Yesterday the NTSB held a public meeting to issue the probable cause of a Learjet 60 runway overrun accident on Sept. 19, 2008. Both pilots and two of four passengers were killed when the Learjet, operated by Global Exec Aviation, overran Runway 11 during a rejected takeoff at Columbia Metropolitan Airport in South Carolina. One key issue in the accident was tire pressure. (The other main issue was captain’s execution of a rejected takeoff after V1.) According to the NTSB, the Learjet 60’s tires lose 2 percent of pressure every day. The full inflation of 219 psi drops to 185 psi after eight days and 140 psi after three weeks, the level determined in the accident airplane. During the takeoff sequence, the outboard right tire burst first at 137 knots followed by the other tires, due to excessive flexing of the tires and heat damage caused by underinflation.
At the hearing, the NTSB concluded, “All four main landing gear tires on the airplane were operating while severely underinflated during the takeoff roll, which resulted in the tire failures.” In its conclusion, the NTSB added, “The accident airplane’s insufficient tire air pressure was due to Global Exec Aviation’s inadequate maintenance.” Tire fragments damaged the squat switches, and the logic switched to air mode, which automatically stowed the thrust reversers and resulted in high forward thrust when the pilots were trying to slow the airplane.
Checking tire pressure is “technically simple,” the NTSB stated, but the FAA is inconsistent about whether pilots can do this. The FAA has issued a legal interpretation that checking tire pressure is preventive maintenance covered by Part 43 Appendix A, according to the Safety Board. Pilots are allowed to perform preventive maintenance on aircraft operated under Part 91, but not 135.
According to the NTSB, “Some operators are not sufficiently aware of the appropriate tire pressure check intervals for the airplanes in their fleets and are operating their airplanes with tires inflated below the aircraft maintenance manual replacement specifications.
“Aircraft maintenance manual (AMM) formats that refer to tire pressure checks as guidance information rather than required maintenance intervals and the lack of standardization of AMM formats with respect to the location of tire pressure check interval information do not provide sufficient emphasis on the criticality of checking and maintaining tire pressure.
“The probable cause of this accident,” according to the NTSB, “was the operator’s inadequate maintenance of the airplane’s tires, which resulted in multiple tire failures during takeoff roll due to severe underinflation, and the captain’s execution of a rejected takeoff after V1, which was inconsistent with her training and standard operating procedures.
“Contributing to the accident were (1) deficiencies in Learjet’s design of and the [FAA] certification of the Learjet Model 60’s thrust reverser system, which permitted the failure of critical systems in the wheel well area to result in uncommanded forward thrust that increased the severity of the accident; (2) the inadequacy of Learjet’s safety analysis and the FAA’s review of it, which failed to detect and correct the thrust reverser and wheel well design deficiencies after a 2001 uncommanded forward thrust accident; (3) inadequate industry training standards for flight crews in tire failure scenarios; and (4) the flight crew’s poor crew resource management.”
With regard to the tire issue, the NTSB issued the following recommendations:
1. Provide pilots and maintenance personnel with information that (1) transport-category aircraft tires can lose up to 5 percent pressure per day, (2) it may take only a few days for such tires to reach an underinflation level below what the aircraft maintenance manual specifies for tire replacement, and (3) the underinflation level that would require tire replacement is not visually detectable.
2. Require that all Part 121, 135 and 91 Subpart K operators perform tire pressure checks at a frequency that will ensure that the tires remain inflated to within aircraft maintenance manual-specified inflation pressures.
3. Require that aircraft maintenance manuals specify, in a readily identifiable and standardized location, required maintenance intervals for tire pressure checks (as applicable to each aircraft).
4. Allow pilots to perform tire pressure checks on aircraft, regardless of whether the aircraft is operating under Part 91, Part 91 Subpart K or Part 135.
5. Require tire pressure monitoring systems for all transport-category airplanes.
6. Require that tire testing criteria reflect the actual static and dynamic loads that may be imposed on tires both during normal operating conditions and after the loss of one tire and consider less-than-optimal allowable tire conditions, including, but not limited to, the full range of allowable operating pressures and acceptable tread wear.
“This accident chain started with something as basic as inadequate tire inflation and ended in tragedy,” said NTSB chairman Deborah Hersman. “This entirely avoidable crash should reinforce to everyone in the aviation community that there are no small maintenance items because every time an airplane takes off, lives are on the line.”