Aircraft tires are sophisticated structures built to withstand enormous stress. For example, a Boeing 737 nose tire is nearly identical in size to a typical passenger car tire, but it is designed to operate under much heavier loads and stresses. During the high speeds of takeoff and landing, the nose tire must tolerate the centrifugal forces of 195 kt. Aircraft tires need to absorb the impact shock upon landing while also transmitting deceleration braking forces to the runway. The tires must also provide a stable, cushioned ride while resisting heat generation, abrasion and wear when rolling on sunbaked taxiways.
The tread rubber compound on aircraft tires is formulated to resist wear, abrasion, cutting, cracking and heat accumulation. When properly maintained and operated according to the recommendations from their manufacturers, aircraft tires are designed to last through the stresses involved with several hundred landings. In contrast, when operators don’t follow those recommendations, the consequences can range from early tire replacement to a potentially dangerous high-speed tire failure.
Most aircraft tires are designed with circumferential grooves molded into the tread surface. These grooves act as a visual indicator of tread wear by allowing easy depth measurement. A tire with normal wear will show even wear of the center grooves. The shoulder grooves may still be visible. Removal criteria for normal wear is based on the remaining tread rubber as determined by groove depth or exposure of textile or steel ply material. If your preflight inspection notices the amount of wear reaching the bottom of any groove along more than one-eighth of the circumference on any part of the tread, this needs to be brought to the attention of your maintenance technician. This amount of wear normally requires removal of the tire. Continued operation of a tire after the top belt plies or bias have been exposed increases the possibility of chunking of the tread.
Underinflation Is the Tire’s Biggest Enemy
During the busy wildfire season of 1996, the pilots of a U.S. Forest Service Sabreliner NA-265-80 failed to notice that the tires were underinflated while the aircraft was undergoing a progressive maintenance inspection. On the warm evening of May 1, 1996, the flight crew attempted takeoff from Runway 21 at Albuquerque International Sunport (KABQ). When the airplane had achieved approximately 120 kt. and was about 5,000 ft. down the runway, there was a loud noise followed by a severe vibration, and the airplane pulled hard to the right. The captain initiated abort takeoff procedures by applying brakes and reverse thrust. The aircraft tracked across and departed the left side of the runway. Its nose landing gear collapsed, and the aircraft came to a halt 300 ft. beyond the departure end of the runway.
FAA inspectors reported there was evidence that the left main tires blew out at the 3,800-ft. mark. At the 5,000-ft. mark, the Sabreliner went onto the right shoulder of the runway. At the 9,800-ft. mark, the airplane departed the left side of the runway. In addition to the blown left tires, the right tires were found flattened.
The NTSB determined the probable cause of this accident was the over-deflection of the left outboard main landing gear tire, probably due to underinflation and inadequate inspection, which resulted in fatigue failure of that tire and subsequent failure of the left inboard tire.
This tire-failure accident is not alone in the NTSB files involving business jets. A Flight Safety Foundation study on business jet safety found that underinflated tires were blamed in 40% of the tire-failure aborted takeoffs in business jets.
Correct tire inflation is essential in assuring that tires can withstand the centrifugal forces and heat of normal operations with an adequate margin of safety for operating conditions such as rejected takeoffs and hard landings.
Underinflation causes a cacophony of effects that will lead to premature failure. Underinflated tires lead to excessive heat generation, which is the most detrimental factor affecting tire reliability. As tires rotate, heat is generated by friction during tire deflection and by distortion of tires resuming their normal shape after deflection. Incidentally, the deflection of aircraft tires is far more than that of automobile tires. An aircraft tire typically runs at 32% deflection while automobile tires are 11%.
Underinflation or overloading increases the shear force between the outer and inner plies, contributing to internal heat creation that weakens the bonding between these important layers, possibly resulting in ply separation. This is the most detrimental factor affecting tire reliability.
Heavy loads and improper inflation increase the traction wave. Due to centrifugal force and inertia, the tread surface doesn’t stop at its normal periphery but overshoots, briefly distorting the tire from its natural shape. This sets up a traction wave in the tread surface. Traction wave is mostly affected by two factors--speed and underinflation. Excessive traction wave exposure can cause cracking and tread separation.
Heat buildup due to underinflation causes other undesirable effects. It can cause a breakdown of the rubber compound, ply separation and/or rupture of the plies. Fuse plugs cannot prevent tire failures from the rapid internal heat buildup associated with taxiing on an underinflated tire. This internal damage may not be obvious by visual inspection and might not cause immediate tire failure. However, the weakened tire is more prone to failure on a subsequent flight.
Underinflated tires may creep or slip on the wheels during landing or when brakes are applied, possibly leading to shearing of a valve or destruction of the entire assembly. High lateral loads or landing impact may cause the wheel to pinch the tire or strike the runway. Tires may flex over the wheel flange, causing greater possibility of damage to the bead and lower sidewall areas.
In Part 2, we’ll discuss how hot-and-high operations can cause additional tire stress.
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