An AirAsia flight from Perth to Bali that suddenly lost cabin pressure - plummeting 20,000 feet - dropped at a rate of 11 metres per second, a report has revealed.

Flight QZ535 from Perth turned back just 25 minutes after take-off on October 15 when crew had to make a mid-air emergency decision, terrifying and panicking the 146 passengers.

An investigation by the Australian Transport Safety Bureau (ATSB) has revealed exactly what went wrong when the low-budget airline's Airbus conducted an "emergency descent" with 146 passengers, two flight crew and four cabin crew on board.

The aircraft plunged from 32,000ft to 10,000ft, with dramatic footage captured by passengers showing crew members instructing passengers to "get down, get down" as oxygen masks drop and they are ordered to adopt the brace position.

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Crew members attempted to switch the controls to manually close the valves and stop the cabin's pressurisation rate from climbing, however the caution light disappeared and pilots returned to normal service.

But three minutes later a more serious warning appeared when the altitude warning alarm sounded, requiring crew members to perform an emergency descent.

An Air Asia flight from Perth to Bali that was forced to make an emergency landing was plummeting at a rate of 11 metres per second, according to an investigation. Photo / 9 News Perth
An Air Asia flight from Perth to Bali that was forced to make an emergency landing was plummeting at a rate of 11 metres per second, according to an investigation. Photo / 9 News Perth

Many passengers thought they were going to die during the terrifying plunge.

"I actually picked up my phone and sent a text message to my family, just hoping that they would get it," Perth woman Leah told Nine News.

"We were all pretty much saying goodbye to each other. It was really upsetting.

The plane suddenly dropped 20,000ft, terrifying and panicking the 146 passengers. Photo / 9 News Perth
The plane suddenly dropped 20,000ft, terrifying and panicking the 146 passengers. Photo / 9 News Perth

"The masks fell down and everybody started panicking. Nobody told us what was going on."

At the time, AirAsia apologised for inconvenience caused to passengers.

A final report is expected to be published by the Australian Transport Safety Bureau in May 2018.

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