Six months of AI-171 crash | Probe so far: ‘Technical snag’ being analysed; No evidence yet of ‘pilot error’

The Aircraft Accident Investigation Bureau (AAIB) released its 15-page preliminary report of the investigation on July 12 – a month after the crash that killed 260 persons, including 19 on ground. The report provided a detailed account of the crash and stated that the aircraft’s engine fuel control switches “transitioned” from ‘RUN’ to ‘CUTOFF’ position within a second of each other moments after lift-off, without clarifying if the switches were moved physically.

Although the report did not state these switches were moved by either of the pilots, it stated that one of the two pilots asked the other why he cut off and the other pilot responded that he did not do so. While this aspect of AAIB’s report has led to theories of pilot action, officials in know of the investigation have told this newspaper that the technical snag experienced by the aircraft in days leading up to the crash as well as on June 12 in the incoming flight from New Delhi to Ahmedabad are under the scanner.

An official privy to the investigation, told The Indian Express that analysis of the wreckage and the flight data has been conducted to analyse a ‘specific’ electrical snag that “appears to have caused a cascading impairing of the critical power distribution and flight systems”, including the cockpit instrumentation, after the flight became airborne. The snag under the scanner is said to have originated in the aft electrical system of the aircraft, which controls the most critical parts such as the hydraulic pumps, APU, engine power, the fuel pumps of the central and the main tank as well as the nitrogen generation systems, the official said.

The official added that the power failure led to the pilot instrumentation control being briefly cut off until the Ram Air Turbine (RAT) came on and began supplying hydraulic power– 13 seconds before the aircraft made the chilling Mayday call to the ATC. The Preliminary report mentions that the RAT was deployed, “during the initial climb, immediately after lift off” — began generating power about eight seconds after the take off.

“Till the time that RAT began supplying hydraulic power, it was impossible for the pilots to attempt a dual engine relight as the battery back up does not support the same. Therefore, while the pilots continued to steer the aircraft, attempting to achieve thrust, the engines were relit by recycling the fuel switches in the cockpit, one after the other, in the mandatory sequence after RAT began supplying power. There was barely any altitude or time to make it safely,” the official adds.

The official said that the investigators have not found any untoward actions by either of the pilots on board the aircraft. While the cockpit recorder had led to speculation on the senior pilot’s actions.The official added that the testimony of the survivor Vishwas Kumar about “flickering cabin lights” also supports the possibility of electrical malfunction, with systems switching between the main power, back up generator and attempted relight of systems within a few seconds.

As per the preliminary report, the investigators have recovered the APU from its compartment located in the tail of the aircraft in an “intact condition”, according to the preliminary report. The Preliminary Report also stated a critical detail of the crew of the previous flight AI423 logging a Pilot Defect Report for a status message “STAB POS XDCR” in the technical log, which was troubleshooted by the Aircraft Maintenance Engineer in Ahmedabad before clearing the flight for AI171. On July 17, The Indian Express had reported that the investigators were examining the history of technical snags and the possibility of system malfunction that can impact the FADEC’s (Full Authority Digital Engine Control) Engine Control Unit (the brain of the aircraft) to trigger uncommanded actions.

Post crash even the Emergency Locator Transmitter (ELT) that transmits distress signals in the event of a crash was also found to be inactive according to the preliminary report.

Officials said the investigators are hoping to wrap the probe within six months. “ The data is being analysed by multiple stakeholders in the investigations as well as experts from human psychology, forensic and aeroengineers, Boeing pilots as well as experts on maintenance and operation of the Dreamliner aircraft,” the official said.

The AAIB is yet to release its final report in the crash investigation.