A fan blade missing from the shredded engine of a Southwest Airlines Boeing 737-700 is the first clue to the catastrophe that saw a woman partially sucked out of an aircraft window and seven people injured. National Transportation Safety Board chairman Robert Sumwalt told a briefing in Philadelphia Tuesday night that one of the left engine’s 24 fan blades - the number 13 blade - was missing. "This fan blade was broken right at the hub and our preliminary examination of this was that there's evidence of metal fatigue where the blade separated,'' he said, Sumwalt, who estimated the extensive investigation would take 12 to 15 months, said that photographs of metal fatigue had been sent to the NTSB's materials lab in Washington, DC. "There's much more to be done on this,'' he added.

Questions have already been raised about the similarity between this engine failure and one in 2016 which punched a 5-inch by 16-inch hole in the left fuselage of a Southwest flight traveling between New Orleans and Orlando, Florida. In that case, one of the engine's fan blades also separated from its hub during flight and the NTSB found cracking consistent with fatigue. Sumwalt said in an earlier briefing that it was unclear whether the two were related and the NTSB first needed to understand what had happened in the latest incident. At a separate briefing, Southwest chief executive Gary Kelly told reporters he was not aware of any issues with the aircraft or the engine involved. "The aircraft was delivered in July of 2000 and the last date that it was inspected was April 15," Kelly said, "So I don't have in front of me what inspection or what maintenance procedures were done but there's no information that there were any issues with airplane or the engines.'' Southwest Flight 1380 was travelling between New York’s La Guardia airport to Dallas Love and passing  through 32,500 ft about 20 minutes after take-off when shrapnel from the engine smashed through a window, causing the aircraft to depressurize and oxygen masks to deploy. Mutiple alerts sounded in the cockpit, prompting the crew to don oxygen masks and report to air traffic control that they had a number 1 engine fire and that they were operating on one engine and initiating an emergecy descent. They declared an emergency and were directed to Philadelphia International Airport where they safely made an extended final approach with the flaps set at 5 instead of the usual 737 setting. In the cabin, passengers struggled to pull the woman partially sucked through the window back into the cabin but witnesses said she was hanging outside the aircraft for some time before two men managed to pull her back in and a nurse administered CPR. She was later identified as Jennifer Riordan, a bank executive and mother of two from New Mexico. READ: Southwest passenger death ends 47-year record. "People in the other rows are — just trying to plug the hole, which sounds ridiculous because you know people are using jackets and things, and it's just being sucked right out," passenger Marty Martinez told CNN from the plane.

Martinez said a colleague started writing a final note to his loved ones as the plane made the emergency landing. “My colleague is sitting right next to me, and he's focused just looking down at his phone, and I glance over, writing his last words to his wife and his unborn son," he told the news network. Another passenger, Amanda Bourman, told CBS News: “I just remember holding my husband’s hand, and we just prayed and prayed and prayed.’’ Bourman said she was seated near the back of the plane when she heard a loud noise and oxygen masks dropped from the ceiling. “Everybody was crying and upset,’’ she said. “You had a few passengers that were very strong and kept yelling to people, you know, 'It’s Ok, we’re going to do this.'.” The fatality was the first in a US airline accident since 2009, according to the NTSB’s Sumwalt.  

The NTSB has already listended to cockpit voice recorder and was due to put together transcript Wedneday US time. Engine-maker CFM, a joint venture between General Electric and French company Safran Aircraft Engines,  said it had sent a team of technical representatives to the site to help investigators. It said the model CFM56-7B engine had compiled an “outstanding safety and reliability record since entering revenues service in 1997 while powering more than 6,700 aircraft worldwide". "The engine has accumulated more than 350 million flight hours as one of the most reliable and popular jet engines in airline history,'' it said. Southwest said in a statement:  "We are deeply saddened to confirm that there is one fatality resulting from this accident. The entire Southwest Airlines Family is devastated and extends its deepest, heartfelt sympathy to the Customers, employees, family members and loved ones affected by this tragic event. "We have activated our emergency response team and are deploying every resource to support those affected by this tragedy." ..