Passengers have been left in shock as a plane plunged 30,000 feet midair, with no crew explanation before landing.

An experienced pilot has now weighed in to explain why.

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Virgin flight VA993 was flying from Sydney to Brisbane on Wednesday night when the plane suddenly dropped from 37,000 feet to 8775 feet about 9.10pm.

In the space of five minutes, the aircraft plunged about 30,000 feet — roughly the height of Mount Everest — as oxygen masks deployed from the ceiling.

The flight eventually landed safely, but passengers were not briefed by the crew before then.

Virgin Australia confirmed the flight lost cabin pressure in the air.

“We can confirm that VA933 Sydney to Brisbane arrived safely at Brisbane Airport after experiencing a depressurisation event on Wednesday, 13 August,” a spokesperson said in a statement.

“The flight crew took the appropriate steps, following standard operating procedures, to descend to a lower altitude. As part of that process, a PAN call was transmitted to Air Traffic Control.”

Former Qantas pilot David Oliver described such depressurisation events as rare.

“It’s not common at all. It’s not unheard of, but it’s not common,” Oliver told Sunrise.

He added that the pilots followed the correct procedures to descend the aircraft as quickly as possible.

Oliver explained that when a plane is cruising at 35,000 feet, the cabin is normally pressurised to feel like 6000 feet.

But if the cabin suddenly loses pressure, the air thins rapidly, making it feel much higher.

Once the cabin altitude rises above 14,000 feet, oxygen masks automatically deploy.

“It’s important for the aircraft to descend because there’s only a fixed amount of oxygen that’s available through those masks,” Oliver said.

“So I have to get the aircraft down to an altitude where people can breathe normally without the assistance of the oxygen masks.”

Under such circumstances, pilots are under a high workload, managing the emergency descent and other tasks on the flight deck, which Oliver said explains why the crew didn’t have time to make a public address announcement.

No passengers or crew were injured, and the aircraft is currently under review by engineers.