Not Bad Weather But Loss Of All Three Hydraulic Systems Caused AI Flight to Drop Altitude, Says Preliminary Report

New Delhi: The 292-feet drop that an Air India flight suddenly experienced while cruising at 36,000 feet on August 4, injuring 24 people on board, was because it lost all three of its hydraulic systems, according to a preliminary report by the Aircraft Accident Investigation Bureau released on Friday (September 4), reported Business Standard.

This loss of hydraulic systems that the Phuket-Delhi flight experienced triggered autopilot disengagement, flight-control warnings, and a brief “stall warning” (which alerts pilots if an aircraft’s wings may not generate enough lift), per the report. 

In its preliminary report, the AAIB also ruled out the possibility of adverse weather having caused the drop, while recommending that the Directorate General of Civil Aviation (DGCA) take appropriate action about the pilot-in-command testing positive for psychoactive substances, BS reported.

Loss of all hydraulic systems

The flight, an Airbus 320, took off from Phuket at 7.11 am Indian Standard Time (IST) and was asked to climb to 36,000 feet by the Kolkata air traffic controller. The A320 has three separate hydraulic systems, each with its own reservoir and sources of pressure. Hydraulic systems use high-pressure fluid to power important aircraft controls.

Per the Business Standard report, the aircraft detected a loss of pressure in its green hydraulic system at 9.32.43 am while cruising at 36,000 feet. Four seconds later, it detected a loss of pressure in the blue and yellow hydraulic systems too. 

The report says that at 9.32.48 am, the autopilot disengaged and a continuous warning chime sounded. Three seconds later, the “stall warning” was triggered for two seconds. The co-pilot, who was flying the aircraft, responded to the situation before the pilot-in-command took over the controls, the preliminary report of the AAIB noted.

This was when the A320 experienced the altitude drop, initially gaining 372 feet and then falling 292 feet. The Blue hydraulic system recovered at around 9.32.52 am, followed a few seconds later by the Yellow and Green systems. These recoveries restored the flight-control surfaces and the aircraft returned to normal operating conditions, BS reported.

“Consequently, this led to the recovery of the flight control (F/CTL) surfaces. Subsequently, the aircraft operation returned to its normal operating condition,” the newspaper quoted the AAIB’s report as saying. 

However, at the time of the incident, the seat-belt signs were off. This resulted in injuries to passengers and cabin crew. The cabin supervisor informed the cockpit about the injuries, after which the crew assessed the situation and decided to continue to Delhi instead of diverting to the nearest airport, the report mentioned. The pilots later sought medical priority from Delhi ATC. The aircraft landed safely at the Delhi international airport at 11 am IST and taxied to its parking bay on its own power, BS reported.

However, Hindustan Times quoted a pilot as saying that two important questions still remain unanswered: what the real cause of the hydraulic system failure was, and how the three systems failed in such fast succession. Another pilot quoted by HT said that the preliminary  report had not incorporated data from the Cockpit Voice Recorder, does not explain the pilot’s side-stick inputs or reveal what caused the aircraft to move so violently despite the reported failure of all three hydraulics.

Pilot tests positive for psychoactive substances 

Meanwhile, the preliminary report also noted that the pilot-in-command tested “non-negative” for a psychoactive substance in a confirmatory test and recommended that the Directorate General of Civil Aviation (DGCA) take appropriate action to prevent abuse of such substances. 

“The confirmation of use of psychoactive substance is a serious concern and AAIB recommends appropriate actions by DCA on priority,” the preliminary report said.

The co-pilot tested negative. 

Per the Business Standard report, Air India stated after the release of the report that it has terminated the pilot-in-command's employment with immediate effect in line with the airline's “zero tolerance approach” to violation of safety, fitness and regulatory require-ments. 

“Safety remains Air India's highest priority,” the BS report quoted an Air India spokesperson as saying in a statement. “Our pilots undergo rigorous training, regular proficiency checks, medical examinations, and regulatory assessments in accordance with applicable national and international requirements. Following the incident, Air India has started voluntary testing of 100 per cent of its pilots.”

The aircraft, an Airbus A320-251N bearing registration VT-EXO, was carrying 145 people – 137 passengers, six cabin crew, and two flight crew. Of the 24 people injured during the sudden altitude drop, four people sustained serious injuries and had to be hospitalised.

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