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FAA Glossaries

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Tire Pressure

One of the things that is often overlooked on a pre-flight is tire pressure. Sure, you look at the tires and the look right, but because of the thick sidewalls, even if they look right they may not have the correct pressure. I use use a good dial-type pressure gauge and I have heard that digital gauges can be very accurate as well. The cheap stick-type gauges are handy, but many aren’t accurate. Check out the online reviews before you buy one. Likewise, the pressure indicator on most inexpensive inflators is not very accurate. The tire inflator that I keep in my car shows 10 psi low.

The inflation pressures shown are for unloaded tires. When tires are inflated under load, the applicable pressures should be increased four percent.

According to the Michelin Care and Service Manual, Michelin aircraft tires or tubes have no age limit and may be placed in service, regardless of their age, provided all inspection criteria for service/storage/mounting and individual customer-imposed restrictions are met.… Temperatures should remain between 0°C/32°F and 40°C/104°F. So keeping a spare inner tube in the plane when flying is probably a good idea—but leaving one in a plane that lives on the ramp is probably not the best idea.

Nitrogen

Many regulatory agencies require the use of nitrogen when inflating tires for aircraft above a specified Maximum Take-Off Weight (MTOW ). Michelin recommends the use of a dry, commercial grade nitrogen of at least 97% purity when inflating all aircraft tires. Nitrogen provides a stable, inert inflation gas while eliminating the introduction of moisture into the tire cavity. For light aircraft tires, the amount of moisture introduced is negligible. The temperatures that airline tires reach are substantial so the effects of oxygen on tires is accelerated. Small aircraft tires never get very hot so the oxidation effect is negligible.

Effect of Cold Weather

From Michelin, A drop of 3°C/5°F will reduce inflation pressure 1%. When I last checked the tire pressures on my Cherokee the temperature was 85°. When I checked them recently, the temperature was in the 50s. Even without any leaking, the reduction of 9% from a pressure of 24 psi would be a couple of pounds. On the Cessna 210 it would be around five psi. If you live in a colder climate, the difference in pressure could be substantial.

This implies that tires should always be checked when cold. The effect is minimal on small aircraft tires, but not zero.

Effects of Underinflation

Too little pressure can be harmful to your tires and dangerous to your aircraft and those in it. Underinflated tires can creep or slip on the wheel under stress or when brakes are applied. Valve stems can be damaged or sheared off and the tire, tube, or complete wheel assembly can be damaged or destroyed. Excessive shoulder wear may also be seen.

Underinflation can allow the sidewalls of the tire to be crushed by the wheel’s rim flanges under landing impact, or upon striking the edge of the runway while maneuvering. Tires may flex over the wheel flange, with the possibility of damage to the bead and lower sidewall areas. The result can be a bruise, break or rupture of the cord body. In any case where the bead or cord body of the tire is damaged, the tire is no longer safe to use and must be replaced.

reported on an accident directly attributed to low tire pressure. On September 19, 2008, a Learjet 60 aircraft operating under Part 135 crashed during a rejected takeoff at the Columbia, South Carolina airport.… The National Transportation Safety Board (NTSB) investigation revealed that the accident aircraft’s tire pressure had not been checked in approximately three weeks.

Loaded versus Unloaded Tires

Be sure that it is clear whether operating inflation pressures are given for loaded or unloaded tire conditions. A tire’s inflation pressure when loaded will be 4% higher than when unloaded (Loaded pressure = 1.04 x unloaded pressure).

Properly Inflating Tube-Type Tires

Air is usually trapped between the tire and the tube at the time of mounting. Although initial readings indicate proper pressure, the trapped air will seep out around the valve stem hole in the wheel, and under the beads. Within a few days, as the tube expands to fill the void left by the trapped air, the tire may become severely underinflated. To compensate for this effect, check tire pressure before each flight for several days after installation, adjusting as necessary, until the tire maintains proper pressure.

Hydroplaning

According to the Pilot’s Handbook of Aeronautical Knowledge, Tire pressure is a factor in dynamic hydroplaning. Using the simple formula in Figure 11-18, a pilot can calculate the minimum speed, in knots, at which hydroplaning begins. In plain language, the minimum hydroplaning speed is determined by multiplying the square root of the main gear tire pressure in psi by nine. For example, if the main gear tire pressure is at 36 psi, the aircraft would begin hydroplaning at 54 knots.

If, like on my Cessna 210, the main gear tire pressure is 55 psi then normally hydroplaning would start at 67 knots. However, if they are severely under-inflated, say 40 psi, then hydroplaning would start at 57 knots. And you can’t tell just by looking at them that they have lost that much pressure—you need to use a good tire gauge to measure the pressure.

Leakstop tubes

Normal tubes lose a few percent of their pressure every week. According to both Michelin and Goodyear a tire/wheel assembly can lose as much as five percent (5%) of the inflation pressure in a 24-hour period and still be considered normal. Leakstop or Airstop® tubes are designed to hold pressures for much longer and seem to work.

Replacement

The wheels on the Cherokee are pitched inward a bit so they tend to wear on the inside edge before the outside. We replace tires when the wear level reaches the bottom of any groove along the middle of the tire or the reinforcing ply is exposed.

Per FAR Part 43 you can change the tires yourself.
(c) Preventive maintenance. Preventive maintenance is limited to the following work, provided it does not involve complex assembly operations:
(1) Removal, installation, and repair of landing gear tires.

You need training from your A&P before you perform maintenance, but this video can refresh your memory.

According to the Goodyear Aircraft Tire DataBook, A new tube should be used when installing in a new tire. Tubes, like tires, grow in service, taking a permanent set of about 25% larger. This makes a used tube too large to use in a new tire which would cause a wrinkle and lead to a leak.

Goodyear concurs It is recommended that tubes not be reused; they can grow as much as 25% in service. Reusing them can result in folded, pinched tubes which can fail or create an imbalance.

Here’s an interesting video showing what happens to airline tires when they get too hot.

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