AN aviation authority has completed an investigation into why two aircraft from Asia flew below the minimum altitude threshold on their approaches into Gold Coast Airport.

Two AirAsia X flights from Malaysia's capital Kuala Lumpur, May 4 and 29 2010, were not in immediate danger but exposed the aircraft to non-controlled airspace, which sparked the investigation.

According to Australian Transport Safety Bureau investigators the aircraft flew so low that "separation from terrain and other aircraft operating in non-controlled airspace was no longer assured."

Pilots on the flight from Kuala Lumpur on May 4 with 258 passengers onboard tried to activate the autopilot but a fault meant they had to manually input navigational information.

As a result the pilots were flying too high on the first approach and had to abort the landing.

It was the second approach that was the cause of the investigation with the pilot descending to 1300ft 13km from the runway.

After two more aborted landings due to poor visibility the aircraft was diverted to Brisbane.

"It was not clear why the flight crew commenced descent prior to the aircraft intercepting the recommended profile for the approach," investigators said.

"Being above the recommended flight path on the first approach might have influenced the crew to descend earlier to avoid being similarly high during the second approach."

In the second incident in the same month the AirAsia X flight with 260 passengers seemed to ignore minimum altitude limits according to investigators.

"However, unfamiliarity with the conduct of VOR (VHF Omnidirectional Range) approaches that included intermediate segment minimum safe altitudes was again a possibility," investigators said.

As a result of the investigation, all AirAsia pilots were required to complete training prior to further operations into the Gold Coast.

The investigators also recommended a maximum of two approaches to the Gold Coast airport before being diverted to another airport.

AirAsia X was unavailable for comment on the findings.