An Air India Airbus A320 carrying 145 people suffered the near-simultaneous loss of all three hydraulic systems while cruising at 36,000 feet on a Phuket-Delhi flight on August 4, triggering an autopilot disconnect, a brief stall warning and an altitude upset that injured 24 people, according to a preliminary report by the Aircraft Accident Investigation Bureau (AAIB).
The aircraft, operating as flight AI 2379, recovered its hydraulic systems within seconds and continued to Delhi, where it landed safely. The AAIB said its investigation into the technical and other aspects of the incident was still underway.
The report did not establish any link between the hydraulic failure or altitude upset and the pilot-in-command testing positive for a psychoactive substance after the flight. However, the AAIB recommended that the Directorate General of Civil Aviation (DGCA) take action to prevent the abuse of psychoactive substances.
“The confirmation of use of psycho-active substance is a serious concern and AAIB recommends appropriate actions by DGCA on priority,” the report said.
The aircraft, an Airbus A320-251N registered as VT-EXO, had departed Phuket International Airport at 7.11 am IST with 137 passengers, including three infants, six cabin crew and two pilots.
At 9.10 am, while in the Kolkata Flight Information Region, the aircraft was instructed to climb from 34,000 feet to 36,000 feet.
At 10.02 am, while cruising at 36,000 feet, the aircraft detected a loss of pressure in its green hydraulic system. Four seconds later, its flight-control system detected the loss of pressure in the blue and yellow systems as well.
The autopilot disconnected almost immediately and a continuous chime sounded. Three seconds later, the aircraft generated a stall warning that lasted about two seconds.
The co-pilot, who was initially flying the aircraft, responded to the event before the pilot-in-command took over. During the episode, the aircraft climbed about 372 feet before subsequently losing about 292 feet from its assigned altitude.
The blue hydraulic system recovered within seconds, followed by the yellow and green systems. Flight-control surfaces then returned to normal operation.
The seat-belt signs had been switched off when the incident occurred, leaving passengers and cabin crew vulnerable to the sudden aircraft movement. The cabin supervisor later informed the cockpit that several passengers and crew members had been injured.
The pilots decided to continue to Delhi instead of diverting to the nearest airport and later requested medical priority from Delhi air traffic control.
The aircraft landed safely at Indira Gandhi International Airport at 11 am. Medical teams treated 24 people — 20 passengers and four cabin crew. Twenty were discharged after treatment for minor injuries, while four people with serious injuries were admitted to hospitals.
The aircraft sustained extensive damage inside the cabin, including cracked ceiling panels, dislodged overhead-bin doors, broken handrails, a damaged emergency-exit handle and a lavatory commode that had been uprooted from its position.
Investigators are also examining the aircraft's maintenance history. Before departure from Phuket, maintenance personnel had recorded no new discrepancies apart from an existing Minimum Equipment List item relating to the Power Transfer Unit (PTU), which had been invoked on July 28.
The A320 has three independent hydraulic systems — green, blue and yellow — that provide power to major flight-control and other aircraft systems. The PTU provides auxiliary power between the green and yellow systems when there is a significant pressure difference.
AAIB said relevant hydraulic components had been removed for further analysis. The cockpit voice recorder and flight-data recorder were also recovered and downloaded.
The post-flight medical tests found the co-pilot negative for psychoactive substances. The pilot-in-command's initial sample was non-negative, and a subsequent blood test sent to a laboratory for confirmation also returned a non-negative result.
The AAIB did not identify the substance detected or state whether it had any operational effect on the flight.
Both pilots had rested for nearly 22 hours before the flight. The pilot-in-command, 34, held a valid Airline Transport Pilot Licence and had around 5,055 hours of flying experience, all on the A320. The 32-year-old co-pilot had about 1,394 hours of total and A320 flying experience.
No adverse weather was reported by the crew or air traffic control at the time of the incident. Investigators are examining the aircraft's technical systems and other operational factors.
France's Bureau of Enquiry and Analysis for Civil Aviation Safety (BEA), representing the state of design and manufacture, has appointed an accredited representative to assist the investigation. Airbus technical experts inspected the aircraft in India between August 13 and 15, following which investigators collected hydraulic components and fluid samples for testing.
AAIB said the preliminary report was intended to establish the probable causes and contributing factors and not to apportion blame or liability. A final report will be issued after the investigation is completed.