Mounting Procedures
Bead lubrication in mounting both tubeless and tube-type tires is often desirable to facilitate mounting and seating of the beads against the wheel flanges. A light coat of talc or approved liquid bead lubricant can be used.
Mounting Guidelines:
- Use a clip-on chuck, an extension hose, and a safety cage for inflation
- Use a direct reading or dial type pressure gauge with 5 psi increments that is calibrated on a regular basis
- When inflating a tire/wheel assembly, regulate the supply line to a pressure no more than 50 percent higher than the tire service pressure
- Do not inflate a tire above rated pressure to seat beads
Tube-Type
- Use the correct tire and tube for the wheel assembly
- Clean inside of tire, then lubricate lightly with talc
- Inflate tube to slightly round, and insert in tire
- Align yellow stripe on tube with red balance dot on tire. Align red dot with valve if no stripe on tube
- When mounting tire and tube on wheel, be sure that wheel bolts are torqued to wheel manufacturer’s instructions before inflating
- Inflate tire in a safety cage to rated pressure
- Deflate assembly to equalize stretch
- Re-inflate to rated pressure
- After 12 hour period, re-inflate to rated inflation pressure
NOTE: Aircraft tubes are made of 100% natural rubber and will diffuse limited amounts of inflation gas. Check inflation pressure prior to each flight.
If pressure drops more than 5 percent in the next 24 hours:
- Check valve core for leakage
- If OK, disassemble tire/tube from wheels and check tube for leaks. Replace tube if necessary
Tube Inspection and Repair
Since there are three reasons for air loss in a tube-type tire (a hole in the tube, a defective valve stem or valve core), finding an air leak is usually simple. The first step is to check the valve and replace the core if it is defective. If the valve is airtight, demount the tire, remove the tube, locate the leak (by immersion in water if necessary). Repair of aircraft tubes is not recommended.
CAUTION !
Use only enough pressure to round out tube. Excessive inflation strains splices and may cause fabric separation of reinforced tubes.
Reuse of Tubes
A new tube should be used when installing a new tire. Tubes grow in service, taking a permanent set of about 25% larger than the original size. This makes a used tube too large to use in a new tire, which could cause a wrinkle and lead to tube failure.
Tubeless Tires
A new O-ring seal with the correct part number should be used at each tire change following the wheel manufacturer’s specifications.
- Check for word “Tubeless” on sidewall
- Make sure tire is clean inside. Clean the bead base with a cloth dampened with denatured alcohol. Allow bead seat area to dry
- Align red balance dot on tire with wheel valve or wheel heavy point, if indicated on wheel. If no balance dot appears on the tire, align the tire serial number and valve stem
- Be sure that wheel bolts are properly torqued per the wheel manufacturer’s instructions
- Inflate tire in a safety cage to rated pressure
- After 12 hour stretch period, reinflate to rated inflation pressure
If pressure drops more than 5 percent in the next 24 hours:
- Check with water or soap solution for loose or defective valve, valve core, valve seal, fuse plug, pressure release plug, O-ring seal, wheel base and flanges
- If no leaks are found, rerun 24 hour diffusion check. If pressure still drops more than 5%, disassemble tire/wheel assembly
- Check wheel O-ring seal for condition, proper size and type, and lubricant
- Check wheel for cracks, porosity, fuse plug or pressure release plug malfunction
Valve
Before deflating and removing tire, check the valve. Put a drop of water or soap solution on the end of the valve and watch for bubbles indicating escaping pressure. Tighten valve core if loose. Replace valve core if defective and repeat leak test to check. Check the valve stem and its mounting for leaks with a soap solution. If a leak is detected, deflate the tire/wheel assembly and replace the valve core and/or valve assembly.
Make certain that every valve has a cap to retain inflation and prevent dirt, oil, and moisture from damaging the core.
Inflation Pressure Loss in Tubeless Assemblies
Since there are many causes for inflation pressure loss with a tubeless assembly, a systematic troubleshooting approach is advisable for minimum maintenance costs. Moreover, when chronic but not excessive inflation pressure loss exists, other factors such as inaccurate gauges, air temperature fluctuations, changes in maintenance personnel, etc., may be the source. If a definite physical fault is indicated, a troubleshooting procedure similar to the one outlined below is recommended. (See wheel manufacturer’s maintenance/overhaul manual for details pertaining to specific wheels.)
Fusible Plug
The fusible plug may also be defective or improperly installed. Use a soap solution to check fusible plugs for leaks before removing tire. Leaks can usually be pinpointed to the plug itself (a poor bond between the fusible material and the plug body) or to the sealing gasket used. Be sure the gasket is one specified by the wheel manufacturer and that it is clean and free of cuts and distortion.
If excessive heat has caused a fusible plug to blow, the tire may be damaged and should be replaced. After a fuse plug in a wheel blows, the wheel should be checked for soundness and hardness in accordance with the applicable wheel maintenance/overhaul manual. If the tire has not rolled, it can be sent to a retreader for inspection and retreading.
Release Plug
The inboard wheel half may contain a pressure release plug, a safety device that prevents accidental overinflation of the tire. If the tire is overinflated, the pressure release plug will rupture and release the tire pressure. A soap solution can be used to check a release plug to determine whether or not it is defective.
Wheel Base
Gas escaping through a cracked or porous wheel base is usually visible in an immersion test. Consult the wheel manufacturer’s manual for rim maintenance and repair.
O-Ring Seal
A defective o-ring seal can usually be detected in an immersion test. Check to see that wheel bolts are properly torqued.
Beads and Flanges
Check the bead and flange areas of a tire for leaks before demounting. This can be done either by immersion or by using a soap solution.
Any of the following factors can cause gas loss:
- Cracks or scratches in wheel bead ledge or flange area
- Exceptionally dirty or corroded wheel bead seating surfaces
- Damaged or improperly seated tire bead
Tire
Before demounting, use an immersion test or soap spray to determine if the tire itself has a puncture. If a puncture is found in the tread or sidewall, the tire must be scrapped.
Casing Vents
All tubeless tires have been vented in the lower sidewall area. These vents prevent separation by relieving pressure buildup in the casing plies and under the sidewall rubber. These vent holes (marked by green dots) will not cause undue pressure loss. Covering them with water or a soap solution may show an intermittent bubbling, which is normal.
Pressure Retention Test
When no leaks can be found on the prior checks, a pressure retention test must be performed. The tire should be inflated to operating pressure for at least 12 hours before starting the test. This allows sufficient time for the casing to stretch, but can result in apparent inflation pressure loss. The tire must be reinflated after the stretch period to operating pressure. Allow the tire to stand at constant temperature for a 24-hour period and recheck pressure.
