance exists within the spinner.
8-95. TACHOMETER INSPECTION. Due to the exceptionally high stresses that may be generated by particular propeller/engine combinations at certain engine rpm, many propeller and aircraft manufacturers have established areas of rpm restrictions and other restrictions on maximum rpm for some models. Some rpm limits do not exceed 3 percent of the maximum rpm permitted, and a slow-running tachometer can cause an engine to run past the maximum rpm limits. Since there are no post-manufacture accuracy requirements for engine tachometers, tachometer inaccuracy could lead to propeller failure, excessive vibration, or unscheduled maintenance. If the tachometer exceeds 2 percent (plus or minus) of the tested rpm, replace it.
8-96.–8-106. [RESERVED.]
Par 8-107 Page 8-50 SECTION 6. PROPELLER TRACKING AND VIBRATION
8-107. GENERAL. To ensure smooth powerplant operations, first start with a properly installed propeller. Each propeller should be checked for proper tracking (blades rotating in the same plane of rotation). Manufacturer’s recommendations should, in all cases, be followed.
8-108. PROPELLER TRACKING CHECK. The following is a simple procedure that can be accomplished in less than 30 minutes:
a. Chock the aircraft so it cannot be moved.
b. Remove one spark plug from each cylinder. This will make the propeller easier and safer to turn.
c. Rotate one of the blades so it is pointing down.
d. Place a solid object (e.g., a heavy wooden block that is at least a couple of inches higher off the ground than the distance between the propeller tip and the ground) next to the propeller tip so that it just touches (see Figure 8-30) or attach a pointer/indicator to the cowling itself.
FIGURE 8-30. PROPELLER TRACKING (WOOD BLOCK OR COWLING FIXTURE SHOWN)
Par 8-108 Page 8-51
e. Rotate the propeller slowly to see if the next blade “tracks” through the same point (touches the block/pointer). Each blade track should be within 1/16-inch (plus or minus) from the opposite blade’s track.
f. If the propeller is out-of-track, it may be due to one or more propeller blades being bent, a bent propeller flange, or propeller mounting bolts that are either over or under-torqued.
An out-of-track propeller will cause vibration and stress to the airframe and engine and may cause premature propeller failure.
8-109. VIBRATION. Although vibration can be caused by the propeller, there are numerous other possible sources of vibration which can make troubleshooting difficult.
a. If a propeller vibrates, whether due to balance, angle, or track problems, it typically vibrates throughout the entire rpm range, although the intensity of the vibration may vary with the rpm. If a vibration occurs only at one particular rpm or within a limited rpm range
(e.g., 2200–2350 rpm), the vibration is not normally a propeller problem but a problem with a poor engine/propeller match.
b. If a propeller vibration is suspected but cannot be positively determined, if possible, the ideal troubleshooting method is to temporarily replace the propeller with one which is known to be airworthy and test fly the aircraft.
c. There are numerous allowable tolerances in blade angles, balance, track, and blade width and thickness dimensions. These tolerances have been established through many years of experience. The degree to which these factors affect vibration is sometimes disputed and can involve significant repair bills, which may or may not cure a vibration problem. Reliance upon experienced, reputable propeller repair stations is the owner’s best method of dealing with these problems.
d. Blade shake is not the source of vibration problems. Once the engine is running, centrifugal force holds the blades firmly (approximately 30–40,000 pounds) against blade bearings.
e. Cabin vibration can sometimes be improved by reindexing the propeller to the crankshaft. The propeller can be removed, rotated 180 degrees, and reinstalled.
f. The propeller spinner can be a contributing factor to an out-of-balance condition. An indication of this would be a noticeable spinner “wobble” while the engine is running. This condition is normally caused by inadequate shimming of the spinner front support or a cracked or deformed spinner.
8-110.–8-129. [RESERVED.]
Par 9-1 Page 9-1 CH APTER 9. AIRCRAFT SYSTEMS AND COMPONENTS S ECTION 1. INSPECTION AND MAINTENANCE OF LANDING GEAR
9-1. GENERAL.
a. The landing gear on aircraft may be fixed or retractable. A fixed gear may be wheels, floats, or skis; and for amphibians a combination of floats and wheels.
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 gas bottles that can be directed into actuating cylinders, mechanical systems that can be operated manually, or freefall gravity systems.
9-2. GENERAL INSPECTION. A thorough inspection of the landing gear 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.
9-3. CLEANING AND LUBRICATING.
a. 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.
b. Wear points, such as landing-gear up-and-down latches, jack-screws, door hinges, pulleys, cables, bellcranks, and all pressure-type grease fittings, should be lubricated after every cleaning operation.
c. 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 Federal Aviation Administration (FAA)-approved process for cleaning, purging, and lubricating of the component.
