The airline's decision to send its 737s to Malaysia for maintenance checks has come under intense scrutiny after the first plane sent there two months ago came back with 95 defects. It was grounded in Melbourne on Thursday because of noise from an air-conditioning fault. Malaysia Airlines issued a statement yesterday defending its checks and calling Australian reports on defects unsubstantiated.

Two other planes were earmarked for heavy "C" checks - a regular procedure lasting more than a week, in which engineers have to check most of the airplane's parts - in Malaysia. But the airline's monthly maintenance schedule put out last week showed the planes were rescheduled to be checked at Tullamarine in Melbourne. As a result, checks on two other planes that were to take place at Tullamarine will now take place at Avalon in Victoria, and two planes that were to be checked at Avalon will be sent to a third party, John Holland Aviation Services, in Tullamarine.

"We don't know why it changed, but it's likely tied to the fact that CASA are yet to finish their investigation [into maintenance procedures]," a source said. The executive general manager of engineering at Qantas, David Cox, confirmed the maintenance work will now be done in Australia. "We only have overflow heavy maintenance work undertaken overseas," he said. "We explored options for checks on two 737-400 aircraft. Once space became available at our Tullamarine facility, the decision was taken to have the work done there."

A CASA spokesman said the decision was made by the airline and was not the result of an order made by the authority. He confirmed that the airline has regulatory approval to conduct maintenance checks at the Malaysian base but investigations into the aircraft that returned from that facility earlier this year were continuing. "It's too early to say whether [the aircraft's grounding in Melbourne] was related to the maintenance check in Malaysia or not," the spokesman said. The senior general manager of Malaysia Airlines, Mohammed Roslan Ismail, defended the checks in a statement yesterday, saying Qantas had 12 personnel attached to its maintenance team.

"All the highlights were rectified, to the satisfaction of the Qantas team, before aircraft delivery to Australia," he said. "With regards to the 'string of faults' that were reported in the media, [Malaysia Airlines] investigated and established that these were unsubstantiated.

"This is based on the fact that all these aspects were originally checked and found to be free from defect during the maintenance check and test flight, with the concurrence from the Qantas team."