Indian investigators have cited a lethal combination of sleep inertia and over-dominance by the captain as having led to the fatal overrun by an Air India Express Boeing 737-800 at Mangalore.
Cockpit voice recordings recorded typical breathing patterns of deep sleep from the Serbian captain, lasting for 1h 28min, until just 21min before the accident. The Indian ministry of civil aviation's final inquiry report indicates that the captain slept for at least 1h 40min.
Because Mangalore radar was unavailable, the aircraft had to follow a shorter descent. But the ministry says the crew failed to prepare the descent profile properly, and the jet was "much higher" than it should have been for the instrument landing system approach to runway 24.
But while the Indian co-pilot suggested three times that the aircraft should execute a go-around, the captain instead attempted to correct the badly-misaligned approach.
In a bid to lower the aircraft's altitude, and capture the correct glide path for a visual landing, the captain increased the descent rate to nearly 4,000ft/min and continued the approach despite several automated 'sink rate' and 'pull up' warnings from ground-proximity systems.
Mangalore has a table-top runway and Air India Express permitted only the captain to conduct the landing. The captain on the fatal flight, IX812 from Dubai, had made 16 landings at the airport.
Flight-data recorder information shows that, as a result of the instability, the aircraft crossed the runway 24 threshold at a height of 200ft - four times higher than it should have been - and travelling at 160kt.
It touched down about 5,200ft beyond the threshold, leaving just 2,800ft of paved surface on which to stop. Despite the long landing the captain "did not apply the brakes appropriately" and even attempted a go-around after activating reverse thrust - in contravention of standard operating procedures.
The 737 failed to stop before overrunning the runway. Its right wing collided with the localiser antenna and the aircraft dropped off the edge of a steep gorge. Just eight of the 166 occupants survived the 22 May 2010 accident, none of whom was among the flight or cabin crew.
In its inquiry report the ministry says the prolonged sleep by the captain, particularly during the overnight circadian low period, could have led to "sleep inertia" and possibly "impaired judgement" over the approach shortly after he woke.
The ministry highlights regulations from another carrier which allow only a controlled 45min rest period, which must be completed at least 30min before top of descent.
But it also emphasises that a steep authority gradient existed in the cockpit, with an assertive captain paired with a submissive first officer.
The ministry points out that, even though the aircraft was not correctly positioned on the ILS glide path, the captain had "somewhat forced" the first officer to call "affirmative" when air traffic control queried whether the flight was established on the ILS approach.
"With the first officer not showing any signs of assertiveness, the captain had continued with the faulty approach and landing, possibly due to incorrect assessment of his own ability to pull off a safe landing," the ministry says. The first officer did not attempt to take over the controls to discontinue the ill-fated approach.