Severity was understated in test of jet engine at LAX

Engine on American Airlines Boeing plane blew apart during a check nearly two weeks ago, causing a fire and sending metal half a mile away.

Thursday June 15, 2006

US National Transportation Safety Board is investigating an uncontained engine failure on an American Airlines 767 that occurred during a ground maintenance test run on June 2 at Los Angeles International. According to NTSB, the HPT stage one disk on the number one GE CF6-80A2 broke into several pieces that "were found embedded in the fuselage, the number two engine, and scattered as far 3,000 feet from the airplane." The ensuing holes in the wings caused fuel leaks that resulted in a ground fire that was extinguished by airport firefighters. There were no reported injuries. Initial examination of the disk pieces found indications of fatigue cracking, the board said.

An explosion that ripped apart the engine of an American Airlines jet during a ground test at Los Angeles International Airport this month was far more dangerous than first reported, new details suggest.

The blast was strong enough to hurl an 18-inch chunk of metal more than half a mile -- across taxiways, service roads and two active runways. Airport workers found the piece two days later, not far from the airport's perimeter fence.

The investigation into what caused the engine to explode has focused in part on tiny cracks found on a key piece of the turbine. An Air New Zealand jetliner lost the same kind of engine to similar cracks in late 2002 and was forced to make an emergency landing.

"It was pretty fortunate that no one was hurt," LAX spokeswoman Nancy Castles said, "and that no planes were taking off or landing at that time."

The Boeing 767 jet had arrived in Los Angeles on a regular flight from New York City. Its flight crew had reported some kind of mechanical problem, so the airline sent the plane to maintenance after it landed at LAX.

An airline spokesman would not discuss that initial problem in any detail, saying only that it was unrelated to the engine failure that followed.

Workers were still trying to figure out what was wrong with the airplane when they pushed the throttle for both engines, and one of them blew apart.

The explosion outside the American Airlines maintenance hangar on June 2 sparked a small fire that sent a column of dark smoke over the passenger terminals at LAX and drew most of the initial attention. The explosion -- officially an "uncontained engine failure" -- gutted the engine and blackened part of the airplane's fuselage.

It also blasted pieces of the engine onto a nearby runway -- and, in one case, clear across the southern airfield. The workers who found that piece half a mile away described it as a wedge of metal, 2 inches thick, and heavy.

The NZ 767 Incident

The early built turbine disks were cut by cutting tools which then made about 10 or so turbine disks. Then the cutting tools were replaced and another 10 or so turbine disks were cut from stock.
Apparently the disks that have been failing are the ones that were made/cut last before changing the cutting tools. So now when turbine disks are made, the cutting tools are replaced sooner. This apparently made for a better quality turbine disk. This was on the early CF6-80 A series engines.
When the disk let go on Air NZ's 767 out of Brisbane, Australia, the engine stopped rotating, from climb power, in 42 revolutions. We counted this on the engine casing.
The force of this sudden stop made the aircraft yaw so hard that the co-pilot hit his head against the pillar between the windows. There were rumors that he was knocked out, but I'm not sure about that one.
Boeing was glad they strengthened up the pylon struts coz the engine would have fallen off the wing and I guess the airframe would have been lost. They knew this because a few years before Air NZ's incident, a similar disk departure happened during a maintenance ground run where the entire engine fell off the wing and sat on the ground.
Ever wondered why there is a 'dry bay' area in the wing just above the turbine's? It's so that an uncontained turbine failure, should it happen into the wing, can be survivable.

Other pieces punched through the airplane's fuselage and wings and embedded themselves in its other engine, according to a statement released Tuesday by the National Transportation Safety Board. Robert Ditchey, an aviation consultant in Marina del Rey who once oversaw maintenance for Pan American World Airways, called that especially worrisome.

Airline engine systems, he explained, have extra shielding that's supposed to protect the rest of the airplane from that kind of damage. Pieces of the engine, he said, "are not supposed to penetrate the fuselage" under any circumstance.

Three maintenance workers who were on the plane when the engine blew escaped without injury. Nobody on the ground was hurt, either -- despite the blast of metal pieces across taxiways and service roads usually swarming with airport workers.

Two federal agencies are looking into the explosion: the transportation safety board and the Federal Aviation Administration. Both have offices here but, in a sign of how important this investigation is, both sent special investigators from their Washington, D.C., headquarters.

Their investigations will determine whether the flaw that destroyed the engine was an isolated problem or could affect other airliners still in service. "I don't think there's any indication right now that this is a systemic problem," FAA spokeswoman Laura Brown said.

Investigators have found evidence of "fatigue cracking" on pieces of a disk from the engine's turbine, the safety board said in its brief statement.

Such cracks can develop from a microscopic flaw in the metal and weaken engine parts against the extraordinary stress they must endure.

The same kind of engine, built by General Electric, was the subject of an FAA order in 2003 that required regular inspections for fatigue cracks. The order was prompted by the emergency landing of the Air New Zealand plane after one of its engines spun apart and damaged a wing.

Such cracks are rare, Ditchey said, and should be caught by the rigorous inspections -- "down to the last nut and bolt" -- that airlines put their engines through. The materials used to build an aircraft engine, he added, are "the highest tech of the high-tech. It doesn't get any fancier than that."

Investigators have shipped pieces of the turbine disk shattered in the LAX explosion to the transportation safety board's laboratory in Washington, D.C., for further tests. The engine itself was sent to an American Airlines maintenance center in Oklahoma.

Airline spokesman Tim Wagner said he did not know how long the airplane had been in service. He said workers were conducting a routine maintenance check known as a run-up to diagnose an unrelated issue when the engine broke apart.

The airline, Wagner said, is "still in the process of refurbishing the aircraft."

http://www.dailybreeze.com/news/articles/3048421.html?page=2&c=y

Quote:
Originally Posted by Volume
Scary to see a full turbine disk having left the engine. During certification normally one third of a disk is seen as the worst case scenario, and even for such event a small risk of a catastrophic result is accepted.
I wonder whether the NTSB will analyse this event, as it was not during operation. I would be really interested in the chain of events of the breakup, it it is possible to determine it. Normally you assume that the fracture of a turbine disk causes the failure, but the disk is in one piece this time.

It looks that way in the LAFD pictures, but one of the shots that the person

 kindly provided here provides a different view:


There was an AD on the subject of cracking on the dovetail slots of HPT disks on the CF6-80, along with service bulletins from GE. Interesting as related reading: see link

 

 

N330AA (Friday's fire) was a 767-200 (-223 if memory serves) with CF6-80A2 donks - the baseline 767 for primarily domestic routes. N654US (US Airways PHL 9/2000) was a -200ER with -80C2B2's; bigger fan, an extra LPT and booster (LPC) stage for better SFC for international routes.
 

  

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