Essentials of Tire Wear and Care, Part 2
A tragic loss of tire pressure led to a fatal accident involving a Learjet 60 on Sept. 19, 2008, during a rejected takeoff at Columbia, South Carolina. The NTSB’s investigation revealed that the tires had experienced approximately 2% pressure loss per day in the previous three weeks since the pressures had last been checked. At the time of the accident the tire pressure was approximately 140 psi, dangerously below the recommended pressure of 219 psi.
The FAA and tire manufacturers recommend checking that a tire’s pressure is within limits before the first flight of each day. A visual inspection by a pilot can determine if wear or foreign object damage (FOD) has occurred to a tire but is not sufficient to adequately determine a tire’s pressure.
The pressure check must be done with a properly calibrated gauge. A “cold” tire is defined as a tire that is the same temperature as the surrounding air. This includes a tire that has not taxied, taken off or landed for a minimum of three hours. It is important to keep a written record to track the changes in a tire’s inflation. The temperature at the time of the inflation check should also be recorded.
The pressure within a sealed tire can vary substantially depending on the temperature. For example, if the aircraft is sitting on a 100F ramp in Palm Springs, California, then flies to Aspen, Colorado, where the temperature is 50F cooler, the pressure within the tire will be 10% less. For each 5F decrease in temperature there will be a 1% decrease in inflation pressure. Conversely, an increase in temperature will cause a corresponding increase in the inflation pressure.
Airport ramps are a natural “heat island.” The amount of solar radiation absorbed by the ramp depends on various factors, such as the angle of the sun with respect to the ramp (the noontime sun directly overhead bombards the ramp with the highest ratio of sunshine), clear-vs.-cloudy days, etc. Dark surfaces such as asphalt absorb more radiation than lighter colored surfaces, which tend to reflect some of the radiant energy. It takes a lot of incoming radiation to “heat up” concrete but once it does reach a warm temperature, it tends to retain that heat for quite some time.
Heat radiating off the ramp can be considerable. How hot can ramp temperatures get? Much hotter than the temperature reported on the ATIS. On an August 2012 day at Nebraska’s Lincoln Airport (KLNK) with the ATIS reporting 108F, mechanics showed me their recently acquired infrared temperature detector. Their “temperature shot” from the cement showed a temperature of 127F. The blacktop was even worse. It showed 143F. Will the heat from the ramp’s surface affect a tire’s pressure? Definitely.
It is not uncommon for a business aircraft to travel from a resort location in Mexico and land hours later in Chicago where the temperatures can be near 0F, resulting in nearly a 90F temperature difference. The tire pressure specified by the airframe manufacturer for each aircraft configuration is required to carry the load of the aircraft. This pressure value is needed regardless of the ambient temperature. Since it is not practical to make small pressure changes, the pressure should be set for the most adverse conditions (i.e., the coldest temperature) expected for operation.
In general, if the tire’s pressure is between 95% and 100% of the service pressure, the manufacturers recommend reinflating it to the specified service pressure. If the tire’s pressure is between 90% and 95% of the service pressure, it is recommended that the tire and wheel be inspected, reinflated and recorded in the logbook. If the pressure is found to be between 80% and 90%, remove the tire and wheel assembly from the aircraft.
Tires on the same axle should be operated at the same pressure. The margin of difference between the pressure is 5%. When adjacent tires are operated at unequal pressures the tire with the higher pressure will carry a greater load. This can cause shorter life in both tires due to casing fatigue or abnormal wear.
Hot and High = Additional Tire Stress
Takeoffs attempted in thinner air caused by hotter temperatures and/or higher altitudes will require higher rotation speeds, thus placing considerable additional stress on tires. The stress on a tire rises markedly as speed increases. For example, the centrifugal force on a 30-in. tire at 100 mph is 500 Gs, but when traveling at 200 mph the force increases to 2,000 Gs.
A combination of heat and high speed increases the possibility of tread loss. Conditions that contribute to the stress on the tires include takeoffs at close to tire-speed-limit weight, quick turnarounds, hot and long taxi distances, high density altitude, slower-than-normal rotation rate, late rotation or a tailwind. The consequences are likely to be an increased probability of high-speed tire failures and increased maintenance costs for earlier-than-predicted tire replacement.
Many of these were factors on Aug. 28, 1998, when a Dassault Falcon 20C was substantially damaged as it overran the departure end of Runway 22 following an aborted takeoff from El Paso International Airport (KELP) in Texas. After the flight crew completed the preflight checks, they taxied the aircraft for about 2 mi. from the FBO ramp to Runway 22 for departure.
The first officer, who occupied the right seat, was the pilot flying for this leg of the flight. The PIC stated that when they were cleared for takeoff, they held the brakes, ran the engines to full power, and released the brakes. At approximately 120 kt., they heard a loud “bang” noise followed by a vibration, and the PIC called for the first officer to abort. The crew stated that they thought they had enough runway to stop but reported that application of both pilot and copilot brake pedals was not effectively slowing the aircraft. The PIC stated that he moved both engine throttle levers to the “flight idle” position during the abort. He said that he moved the levers to the “idle cut off” position when he realized that the aircraft was going to overrun the departure end of the runway.
After overrunning the runway, the Falcon traveled over the 800-ft. paved stop-way, across approximately 1,000 ft. of sandy terrain, through a steel fence (airport perimeter), over embedded railroad tracks, through a concrete curb, across a four-lane highway (impacting three moving vehicles), through a second concrete curb, and through another steel fence, before finally coming to a stop.
The distance from the departure end of the runway to where the aircraft stopped was about 2,010 ft. Thankfully, both aircraft occupants were unhurt after this significant trip from the end of the runway. They evacuated the airplane unassisted through the two emergency escape windows in the cockpit.
Accident investigators found remnants of a blown tire approximately 7,200 ft. from where the aircraft had commenced its takeoff roll. The rubber debris constituted about 90% of a tire's “recap” material. The NTSB determined the probable cause of this accident was the captain's decision to abort the takeoff at an airspeed above V1, which resulted in a runway overrun. Contributing factors were the loading of an excessive amount of cargo by the shipper, which resulted in an over-gross-weight airplane, the high density altitude, the separation of tire retread on takeoff roll and the flight crew’s lack of experience in the make and model of the accident aircraft.
In the final part of this series, we’ll discuss causes of earlier-than-expected tire replacement. Essentials of Tire Wear and Care, Part 1, can be found at https://aviationweek.com/business-aviation/maintenance-training/essenti…