Tuesday, April 07, 2009

NTSB: American Airlines MD-82 engine fire caused by improper maintenance procedures

NTSB logoThe U.S. National Transportation Safety Board (NTSB) has just released a synopsis of its forthcoming final report on its investigation of a 2007 engine fire on an American Airlines MD-82 aircraft. The report cites "American Airlines’ maintenance personnel’s use of an inappropriate manual engine‑start procedure" as the probable cause for the engine fire, and says that the fire "was prolonged by the flight crew’s interruption of an emergency checklist to perform nonessential tasks." The Board also found that the airline's "flawed internal safety management system, which could have identified the maintenance issues that led to the accident" was a contributing factor.

The Accident

In the early afternoon of September 28, 2007, an American Airlines MD-82 aircraft (registration N454AA) departed Lambert-St. Louis International Airport (STL), operating as American Airlines Flight 1400. The aircraft's number one engine caught fire during climb-out. The crew returned to STL, but during approach the nose gear failed to extend. A go-around was executed, and the crew then extended the nose gear using emergency procedures and successfully landed the aircraft.

The two pilots, three flight attendants, and 138 passengers deplaned on the runway. There were no injuries, but the aircraft was substantially damaged.

Probable Cause

Quoting directly from the NTSB Report Synopsis:
The National Transportation Safety Board determines that the probable cause of this accident was American Airlines’ maintenance personnel’s use of an inappropriate manual engine‑start procedure, which led to the uncommanded opening of the left engine air turbine starter valve, and a subsequent left engine fire, which was prolonged by the flight crew’s interruption of an emergency checklist to perform nonessential tasks. Contributing to the accident were deficiencies in American Airlines’ Continuing Analysis and Surveillance System program.

The NTSB investigation found that a component in the manual start mechanism of the engine was damaged when a mechanic used an unapproved tool to initiate the start of the #1 (left) engine while the aircraft was parked at the gate at STL. The deformed mechanism led to a sequence of events that resulted in the engine fire, to which the flight crew was alerted shortly after take-off.

The origin of the problem was traced to a faulty air turbine starter valve (ATSV) air filter. The NTSB says the filter had not been cleaned by maintenance personnel in accordance with C check cleaning procedures, thus, an opportunity to identify and replace the damaged filter was missed.

Investigators found that in the 13 days prior to the accident flight, the aircraft's left engine air turbine starter valve had been replaced a total of six times in an effort to address an ongoing problem with starting the engine using normal procedures. None of valve replacements solved the engine start problem and the repeated failures to address the issue were not recognized or discovered by the airline's Continuing Analysis and Surveillance System (CASS).

Then, says the NTSB, the following sequence of events occurred on the day of the accident:
  • The filter element of the air turbine starter valve-air filter disintegrated, allowing the end cap to become free, which blocked the air flow and caused the engine no-start condition.
  • American Airlines’ maintenance personnel’s troubleshooting efforts for the engine no-start condition incorrectly focused on the air turbine starter valve (ATSV) and engine start system wiring because of the intermittent nature of the condition, the history of ATSV electrical circuit problems, and the lack of a history of ATSV-air filter failures for which no troubleshooting guidance existed.
  • American Airlines’ maintenance personnel repeatedly used an unapproved maintenance procedure, which included using a prying device to push the air turbine starter valve manual override button, to manually start the accident engine, which resulted in bending the internal pin in the override button.
  • The internal pin in the left engine air turbine starter valve (ATSV) override button was bent, which resulted in the uncommanded opening of the ATSV during high‑power engine conditions at the beginning of the takeoff roll and caused the air turbine starter to freewheel until it sustained a catastrophic internal failure.
  • The open air turbine starter valve and resulting failed air turbine starter allowed a hotter than typical airstream and/or incandescent particles to flow into the engine nacelle area and likely provided the ignition source for the in‑flight fire.
The NTSB notes that fire damage to the engine precluded the determination of the specific source of the combustible fluid (oil, hydraulic fluid, or fuel) that fed the fire, once ignited.

The report suggests that, during take-off, the pilots may not have immediately noticed that the air turbine starter valve (ATSV)-Open light was illuminated "because of its location, static appearance, and color." Once they did detect the light, "the pilots did not immediately respond to it because an open ATSV was considered an abnormal situation that did not require immediate action and they were involved in air traffic control communications and airplane configuration changes."

The NTSB report comments on  what it calls the pilots' "poor performance" during the emergency. A press release that accompanied the report stated that the Board was particularly concerned with how the crew repeatedly interrupted their completion of the emergency checklist items with lower priority tasks.

Specifically, the report says, the pilots failed to properly allocate tasks, including checklist execution and radio communications, and "they did not effectively manage their workload, which adversely affected their ability to conduct essential cockpit tasks, such as completing appropriate checklists."

"Here is an accident where things got very complicated very quickly and where flight crew performance was very important," said NTSB Acting Chairman Mark V. Rosenker.  "Unfortunately, the lack of adherence to procedures ultimately led to many of this crew's in-flight challenges."

Among the Board's conclusions regarding the crew members' behavior:
  • The pilots’ interruption of the emergency Engine Fire/Damage/Separation checklist at a critical point prolonged the fire and led to additional problems, including the loss of hydraulic pressure, which caused the nose landing gear to fail to extend.
  • Given the airplane’s altitude and the lack of time to prepare for a nose landing gear up landing, the captain’s decision to go around was a reasonable choice.
  • The captain’s decision not to conduct an emergency evacuation after the airplane landed was in accordance with company guidance and was appropriate because the fire was not severe and aircraft rescue and firefighting personnel were actively responding to the residual fire.
  • The Safety Board concludes that the incident commander’s decision to deplane the passengers after fuel spilled out of the engine area was prudent.
  • The first officer did not have a clear understanding of the relationship between the pneumatic crossfeed handle and the engine fire handle, most likely because of inadequate company guidance and training on the issue, which resulted in the first officer inadvertently reintroducing fuel to the left engine, creating potential unnecessary risk of fire.
  • The casual atmosphere in the cockpit before takeoff affected and set a precedent for the pilots’ responses to the situations in flight and after landing, eroded the margins of safety provided by the standard operating procedures and checklists, and increased the risk to passengers and crew.
  • During the emergency situation, the flight attendants did not relay potentially pertinent information to the captain in accordance with company guidance and training.
The NTSB has issued nine safety recommendations arising from the findings of this investigation. Eight of those recommendations, mostly related to maintenance issues and crew training standards, were made to the Federal Aviation Administration (FAA).

The final recommendation was directed to American Airlines:
Evaluate your Continuing Analysis and Surveillance System program to determine why it failed to (1) identify deficiencies in its maintenance program associated with the MD‑80 engine no‑start failure and (2) discover the lack of compliance with company procedures. Then, make necessary modifications to the program to correct these shortcomings.
The NTSB Acting Chairman said, "The airline's own internal maintenance system, the purpose of which is to catch maintenance and mechanical issues that could lead to an incident or accident, failed to do what it was designed to do. And that allowed this sequence of events to get rolling, which ultimately resulted in the accident. Following the appropriate maintenance procedures would have gone a long way toward preventing this mishap."

Here is the link to the Report Synopsis: NTSB/AAR-09/03

RELATED: American Airlines MD-82 engine fire and emergency landing at St Louis