annular actuating piston. Hydraulic pressure forces the annular piston to move outward, compressing the rotating disks, which are keyed to the landing wheel and compressing the stationary disks, which are keyed to the bearing carrier. The resulting friction causes a braking action on the wheel and tire assembly.
When the hydraulic pressure is relieved, the retracting springs force the actuating piston to retract into the housing chamber in the bearing carrier.
The hydraulic fluid in the chamber is forced out by the returning of the annular actuating piston, and is bled through the automatic adjuster to the return line. The automatic adjuster traps a predetermined amount of fluid in the brake, an amount just sufficient to give correct clearances between the rotating disks and stationary disks.
Maintenance of the multiple-disk brake may include bleeding, checking disks for wear, replacing disks, and performing operational checks.
Bleeder valves are provided, making it possible to bleed the brake in any position. Bleeding should be accomplished according to the instructions for the specific aircraft.
The disks are checked for wear using a gage equipped with a movable slide, a stop pin, and an anvil.
Segmented Rotor Brakes Segmented rotor brakes are heavy-duty brakes, especially adapted for use with high-pressure hydraulic systems. These brakes may be used with either power brake control valves or power boost master cylinders. Braking is accomplished by means of several sets of stationary, high-friction type brake linings making contact with rotating (rotor) segments. A cutaway view of the brake is shown in figure 9-36.
The segmented rotor brake is very similar to the multiple-disk type described previously. The brake Adjuster pin nut Tire Wheel Piston return spring Adjuster pin sleeve Piston Adjuster clamp
• Rotor Carrier assembly assembly
- Piston cup Brake assembly pin Piston Pressure plate Brake Stator Drive sleeve bolts Adjuster pin lining assembly disk
- Hydraulic line
- Rotor assembly sleeve Rotor assembly Stator plate - Brake lining disk (typical) - FICURE 9-36. Segmented rotor brake assembly unita.
assembly consists of a carrier, two pistons and pis ton cup seals, a pressure plate, an auxiliary stator plate, roto segments, stator plates, a compensating shim, automatic adjusters, and a backing plate.
The carrier assembly is the basic unit of thr brake. It is the part which is attached to the landing gear shock strut flange upon which the other components are assembled. Two grooves, or cylinders, are machined in the carrier to receive the piston cups and pistons. Hydraulic fluid is admitted to these cylinders through a line which is attached to a threaded boss on the outside of the carrier.
The automatic adjusters are threaded into equally spaced holes (figure 9-36) located in the face of the carrier. The adjusters compensate for lining wear by maintaining a fixed running clearance with the brake in the "off" position. Each automatic adjuster is composed of an adjuster pin, adjuster clamp, return spring, sleeve, nut, and clamp hold- ‹own assembly.
The pressure plate is a flat, circular, nonrotating plate, notched on the inside diameter to fit over the atator drive sleeves.
Following the pressure plate is the auxiliary stator plate. This is also a nonrotating plate, notched on the inside diameter. Brake lining is riveted to one side of the auxiliary stator plate.
The next unit in the assembly is the first of several rotor segments. Each rotor plate is notched on the outside circumference. This enables it to be keyed to the landing gear wheel and rotate with it.
This particular model of the segmented rotor brake has four sets of these rotor segments.
Mounted between each of the rotor segments is a stator plate (figure 9-37). The stator plates are non-rotating plates and have brake linings riveted to both sides. The linings are in the form of multiple blocks, separated to aid in the dissipation of heat.
Following the last rotor segment is the compensating shim. The compensating shim is provided so that all the brake lining available for wear can be used. Without the shim, only about one-half of the lining could be used due to the limited travel of the pistons. After approximately one-half of the brake lining has been used, the shim is removed. The adjuster clamp is then re-positioned on the automatic adjuster pin, restoring piston travel so that the remaining lining can be used.
The backing plate (figure 9-38) is the final unit in the assembly and is a non-rotating plate with brake linings riveted to one side. The backing plate receives the ultimate hydraulic force resulting from brake application.
Backing plate and lining assy, Drive sleeve bolt nuts Washer Clamp, Friction block Shoulder FIcURE 9-38. Backing plate installed.
