A seemingly minor uncompleted maintenance task spawned a "perfect storm" accident chain that led to disastrous consequences for a Bombardier Learjet 60 crew and passengers during a late night departure from Columbia, South Carolina in September 2008.
Recommendations issued by the US National Transportation Safety Board in its conclusion following its investigation into the 6 April accident include alerting the industry to the dangers of underinflated tyres.
The NTSB determined that the probable cause of the crash, which killed the pilot and two of the four passengers, was partly the operator's inadequate maintenance of the aircraft's tyres, specifically the four Goodyears on the main landing gear, which had been "severely underinflated".
The board also determined that the captain's decision to perform a rejected take-off at a speed higher than "V1", the point beyond which crews are generally taught to continue the take-off, was equally at fault.
The chain of events, described as a "perfect storm" by board member Robert Sumwalt, included the operator not checking the tyre pressure, which led to all four main gear tyres rupturing at just above V1 speed. The captain, hearing a rumbling noise when the tyres burst, rejected the take-off though above V1.
However, debris from the ruptured tyres punctured hydraulic lines and disabled a microswitch in the landing gear well, causing the twinjet's thrust reverser logic to believe the aircraft was airborne, retracting the reverse thrusters.
The Learjet 60's thrust reverser design does not provide mechanical feedback to the pilots to let them know that forward thrust is being applied rather than reverse thrust. The accident aircraft, with forward thrust, no main tyres and possibly no hydraulic braking, overran the runway and burst into flames after crossing a highway.
A key finding in the investigation was that operators are not generally aware of the importance of proper tyre maintenance, nor are pilots allowed by Part 135 regulations to check the pressure themselves; only certificated mechanics can legally check it. When underinflated, the NTSB says the tyres begin to flex, causing heat that can melt the nylon fibre materials that strengthen the structure.
Investigators found that the Learjet's tyres, which are supposed to be inflated to 15.1bar (219lb/in2), had progressively lost about 2% of their pressure a day over a period of about three weeks before the accident, a typical leak rate for high-pressure tyres. Federal Aviation Administration certification rules allow for the tyres to lose as much as 5% a day.
Investigators determined that the tyres on the accident aircraft were inflated to about 9.66bar, 36% below the optimum pressure and well below the point at which they must be changed because of heat damage.
Recommendations for the FAA include requiring fractional operators, air taxi and airline operators to provide information and required intervals for measuring tyre pressure, allowing pilots as well as mechanics to check tyre pressure for Part 135 operations, requiring simulator upgrades to realistically emulate tyre failures on take-off and requiring Bombardier to make changes to the Learjet 60 thrust reverser design.
Source: Flight International
