American Airlines failed to catch repeated errors by mechanics before a September 2007 flight that made an emergency landing after one of its engines caught fire during departure.
The 143 people onboard weren't injured, but the incident could have become catastrophic because of additional mistakes by the flight crew, members of the National Transportation Safety Board said Tuesday.
The NTSB chairman said the airline "failed to do what it was supposed to do" in the weeks leading up to the incident.
Investigators found that in 13 days prior to the incident, the engine's starter valve had been replaced six times. None of the replacement valves solved the engine troubles.
The board said that repeated failures to address the problems were not uncovered by the airline's internal system designed to monitor ongoing mechanical problems that could lead to an accident.
The investigation discovered that, as a result of the faulty starter, mechanics damaged the engine's manual start system prior to take-off in St. Louis, when they used an unapproved tool to start the engine. That damage ultimately led to the fire.
The four-member board recommended changes in pilot training programs to take into account simultaneous emergencies.
The findings come as American faces heightened scrutiny by the Federal Aviation Administration.
The agency recently assigned a special team of 17 inspectors to examine American's aircraft maintenance and other operations. The special audit is expected to take about three months.
American spokesman Tim Wagner said the airline is changing training procedures for mechanics and pilots as a result of the incident, and hiring more auditors to review maintenance work.
NTSB's issues "were with our personnel not following our procedures rather than any problem with our procedures," he said.
Wagner said the flight crew, "overcame numerous difficulties to safely land the plane with no injuries to anyone on board."
During the fire, the flight crew made several mistakes that acerbated the problem and could have led to a more serious accident, investigators said.
In a statement, the NTSB said the performance of the pilots on the flight was "lacking." Investigators found the captain did not divide tasks between himself and the first officer to "most effectively deal with the emergency in a timely manner."
"Unfortunately, the lack of adherence to procedures ultimately led to many of this crew's in-flight challenges," the NTSB chairman said.
The pilot interrupted his emergency checklist to inform passengers of the trouble, which delayed his shut-off of fuel to the fire and allowed the fire to burn longer, investigators said. That led to the damage to the hydraulic system, they said.
"We probably wouldn't be here talking if he had done that checklist in a timely manner," investigator Dave Tew said.
The co-pilot was engaged in trying to wrestle the cockpit door closed after the fire partially shutdown the aircraft's electrical system, which released the automatic door lock, they said.
"It seems to me it was a series of people taking shortcuts that accumulated on this particular day into what could have been much more catastrophic," said safety board member Kitty Higgins.
As a result of the investigation, the NTSB has issued nine safety recommendations to the FAA, including a recommendation that the agency look into the history of similar starter problems on other MD-80 airplanes, the most common type of plane in American's fleet.