Sleeve hold-down Clearance nut plate Adjusting rod Hydraulic pressure released from the brake con- Compensating trol unit enters the brake cylinders and acts on the shim piston cups and pistons forcing them outward from Torque the carrier. The pistons apply force against the plate pressure plate, which in turn pushes against the Backing auxiliary stator. The auxiliary stator contacts the Piston plate first rotor segment, which in turn engages the first cup Auxiliary stator Rotor assembly stator plate. Lateral movement continues until all Annular ring piston Pressure plate - Lining - Stator plate the braking surfaces are in contact. The auxiliary stator plate, the stator plates, and the backing plate are prevented from rotating by the stator drive FIGURE 9-37. Cross section of a segmented rotor brake. sleeves. Thus, the non-rotating linings are all forced 370
in contact with the rotors, creating enough friction to stop the wheel to which the rotors are keyed.
The function of the automatic adjuster is dependent upon the correct friction between the adjuster pin and the adjuster clamp. Adjustment of brake running clearance is governed by the distance obtained between the adjuster washer and the end of the adjuster nut when the brake is assembled.
During brake application; the pressure plate moves toward the rotors. The washer moves with the pressure plate, causing the spring to compress.
As piston travel increases and as the pressure plate moves farther, the lining then comes in contact with the rotor segments. As the linings wear, the pressure plate continues to move and eventually comes into direct contact with the adjuster sleeve through the adjuster washer. Thus, no further force is applied to the spring. Additional travel of the pressure plate caused by lining wear will force the adjuster pin to slide through the adjuster clamp.
When hydraulic pressure in the brake is released, the return spring forces the pressure plate to return until it bottoms on the shoulder of the adjuster pin.
As this cycle is repeated during brake application and release, the adjuster pin will advance through the adjuster clamp due to lining wear, but the running clearance will remain constant.
Brake block Frame Torque bar Torque bar bolt Brake frame bolt Adjuster Brake spider FIcURE 9-39. Assembled expander tube brake.
Expander Tube Brakes The expander tube brake (figure 9-39) is a lowpressure brake with 360° of braking surface. It is light in weight, has few moving parts, and may be used on large aircraft as well as on small aircraft.
An exploded view of the expander tube brake is shown in figure 9-40. The main parts of the brake are the frame, expander tube, brake blocks, return springs, and clearance adjuster.
The brake frame is the basic unit around which the expander tube brake is built. The main part of the frame is a casting which is bolted to the torque flange of the landing gear shock strut. Detachable metal sides form a groove around the outer circumference into which moving parts of the brake are fitted.
The expander tube is made of neoprene reinforced with fabric, and has a metal nozzle through which fluid enters and leaves the tube.
The brake blocks are made of special brake lining material, the actual braking surface being strengthened by a backing plate of metal. The brake blocks are held in place around the frame and are prevented from circumferential movement by the torque bars.
The brake return springs are semi-elliptical or half-moon in shape. One is fitted between each separation in the brake blocks. The ends of the return springs push outward against the torque bars, while the bowed center section pushes inward, retracting the brake blocks when the brakes are released.
When hydraulic fluid under pressure enters the expander tube the tube expands. Since the frame prevents the tube from expanding inward and to the sides, all movement is outward. This forces the brake blocks against the brake drum, creating friction. The tube shields prevent the expander tube from extruding out between the blocks, and the torque bars prevent the blocks from rotating with the drum. Friction created by the brake is directly proportional to brake line pressure.
The clearance adjuster (figure 9-40) consists of a spring-loaded piston acting behind a neoprene diaphragm. It closes off the fluid passage in the inlet manifold when the spring tension is greater than the fluid pressure in the passage. Tension on the spring may be increased or decreased by turning the adjustment screw. Some of the older models of the expander tube brake are not equipped with clearance adjusters.
For brakes equipped with adjusters, clearance between the brake blocks and drum is usually set to a 371
Expander tube - Torque bar Brake Return spring block Retur spring shield Torque bar bolt Tube shield - Adjuster bolt Frame Nozzle Torque flange Frame Adjuster Brake frame bolt FIcURE 9-40. Exploded view of expander tube brakes.
minimum of 0.002 to 0.015 in., the exact setting depending on the particular aircraft concerned. All brakes on the same aircraft should be set to the same clearance.
To decrease clearance, turn the adjuster knob clockwise; to increase clearance, turn the adjuster knob counterclockwise. It should be kept in mind, however, that turning the adjuster knob alone does not change the clearance. The brakes must be applied and released after each setting of the adjuster knob to change the pressure in the brake and thereby change the brake clearance.
