They got lucky. 138 passengers continued on with their day even after an engine caught fire shortly after American Airlines Flight 1400 departed Lambert-St. Louis International Airport back in 2007. The pilots, while thankfully bringing the McDonnell Douglas MD-80 jet safely back to the airport without injury, were found to have prolonged the fire and danger by failing to follow a checklist that walks them through the procedures to deal with such emergencies.

It would have been as simple as shutting off the supply of fuel to the burning engine. But for all the confusion they didn't. In this case it didn't cost anything more than money. It could have turned out much worse.

The pilots aren't the only one's to blame, though. There were also records from the National Transportation and Safety Board (NTSB) indicating that the jet's left engine failed to start. More than once. By forcing the engine to start by circumventing normal starting procedures, the mechanics doing maintenance on the engine allowed it to fly even though they started it using unapproved methods. By using a screwdriver to open a valve, they damaged a pin which triggered a spark inside the engine cover that caused the fire.

The safety board determined that American mechanics replaced the start valve six times but missed the cause of the failed engine starts: a worn-out, stainless steel air filter. American's audit team also failed to identify the cause of the problem, the NTSB said.

"Where they have found deficiencies in how our personnel performed, we are going to strengthen our training," said Tim Wagner, an American Airlines spokesman.