The US National Transportation Safety Board is calling on the Federal Aviation Administration to mandate significant changes to the Bombardier Learjet 60's reverse thrust system and pilot training, both for future builds and the existing fleet.
The recommendations, issued on 17 July, stem from the NTSB's investigation of a fatal Learjet 60 runway overrun in Columbia, South Carolina in September 2008.
Investigators in that case have determined that the aircraft's right outboard tyre failed due to under-inflation after the aircraft reached its "V1" speed of 136kt (252km/h) - the threshold at which pilots generally will take off rather than abort on the ground - followed by blow-out of the other three tyres in quick succession.
The FAA in May proposed an airworthiness directive that would require Learjet 60 operators to include flight and maintenance manual procedures highlighting proper tyre inflation, again based on early findings from the Columbia accident.
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NTSB-requested changes to the Learjet, however, centre on what happened after the tyres failed and the pilots began the rejected take-off procedure, which calls for reducing the throttles to idle, pulling the reverse levers upward and increasing engine thrust, now deflected, to slow the aircraft.
Reversers will deploy if certain conditions are met, including weight-on-wheels as sensed by an air/ground squat sensor on the main landing gear.
If certain conditions are no longer met, for instance if the squat switch indicates the aircraft is now off the ground, the reversers will stow but the reverse levers on the Pratt & Whitney PW305-powered Learjet 60 will stay in the upward position, as there is no mechanical or cable connection between the elements.
Although the board has not yet reached its final conclusions as to the probable cause(s) of the accident, investigators have found evidence that the thrust reversers were stowed rather than deployed as the aircraft crossed a road past the end of the runway and hit an embankment with both engines running at high power.
NTSB says a "nose gear steering disconnect" tone picked up on the cockpit voice recorder indicates that the squat switch went to the "air" mode during the accident sequence, likely triggered by damage to the squat switch wiring caused by the tyre blow-outs.
"Thus with the loss of the on-ground signal, the system would have moved both thrust reversers to the stowed position [the position in which the accident aircraft's thrust reversers were found]," says the agency in the recommendation.
"Accordingly, the engines would have provided forward thrust commensurate with the angle of the thrust level position commanded by the pilots, even though the reverse levers were in the reverse position."
As a result, the NTSB is asking that FAA require Bombardier to design and install several modifications to the Learjet 60 thrust lever system for new and existing aircraft, including a reverse lever that actively returns to the normal position when the reversers stow and improved aural or visual cues that would allow the pilots to recognise an inadvertent thrust reverser stowage.
The NTSB also wants all Learjet 60 pilots to receive training for inadvertent reverser stowage for take-off as well as landing scenarios.
The board is also asking the FAA to analyse the PW305-powered Hawker 1000, which uses a similar thrust reverser design, to evaluate the potential for similar failures.
Source: Flight International