INSPECTION AND MAINTENANCE OF BRAKE
SYSTEMS Proper functioning of the brake system is of the utmost importance. Hence, conduct inspections at frequent intervals, and perform needed maintenance promptly and carefully.
When checking for leaks, make sure the system is under operating pressure. However, tighten any loose fittings with the pressure off. Check all flexible hoses carefully for swelling, cracking, or sott spots, and replace if evidence of deterioration is noted.
Maintain the proper fluid level at all times to prevent brake failure or the introduction of air into the system. Air in the system is indicated by a spongy action of the brake pedals. If air is present in the system, remove it by bleeding the system.
There are two general methods of bleeding brake systems-bleeding from the top downward (gravity method) and bleeding from the bottom upward (pressure method). The method used generally depends on the type and design of the brake system to be bled. In some instances it may depend on the bleeding equipment available. A general description of each method follows.
Gravity Method of Bleeding Brakes In the gravity method, the air is expelled from the brake system through one of the bleeder valves provided on the brake assembly (figure 9-41).
A bleeder hose is attached to the bleeder valve, and the free end of the hose is placed in a recepta- External reservoir Whoel brake Bleed hose Air bubbles Brake shor Fluid cu FICURE 9-41. Gravity method of bleeding brakes.
cle containing enough hydraulic fluid to cover the end of the hose. The air-laden fluid is then forced from the system by operating the brake. If the brake system is a part of the main hydraulic system, a portable hydraulic test stand may be used to supply the pressure. If the system is an independent master cylinder system, the master cylinder will supply the necessary pressure. In either case, each time the brake pedal is released the bleeder valve must either be closed or the bleeder hose pinched off;
otherwise, more air will be drawn back into the system. Bleeding should continue until no more air bubbles come through the bleeder hose into the container.
Pressure Method of Bleeding Brakes In the pressure method, the air is expelled through the brake system reservoir or other spe.
cially provided location. Some aircraft have a bleeder valve located in the upper brake line. In using this method of bleeding, pressure is applied using a bleed tank (figure 942). A bleed tank is a portable tank containing hydraulic fuid under pressure. The bleeder tank is equipped with an air valve, air gage, and a connector hose. The connector hose attaches to the bleeder valve on the brake assembly and is provided with a shutoff valve.
Wheel brake ing the entire operation. A low fluid supply will allow air to be drawn into the system.
(3) Bleeding should continue until no more air bubbles are expelled from the system, and a firm brake pedal is obtained.
(4) After the bleeding operation is completed, check the reservoir fluid level. With brake pressure on, check the entire system for leaks.
Brakes which have become overheated from excessive braking are dangerous and should be treated accordingly. Excessive brake heating weakens the tire and wheel structure and increases tire pressure.
AIRCRAFT LANDING WHEELS Aircraft wheels provide the mounting for tires which absorb shock on landing, support the aircraft on the ground and assist in ground control during taxi, takeoff and landing. Wheels are usually made from either aluminum or magnesium. Either of these materials provides a strong, light weight wheel requiring very little maintenance.
1. Split wheel-the most popular type. (Figures 9-43 and 9-44 Heavy aircraft wheel, Figures 9-45 and 9-46 Light aircraft wheel.)
2. Removable flange type Figure 9-47.
3. Drop center fixed flange type Figure 9-48.
The split wheel is used on most aircraft today.
Illustrations of wheels used on light civilian type and heavy transport type aircraft are shown to illustrate the similarities and differences.
Split Wheels hate abso trated in figure 9-43 is used on the Boeing B-727 transport aircraft.
FIGURE 9-42. Preasure method of bleeding brakes, Perform this method of bleeding strictly in accordance with the specific manufacturer's instructions for the aircraft concerned.
Although the bleeding of individual systems presents individual problems, observe the following precautions in all bleeding operations:
(1) Be certain that the bleeding equipment to be used is absolutely clean and is filled with the proper type of hydraulic fluid.
(2) Maintain an adequate supply of huid dur- Description-The numbers in parenthesis refer to figures 9-43 and 9-44.