Par 9-3 Page 9-2
d. To obtain proper lubrication of the main support bushings, it may be necessary to jack the aircraft.
NOTE: Any time the aircraft is on jacks, check the landing gear’s main support bushings for wear. Consult the aircraft manufacturer’s overhaul manual for specific wear tolerances.
e. 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.
9-4. FIXED-GEAR INSPECTION. 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.
a. Old aircraft landing gear that employs a rubber shock (bungee) cord for shock absorption must be inspected 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-5651, the color code for the year of manufacture is repeated in cycles of 5 years. Table 9-1 shows the color of the code thread for each year and quarter year.
TABLE 9-1. BUNGEE CORD COLOR CODES
YEARS ENDING WITH COLOR QUARTER COLOR 0 or 5 Black 1st Red 1 or 6 Green 2nd Blue 2 or 7 Red 3rd Green 3 or 8 Blue 4th Yellow 4 or 9 Yellow 1st Red
c. The color coding is composed of threads interwoven in the cotton sheath that holds the strands of rubber cord together. Two spiral threads are used for the year coding and one thread is used for the quarter of the year sheath, 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
Par 9-4 Page 9-3 the struts should be checked to make sure that the springs are not worn or broken. The piston section of the strut 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 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 valve core, attach a clean 2-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 2-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 the valve 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-oleo 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 strut lower 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 security and condition. Bolts in the torque links and shimmy damper tend to wear and become loose due to the operational loads placed on them. The nosewheel 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.
9-5. INSPECTION OF RETRACTABLE LANDING GEAR. 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 inspected for wear looseness in any joint, trunnion, or bearing; leakage of fluid from any hydraulic line or unit; and smoothness of operation. The
Par 9-5 Page 9-4 operational check is performed by jacking the aircraft according to the manufacturer’s instructions and then operating the gear retracting and extending system.
a. During the operational test, the smoothness of operation, effectiveness of up-and-down locks, operation of the warning horn, operation of indicating systems, clearance of tires in wheel wells, and operation of landing-gear doors should be checked. Improper adjustment of sequence valves may cause doors to rub against gear structures or 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 strut cylinder, play in wheel bearings, and play in actuating linkages.
Emergency blowdown gear bottles should be inspected for damage and corrosion and weighed to see if the bottle is still retaining the charge.
b. Mechanics should be aware that retread tires can be dimensionally bigger than a “new” tire. While this does not pose a problem on fixed landing-gear aircraft, it may present a serious problem when installed on retractable landing-gear aircraft. It is strongly recommended that if a retread tire is installed on a retractable landing-gear aircraft, a retraction test be performed. With the gear in the up-and-lock position, the mechanic should determine that if the tire expands due to high ambient temperature, heat generated from taxi and takeoff, repeated landings, or heavy braking, the tire will not expand to the point that it becomes wedged in the wheel well.
c. 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 switch closing or opening can be checked by removing all air from 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.
9-6. EMERGENCY 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 employ either mechanical, pressure-bottle, or freefall extension capabilities. Check for the proper safeties on triggering mechanisms, and for the presence of required placards, and necessary accessories such as cranks, levers, handles, etc.
Emergency blowdown bottles should be checked for corrosion damage, and then weighed to see if the bottle is still retaining the charge.
9-7. LANDING-GEAR COMPONENTS. The following items are susceptible to service difficulties and should be inspected.
a. Shock Absorbers. Inspect the entire shock strut for evidence of leaks, cracks, and possible bottoming of the piston, as this condition causes overloading of landing-gear components and contributes to fatigue cracks. Check all bolts, bolt holes, pins, and bushings for condition, lubrication, and proper torque values. Grease fitting holes (pressure-type) are especially vulnerable to cracks and cross-threading damage. Check all safety wire and other locking devices, especially at the main packing gland nuts.
Par 9-7 Page 9-5
(1) When assembling shock struts, use the correct type and number of new O-rings, Chevron seals, and backup rings. Use only the correct filler valve core assembly and follow the manufacturer’s instructions when servicing with fluid and air. Either too much or too little air or oil will affect aircraft handling characteristics during taxi, takeoff, and landing, and can cause structural overloads.
(2) Shock cords and rubber discs deteriorate with age and exposure. When this type of shock absorber is used, inspect for general condition; i.e., cleanliness, stretching, fraying, and broken strands. These components should be kept free of petroleum products as they accelerate deterioration of the rubber.
b. Nose Gear Assembly. Inspection of the steering mechanism should include torque links (scissors), torque tubes, control rods and rod end bearings, shimmy dampers, cables, and turning stops. In addition, check all nose landing-gear components, including mud scrapers and slush deflectors, for damage.
