Debris missed control cables

The tyre of an Air Malta jet that burst on take-off from Glasgow airport last September was "close to its fully worn condition" and could have potentially resulted in a control restriction during a critical phase of flight, accident investigators have concluded.

The report by the UK Department of Transport's air accident investigation unit concluded that debris from the tyre had damaged the Boeing 737's hydraulic system, leading to the loss of fluid and several flaps.

The investigators made no recommendations in the report because the airline made several safety improvements to prevent a recurrence.

The incident had sparked a major airport alert as the Malta-bound jet, with 143 people on board, had to circle over Glasgow for nearly three hours to burn off fuel before landing safely.

Investigators said they were particularly alarmed that debris had just missed crucial control cables. "The most concerning aspect of the tread failure was the piece of debris which had become lodged near to the control cables for the spoilers and ailerons (hinged flaps) on the left wing," the report said.

Preparations for the flight had been carried out with no identified problems and the aircraft taxied uneventfully. During take-off, the left inboard tyre shed its tread, leading to the loss of the A system hydraulic contents, failure of the landing gear to retract and failure of the left main landing gear light to illuminate in the cockpit. The tyre failure was most probably caused by fatigue in the sidewall, the report noted.

The report said that five seconds after take-off, officials at the airport noticed a white stream emanating from the underside of the aircraft and alerted the crew who confirmed that they had lost their A hydraulic system.

The crew calculated that the plane was too heavy for an immediate landing and so decided to remain in the vicinity of the airport to burn off fuel.

Tyre debris was then spotted on the runway by the crew of an arriving plane and the strip was closed for inspection and clearance.

The Air Malta plane pilots were unable to assess the extent of the damage and arranged for a flypast of the airport control tower. An engineer on the ground saw that the tyre was either missing or damaged and the main landing gear was down.

The report said that tyre debris had been flung upward and rearward, becoming lodged in the left spar close to the aileron and spoiler control cables; debris was also found wrapped around the main landing gear actuator.

"Hydraulic lines feeding the gear transfer unit, which is supplied by A hydraulic system, were damaged and allowed the leakage of hydraulic fluid. It also led to a subsequent failure of the No. 1 engine hydraulic engine drive pump. Structurally, the left inboard trailing edge flap, mid flap and fore flap also suffered damage from the debris."

The air accident investigators said the burst tyre had been on its sixth and final retread "and may have reached its ultimate fatigue life prematurely for an undetermined reason".

The report said that a detailed examination of the limited amount of retrieved tread pieces had shown that the tyre was "close to its fully worn condition". It added: "The tyre had probably reached its ultimate fatigue limit at an earlier age than predicted by the sample testing of other tyres".

The tyre was manufactured in 2000 and was re-treaded for a sixth time on February 5, 2004. It was fitted to the plane in question on July 22, 2004 and had completed 230 cycles before the accident.

The report notes that Air Malta has made several improvements since the incident including limiting aircraft tyres to three retreads, improving tyre checks and briefing flight crews.

Speaking to The Times after their arrival in Malta a day later, some of the passengers had recounted what they described as the "longest three hours" of their lives.