MUMBAI: Aircraft Accident Investigation Bureau’s preliminary report has confirmed what pilots and engineers call an exceptionally rare event, the near-simultaneous loss of all three hydraulic systems on the A320neo aircraft at 36,000 feet. Hydraulic systems are the fluid-pressure circuits that physically move an aircraft’s wing and tail surfaces. The green, blue and yellow systems dropped out within seconds of each other, triggering an autopilot disengagement, a stall warning and a brief altitude upset that injured 24 flyers.The accident occurred when the eight-year-old A320neo (VT-EXO) carrying 145 people had just been cleared to climb from 34,000 feet to 36,000 feet after entering the airspace controlled by Kolkata ATC. At 9:32:43am IST, within four seconds the flight control system detected loss of all the three hydraulic systems.‘Triple hydraulic failure close to unthinkable’In the next few seconds, the autopilot disengaged, a continuous repetitive chime sounded, and the stall warning triggered for two seconds. “The aircraft encountered altitude upset. Initially, it gained 372 ft and fell by 292 ft from its assigned 36,000 feet flight level during the event,” the report said.It did not give information on the cause of the triple hydraulic failure, nor did it put forth any safety recommendations for the A320 aircraft type. It said the investigation is in progress and hydraulic components, fluid samples and recorder data have been collected and are still being analysed.The critical information on the probable cause and contributory factors is likely to be released in the final report, expected to be published on or before Aug 4, 2027.In simple terms, hydraulic pressure is what physically moves an aircraft’s control surfaces, the ailerons on the wings that make it roll, the elevators on the tail that make it pitch up or down, and the rudder that makes it yaw left or right. Pilots move the stick and pedals, but it is hydraulic power that pushes the actual surfaces. An A320 carries three independent hydraulic circuits so that losing one, or even two, still leaves enough pressure to fly and manoeuvre the aircraft normally.“Losing all three at once is considered close to unthinkable, because the A320 has a triple-redundant hydraulic design which ensures no combination of failures can disable flight controls entirely. It is the kind of failure engineers cite alongside a handful of decades-old incidents involving other aircraft types, none of them an A320,” said a senior aircraft maintenance engineer. The notable total hydraulic loss events that occurred globally are the 1985 JAL 123 (Boeing 747), the 1989 United Airlines DC-10, and a DHL A300 that was hit by a missile over Baghdad in 2003.
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Following the triple hydraulic failure, the co-pilot, who was flying the aircraft, responded first to the upset before the captain took over the controls. Recovery was swift, the blue system came back within seconds, followed shortly by yellow and green, restoring normal flight control surfaces, the report said.