On 26 May 2010, a Boeing 737-800 being operated by Air India Express on a passenger flight from Dubai UAE to Pune, India was in the cruise at night at FL370 near PARAR when a sudden high speed descent occurred without ATC clearance during which nearly 7000 feet of altitude was lost in a little over 30 seconds before recovery was made. The remainder of the flight was uneventful. Despite the abnormal pitch, pitch change and ‘g’ variation, none of the 113 occupants had been injured.
An Investigation was carried out by the Indian DGCA. Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) data were available. The cabin crew reported that during the excursion, there had initially been “commotion in the cabin for a couple of minutes (with) the (senior Cabin Crew) asking the passengers to fasten (their) seat (belts) and making an attempt to secure the cabin”.
It was established that when the aircraft commander had left the flight deck with the intention of using the toilet, he had discovered that it was occupied and had immediately attempted to return. However, his door entry signal had not been answered and he had then activated the emergency flight deck entry system. By the time it was possible to get back into the flight deck as a result of this action, the overspeed warning was sounding and the aircraft was in a steep descent with thrust levers fully retarded. A recovery was then commenced.
FDR data showed that whilst the commander was out of the flight deck and the maintaining FL 370 and Mach 0.76 with AP and A/T engaged, a nose-down and slightly left input had been made to the First Officer's control column which had activated CWS pitch mode. After a brief cessation, the pitch down input had continued with a consequent speed increase occurring until Mach 0.82, Mmo, was exceeded passing FL 349 when, with the pitch increasing through -13° and thrust levers at flight idle, the overspeed warning was anunciated. As the aircraft continued to descend and reached approximately -22° pitch down, a two second split in the column sensor force occurred as the commander took, and then retained, pitch control still with the AP engaged. disconnecting the AP shortly afterwards. With thrust restored, a recovery climb from the minimum level reached of FL 302 was continued back to FL370 with a maximum speed of Mach 0.89 occurring before airspeed began to decrease in the climb with AP now disconnected. The recorded time above Mmo was 37 seconds, the normal load factor during the excursion was found to have ranged from a minimum of -0.2g to a maximum 2.1g. The total duration of the excursion including the recovery climb to FL370 was nearly 7 minutes.
It was noted that the commander had re-entered the flight deck at about FL 350 but had not then initiated the recovery “as per standard procedure”. As a result, a further 5000 feet of altitude had been lost. In particular, the AP was left engaged and the manual pitch input to cease descent had been abrupt, with the control column “yanked…..with approx 125 lb pull force in 2 second(s)”.
It was established that the actions of the First Officer had been inadvertent and had begun when he had inadvertently pushed the control column forward whilst adjusting his seat. When the warnings began to activate, he stated that he had “got into (a) panic situation and couldn’t control the aircraft (or) open the cockpit door”. At one point, he stated that he had tried to leave his seat to open the door “but the aircraft pitch increased further and altitude was losing rapidly”. Then the commander gained access via the emergency entry process.
It was noted that, once the aircraft was back at FL 370 and on the flight planned track, from which there had been a brief deviation during the altitude excursion, the rest of the flight had been uneventful. The Investigation found no evidence of any fluctuations in angle of attack, normal load factor or recorded wind velocity before or during the excursion which would have indicated the presence of relevant atmospheric disturbances.
The Probable Cause was determined as “inadvertent handling of the control column in fully automated mode by the copilot which got compounded as he was not trained to recover the aircraft in automated mode”. It was further concluded that the Contributory Factor was the “subsequent recovery actions by the PIC without coordination with copilot”.
Two Safety Recommendations were made as result of the Investigation:
- The appropriate action shall be taken against the involved crew.
- In view of the incident, (the aircraft operator) should review the training curriculum including the simulator training of the pilots to include such in-flight emergencies.
The Final Report was completed on 6 August 2010 and published on 23 November 2010: Final Investigation Report on Serious Incident to AirIndia Charters ltd.