A. The main landing gear wheel is a tubeless, split-type assembly made of forged aluminum.
B. The inner and outer wheel half assemblies are fastened together by 18 equally spaced tie bolts (11), secured with nuts (9). A tubeless tire valve assembly installed in the web of the inner wheel half (48), with the valve stem (7) protruding through a vent hole in the outer wheel half (30), is used to inflate the tubeless tire used with 373
43 11 12 34 35 36 38 48 5 14 13 30 23 24 25 22 21 FIcURE 9-43. Split wheel-heavy aircraft.
Index No. Description WHEEL, LANDING GEAR, 49 x 17, TUBELESS, MAIN CO tO M CONE BEARING RING, RETAINING IDENTIFICATION PLATE INSTRUCTION PLATE
SEAL PLATE, IDENTIFICATION
CONE, BEARING INSERT, HELI-COIL
VALVE ASSY, TUBELESS TIRE CUP, BEARING
CAP, VALVE WHEEL HALF, OUTER
VALVE, INSIDE WHEEL HALF, ASSY, INNER
STEM, VALVE NUT GROMMET, RUBBER (TIRE AND RIM ASSOC.) WEICHT, WHEEL BALANCE, 1/4 oz.
NUT 33 BOLT, MACHINE WASHER BOLT 34 35 NUT WASHER, FLAT WASHER 36 IDENTIFICATION PLATE
PACKING, PREFORMED INSTRUCTION PLATE PACKING, PREFORMED 38 BOLT, MACHINE PLUG, MACHINE THD, THERMAL PRESSURE 39 NUT RELIEF, ASSY OF 40 WASHER, FLAT 16 PACKING, PREFORMED 41 BOLT, MACHINE WHEEL HALF ASSY, OUTER 42 BRACKET NUT 43 SHIELD, HEAT WEIGHT, WHEEL BALANCE, 1/4 oz. 44 SCREW BOLT, MACHINE 45 INSERT
WASHER INSERT, HELI-COI
NUT CUP, BEARINC WASHER, FLAT 48 WHEEL HALF, INNER
BOLT, MACHINE
WASHER, FLAT FICURE 9-44. Parts list-split wheel, heavy aircraft.
this wheel. Leakage of air from the tubeless tire through the wheel half mating surfaces is prevented by a rubber packing
(14) mounted on the register surface of the inner wheel half. Another packing
(13) mounted on the inner register surface of the inner wheel half seals the hub area of the wheel against dirt and moisture.
C. A retaining ring (2) installed in the hub of the inner wheel half holds the seal (3) 374 and bearing cone (4) in place when wheel is removed from axle. The seal retains the bearing lubricant and keeps out dirt and moisture. Tapered roller bearings (1, 4, 29, 47) in the wheel half hubs support the wheel on the axle.
D. Inserts (45) installed over bosses in the inner wheel half (48) engage the drive slots in the brake disks, rotating the disks as the wheel turns. A heat shield (43), mounted underneath and between the in-
22 11 10 10 9 FIcURE 9-45. Split wheel--light aircraft.
Index No. Description Fixed Flange Wheels WHEEL ASSEMBLY (With Valve Assy) SEAL ASSY 9-48) are special use wheels such as military, for Drop center fixed fange aircraft wheels (figure CONE, Bearing high pressure tires. Some may be found installed VALVE ASSY, Tubeless tire on older type aircraft.
CODE, Valve NUT CAP, Valve give added support to the rim at the outboard Outboard radial ribs are provided generally to SPACER bead seat. The principal difference between wheels 7 GROMMET used for streamline tires and those used for smooth & STEM, Valve contour tires is that the latter are wider between 10 NUT WASHER the flanges.
11 BOLT 12 PACKING 13 WHEEL-HALF ASSY, Outer 14 PLATE, Identification 15 PLATE, Instruction 16 CUP, Bearing Grease WHEEL-HALF ASSY, Inner retainer - 17 WIRE, Lock 18 SCREW Axle- 19 20 INSERT, Heli-coil KEY, Torque Bearing nut 21 PLATE, Identification 22 PLATE, Instruction 23 24 WHEEL-HALF, Inner CUP, Bearing Fairing, disc FIcURE 9-46. Parts list-split wheel, light aircraft.
A brake-drum liner may be held in place by means of steel bolts projecting through the casting with lock nuts on the inner side. These can be tightened easily through spokes in the wheel.
