On 8 August 2014, Turkish ATC observed that a Boeing 777-300ER (VT-JEL) being operated at night by Jet Airways on a scheduled international passenger flight from Mumbai to Brussels as 9W-228 had departed its cleared cruise altitude of FL340 and queried the pilots. The descent was arrested at FL317 and the aircraft re-cleared to continue at FL320.
An Investigation was carried out by the Indian DGCA in accordance with Regulations based on the principles of ICAO Annex 13. DFDR data were used in support of the Investigation.
The 40 year-old Captain was found to have 12,845 hours total flying experience including 4785 hours on type, almost all in command. The 46 year-old First Officer was found to have 7388 hours total flying experience including 4332 hours on type. In 2001, the First Officer was found to have failed an initial attempt to gain an ATR 72 type rating and during further training for the eventually successful second attempt to obtain one to have been recorded as "lacking judgement". However, it was found that there were no further adverse remarks in respect of competency on her training records on the ATR 72 or in respect of subsequent transition to and service on the Boeing 737 NG and Boeing 777.
It was established that the aircraft had been in the cruise at FL340 when, with the aircraft commander asleep during in a period of Controlled Rest and the aircraft at waypoint CRM in Turkish airspace, the First Officer inadvertently input and activated what should have been a change of track to 292 into the MCP as the selected altitude 29,200. She did not detect the error during the subsequent descent over nearly 2 minutes at up to 1664 fpm and only became aware of the situation when ATC noticed the departure from the cleared altitude and queried this observation. She then arrested the descent at FL317 and, after re-clearance by ATC to cruise at FL320, adjusted the altitude to that level without waking the commander. Once at FL320, she woke the Captain who stated that he had seen an altitude of 29,200 on the FMC. DFDR data showed that the AP had remained engaged throughout the excursion and was used to confirm that the excursion had been caused not be any malfunction but by flight crew action. It was noted that the First Officer had claimed to the Investigation that "the aircraft descended from FL340 without any pilot input" until she arrested the descent at FL317 using 'ALT HOLD' after being alerted to the loss of altitude by ATC.
It was explicitly noted that, having examined all the available data, Boeing "was unable to identify an aircraft system failure that caused this descent" and had concluded without equivocation that "the available data does not (indicate) the need to perform any additional aircraft maintenance in response to this event.”
The procedures for Controlled Rest were noted as contained in DGCA Operations Circular 8/2013. These included:
- controlled rest shall not be longer than 40 minutes with another 20 minutes for operational orientation before resuming flight deck duties
- any flight crew depending on the circumstances may terminate the rest at any time.
- altitude changes are not permitted during controlled rest.
- in the event of any abnormal condition, the non-resting pilot shall wake the resting pilot.
- any system intervention which would normally require a cross check according to multi crew principles must be avoided until the resting crew member resumes his duties.
It was considered by the Investigation that "the presence of cabin crew (on the flight deck) during controlled rest would enhance the alertness of the remaining crew" and that "this should also reduce (the chances of) inadvertent action to some extent".
The Probable Cause of the event was formally determined as "the First Officer inadvertently fed the Altitude window on the Mode Control Panel with the corrected track of 292º and activated it thereby causing the altitude of the aircraft to drop."
Two Safety Recommendations were made as follows:
- that the Co-Pilot (involved) should be given corrective training on fight automation systems.
- that the DGAC should take any other action deemed appropriate.
The Final Report was released on 28 December 2015.