On 24 November 2006, an AIRBUS A-310 descended significantly below cleared altitude during a radar vectored approach positioning, as a result of the flight crew's failure to set the Altimeter Pressure Settings, which was unusually low.
The aircraft was being radar vectored towards a night ILS approach to Runway 15 at Birmingham Airport. The radar controller had cleared the crew to descend to an altitude of 2,500 ft, but noticed that the aircraft had descended to almost 1000 feet below the cleared altitude at a range of 11-12nm, in the presence of an obstruction in the vicinity. The crew were instructed to climb and given the QNH, which they had not set. With the correct QNH set, the aircraft climbed and levelled as instructed at 2,000 feet and were then cleared to intercept the localiser and continue descent with the glideslope after which a normal landing was completed.
The AAIB Report includes the following analysis:
"The crew had not changed the altimeter setting from the standard setting of 1013 hPa to the Birmingham QNH of 982 hPa when first cleared to descend from a flight level to an altitude. Based on an average height of 30 ft per hPa, a height difference of 930 ft existed between the aircraft actual altitude and that indicated on the altimeters. Consequently, thus when the altimeters were indicating 2,500 ft the aircraft had actually descended to 1,570 ft. As the aircraft continued its descent below its cleared level of 2,500 ft the radar controller notified the crew and warned them of the mast ahead. Having realised that the altimeter sub scale setting was incorrect the crew initiated an immediate climb, re-set the altimeters to the correct QNH and followed the controller’s instructions. The crew could not recall any distractions or unusual flight deck activity at the point at which they would normally have adjusted the altimeter sub-scales."