The US Federal Aviation Administration has issued a proposed airworthiness directive mandating that operators of 63 US-registered Bombardier Q400 turboprops make changes to the aircraft's nose landing gear mechanical linkage.
The directive, originally issued by Transport Canada on 29 June 2009, was spawned by two in-service incidents where nose gear tyres were punctured by a pivot pin that had moved from its normal position because of a damaged retention bolt.
Investigators determined that the bolts were damaged by repeated contact with towbars or towbarless rigs on the ground.
The FAA warns that along with flat tyres, the loss of the pivot pin could result in a loss of nose wheel steering.
If finalised as written, the AD will require operators to modify the landing gear system by one of two Bombardier service bulletins issued after the incidents within 2,000h.