The Aircraft Accident Investigation Bureau (AAIB) has identified a brief failure of all three hydraulic systems and a non-negative confirmatory drug test involving the pilot-in-command as two key areas in its preliminary investigation into an Air India Phuket-Delhi flight incident.

The Airbus A320, operating as flight AI 2379 on August 4, experienced an altitude upset while cruising at 36,000 feet. The incident injured 20 passengers and four cabin crew members. Seventeen of those injured were hospitalised, according to the preliminary report.

The aircraft was carrying 137 passengers, six cabin crew members and two pilots.

The AAIB has not determined a definitive cause of the incident. It said technical and other aspects of the investigation remain under examination.

AAIB recommends action over psychoactive substance test

The preliminary report said the pilot-in-command was found non-negative for a psychoactive substance in a confirmatory test.

The AAIB has recommended that the Directorate General of Civil Aviation (DGCA) take appropriate action on priority to prevent the abuse of psychoactive substances among aviation personnel.

According to sources cited in the report, the pilot had tested positive for marijuana. However, the AAIB has described its finding in the preliminary report as a non-negative confirmatory test and has not presented it as the cause of the aircraft incident.

The investigation into the circumstances surrounding the test and the aircraft's technical problems remains underway.

All three hydraulic systems briefly lost pressure

The aircraft's hydraulic systems have emerged as a major technical focus.

The Airbus A320 uses three independent hydraulic systems, known as green, blue and yellow. The systems provide redundancy, allowing the aircraft to retain hydraulic control even if one system becomes unavailable.

According to the preliminary report, the green hydraulic system was lost first at around 9:32:43 am India time while the aircraft was cruising at FL360.

Around four seconds later, the blue and yellow systems were also detected as lost. The autopilot then disengaged, followed shortly by a two-second stall warning.

The simultaneous loss of all three hydraulic systems is considered an extremely unusual event on the A320, according to aviation experts cited in the report.

Aircraft had been operating under an MEL provision

The aircraft had been dispatched under a Minimum Equipment List (MEL) item associated with its power transfer unit, or PTU. The provision had reportedly been invoked around a week before the incident.

Operating under an MEL does not itself constitute a safety violation. Airlines can legally operate aircraft with certain inoperative equipment when the relevant regulatory requirements and procedures are met.

The PTU serves as an auxiliary source of hydraulic power for two of the A320's three hydraulic systems.

Investigators will have to determine whether the earlier PTU issue had any connection with the hydraulic failures during the flight. The preliminary report, however, has not established such a link.

Pilots regained control as hydraulic systems recovered

The co-pilot, who was serving as the Pilot Flying, initially responded to the situation before the pilot-in-command took control.

During the event, the aircraft first climbed about 372 feet and then descended about 292 feet from its assigned flight level.

The blue hydraulic system recovered at approximately 9:32:52 am India time. The yellow and green systems recovered a few seconds later, allowing the aircraft's flight-control surfaces to return to normal operation.

The flight subsequently continued towards Delhi.

Because the seatbelt signs were off when the altitude upset occurred, several passengers and cabin crew members were injured.

The flight crew assessed the cabin situation and decided not to divert to an en-route airport. The aircraft continued to Delhi and the crew later requested medical priority from air traffic control.

The aircraft landed safely at Delhi's Indira Gandhi International Airport at around 5:30 UTC.

Air India begins voluntary testing of all pilots

Following the incident and the AAIB findings, Air India said it was cooperating fully with the investigation and providing access to operational, maintenance and technical records.

The airline said its pilots undergo training, proficiency checks, medical examinations and other regulatory assessments under applicable requirements.

Air India has also started voluntary testing of 100% of its pilots as an additional safety measure.

The airline said it maintains zero tolerance for breaches involving safety, fitness or regulatory requirements and would take appropriate action where necessary.

The AAIB investigation is continuing. Its final report is expected to provide a more detailed assessment of the aircraft's hydraulic failure, pilot actions and other factors surrounding the incident.