Maintenance failure probable cause of Alaska Air 737 engine accident

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For a passenger looking out the window of a jet, it is alarming to see the metal shroud in the center section of the engine casing smash open, shred like thin paper, then rip off, throwing debris back into the wing and fuselage.

That was the unsettling view from windows Monday for passengers aboard an Alaska Airlines Boeing 737. When the flight to San Diego took off from Seattle-Tacoma International Airport, the covers on either side of the left engine housing — called the nacelle — came loose.

When the pilots immediately returned to Seattle, the covers tore off completely on landing and parts hit the fuselage.

The jet landed safely and none of the 182 passengers and crew on board were injured. The passengers were rebooked on another flight.

Dozens of such accidents have occurred over the past 30 years, much more frequently on the Airbus A320 than on the Boeing 737. Although some of these incidents have become more serious, none have resulted in injuries.

This type of failure is very different from the rarer but much more dangerous accidents in which an engine fan blade breaks off, causing catastrophic damage to the engine and throwing heavy metal fragments into the airframe. These are often caused by undetected long-term metal fatigue in the fan blades.

In contrast, in almost every instance of engine cowling flipping open and disintegrating, the cause has been attributed to error by the pre-flight maintenance mechanic and subsequent failure to observe that error during pre-flight checks.

The Alaska incident was the second such failure of a Boeing 737 in the United States this month. On a Southwest Airlines flight from Orlando, Fla., Aug. 12, the same covers ripped off as the jet landed in St. Louis. Again nobody was hurt.

Airbus was forced to change its A320 design after a series of such incidents.

Kyriakos Kourousis, a senior lecturer at the University of Limerick’s School of Engineering in Ireland who conducted a comprehensive investigation into the Airbus accidents and authored a paper examining the role of human error and the effectiveness of the modification, said it ” might be a good idea to do a follow-up survey on the 737 at this point.”

The parts that failed in all of these incidents are called fan shroud doors, covers on each side of the nacelle around the engine that flap up like gull wings to allow service mechanics access to the engine.

In the dozens of incidents where the fan shroud door was ripped off, it was mostly because a mechanic failed to relatch the doors properly before takeoff and the pre-flight inspections by the pilots and maintenance supervisors missed the error.

Contributing to such errors is that this maintenance work is sometimes performed in the dark or in a hurry because the next flight is behind schedule.

The maintenance and pre-flight inspection procedures are sure to be the first focus of an ongoing investigation by Alaska Airlines and the Federal Aviation Administration into Monday’s incident. The FAA said it is also investigating the earlier incident in St. Louis.

Aside from confirming their ongoing investigations, Alaska and the FAA declined to comment Tuesday on possible causes.

Airbus forced to remedy the situation

Fan cowl door failure in flight has historically been more of a problem on the Airbus A320 jet family.

At least 45 such incidents on its A320s over three decades – some involving jets with the same engines as the 737 – prompted Airbus to develop a modification that was then commissioned by both the European Aviation Safety Agency and the FAA.

Kourousis said the reason the 737 has a better record is simply because the latch for the fan shroud doors is close to the edge of the underside of the engine case and is therefore easier to see from the side.

In contrast, the latch of the Airbus fan cww doors is located in the middle of the underside of the nacelle and can only be inspected by crawling under.

“It’s easier for the Airbus jet if it’s not noticed,” Kourousis said.

Unlike the heavy-duty material used for the inlet shroud at the front end of a jet engine pod, which is designed to accommodate a fan blade that breaks off, the fan shroud doors are made of relatively thin aluminum.

Normally, when parts of these doors break off, all they can do is scratch or dent the wings, fuselage, and tail of the aircraft.

In some Airbus incidents, both coincidentally involving British Airways aircraft, the metal debris punctured a fuel line and caused an engine fire.

Following the second of these more serious incidents in 2015, Airbus was finally forced to act when the fan shroud doors of both engines became detached from an A319 as it took off from London Heathrow.

The fuel leak ignited an engine fire in flight. After landing, the passengers were evacuated on evacuation slides.

As this was the culmination of a series of incidents, Airbus responded by designing a modification to the door latches, which required locking and unlocking with a specific key. This key had a “remove before flight” flag affixed to it and had to be kept in a designated area in the cockpit.

Some airlines, including Air Canada and United Airlines, opposed this solution, arguing that human error could still be made with this arrangement.

United said it implemented a double sign-off procedure for fan cowl door latches and had no further incidents thereafter.

Despite these objections, the FAA and EASA ordered the Airbus modification in 2018 at an estimated cost of US$4.3 million for US airlines to repair 400 aircraft over three years.

Kourousis’ 2018 paper concludes that the Airbus A320 modification will “help positively” to reduce fan shroud door failures. However, it also recommended other procedural changes.

This included a double signoff process, as introduced by United; handover logs for maintenance shifts; and focused training to encourage better communication and a collaborative attitude in the airline’s maintenance culture.

Following the investigation into Monday’s incident, such “human factor” recommendations will likely be high on Alaska’s agenda to prevent future failures.

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