Requirements for Engine Containment Failure Clearly Exceeded in A380 Incident

Wed, Nov 10, 2010 — David Evans


Qantas’ grounding of the A380 until more can be sorted out about what happened during the uncontained engine failure 4 November may be fully justified as more details emerge about the seriousness of the event. (See Aviation Safety Journal, “A380 Engine Failure Raises Question About Tolerance of Uncontained Failure”)

Photographs taken by passengers reveal what has been identified as part of a turbine blade passed through the wing above the No. 2 engine and lodged a short distance behind the leading edge. The puncture was ahead of the fuel tank in the wing, but smaller holes further aft suggest that some of the flying debris from the engine may have penetrated the fuel tank, which would explain accounts of fuel and fuel vapor trailing off the wing after the engine failure.

If the wing tank was indeed penetrated, the Airbus argument against inerting the tank with nitrogen enriched air, as called for by the U.S. National Transportation Safety Board (NTSB), seems bogus. In the wake of the TWA 800 explosion of the B747-100’s center wing tank, the NTSB recommended that all airplanes with heat sources near center wing tanks be inerted. Airbus argued against this requirement for the A380 on the grounds that the wing did not feature any nearby heat sources and therefore inerting was not necessary.

If the wing tank was punctured by engine debris, however, the fuel tank – under the right conditions of temperature and fuel tank vapor — may be explosive, requiring only an ignition source to ignite. A piece of hot metal may provide this ignition source.

There is also indirect evidence that the major puncture in the wing leading edge severed some of the wiring to the outermost number 1 engine.

Puncture aft of the "no step" line probably penetrated the wing fuel tank.

Puncture aft of the "no step" line probably penetrated the wing fuel tank.

The evidence comes from a photograph of fire trucks dousing the engine after the airplane came to a stop at Singapore’s Changi airport. Apparently, as long as fuel was supplied to the engine, it could not be shut down. If the engine could not be shut down from the cockpit, or the fuel flow stopped, the wiring in the leading edge may require separation and additional protection from shrapnel.

Photos also show that at least a part of the hydraulic functions of the A380 had been rendered inoperative by the burst engine. Visual clues to this effect are twofold. One, that the nosewheel may have required a gravity drop rather than the hydraulics to power it into the locked down position. When the nosewheel descends normally, the doors to the nose landing gear normally return to the closed position after the gear extends. That is not the case in the A380 emergency landing, where the doors are shown in the open position, as would be the case if gear extension was by gravity.

Deployed nose gear doors suggest that some hydraulics were not functioning.

Deployed nose gear doors suggest that some hydraulics were not functioning.

Two, it is possible that leading edge slats on the wing were not deployed before landing, resulting in a faster and longer landing roll. However, leading edge slats and traling edge flaps are normally retracted after landing.

The notion of triple or quadruple redundancy to enable the pilots to handle a localized failure is in need of serious inquiry. If an uncontained engine failure can lead to electrical and hydraulic failure – despite redundancies – we have only the illusion of redundancy.

The pilots evidently took their time dumping and consuming fuel while they assessed the situation and discussed it with Qantas operations. Most likely, Airbus and engine manufacturer Rolls-Royce was consulted, as well as the airport authorities.

There would have been careful consideration of everything that could go wrong in an emergency landing. It seems probably that a Qantas response team may have realized how potentially catastrophic was the engine failure. Qantas faced a major problem rebooking literally thousands of passengers brought on by the decision to ground the carrier’s six A380s. An estimated 2,400 passengers were affected.

A Qantas statement said, “We have commenced our own investigation as to how this incident occurred and have notified the ATSB [Australian Transport Safety Bureau]. We will continue to work with them as they investigate this issue.”

Certification of the Trent 900 series engine will doubtless come under scrutiny. FAA type certification data sheets indicated that the engine provides about 80,000 pounds of takeoff thrust. The engine contains 35 quarts of lubricating oil; the low-pressure (LP) and intermediate-pressure (IP) assemblies rotate independently. The blade containment requirement was revised in 1984 to reflect the following:

Part 33 Airworthiness Standards; Aircraft Engines

Sec. 33.27 General design and construction

… The design and functioning of engine systems, instruments, and other methods … must give reasonable assurance that those engine operating limitations that affect turbine, compressor, fan, and turbocharger rotor structural integrity will not be exceeded in service.

Sec. 33.94 Blade containment and rotor unbalance tests

… it must be demonstrated by engine tests that the engine is capable of containing damage without catching fire and without failure of its mounting attachments when operated for at least 15 seconds …

A Trent 900 design failing prefigured by problems with the Trent 700 and 800 series engines?

A Trent 900 design failing prefigured by problems with the Trent 700 and 800 series engines?

Obviously, “in service” failure exceeded the certification requirements, raising significant questions about the rigor and realism of those standards – and the false redundancy on which the electrical and hydraulic systems were certified.

Over this last weekend, Qantas engineers conducted eight hours of extensive tests on the A380 engines; oil leaks were found in three of them. Qantas CEO Alan Joyce said, “The oil leaks were beyond normal tolerances.”

Thus, excessive oil leaks in the turbine area have now been found. The source of the uncontained shrapnel was in the Trent 900’s turbine area.

It is important to note that engines carry forward design practices found in earlier variants – in this case the R-R Trent 700 series, which is found in Airbus A330 airplanes, and the Trent 800 series, found on the Boeing 777.

Both the 700 and 800 series engines have an oil vent tube clogging problem. Could the same oil vent tube construction and problem carry forward to the 900 series engines. Consider the following airworthiness directives (ADs):

AD 2007-02-09, issued 2 April 2010: Refers to R-R Trent 800 series mounted on Airbus A330 airplanes. Requires initial and repetitive borescope inspections of the high pressure and intermediate pressure (HP-IP) turbine internal and external oil vent tubes for coking and carbon buildup, and cleaning or replacing the vent tubes as necessary.

AD 2010-06-14, issued 29 March 2010, The AD regarding the R-R Trent 800 states:

“The Trent 800 has a similar type design standard to that of the Trent 700 and has also been found in service to be susceptible to carbon deposits in the oil vent tube. We are issuing this AD to prevent internal oil fires due to coking and carbon buildup in the HP-IP turbine bearing oil vent tube that could also cause uncontained engine failure and damage to the airplane.”

AD 2010-07-09, supersedes the first AD:

“Rolls-Royce requests that we incorporate by reference the latest alert service bulletin (ASB) … which is ASB No. RB.211-72-AE302, Revision 8, dated October 21, 2009. We agree.”

The ASB tells operators how to inspect the engines and correct any findings.

Again, this is a certification issue. There is a high probability that the experience with Trent 700 and 800 engine vent tube coking and carbon buildup did not figure into certification of the Trent 900 engine for the A380. The investigation of the 4 November incident should take a sweeping, in-depth look at certification standards.

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