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UNCONTAINED ENGINE FAILURES
An airworthiness directive (AD) calling for removal, inspection and reworking of high pressure turbine (HPT) disks beginning at 6,900 cycles is not good enough, declares the National Transportation Safety Board (NTSB). Disks with more than 3,000 cycles since new or since the last inspection should be removed immediately for maintenance, the NTSB says.

"This significantly more stringent standard would not permit disks to remain in service without inspection beyond the earliest know number of cycles at which cracks have been detected or failure has occurred," the NTSB intoned in a 12-page letter of August 28 to the Federal Aviation Administration (FAA).

The immediate subject of the Safety Board's concern was the June 2 uncontained failure of the HPT stage 1 disk on a General Electric CF6-80A engine mounted on the left side of an American Airlines B767-200. The incident occurred at Los Angeles International Airport during a ground test of the engine.

The engine was a total loss, and the airplane was damaged substantially. The three technicians aboard the airplane at the time, and a fourth technician observing on the ground, were not injured.

The maintenance technicians told investigators they were performing a high-power run-up when the failure occurred. They heard a loud explosion, observed fire under the fuselage and aft of the wing, and shut the engine down.

According to the NTSB, "Post-incident examination of the No. 1 engine revealed that the HPT stage 1 disk had ruptured and completely split the engine, with the fan, booster, high pressure compressor, and combustor hanging from the forward-engine mount and the low pressure turbine and exhaust hanging from the rear-engine mount."

One piece of the shattered disk bounced off the ground and sliced into the airplane, completely severing the left-hand keel beam and partly severing the right-hand keel beam before exiting the fuselage and striking the No. 2 engine, lodging in the exhaust. One piece from the ruptured engine was found some 2,500 feet away, lying against the airport perimeter fence after crossing two active runways and taxiways. An Air New Zealand B747 had just landed on one of these runways and was taxiing to the terminal at the time of the American engine failure.

Of interest is the damage caused to the right engine by the ruptured disk on the left engine. The left side of the right engine's nacelle was peppered with holes, in addition to the piece of the HPT disk that was lodged in the exhaust duct. In addition, the engine rupture caused numerous holes in the left and right wing's fuel tanks. The fuel leaking out of the left wing fed a ground fire that burned the left wing and a portion of the fuselage aft of the wing.

More than 100 firefighters responded to the event, which took some seven hours to resolve. The fire had to be put out (reports indicate the fire was brought under "control" in 16 minutes), then the fuel on the ground had to be sopped up and disposed of. The firefighters were confronted with a significant fuel leak from the left engine. Foam was used to cover the fuel on the ground to prevent ignition and to forestall the possibility of a larger conflagration. Firefighters and airport personnel offloaded the remaining fuel, limited the spilled fuel from reaching storm drains, and rendered the situation safe. Using dikes, vacuum trucks, and de-fuelers, some 10,000 gallons of fuel were successfully offloaded and contained.

"The American Airlines incident raises serious safety concerns because, if it had occurred during flight rather than on the ground during maintenance, the airplane may not have been able to maintain safe flight," the NTSB cautioned.
One commentator offered the following:
"Let me see if I got the picture:
During a power run for maintenance, cracking and rupture of a HPT stage 1 disk, No. 1 engine.
Split the engine in half.
Fire under the wing.
Rim to bore fracture.
Disk separated from the shaft and completely missing from the engine.
Widespread damage beyond the affected engine.

A threat to anyone if it happens in the air. ETOPS (extended range twin engine operations) involved.

But I'm not talking about this latest incident, but another: a US Air 767-200(ER) back in September 2000 at Philadelphia. And in December 2002 an Air New Zealand 767-200(ER) climbing through 11,000 feet experienced a similar failure and returned to Brisbane.

So let's stop talking about a mere 'fire' at Los Angeles and ask what's been done since 2000 about exploding engines every bit as powerful as a bomb in a cargo hold."

Following the US Air incident, the NTSB in its safety recommendations expressed its concern and also asked the FAA for a design safety review.

To be sure, there is a history of service bulletins from the engine manufacturer and a history of ADs from the FAA.

The NTSB now says, "The fact that an uncontained failure of an HPT stage 1 disk recurred indicates that further actions are necessary."

OTHERWISE SURVIVABLE
Over 125 accidents involving light planes, in which 205 people were killed, were otherwise survivable, but post-impact fire had a lethal effect.

