NTSB: underinflated tires caused Learjet 60 crash
Greatly underinflated tires and a rejected takeoff after V1 were the probable causes of the 2008 crash of a Learjet 60 while attempting to depart the Columbia (S.C.) Metropolitan Airport (CAE) for a nighttime flight to Van Nuys, Calif., according to the NTSB.
As the tires failed in a sequence from right to left, pieces from the disintegrating tires damaged a squat switch in a wheelwell and caused the airplane’s thrust reversers to return to the stowed position.
While the captain was trying to stop the twin-engine jet by commanding reverse thrust, forward thrust was being provided at near-takeoff power because the disabled microswitch “made the aircraft think it was in the air.” The Safety Board determined that the inadvertent forward thrust acerbated the severity of the accident.
Contributing to the accident, the NTSB said, were deficiencies in Learjet’s design of the Model 60’s thrust reverser system, the inadequacy of Learjet’s safety analysis to detect and correct the thrust reverser and wheelwell design deficiencies after a 2001 uncommanded forward thrust accident in Alabama, inadequate industry training standards for flight crews in tire-failure scenarios, and the flight crew’s poor crew resource management.
The West Coast-bound Learjet, operated by Global Exec Aviation, was departing CAE shortly before midnight on Sept. 19, 2008, when it overran the departure end of Runway 11 during the rejected takeoff. It struck airport lights, crashed through a perimeter fence, crossed a roadway and came to rest on a berm.
The captain, the first officer and two passengers were killed; two other passengers–the drummer for a famous rock band and a well-known Los Angeles DJ–were seriously injured.
The NTSB investigation revealed that the aircraft was being operated while the main landing gear tires were severely underinflated because of Global Exec Aviation’s inadequate maintenance. The under- inflation compromised the integrity of the tires, which led to the failure of all four during the takeoff roll.
“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.”
Board member Robert Sumwalt, a former airline and corporate pilot, said “this accident was the perfect storm” because when the failing tires disabled the microswitch, it caused the Learjet 60’s systems to operate as though the jet was already airborne.
NTSB investigator Bob Swaim, who was in charge of the tire and maintenance aspects of the crash, said the number-four tire burst at about 137 knots. The fragments struck the aircraft and, as the other tires came apart, the Learjet lost braking capability.
He testified that full inflation for the tires should have been 219 psi. At a normal loss of inflation of 2 percent per day, after eight days it would be 185 psi, at which point maintenance manuals call for replacement.
Investigators determined that the tire pressure on the accident airplane had not been checked for three weeks. By that time, pressure would have been about 140 psi, according to investigators’ estimates. Board investigator Capt. David Tew said pilots flying in Part 135 operations are expected only to check the condition of tires visually, and are prohibited by the FAA from checking the pressure.
NTSB investigator-in-charge Bill English testified that the cockpit voice recorder picked up sounds of the disintegrating tires hitting the undercarriage about one-and-a-half seconds after reaching V1. While the first officer indicated that the takeoff should be continued, the captain decided to reject the takeoff and deployed the airplane’s thrust reversers.
But the investigation found no evidence that the accident airplane was uncontrollable or unable to become airborne. When the first officer recognized the need to shut down the engines, he called ATC to “roll the equipment.”
“The captain in this accident was required to make a decision– in an instant–about whether to continue with the takeoff–consistent with her training–or to abort the takeoff after she had reached a speed at which she could not stop the airplane on the remaining runway,” said Hersman, who added the decision was made in the span of about two seconds.
While the captain had 3,140 hours total time, she had only 35 hours in the Learjet 60 and only eight hours as pilot-in-command (PIC). Overall, she had “limited experience” as PIC in two types of aircraft. The pilots had some training in crew resource management, although neither had received training on tire failure. In this accident, “the startle factor played a role,” and the crew was described as being surprised and confused.
One of the recommendations in the Board’s final report is that simulators be developed to train for tire failure scenarios, providing realistic sound and motion cueing. Hersman said that an NTSB Special Investigation Report on rejected takeoffs in 1990 highlighted the severity of the sound and the startle effect. She said the captain’s inexperience as PIC “contributed to her indecision,”
as did switching between two different airplanes.
The Safety Board also found that neither the FAA nor Learjet adequately reviewed the Model 60’s design after the uncommanded forward thrust accident that occurred in 2001 in Alabama. While the modifications put into place after that accident provided additional protection against un-commanded forward thrust upon landing, no such protection was provided for a rejected takeoff.
The NTSB issued 14 safety recommendations as a result of its investigation into the Sept. 19, 2008, Learjet 60 aborted landing crash in Columbia, S.C, including these operational recommendations:
- Make sure pilots and mechanics know that tires can lose up to 5 percent of pressure per day and that the underinflation level that would require tire replacement is not detectable visually.
- Require all operators to do tire pressure checks often enough to ensure tires stay within specified pressure limits.
- Require the maintenance manuals clearly specify tire pressure check intervals.
- Allow all pilots to perform tire pressure checks (Part 91, 91K or 135 operations).
- Require tire pressure monitoring systems for all transport-category airplanes.