How Does Tire Pressure Maintenance Impact Aircraft Safety?
With good reason, aircraft safety concerns are paramount in maintenance operations. No one wants to see loss of life, injuries, or serious property damage as a result of poor aircraft maintenance. Aircraft maintenance manuals have very detailed, step-by-step procedures, tool, and parts lists. Certified inspectors must sign off on work cards, validating that the mechanic completed the job and completed it correctly. These procedures and checks and balances are all designed to insure the aircraft remains safe to operate. Making sure that the aircraft tires are properly inflated is a crucial part of preflight and routine service checks which can go a long way to maximizing tire utilization and minimizing risks to safe operations. At Michelin, we are committed to aircraft safety and we are proud to share our expertise with pilots, A&P mechanics, and the FAA.
Aircraft tires are not the same as passenger car or truck tires. As a result, their care and service requirements differ. Figure 1 below compares key parameters between a passenger car tire and two aircraft tires of about the same dimensions:
Figure 1: Comparison of Car to Aircraft Tires
Car Tire Aircraft Tire Aircraft Tire
(Typical) (737 NLG) (F-16 Blk 40 MLG)
Size P205/75 R15 27x7.75 R15 27.75x8.75R14.5
Outside Diameter 27.2 inch 27 inch 27.75 inch
Selection Width 8.1 inch 7.75 inch 8.75 inch
Max Inflation 35 psi 200 psi 320 psi
Max Load 1,598 lbs. 9,650 lbs. 21,500 lbs.
Speed Rating 106 mph 225 mph 259 mph
Operation Continuous Intermittent Intermittent
Temperature Equilibrium No Equilibrium No Equilibrium
Note that although the tires are dimensionally similar, they differ significantly in rated inflation pressure, load, and speed. Additionally, the passenger car and truck tires are able to operate continuously because they reach thermal equilibrium while aircraft tires do not. An aircraft tire which taxis continuously at rated load and 40 mph will continue to heat up until the tire fails. The failure to reach thermal equilibrium is a consequence of the high deflections (the difference between the unloaded and loaded tire section height) at which aircraft tires operate. High inflation pressure and high deflection rates – 2.5 times greater than a car tire – allow relatively small tires to support the high loads of an aircraft.
Because aircraft tires operate at such high extremes of pressure, load, and speed, their care and service is critically important. The most important action an operator can take to prevent tire-related events is to maintain proper tire inflation pressure. Failure to keep aircraft tires properly inflated can lead to very serious consequences. The most serious of these is the structural failure of the tire. If the tire operates underinflated or overdeflected/overloaded, the nylon cords which form the structure of the tire go in and out of compression as the tire rotates. This weakens the cords – much like a paperclip which is bent back and forth – until eventually the cords break. If enough cords break, the entire structure will eventually fail. Figure 2 shows what a compressive break in an interior ply looks like. This particular break damaged the inner liner, allowing air to migrate through the tire to the atmosphere.
Another serious consequence of under-inflation is the thrown tread. Overdeflection increases shear between components in the tire as it rolls in and out of contact and deforms. This results in a more rapid build‐up of heat within the tire than would be evident if the tire were properly inflated. At some point the excessive heat will cause the rubber to revert, or reverse cure. This reverted rubber is like grease with no strength or adhesion to contain the structure. This can allow the tire to decompose, throwing the entire tread or pieces of it. In addition to destroying the tire, thrown treads often result in expensive aircraft damage. Figure 3 is an example of reversion.
How can an operator know what the proper pressure is for their tires? Aircraft tires are designed and tested to “rated” conditions. These rated conditions are specified by industry standard’s bodies like The Tire and Rim Association (T&RA) and The European Tyre and Rim Technical Organisation (ETRTO). However, most aircraft are not operated at the rated limits. This means that an operator must consult the aircraft’s Aircraft Maintenance Manual (AMM) or Pilot Operating Handbook (POH) to find out the recommended operating pressure range.
In order to ensure that an aircraft’s tires are properly inflated, Michelin recommends checking the inflation pressure, with a calibrated gauge, before the first flight of the day or before each flight if not flown daily.
Why is it important to check the pressure every day? Because of the high pressures and extreme temperatures at which aircraft tires operate, they do not hold air perfectly. In fact, an aircraft tire can lose up to 5 percent of its pressure in 24 hours and still be perfectly serviceable.