Accordingly, the Transportation Safety Board (TSB) of Canada has called on regulatory authorities to change how small planes are built in order to make them more resistant to post-crash fire, and to retrofit existing planes to make them safer.

By small airplanes is meant those with a take-off weight of 5,700 kilograms (12,566 pounds) or less. By virtue of its call for retrofit, the TSB recommendation could affect hundreds of thousands of general aviation (GA) airplanes.

In a safety issues investigation report (SII A05-01) announced August 29, the TSB said its study of accidents involving these aircraft found that post-impact fire (PIF) "contributes significantly to injuries and fatalities in accidents that are otherwise potentially survivable."

The TSB examined 521 accidents that took place between 1976 and 2002. Of these, 128 were accidents in which fire or smoke inhalation was identified as either partly or solely the cause of death or serious injury (see box).

"If people can survive the impact, the airplane should not burn," said Bill Kemp, the author of the TSB report. "What we're suggesting is to design the airplane so that it won't burn when the impact forces are within the range of human survivability."

The TSB study was prompted by the May 2000 death of a Cessna 177B Cardinal pilot, who was attempting to take off from a grass airstrip in Alberta. The aircraft hit trees during the initial climb, struck the ground and burst into flames. The pilot died from the effects of the fire, and the passenger was seriously injured as well as burned attempting to extricate the pilot from the wreckage.

In the report of investigation into that crash, the TSB noted that the U.S. National Transportation Safety Board issued recommendations regarding the vulnerability and susceptibility of light airplanes to post-impact fire, that the Federal Aviation Administration studied the issue, and that no tangible action resulted.

Kemp's study noted that Canada and the U.S. have adopted stringent safety standards for highway motor-vehicle design, resulting in crash-worthy standards for automobiles and trucks that "surpassed those of small aircraft, especially in control of ignition sources, fuel system integrity and occupant protection in crash conditions."

"The race-car industry has applied appropriate technology to their cars. We have many examples of cars that crashed at close to the same cruise speed of a small aircraft and there's been no fire," Kemp said. "We're suggesting this technology is transferable to small aircraft."

For example, fuel tanks placed at the front of a wing, rather than behind the protective spars, are at extreme risk of rupturing during a crash.

"None of this stuff is perfectly cut and dried," Kemp said. "If you relocate the fuel tanks, you have to reconsider weight and balance issues as well." Kemp argued that airplanes should be designed with an eye to reducing the risk of post-impact fire.

Retrofitting technologies for existing aircraft is also possible, he said.

For example, leaked fuel is a danger if it ignites. To prevent this ignition, one approach might involve turning off the battery and electrical systems to eliminate the potential for electrical arcing, which could ignite fuel-air vapors.

"If you have damaged wiring in a small aircraft accident and the battery remains on line, you can get high temperature arcing. If that arcing happens in close proximity to leaking fuel, you're going to have a fire, guaranteed," Kemp said.

For both new and existing aircraft, there are maintenance implications to the TSB recoimmendations.

The Canadian Study
Results
In all 128 accidents in which PIF contributed to serious injuries or fatalities, the aircraft occupants were in close proximity to fire or smoke for some time following the impact. The investigation identified four conditions that were essential for this to occur:

• There was an ignition source in proximity to a combustible material, such as fuel.
• There was combustible material in close proximity to the occupants.
• Occupant egress was compromised.
• The fire was not suppressed in time to prevent fire-related injuries or fatalities.

 

Conclusions
PIF presents a great risk to the occupants of small aircraft because of

• The high volatility of aviation fuel;
• The close proximity of fuel to occupants;
• The limited escape time;
• The limited energy-absorption characteristics of small-aircraft airframes in crash conditions;
• The high propensity for immobilizing injuries, and;
• The inability of airport firefighters and emergency response personnel to suppress PIFs in sufficient time to prevent fire-related injuries and fatalities.

Considering the propensity for rapid propagation and the catastrophic consequences of fuel-fed PIF, the most effective defense against PIF is to prevent the fire from occurring at impact, either by containing fuel or preventing ignition, or both.

The benefits of PIF-resistant fuel system technology have been proven in land vehicle applications and, recently, in certified helicopter applications. A requirement for similar engineering countermeasures in existing, newly manufactured and newly certified [aircraft] … would reduce the incidence of fire-related serious injuries and fatalities in otherwise survivable accidents, and could significantly increase the rate of occupant survival.
The implication of design improvements on new aircraft will be significant, and even more significant on existing designs. Source: TSB, SII A05-01