Essentials of Tire Wear and Care, Part 3

Some aspects of the operational use of business jet tires can lessen their overall life. Daily pressure checks are difficult to perform when an aircraft is not at its home base with access to the operator’s maintenance technicians. Quick turnarounds don’t allow tires to cool after a landing and long taxi to the ramp. Foreign objects on ramps or crumbling ramp surfaces create the potential to damage a tire.

The smaller tires on business aircraft experience greater stresses than the larger tires on airliners. Smaller tires have to rotate much more to cover the same distance, and each rotation causes a deflection in the tire’s treads that contributes to heat buildup. Additionally, since centrifugal force on a tire is inversely proportional to the radius, tires half the size will have twice the stress of a larger tire.

An operator with a large fleet of aircraft reviewed its tire duration statistics after observing that its tires were being replaced prior to the expected number of landings. The expected number of landings for the main gear was 195; however, the carrier needed to replace these tires after just 166 landings. The nose-gear expectation was 225 landings, but these rendered only 179 (roughly 80% of the predicted life).

What were the causes for the earlier-than-expected replacement of these tires? Fifty-eight percent of them were removed for excessive wear. High-speed braking, sharp and fast turns, long rollouts and pivoting increase ground contact pressure and tread wear. Flight crews can minimize the possibility of tire failures due to overheating by using low taxi speeds and minimizing taxi braking whenever possible.

According to the FAA’s Advisory Circular “Aircraft Tire Maintenance and Operational Practices” (AC 20-97B, dated April 18, 2005), tight turns place high lateral loads on tires, which can cause external damage to the tread or sidewall, internal damage to the casing, or bead unseating with consequent pressure loss. Therefore, pilots should make the largest feasible turn radius when possible. For nose-gear tires, the aircraft should begin rolling forward before the turn is initiated.

• Sixteen percent of the earlier-than-expected tire replacements were due to lower-than-allowed tire pressure. Could these have been detected when the pressure was still within limits to allow reinflation? It’s likely. Unfortunately, in fractional, corporate or non-scheduled aviation an aircraft can remain away from its maintenance base for days if not weeks. Logistically it is difficult for aircraft under these scheduling practices to have tire pressures checked at sufficient intervals.
• Fifteen percent of the tires were removed from service for cuts. This important finding deserves additional explanation. The sample used in this study was based largely on straight-wing business jets that have the capability to operate into airports with nominal runway lengths. The condition of “lesser maintained” runways, taxiways and ramp surfaces can sometimes be marginal with hazards created by crumbling asphalt or FOD, all of which can cause significant damage to a tire.

Tires can be easily damaged as they roll over hard objects that protrude from a paved surface. Tire damage can range from a superficial mark to a penetration of the casing. Total penetration will result in loss of inflation and over-deflection of the tire.

AC 20-97B recommends an inspection of tires after each duty cycle. At a minimum, they should be inspected daily. The advisory circular also recommends a daily walkaround inspection and pickup as part of a FOD-control program, as well as mechanical sweeping every three days.

• Eight percent of the tires were removed due to vibrations. Again, this is worthy of an additional comment. The vast majority of this sample utilized aircraft that are known to develop a nosewheel shimmy. There are numerous causes of gear vibration, including gear alignment, worn or loose gear components, out-of-balance tires from flat spotting, improperly assembled tires, improperly installed tubes, and improperly torqued axle or wheel nuts. In addition to severe vibration, an unbalanced assembly will cause irregular and localized tread wear that can reduce the overall performance life of the tire. A properly balanced tire/wheel assembly improves the tire’s overall wear.
• Three percent of the tires were removed from service due to a flat spot. Flat spots can be caused by excessive skidding from overzealous braking. Flat spots can also be created when a tire hydroplanes on water, resulting in scalding steam that literally melts the outer layer of the tire. The removal criteria for both of these types of flat spots are the same: Remove the tire only if fabric is exposed or unacceptable vibration results.

Flat spots can also result from parking the aircraft with a load on the tires. The severity depends on several factors including temperature, inflation, aircraft weight and length of time under these conditions. Flat spotting is more severe and more difficult to work out during cold temperatures. Nylon sets usually disappear by the end of the taxi roll. They can be prevented by occasionally moving a parked aircraft or by jacking up an aircraft expected to be parked over 30 days to remove weight from the tires.

Failure At a Remote Location
One of business aviation’s greatest advantages—the ability to fly into airports closer to a business or recreational interest—also has additional disadvantages in terms of maintenance. Flying a client who owns a ranch in Montana or Wyoming and hitting FOD on the runway can leave the aircraft grounded at a remote runway without sufficient facilities and equipment to change the tire.

A similar difficulty occurs in fractional flying, where an aircraft can spend many days flying multiple legs into airports that are far from the maintenance facilities. Thus, a tire that has barely sufficient tread depth on day two of a busy tour can have insufficient tread depth by day four. When these events occur in a remote location, the availability of the aircraft will be restricted to “Aircraft On Ground,” probably for several days until the proper replacement components and equipment (such as the right jacks) can arrive on scene.

For return-to-base operation only, do not exceed more than five normal landings when the protector ply (radial) or reinforcing ply (bias) is exposed for more than one-eighth of the tire’s circumference at a given location.

Wide Range of Persons Responsible
The FAA’s Safety Alert for Operators (SAFO) titled “The Importance of Properly Inflated Aircraft Tires” (SAFO No. 11001, dated Jan. 6, 2011) stresses the role of many in our industry to ensure that tires are properly inflated using appropriately calibrated tire pressure gauges and servicing safety precautions. This includes certificate holders, fractional ownership program managers, training center managers and maintenance providers. When aircraft tires are improperly serviced, they are severely compromised, and catastrophic consequences can occur.

Essentials of Tire Wear and Care, Part 1: https://aviationweek.com/business-aviation/maintenance-training/essenti…

Essentials of Tire Wear and Care, Part 2: https://aviationweek.com/business-aviation/safety-ops-regulation/essent…