(1) Towing of some aircraft with the rudder locks installed may cause damage to the steering linkage and rudder control system. Exceeding the steering or towing stop limits should be followed by a close inspection of the entire nose steering assembly. A broken steering stop will allow turning beyond the design limit, transmitting excessive loads to structures, and to the rudder control system. It is recommended that the nose steering arc limits be painted on the steering collar or fuselage.
(2) Inspect shimmy dampers for leakage around the piston shaft and at fluid line connections, and for abnormal wear or looseness around the pivot points. Also check for proper rigging, “bottoming” of the piston in the cylinder, and the condition of the external stops on the steering collar.
c. Tail Wheels. Disassembly, cleaning, and rerigging of tail wheels are periodically necessary. Inspect them for loose or broken bolts, broken springs, lack of lubrication, and general condition. Check steerable tail wheels for proper steering action, steering horn wear, clearances, and for security and condition of steering springs and cables.
d. Gear Doors. Inspect gear doors frequently for cracks, deformation, proper rigging, and general condition. Gear door hinges are especially susceptible to progressive cracking, which can ultimately result in complete failure, allowing the door to move and cause possible jamming of the gear. This condition could also result in the loss of the door during flight. In addition, check for proper safetying of the hinge pins and for distorted, sheared, loose, or cracked hinge rivets.
Inspect the wheel wells for improper location or routing of components and related tubing or wiring. This could interfere with the travel of the gear door actuating mechanisms.
e. Wheels. Inspect the wheels periodically for cracks, corrosion, dents, distortion, and faulty bearings in accordance with the manufacturer’s service information. In split-type wheels, recondition bolt holes which have become elongated due to some play in the through-bolt, by the use of inserts or other FAA-approved means. Pay particular attention to the condition of the through-bolts and nuts. Carefully inspect the wheels used with tubeless tires for damage to the wheel flange and for proper sealing of the valve. The sealing ring used between the wheel halves
Par 9-7 Page 9-6 should be free of damage and deformation. When bolting wheel halves together, tighten the nuts to the proper torque value. Periodically accomplish an inspection to ensure the nuts are tight and that there is no movement between the two halves of the wheel. Maintain grease retaining felts in the wheel assembly in a soft, absorbent condition. If any have become hardened, wash them with a petroleum-based cleaning agent; if this fails to soften them, they should be replaced.
(1) Corrosion of wheels. Remove all corrosion from the wheel half and inspect it to ensure that the wheel halves are serviceable. Apply corrosion prevention treatments as applicable. Prime with a zinc chromate primer or equivalent and apply at least two finish coats.
(2) Dented or distorted wheels. Replace wheels which wobble excessively due to deformation resulting from a severe side-load impact. In questionable cases, consult the responsible Flight Standards office concerning the airworthiness of the wheels. Minor dents do not affect the serviceability of a wheel.
(3) Wheel bearings. When inspecting wheel bearings for condition, replace damaged or excessively worn parts. Maintain bearings and races as matched sets. Pack bearings only with the grease type called for in the manufacturer’s maintenance manual prior to their installation. Avoid preloading the wheel bearing when installing it on the aircraft by tightening the axle nut just enough to prevent wheel drag or side play.
f. Brakes. Disassemble and inspect the brakes periodically and examine the parts for wear, cracks, warpage, corrosion, elongated holes, etc. Discolored brake disks are an indication of overheated brakes and should be replaced. If any of these or other faults are indicated, repair, recondition, or replace the affected parts in accordance with the manufacturer’s recommendations.
g. Hydraulic Brakes. For proper maintenance, periodically inspect the entire hydraulic system from the reservoir to the brakes. Maintain the fluid at the recommended level with proper brake fluid. When air is present in the brake system, bleed in accordance with the manufacturer’s instructions. Replace flexible hydraulic hoses which have deteriorated due to long periods of service and replace hydraulic piston seals when there is evidence of leakage.
h. Micro-Switches. Inspect micro-switches for security of attachment, cleanliness, general condition, and proper operation. Check the associated wiring for chafing, proper routing, and to determine that protective covers are installed on wiring terminals, if required. Check the condition of the rubber dust boots which protect the micro-switch plungers from dirt and corrosion.
9-8. FLOATS AND SKIS. Aircraft operated from water may be provided with either a single float or a double float, depending upon the design and construction; however, if an aircraft is an amphibian, it has a hull for flotation and then may need only wingtip floats. Amphibious aircraft have floats or a hull for operating on water and retractable wheels for land operation.
a. Skis are used for operating on snow and ice. The skis may be made of wood, metal, or composite materials. There are three basic styles of skis. A conventional ski, shown in Figure 9-1, replaces the wheel on the axle. The shock cord is used to hold the toe of the ski up