The bearing races are usually shrink-fitted into the hub of the wheel casting and provide the surfaces on which the bearings ride. The bearings are the tapered roller type. Each bearing is made up of a cone and rollers. Bearings should be cleaned and repacked with grease periodically in accordance with applicable manufacturer's instructions.
Brake drum liner FicURE 9-48. Fixed fange, drop center wheel.
Wheel Bearings The bearings of an airplane wheel are of the tapered roller type and consist of a bearing cone, rollers with a retaining cage, and a bearing cup, or outer race. Each wheel has the bearing cup, or race, pressed into place and is often supplied with a hub cap to keep dirt out of the outside bearing.
Removable flange - Removable Snap ring Lockring flange Wheel casting retainer Grease 00(3) Fairing disc Fairing - retaining screw A. Drop center. B. Flat base.
FIGURE 9-47. Removable flange wheels.
Suitable retainers are supplied inboard of the inner bearing to prevent grease from reaching the brake Felt seals are provided to prevent dirt from fouling multiple-disk brakes. Seals are also supplied on amphibian airplanes to keep out water.
Beads Made of steel wires embedded in rubber and wrapped in fabric, the beads anchor the carcass plies and provide firm mounting surfaces on the wheel.
AIRCRAFT TIRES Aircraft tires, tubeless or tube type, provide a cushion of air that helps absorb the shocks and roughness of landings and takeoffs: they support the weight of the aircraft while on the ground and provide the necessary traction for braking and stopping aircraft on landing. Thus, aircraft tires must be carefully maintained to meet the rigorous demands of their basic job... to accept a variety of static and dynamic stresses dependably--in a wide range of operating conditions.
Aircraft Tire Construction Dissect an aircraft tire and you'll find that it's one of the strongest and toughest pneumatic tires made. It must withstand high. speeds and very heavy static and dynamic loads. For example, the main gear tires of a four-engine jet transport are required to withstand landing speeds up to 250 mph, as well as static and dynamic loads as high as 22 and 33 tons respectively. Typical construction is shown in figure 9-19.
Flippers These layers of fabric and rubber insulate the carcass from the bead wires and improve the durability of the tire.
Layers of fabric and rubber that protect the carcass from damage during mounting and demounting. They insulate the carcass from brake heat and provide a good seal against movement during dynamic operations.
Bead Toe The inner bead edge closest to the tire center line.
Bead Heel The outer bead edge which fits against the wheel flange.
Innerliner Made of rubber compound for toughness and durability, the tread is patterned in accordance with aircraft operational requirements. The circumferential ribbed pattern is widely used today because it provides good traction under widely varying runway conditions.
On tubeless tires, this inner layer of less permeable rubber acts as a built-in tube, it prevents air from seeping through casing plies. For tube type tires, a thinner rubber liner is used to prevent tube chafing against the inside ply.
Tread Reinforcing Ply Tread Reinforcement Rubber compound cushion between tread and One or more layers of reinforced nylon cord casing plies, provides toughness and durability.
fabric strengthens the tread for high speed opera- It adds protection against cutting and bruising tion. Used mainly for high speed tires. throughout the life of the tread.
Breakers Not always used, these extra layers of reinforcing nylon cord fabric are placed under the tread rubber to protect casing plies and strengthen tread area. They are considered an integral part of the carcass construction.
Casing Plies/Cord Body Diagonal layers of rubber-coated nylon cord fabric (running at opposite angles to one another) provide the strength of a tire. Completely encompassing the tire body, the carcass plies are folded around the wire beads and back against the tire sidewalls (the "ply turnups").
Sidewall Sidewalls are primarily covers over the sides of the cord body to protect the cords from injury and exposure. Little strength is imparted to the cord body by the sidewalls. A special sidewall construction, the "chine tire," is a nose wheel tire designed with built-in deflector to divert runway water to the side, thus reducing water spray in the area of rear mounted jet engines.
Apex Strip The apex strip is additional rubber formed around the bead to give a conture for anchoring the ply turnups.
Tread • Breakers Tread Reinforcing ply Inner liner Sidewall Plies Chafers Cord body Bead heel Ply turnups
• Bead toes Flippers Wire beads Apex strip FIGURE 9-49. Aircraft tire construction.
Aircraft Tire Care Tires are as vital to the operation of aircraft as they are to the operation of an automobile. During ground operation tires can be considered as ground control surfaces. The same rules of safe driving and careful inspection apply on the runway as on the highway.
