A collective lawsuit filed against Airbus by 11 families of passengers on AirAsia Flight QZ8501, which crashed last December, received an impetus this week after Indonesian investigators pointed to mechanical failures as a cause for the tragedy.

Veteran aviation lawyer Floyd Wisner, who is representing the families, told The Straits Times yesterday that he is optimistic about obtaining relief for his clients.

"We believe the recent report by the Indonesian authorities confirms our position that this tragic crash was caused, at least in part, by a mechanical defect in the aircraft and certain of its components," said Mr Wisner.

The United States-based lawyer is representing the 11 families that collectively lost 21 of their loved ones after the Airbus A320-200 jetliner went down in the Java Sea.

The writ, first filed in June at a Chicago courthouse, argues that the aircraft in question was "defectively and unreasonably dangerous" in part because Airbus had "negligently breached its duty of care" in the design, manufacturing and assembling of the plane.

Court documents also showed Airbus' suppliers Doric, Honeywell, Thales Avionics, Motorola and Goodrich Corporation and United Technology Corporation were named as defendants.

Among the families' claims was that the European plane-maker had sold the A320-200 aircraft even though its rudder trim limiter was "subject to failure".

The rudder trim limiter, also known as the Rudder Travel Limiter Unit (RTLU), has come under heavy scrutiny after investigators found that the same component on the AirAsia flight had faults 23 times in the 12 months preceding the crash.

An issue with the RTLU surfaced repeatedly when the aircraft took off on its last flight from Surabaya to Singapore on Dec 28, according to Indonesia's National Transportation Safety Committee (KNKT), which made public the findings of its probe into the crash on Tuesday.

All 162 people onboard Flight QZ8501 were killed when the plane plummeted into the sea after the pilots lost control while trying to resolve a problem with the RTLU.

Mr Wisner said AirAsia was not named in the lawsuit because the US has no jurisdiction over the airline under the Montreal Convention, an international treaty that sets out the rights of airline passengers and stipulates rules which govern where lawsuits can be filed. These include factors such as where the carrier operates, where tickets were bought, the destination of the aircraft and the permanent residence of the passengers.

Jakarta-based aviation lawyer Rudhi Mukhtar, who was involved in a similar case against Boeing in 2012, said the families could also file a lawsuit in Indonesia based on civil law.

"But the Indonesian legal system allows many stages of appeals and the whole process can take years," he said. "There is no certainty in the timeframe. This is why most victims' families here go only for insurance claims."

Aviation lawyers said clients in cases where the defendants fall under US jurisdiction are usually advised to file their claims against airlines in the US, where payouts tend to be more generous than Asian jurisdictions. "The timeframe (in the US) is also more certain, two to three years," added Mr Rudhi.

However, Mr Gerry Soejatman, from aviation consulting firm Communicavia, said the KNKT report could not be used as supporting evidence in a lawsuit, but Mr Wisner did not see this as an obstacle in the case against Airbus.

He said: "We are commencing the investigation phase of the litigation by issuing discovery, or requests for information and documents, to the defendants."

As the lawsuit begins its course, other family members, such as Mr Eka Santoso, are still considering their next course of action.

The 52-year-old engineer, who lost his brother who was travelling with his wife and two children, said almost half of the passengers' families have received insurance payouts. He said the process to receive the payout, however, was "not as easy" as it involves a lot of red tape. "Some of the families have engaged a lawyer," said Mr Eka. "But it is too early now. We are still talking."

• Additional reporting Wahyudi Soeriaatmadja