India's Aircraft Accident Investigation Bureau said the aircraft detected a triple-hydraulic failure, with the autopilot disengaging and a stall warning triggered for two seconds.
An investigation into an Air India flight that dropped almost 300 feet last month has found that the plane suffered a temporary hydraulics failure.
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Flight AI2379 was travelling from Phuket, Thailand to the Indian capital of New Delhi on 4 August when the incident occurred.
The Airbus A320, which had 137 passengers, six cabin crew and two pilots on board at the time, suddenly lost altitude while flying over Indian airspace, leaving 24 people injured.
In a preliminary report published on Friday, India's Aircraft Accident Investigation Bureau (AAIB) said the aircraft detected a triple-hydraulic failure, with the autopilot disengaging and a stall warning triggered for two seconds.
"The aircraft encountered altitude upset. Initially, it gained 372 ft and fell by 292 ft," the report said.
The hydraulic systems then recovered, and the "aircraft operation returned to its normal operating condition."
The report added that both pilots were tested for psycho-active substances following the incident.
"The sample of PIC (pilot in command) was found non-negative. Subsequently, blood samples of both the flight crew were sent to the pathological laboratory for confirmatory test. The confirmatory test report of PIC was found non-negative whereas the sample of the Co-Pilot was found negative in both the tests," it said, adding that the "confirmation of use of psycho-active substance is a serious concern."
Air India has since reportedly ordered one-time drug tests for its pilots.
The investigation remains ongoing and a full report is due to be issued at a later date.