They include control of speed, braking, and cornering, and inspection for proper inflation, cuts, bruises, and signs of tread wear. Contrary to what most people think-including many beginning pilots- the toughest demand on aircraft tires is rapid heat buildup during lengthy ground operations, not the impact of hard landings.
Aircraft tires are designed to flex more than automotive tires-over twice as much. This flexing causes internal stress and friction as tires roll on the runway. High temperatures are generated, damaging the body of the tire.
The best safeguards against heat buildup in aircraft tires are short ground rolls, slow taxi speeds.
minimum braking. and proper tire inflation.
Excessive braking increases tread abrasion. Likewise, fast cornering accelerates tread wear. Proper inflation assures the correct amount of flexing and keeps heat buildup to a minimum, increasing tire life and preventing excessive tread wear.
Infiation pressure should always be maintained as specified in the aircraft maintenance manual or according to information available from a tire dealer.
Even though using a tire gage is the only accurate way to spot-check inflation, a quick visual inspection of the tread can reveal if air pressure has been consistently high or low. Excessive wear in the shoulder area of the tire is an indication of under inflation. Excessive wear in the center of the tire suggests over inflation.
Tires should also be carefully inspected for cuts or bruises. The best way to avoid aircraft tire cuts and bruises is to slow down when unsure of runway or taxiing surface conditions.
Since airplane tires have to grip the runway in the same way car tires grip the road, tread depth is also important. Tread grooves must be deep enough to permit water to pass under the tires, minimizing the danger of skidding or hydroplaning on wet runways. Tire treads should be inspected visually or with an approved depth gage according to manufacturers' specifications.
Another inspection goal is detection and removal of any traves of gasoline or oil on the tires.
Such mineral fluids damage rubber, reducing tire lite. Likewise, tires should be inspected for ozone or weather checking. Electricity changes oxygen in the air to ozone, which also prematurely ages rubber.
MATCHING DUAL, TIRES Matching tires on dual wheels, or dual wheels on a O. D. Range of Tires Maximum Tolerance multi-wheel gear configuration, is necessary so that each Permissable tire will have the same contact area with the ground and thereby carry an equal share of the load. tires having inflated diameters within the tolerances Up to 24" 25" to 32" 5/18" 1/4" listed below should be paired together on dual wheels. 33" to 40" 3/8" Tires should not be measured until they have been 49" to 55" 41" to 48" 7/16" 1/2" mounted and kept fully inflated for at least 12 hours, 56" to 65" • 9/16" at normal room temperatures. 86" and up 5/8" FIcURE 9-50. Matching tires on dual wheel installations.
Aircraft tires should be stored in a cool dry place away from electric motors. The manufacturers specifications should be followed at all times when performing tire maintenance.
Tires on dual wheel aircraft will have a longer operational life if matched as suggested in figure
9-50.
AIRCRAFT TIRE MAINTENANCE All aircraft tire manufacturers publish maintenance manuals and instruction manuals.
The following discussion on aircraft tires is excerpted from the B.F. Goodrich publication Care and Maintenance of Aircraft Tires, Fourth Edition, and is published with their permission.
Proper Inflation For Satisfactory Service Proper inflation is undoubtedly the most neces- • в просо sary maintenance function for safe, long service from aircraft tires.
Tire pressures should be checked with an accurate gage at least once a week or oftener, and it is recommended that they be checked before each flight. Otherwise, if a slow leak should develop, it could cause severe loss of air within two or three days, with resulting damage to the tire and tube.
Air pressures should only be checked when tires are cool. Wait at least two hours after a flight before checking pressures (three hours in hot weather).
For New Mountings A newly mounted tire and/or tube should be checked at least daily for several days, after which the regular inflation control schedule may be followed. This is necessary because air is usually trapped between the tire and tube at the time of mounting, giving a false pressure reading. As this trapped air seeps out under the beads of the tire and around the valve hole in the wheel, the tire may become severely under inflated within a day or two.
Allow For Nylon Stretch All aircraft tires are now made with nylon cord, and the initial 21-hour stretch of a newly mounted nylon tire may result in a 5 percent to 10 percent drop in air pressure. Thus, such a tire should not be placed in service until it has been left to stand at least 12 hours after being mounted and inflated to regular operating pressure. The air pressure should then be adjusted to compensate for the decrease in pressure caused by the stretching of the cord body.
