The Airbus A320 has three hydraulic systems, and they are separate on purpose. Green, blue and yellow. Three reservoirs, three pressure sources, no fluid shared between them. The architecture exists so that no single failure can take away the flight controls.

On 4 August 2026, at 36,000 feet over eastern India, an Air India A320neo lost all three of them in four seconds.

India’s Aircraft Accident Investigation Bureau published its preliminary report into the event this week. It answers a great deal about what happened, and almost nothing about why.

Four seconds

The timeline in the AAIB report is unusually tight. At 04:02:43 UTC the flight control system registered low pressure in the green system. Four seconds later the blue and the yellow systems went too. At 04:02:48 the autopilot disengaged. At 04:02:51 a stall warning sounded for two seconds.

The co-pilot was the pilot flying and responded first; the captain then took over the controls. The aircraft gained 372 feet above its assigned FL360 and then dropped 292 feet below it — a total vertical excursion of 664 feet.

Then it fixed itself. At around 04:02:52 the blue system came back, followed within seconds by yellow and green, and the flight control surfaces were restored. From first indication to full recovery the whole event lasted well under half a minute.

The aircraft’s Post Flight Report logged the sequence in the order it happened: HYD GREEN SYS LO PR, followed by blue and yellow, then AUTO FLT AP OFF, then FLT CTL L, R ELEVATOR FAULT, then low-level warnings on the yellow and blue reservoirs.

Why it should not be possible

The AAIB spells out the architecture in the report, and it is worth reading twice. The three systems supply 3,000 psi to the main users. Hydraulic fluid cannot be transferred from one system to another. Each system has its own reservoir. The green system’s normal pressure source is the engine-driven pump on the left engine.

In other words, three independent circuits, each with its own fluid, dropped pressure inside four seconds and then all recovered. That is not a leak. A leak drains one reservoir and stays drained.

One detail the report records without drawing any conclusion from it: the aircraft was dispatched from Phuket under a Minimum Equipment List item, number 29-23-01A, covering the Power Transfer Unit, invoked on 28 July 2026. The PTU is an auxiliary pressure source that can pressurise the green or yellow system from the other, without transferring fluid, and it kicks in automatically when there is a 500 psi difference between them. The AAIB notes the deferred item. It does not link it to the pressure losses.

Hydraulic components and fluid samples have been removed for testing. A team of Airbus technical experts, working through the French BEA as the accredited representative of the state of design, inspected the aircraft in India from 13 to 15 August. Both flight recorders have been downloaded.

What 664 feet does to a cabin

The seat belt signs were off. That single fact is the difference between an alarming technical event and an accident with hospitalisations.

The damage list in the report reads like a demolition inventory. Overhead ceiling panels cracked or deformed at the 5th, 9th, 10th, 11th, 18th, 19th and 20th rows. Passenger service unit panels dislocated or bent. An overhead handrail broken near row 10. The forward right emergency exit handle broken. Overhead bin doors torn off their hinges at rows 20 and 25. A crew seat bent. And, memorably, the commode in lavatory E uprooted from its mounting.

Twenty-four people needed medical attention on arrival. Twenty were discharged after treatment for minor injuries. Four had serious injuries and were admitted to hospital.

The crew assessed the cabin, decided against diverting, requested medical priority from Delhi ATC and landed on runway 11L at 05:30 UTC, taxiing to the stand under the aircraft’s own power. Neither the crew nor air traffic control reported adverse weather. The occurrence had initially been notified to the AAIB as a turbulence encounter; the investigators reclassified it as an accident after seeing the injuries.

The other finding

Because AI2379 originated outside India, both pilots were subject to a post-flight medical on arrival under DGCA rules. The breath analyser results were satisfactory for both.

The psychoactive substance screening was not. The captain’s sample came back non-negative on the detection kit, and a confirmatory blood test at a pathology laboratory returned the same result. The co-pilot was negative in both. The report does not identify the substance.

Air India terminated the captain’s employment with immediate effect on 4 September, hours after the report appeared.

It is worth being precise about what this does and does not mean. The AAIB draws no link between the test result and the hydraulic failure, and the report is explicit that its purpose is prevention rather than blame. A drug test result does not explain three independent hydraulic circuits losing pressure simultaneously and then recovering. That question is still open.

Indian broadcast coverage of the AAIB preliminary report into flight AI2379.

Sources: Aircraft Accident Investigation Bureau (India) Preliminary Report on VT-EXO; AeroTime; FlightGlobal; The Tribune.