New Delhi, Aug. 18 -- Air India flight 2379, an Airbus A320 flying from Phuket to Delhi, suddenly lost altitude on 4 August, injuring 20 passengers and four crew members. The cause remains unclear: investigators are examining whether a hydraulic-system malfunction, pilot actions or both were responsible.

Mint examines what is known and what investigators are looking for.

At about 04:02:44 GMT, the aircraft recorded low pressure in its Green hydraulic system, one of three that power the flight controls. Within about a second, Blue and Yellow were also temporarily affected, depriving the aircraft of hydraulic power. Airbus says the flight-control surfaces were unresponsive for about four seconds.

At 04:02:46, Autopilot 2 disconnected. Th...