Investigators have found that the pilots of a UTair-Ukraine Bombardier CRJ200 descended far below the glideslope on approach to Moscow Vnukovo after forgetting to reset their altimeter.
The aircraft reached a height of 144m (472ft) while turning onto the final approach to runway 19 in an area where the minimum height was 400m.
Russian chart documentation shows obstacles up to 79m in the approach sector and up to 108m on the border of an airspace zone located just west of the glidepath.
Ukraine's state aviation service says the crew failed to adjust the altimeter from the cruise setting of 1013mb to the airport level of 981mb as the CRJ descended through the transition altitude of 6,000ft.
It says the aircraft had been despatched with a non-functioning auxiliary power unit and, at this critical point, the crew was distracted by procedures relating to pressurisation during the descent.
Despite confirming the reduction in pressure, the altimeter setting was not changed, says the Ukrainian authority, which meant it was falsely indicating that the aircraft was 258m above its true height.
The crew failed to carry out the approach checklist, and the aircraft had descended to 410m during a point in the approach where the minimum height was 600m. It continued to lose height until, at 330m, an air traffic controller intervened. He twice received no response from the crew and, at the third attempt, ordered the aircraft to stop descending and climb to 400m.
"According to the explanation from the aircraft's captain, he did not hear the controller's request because of the [pressurisation procedure]," says the state aviation service.
The aircraft, at 273m, began to ascend after the crew increased power for around 30s. But it reached only 350m before descending once again. Although the controller asked the pilots to confirm the altimeter pressure setting, and the crew responded positively, the setting stayed unchanged.
Investigators add that the controller urged the crew to climb by another 200m to reach 400m but the instruction confused the pilots. Only after the aircraft dipped to 170m, and the ground-proximity warning system sounded, did the crew understand that the jet was too low. The pilots aborted the approach.
No-one was injured during the 1 April 2013 incident, which followed a flight from Donetsk. While the pilots had logged some 10,000h flight time between them, the captain had 80h on the CRJ200 while the first officer had 436h.
Investigators believe that non-standard phraseology contributed to the pilots' lack of awareness about their predicament.