Why check the tires before a flight rather than after? It’s important to check tire pressures when they are cold if at all possible. Michelin recommends checking a tire’s pressure no sooner than three hours after it has last rolled. Tires that have rolled under load recently heat up, making it very difficult to determine what the proper pressure should be.
Why is a calibrated gauge important? Pilots and mechanics may think that they can identify under-inflated tires by looking at them, hitting them with a bat, or looking at the wing height. In fact, it is impossible to look at tires and tell if they’re underinflated because, on a two-wheel gear, the properly inflated tire assumes more of the load than the under-inflated tire and both tires deflect the same amount. In Figure 4, the tires on the left are both properly inflated at 203 psi; one of the tires on the right is at 203 psi and one of them is at 121 psi. Can you tell which one is underinflated?
Because the consequences of having improperly inflated tires can be so severe, the acceptable limits of operation are narrow. The chart in Figure 5 shows Michelin’s recommended actions when checking air pressures. (Although most AMM’s and POH’s have the same recommendations, if there is a difference between the AMM and POH and these recommendations, follow the AMM and POH.)
The target pressure range is between 100 and 105 percent of the AMM-defined operating pressure. Michelin recommends always servicing the tires to the top of that range – 105 percent. If the pressure is found to be between 95 and 100 percent of operating pressure, service the tire to 105 percent; this is considered normal pressure loss. If it is between 90 and 95 percent, the pressure loss is no longer normal. Service the tire to 105 percent, make a log book entry, and recheck in 24 hours. If the pressure is again found to be in the 90 to 95 percent range, remove the tire and troubleshoot the reason for the pressure loss. If the tire has operated at less than 90 percent of the targeted operating pressure, it is no longer serviceable and must be removed. Tires that have operated under these conditions have been overdeflected to the point where the structure of the tire may be compromised. Finally, if a tire has operated at less than 80 percent of the operating pressure, it must be removed from service and so must its mate (on dual-wheel gears). In this case, the mate is most likely damaged as well.
Finally, when servicing tires, it is important to think about ambient temperature change. For every 5 F (3 C) change in temperature, there is a corresponding 1 percent change in tire pressure. If, for example, a tire is serviced in Miami where it is 85 F and the plane flies to Minneapolis where it is 25 F, a tire which was serviced to 100% of operating pressure, will be at 88 percent in Minneapolis (after cooling for three hours). If a mechanic checks the pressure in Minneapolis before the plane returns to Miami, the tire will be unserviceable and need to be changed. This is another reason Michelin recommends servicing the tires to the top of the acceptable range (105 percent of operating pressure). If this same tire had been serviced to 105 percent in Miami, it would have been at 93 percent in Minneapolis and still serviceable.
Improper tire pressure maintenance can have serious consequences for aircraft safety. One relatively easy way to help avoid those consequences is to check tire pressures regularly and service them as necessary. Following these three actions is a good way to make sure you get the most value out of your tires and contribute to the safe operation of your aircraft:
1. Perform daily pressure checks with a calibrated gauge.
2. Target the highest pressure recommended in the AMM or POH (105 percent of operating pressure).
3. Compensate for ambient temperature changes.
Michelin has partnered with the Federal Aviation Administration (FAASTeam) to offer pilots and mechanics an online aircraft tire maintenance course which reinforces the concepts discussed in this article. This course, The Impact of Tire Maintenance on Aircraft Safety is available free at www.FAASafety.com. In addition to this course, Michelin has several other ways to get valuable information on aircraft tire care and service: Download Michelin’s Care and Service Manual from www.airmichelin.com and install Michelin’s Aircraft Tire App from iTunes or Google Play (search “Michelin Aircraft Tires”).
Lee Bartholomew joined Michelin in 1987 as an industrial engineer and in 1995 he joined its aircraft tire division. He has held a number of engineering and management positions in the aircraft tire division including green tire design and industrialization engineer, plant controller, production manager, and bias tire developer. In 2013, Lee joined MATC’s marketing and sales group as a customer support engineer. In this role, he provides technical customer support to Michelin’s regional and general aviation customers in North America. This technical support ranges from capturing usage, auditing maintenance practices, conducting tire care and service training, and problem solving.
