Engineers have found cracks in three more Southwest Airlines planes after a hole ripped open in an aircraft carrying 123 people, forcing it to make an emergency landing, the US airline said.
The airline canceled a further 70 flights, after some 300 were taken off the schedule following the mid-air scare on a flight from Phoenix, Arizona to Sacramento, California.
Fifty-seven aircraft have so far been tested and returned to service, Southwest said in an update on inspections on 79 Boeing 737-300 planes in the company's fleet, to address possible "skin fatigue" on ageing airliners.
"Additional inspections have identified three aircraft with the subsurface cracks. Those aircraft will remain out of service until appropriate repairs can be completed," it said.
"The airline continues to inspect aircraft and will put those with no findings back into service," it added, saying the tests were designed to detect "any subsurface fatigue in the skin that is not visible to the eye."
No-one on Flight 812 was seriously hurt when the hole tore open with a loud bang, causing a sudden loss of pressure and triggering passengers to grasp for oxygen masks and frantically text loved ones.
The plane, suffering from a five-foot by one-foot (1.5-meter by 0.3-meter) gash in its roof, landed without incident at an Arizona military base, where the National Transportation Safety Board (NTSB) is examining the damaged jet.
"We did find evidence of widespread cracking across this entire fracture surface," NTSB member Robert Sumwalt told a press briefing broadcast from Yuma via Internet on Sunday.
A further NTSB briefing was scheduled for later Monday.
Southwest said the plane had undergone all necessary Federal Aviation Administration inspections, including the extensive overhaul in March 2010, and was given a routine inspection on Tuesday, according to Fox News.
Southwest's entire fleet of 548 planes is comprised of 737s, including about 170 Boeing 737-300s, the oldest in the fleet. The airline's planes average 11.2 years of age. Boeing 737-300s were produced from 1984 to 1999.
Incident prompts tests order for 175 Boeing planes
The US Federal Aviation Administration said it will order inspections of some 175 older Boeing 737s worldwide after a fuselage rupture forced the Southwest Airlines flight into an emergency landing.
"The FAA will issue an emergency directive tomorrow that will require operators of specific early Boeing 737 models to conduct initial and repetitive electromagnetic inspections for fatigue damage," the aviation watchdog said in a statement.
The action initially will apply to a total of about 175 aircraft worldwide, 80 of which are registered in the US, the FAA said.
Most of those are operated by Southwest Airlines, it added.
"Safety is our number-one priority," said Transportation Secretary Ray LaHood in the statement.
"Last Friday's incident was very serious and could result in additional action depending on the outcome of the investigation."
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