Investigators have detailed an extraordinary event in which an Air India Express Boeing 737-800 was put into a steep dive moments after the captain was locked out of the cockpit.
While the twin-jet was cruising at flight level 370, en route to Pune from Dubai, the captain left the cockpit for the washroom. Almost as soon as he was gone, the aircraft started to pitch nose-down, after forward pressure on the co-pilot's control column.
India's DGAC attributes this to the co-pilot's adjusting his seat forward and inadvertently knocking the control column.
Flight-data recorder information shows that, after momentary relaxation, the forward pressure on the column increased and the jet pitched to 5° nose-down, before the pitch command briefly transitioned to nose-up.
But another "sharp" nose-down command followed, says the inquiry report, and the forward control column force gradually increased.
The 737 passed through 13° nose-down and an 'overspeed' warning showed the jet's airspeed had risen to Mach 0.82. As the airspeed increased the autothrottle reduced thrust in an attempt to keep the aircraft under control.
Outside the cockpit the captain had felt the change in pitch and attempted to re-enter the flight deck. There was no response from the co-pilot to a request from cabin crew to open the secure cockpit door, and the captain had to resort to an emergency code to gain access. He was away for about 40s in total.
Upon entering the cockpit he saw the aircraft was pitched about 26° nose-down. He responded by pulling on his control column - although the flight-data information shows that, while he was pulling with 130lb (580N) nose-up force, the co-pilot's column was experiencing an opposite pressure of 200lb nose-down.
Shortly afterwards the two columns "rejoined" and the aircraft - which reached a maximum speed of Mach 0.888 - began to pitch nose-up, having lost 6,800ft in altitude during the event. Control regained, the flight continued without further incident.
An admission of being "panic stricken" is given as the co-pilot's explanation for his failure to open the cockpit door. The inquiry report states that he attempted to contact the captain four or five times using an attendant call button.
As the aircraft departed from its planned altitude, as a result of the pressure on the control column, the increase in speed and the warning sounds from the aircraft caused a "panic situation".
The co-pilot "couldn't control the aircraft [or] open the cockpit door and answer the cabin call", the report says: "During the pitch-down attitude he tried to leave the control column to open the cockpit door but the aircraft pitch increased further and altitude [was being lost] rapidly."
It adds that the 25-year-old co-pilot - who had 968h on type - claimed to have forgotten the procedure to return to the assigned flight level.
While none of the 113 passengers was injured during the 26 May event, there had been a commotion in the cabin as a result of the upset, with items spilling into the aisle.