AirAsia X has developed a specific simulator training session addressing Australia's Gold Coast airport approach after investigators found that two Airbus A330-300 crews within four weeks had descended below the minimum safe altitude.
In the first instance, on 4 May 2010, the crew had intended to use the autopilot's "managed" mode to follow the correct Runway 32 very high frequency omnidirectional radio (VOR) approach procedure, but switched to a different mode - "selected" - when the autopilot did not intercept the final approach course.
After aborting their first landing attempt, the crew again used the autopilot in "selected" mode to fly a second approach with a 3˚ flightpath angle. From 11nm out the A330 started descending, dipping immediately below the minimum safe altitude of 2,500ft (762m) and only emerging at 5nm when the minimum height fell away to 750ft.
Owing to poor weather this approach was also abandoned and, after a third attempt - this time to Runway 14 - the aircraft diverted to Brisbane, said the Australian Transport Safety Bureau (ATSB).
On 29 May another crew, in the same A330, executed two VOR approaches to Runway 32 in "selected" mode. In each case the descent started from 2,500ft at 10nm, and then, between 9nm and 7nm out, the aircraft descended to 1,500ft - below the safe altitude of 2,000-2,200ft.
"It was apparent that the flight crews were not monitoring the vertical profile between the initial and final approach fixes to ensure compliance with the segment minimum safe altitudes," said the ATSB.
"Although there were no clear reasons for the non-compliances, the three occurrences on two separate flights with different flight crews indicated that there may be a systemic factor in the operator's conduct of non-precision approaches."
It pointed out that AirAsia X's simulator training for VOR approach was based on a scenario with no segment minimum safe altitudes, and crews were probably "not regularly exposed" to the restrictions. As well as amending its training, the carrier has since advised against using low-power "open descent" techniques - which can result in high rates of descent - and limited crews to two approaches before diversion.