Tubeless Air Diffusion Loss Maximum allowable diffusion is 5 percent for any 24-hour period. However, no accurate tests can be made until after the tire has been mounted and inflated for at least 12 hours, and air added to compensate for pressure drop due to normal nylon cord body expansion, and any changes in tire temperature. A pressure drop of over 10 percent during this initial period should be suffcient reason to not place the tire and wheel assembly into service.
For Duals: Equalize Pressures Differences of air pressure in tires mounted as duals, whether main or nose, should be cause for concern, as it ordinarily means that one tire is carrying more of the load than the other. If there is a difference of more than 5 lbs., it should be noted in the log. The log-book should then be referred to on each subsequent inflation check Impending tire or tube failure can often be detected by this method. Should a pressure difference be found, check the valve core by spreading a little water over the end of the valve. If no bubble appears, it can be assumed that the valve core is holding pressure satisfactorily.
Sources of Pressure Data Inflation of nose wheel tires should follow the recommendations of the aircraft manufacturers, because they take into consideration both the extra 379
load transferred to the nose wheel by the braking effect and by the static load. Air pressure in the nose wheel tire, based on the static load only, would result in under inflation for the load carried when the brakes are applied. Tail wheel tires, however, should be inflated in accordance with the axle static load.
When tires are inflated under load, the recommended pressure should be increased by 4 percent.
The reason is that the deflected portion of the tire causes the volume of the air chamber to be reduced, and increases the inflation pressure reading, which must then be offset in accordance with the above Effects of Under Inflation Under inflation results in harmful and potentially dangerous effects. Aircraft tires which are under inflated are much more likely to creep or slip on the wheels on landing or when brakes are applied.
Tube valves can shear off, and the complete tire, tube and wheel assembly can be destroyed under such conditions. Too-low pressures can also cause rapid or uneven wear at or near the edge of the Under inflation provides more opportunity for the sidewalls or the shoulders of the tire to be crushed landing no veron by the wheel's rim flanges on landing or upon striking the edge of a runway while maneuvering the aircraft. Tires may flex over the wheel flange.
with greater possibility of damage to the bead and lower sidewall areas. A bruise break or rupture of the tire's cord body can result.
Severe under inflation may result in cords being loosened and the tire destroyed because of extreme heat and strain produced by the excessive flexing action. This same condition could cause inner tube chafing and a resultant blowout.
Observe Load Recommendations Since the beginning of air transportation, aircraft tires have been doing their required job with increasing efficiency and safety. But there is a limit to the load that any aircraft tire can safely and efficiently carry.
Loading aircraft tires above the limit can result in these undesirable effects:
1. Undue strain is put on the cord body and beads of the tire, reducing the factor of safety and reducing service life.
2. There is greater chance of bruising upon striking an obstruction or upon landing (bruise breaks, impact breaks and flex breaks in the sidewall or shoulder).
3. Possibility of damaging wheels. Under the severe strain of an extra load, a wheel may fail before the tire does.
Note: While additional air pressure (inflation) to offset increased loads can reduce excessive tire deflection, it puts an added strain on the cord body, and increases its susceptibility to cutting, bruises and impact breaks.
Nylon Flat-Spotting Nylon aircraft tires will develop temporary fat spots under static load. The degree of this flatspotting will vary according to the drop in the internal pressure in the tire and the amount of weight being sustained by the tire. Naturally, flatspotting can be more severe during cold weather and is more difficult to work out of a tire at lower temperatures.
Under normal conditions, a flat spot will disappear by the end of the taxi run. If it doesn't, the tire can generally be reshaped by overinflating it 25 percent or 50 percent and moving the aircratt until the low spot is on the upper side. This pres-
PREVENTIVE MAINTENANCE SUMMARY
• Check tire pressures with an accurate gage at least once a week, and before each flight.
should be at ambient temperature.
• Check newly mounted tires or tubes daily for several days.
• Newly mounted tires should not be placed in service until cord body stretch has been compensated with re-inflation.
• Check for abnormal diffusion loss.
• Follow inflation recommendations carefully - guard against underinflation.
• Observe load recommendations.
• Move aircraft regularly or block up when out of service for long periods.
FIcURE 9-51. Preventive maintenance summary.