The Qantas plane forced to make an emergency landing yesterday after a massive hole tore through its side had been plagued by a history of corrosion.

The Daily Telegraph of Australia can reveal engineers discovered a large amount of corrosion in the Qantas jumbo during a major refurbishment earlier this year.

The 17-year-old Boeing 747-400, registration VH-OJK, received a new interior at Victoria's Avalon airport in March.

Aviation sources last night revealed that aircraft engineers had noted a "lot" of corrosion during the refit.

QF Flight 30 from London to Melbourne had left Hong Kong airport after a stopover at 9 a.m. local time when a loud explosion ripped through the plane's underside.

Some of the 346 terrified passengers onboard the flight last night told of how debris — including bits of wood — flew through the first-class cabin and oxygen masks dropped down.

Experts said the hole, measuring about 2mx4m, could have been caused by metal fatigue, an internal explosion or a combination of both.

Any breach of an aircraft's skin above 10,000 feet can lead to a loud explosion, de-pressurization and an immediate loss of cabin pressure.

The plane — under the command of Captain John Bartels — was travelling at 29,000 feet when it lost pressure and descended 20,000 feet before making an emergency landing at Manila airport.

The damage occurred at a point called the wing root fairing, close to where the aircraft's highly explosive oxygen bottles are stored.

Australian investigators traveled last night to Manila to inspect the damaged aircraft, while Qantas said a replacement aircraft had been organized and was expected to leave Manila at 1 a.m. today.

According to aviation experts, investigators would also closely examine the maintenance regime of the ageing 747, registered in 1991.

They would also focus on what happened at Hong Kong airport before the flight departed.

"The aircraft are getting a bit old and the engineers have been on strike so they might be flying with lower maintenance standards," one insider said.

Under regulations, commercial aircraft are permitted to fly with known defects under official waivers.

The team of four investigators from the Australian Transport Safety Bureau will assist local authorities with their investigation into the incident.

When asked if terrorism and explosives could have been involved an ATSB spokesman said that would be "an aspect of the investigation."

However he stressed that the drama yesterday was being treated as an air safety incident.

While the cause of the explosion is yet to be determined, senior Qantas pilots last night expressed their fears the latest safety incident was a result of poor checks done overseas.

A senior Qantas pilot told The Daily Telegraph that yesterday's mid-air calamity could have been the result of the company's outsourcing of maintenance to Malayasia.

"This could be the result of Qantas having stand-in engineers, or from outsourcing to Malaysia," he said.

"It has been talked about a lot here and we have been told to be extra vigilant when you walk around the aircraft. Qantas outsourcing maintenance to Malaysia is certainly worrying a lot of us pilots. There has been aircraft come back with dodgy staples to secure wiring."

Qantas C.E.O. Geoff Dixon confirmed that the aircraft had "a hole in its fuselage" and that it was being inspected.

"The flight, which originated in London, landed in Manila about 11.15am local time," Mr Dixon said.

"All 346 passengers and 19 crew disembarked normally and there were no reports of any injuries to passengers or crew."

Unions have mounted aggressive campaigns against Qantas over its decision to permanently increase the amount of maintenance work it has done offshore.

They believe the overseas standards are inferior to Australia.

About 15-20 per cent of Qantas engineering has been done offshore for the past 50 years.

The latest incident comes several months after another Qantas 747-400 was forced to make an emergency landing in Bangkok.

In 1989 a United Airlines B747 en route to Australia suffered a major door failure which ripped open the fuselage and resulted in nine passengers being sucked out.

However the aircraft landed safely.