Investigators have found that a Mahan Air crew's incorrect interpretation of navigation data, reinforced by poor cockpit communication, led an Airbus A310-300 to descend to an extremely low altitude while attempting to land at Birmingham, UK.

They are recommending that Mahan thoroughly reviews its cockpit resource management training and improves procedures after finding that a third pilot, not formally part of the operating crew, unwittingly caused confusion by intervening during the approach.

Arriving from Tehran as flight 5020 on 23 February last year the Iranian A310 descended at one point to just 164ft (50m) above ground level while still 5nm (9.2km) from the runway.

 © Andy Martin

Controllers had cleared the A310 to perform the Grove 1A standard arrival to Birmingham's runway 33, which takes inbound aircraft over the district of Honiley, where a VOR is sited about 6nm to the southeast of the airport. The runway 33 glideslope was not available at the time and the pilots briefed for a localiser/DME approach.

But crucially, after the briefing, a third pilot entered the cockpit. He was the captain for the second crew due to return the A310 to Tehran. While he was not part of the operating crew for the Birmingham sector his presence - particularly given that he had not been party to the approach briefing - contributed to a breakdown of situational awareness regarding the aircraft's distance from the airport.

In its inquiry into the "serious" incident, the UK Air Accidents Investigation Branch states that the A310's flight-management computer did not contain the chosen approach, so the distance to the airport's DME beacon did not appear on the primary flight display.

But the crew had neither put the aircraft's 'VOR/NAV/ILS' switch into the 'VOR' position, nor selected the ILS/DME frequency for runway 33 in the VOR control panel. This resulted in the flight-management system auto-tuning to the Honiley VOR - a navaid which was not part of the approach being conducted. The DME distance to the Honiley VOR was indicated on the jet's DME/radio magnetic indicator display.

Controllers cleared the aircraft to descend to 2,000ft in preparation for the approach. Shortly before the A310 levelled at this altitude the first officer correctly advised the captain, who was flying the aircraft, that they were 11nm from the runway.

But while the first officer understood the aircraft's location, the captain apparently misinterpreted the reading on the DME/RMI display - which was showing the distance to the Honiley VOR, about 5nm - as the distance to the airport. The third pilot reinforced the captain's error by similarly calling the distance as 5nm, despite the first officer's twice repeating that the aircraft was still 11nm from the airport.

The AAIB says that the third pilot was "not restrained from intervening", despite his not being present for the approach briefing, while the captain "ignored" the first officer's correct information, possibly through a combination of natural confirmation bias and mental acknowledgement of the third pilot's status.

Believing the aircraft to be closer to the airport than it was, the captain initiated a descent. No standard calls were made and the aircraft descended far below the correct approach path, and the minimum descent altitude, reaching a height of just 164ft above terrain before a ground-proximity warning prompted a go-around.

Birmingham controllers also issued instructions to the aircraft to execute an immediate climb. Instead of climbing to 3,000ft straight away, however, the aircraft levelled at 1,750ft and then began to descend again. It reached 1,300ft before climbing again to the assigned altitude.

Mahan 5020's second attempt to land initially appeared to follow the first. The A310 again began to descend about 5nm before the Honiley VOR, and 11nm before the airport, before air traffic control asked the crew to confirm they were still maintaining 2,000ft.

The A310 eventually landed at Birmingham with no injuries to the 88 passengers and 10 crew members.

During both approaches the aircraft's autopilot captured the localiser but failed to track it accurately, which added to the crew's confusion and workload. But the investigators make clear that the "substantial breakdown" in cockpit resource management contributed heavily to the "unnecessary risks" involved in the flight.

This poor CRM was demonstrated, says the AAIB, in the two-minute period before the go-around. "A significant feature of this period is the fact that, although a disagreement about the navigation of the aircraft was apparent, none of the pilots attempted to resolve it," it states. "Their utterances were either questions or simple assertions. The commander did not identify the discrepancy and seek a positive resolution of the disagreement."

Nine months after this incident, on 24 November 2006, a second Mahan Air A310-300 descended far below its cleared altitude while approaching Birmingham, after the crew failed to reset the jet's altimeter to the local pressure figure.

Mahan Air was banned by UK authorities this year following concerns over poor operational control and inadequate safety management. Contrary to European regulations the aircraft involved in the February 2006 event was not equipped with a terrain-awareness warning system.

Mahan Air was subsequently included on the European Commission's airline blacklist and is prohibited from operating to the European Union.