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.
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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
Par 9-8 Page 9-7 when landing. The safety cable and check cable prevent the ski from pivoting through too great an angle during flight.
FIGURE 9-1. A TYPICAL SKI INSTALLATION
b. Th e wheel ski is designed to mount on the aircraft along with the tire. The ski has a portion cut out that allows the tire to extend slightly below the ski, so that the aircraft can be operated from conventional runways with the wheels or from snow or ice surfaces using the ski.
This arrangement has a small wheel mounted on the heel of the ski, so that it does not drag on conventional runways.
c. In retractable wheel ski arrangements, the ski is mounted on a common axle with the wheel. In this arrangement, the ski can be extended below the level of the wheel for landing on snow or ice. The ski can be retracted above the bottom of the wheel for operations from conventional runways. A hydraulic system is commonly used for the retraction-system operation.
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9-9. INSPECTION AND REPAIR OF FLOATS AND SKIS. Inspection of floats and skis involves examination for damage due to corrosion, collision with other objects, hard landings, and other conditions that may lead to failure. Tubular structures for such gear may be repaired as described in the section covering welded repairs of tubular structures.
a. Floats. To maintain the float in an airworthy condition, periodic and frequent inspections should be made because of the rapidity of corrosion on metal parts, particularly when the aircraft is operated in salt water. Examine metal floats and all metal parts on wooden or fiberglass floats for corrosion and take corrective action in accordance with the procedures described in Chapter 6, Corrosion, Inspection, and Protection. Chapter 4, Metal Structure, Welding, and Brazing, outlines methods for repairing damage to metal floats of aluminum and aluminum alloy structures. In the case of wooden floats, make repairs in accordance with general procedures outlined in Chapter 1, Wood Structure. Repair fiberglass floats in accordance with the manufacturer’s instructions.
NOTE: Blind rivets should not be used on floats or amphibian hulls below the water line.
(1) If small blisters are noticed on the paint, either inside or outside the float, the paint should be removed and the area examined. If corrosion is found, the area should be cleaned thoroughly, and a coat of corrosion inhibiting material applied. If the corrosion penetrates the metal to an appreciable depth, replace the metal. Special attention should be given to brace wire fittings and water rudder control systems.
(2) If the hull or floats have retractable landing gear, a retraction check should be performed along with the other recommendations mentioned for retractable landing-gear systems. Sheet metal floats should be repaired using approved practices; however, the seams between sections of sheet metal should be waterproofed with suitable fabric and sealing compound. A float that has undergone hull repairs should be tested by filling it with water and allowing it to stand for at least 24 hours to see if any leaks develop.
b. Skis and Ski Installation. Skis should be inspected for general condition of the skis, cables, bungees, and fuselage attachments. If retractable skis are used, checks in accordance with the general practices for retractable gear should be followed. Ski manufacturers usually furnish acceptable repair procedures. It is advisable to examine ski installations frequently to keep them maintained in airworthy condition. If shock cord is used to keep the ski runner in proper trim, periodically examine to ensure that the cord has enough elasticity to keep the runner in its required attitude and the cord is not becoming loose or badly frayed. Replace old or weak shock cords. When other means of restraint are provided, examine for excessive wear and binding, and replace or repair as required. Examine the points of cable attachment, both on the ski and the aircraft structure, for bent lugs due to excessive loads that have been imposed while taxiing over rugged terrain or by trying to break loose frozen skis. If skis that permit attachment to the wheels and tires are used, maintain proper tire pressure as underinflated tires may push off the wheels if appreciable side loads are developed in landing or taxiing.
c. Repair of Ski Runners. Repair limits are found in the applicable manufacturer’s manual. Fractured wooden ski runners usually require replacement. If a split at the rear end of
Par 9-9 Page 9-9 the runner does not exceed 10 percent of the ski length, it may be repaired by attaching one or more wooden crosspieces across the top of the runner using glue and bolts. Bent or torn metal runners may be straightened if minor bending has taken place and minor tears may be repaired in accordance with procedures recommended in Chapter 4.
d. Ski Pedestals.
(1) Tubular Pedestals. Damaged pedestals made of steel tubing may be repaired by using tube splices as shown in the chapter on welding.
(2) Cast Pedestals. Consult an FAA representative on the repair of cast pedestals.
9-10. TYPES OF LANDING-GEAR PROBLEMS. During inspection and before removing any accumulated dirt, closely observe the area being inspected while the wingtips are gently rocked up and down. Excessive motion between normally close-fitting landing-gear components may indicate wear, cracks, or improper adjustment. If a crack exists, it will generally be indicated by dirt or metallic particles which tend to outline the fault. Seepage of rust inhibiting oils, used to coat internal surfaces of steel tubes, also assists in the early detection of cracks. In addition, a sooty, oily residue around bolts, rivets, and pins is a good indication of looseness or wear.
a. Thoroughly clean and reinspect the landing gear to determine the extent of any damage or wear. Some components may require removal and complete disassembly for detailed inspection. Others may require a specific check using an inspection process such as dye penetrant, magnetic particle, radiographic, ultrasonic, or eddy current. The frequency, degree of thoroughness, and selection of inspection methods are dependent upon the age, use, and general condition of the landing gear.
b. Inspect the aircraft or landing-gear structure surrounding any visible damage to ensure that no secondary damage remains undetected. Forces can be transmitted along the affected member to remote areas where subsequent normal loads can cause failure at a later date.
c. Prime locations for cracks on any landing gear are bolts, bolt holes, pins, rivets, and welds. The following are typical locations where cracks may develop.
d. Most susceptible areas for bolts are at the radius between the head and the shank, and in the location where the threads join the shank, as shown in Figure 9-2.
FIGURE 9-2. TYPICAL BOLT CRACKS
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e. Cracks primarily occur at the edge of bolt holes on the surface and down inside the bore. (See Figures 9-3 and 9-4.)
FIGURE 9-3. TYPICAL CRACKS NEAR BOLT HOLES FI GURE 9-4. TYPICAL BOLT HOLE CRACKS
f. The usual types of failure in riveted joints or seams are deformation of the rivet heads and skin cracks originating at the rivets’ holes.
g. Cracks and subsequent failures of rod ends usually begin at the thread end near the bearing and adjacent to or under the jam nut. (See Figure 9-5.)