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Investigators trying to determine why the roof of a Southwest Airlines jet cracked open in flight have issued preliminary findings suggesting that there may have been flaws in the riveting work when the plane was built 15 years ago.
In its interim report the US National Transportation Safety Board (NTSB) said that some of the rivets used to bind the Boeing 737-300’s aluminium panels together were sunk in holes too large for the rivet shafts. The holes were not lined up correctly and many were misshapen, not round, the board said.
It did not offer any conclusions and said the investigation was continuing.
Southwest flight 812 was 34,000ft over Arizona with 117 passengers on board on 1 April when a 5ft-long hole opened along a row of rivets in the roof. The pilot guided the plane to a safe emergency landing and there were no injuries.
Some experts speculated that metal fatigue due to frequent take-offs and landings over 15 years caused tiny sub-surface cracks in the aluminium skin. But the latest evidence pointed to a likely flaw in the plane’s manufacturing and inspections of the process, said John Goglia, a former NTSB board member and expert on aircraft maintenance.
If rivet holes are even slightly too large, over time the flaw puts too much stress on the surface between holes and causes metal to fatigue or weaken, said Charles Eastlake, a former professor of aeronautical engineering at Embry-Riddle Aeronautical University in Daytona Beach, Florida. If the holes were too big, it would indicate a failure of the inspection process too, he said.
After the incident Boeing told airlines that own about 190 other 737s built in the 1990s to immediately conduct electromagnetic inspections of an area of the roof called the lap joint, where overlapping panels of skin are riveted together. The safety board said at least 136 of the planes, including all those registered in the US, had been inspected.
Boeing said small cracks were found in five other Southwest jets out of 79 grounded by the airline in early April, but not in planes operated by any other airline. All the aircraft with cracks were built within a two-year period in the mid-1990s at the same Boeing plant.
Boeing said it “would not speculate” on what the investigators’ early findings might say about the cause of the hole on flight 812 or cracks found in the other Southwest jets.
A spokesman said that Boeing was analysing portions of panels from those planes “to validate the initial inspection findings”, but added that no conclusions had been drawn.
The flight 812 plane had made about 39,000 flights. A senior Boeing engineer said this month that the company normally did not expect that airlines would need to inspect the lap joints for metal fatigue until an aircraft had completed about 60,000 flights.
Southwest defended its maintenance work, which came under heightened scrutiny because flight 812 was the second Southwest jet to develop a hole in the roof in the past three years. The company said safety was the airline’s top priority and that Southwest was “confident that our aircraft are fully compliant” with a new Federal Aviation Administration safety order requiring detailed inspections of older 737s.