Rotor bursts and single points of failure (Part II)

11/01/2011 — Leave a comment

HPT turbine fragment embedded in engine exhaust

The recent QF32 incident raises questions about a report by the AIA on the risk engine rotor bursts

The 2010 report by an AIA working group on turbine un-contained rotor events found that overall the criteria of FAA advisory circular AC 20-128A delivered acceptable system robustness in the face of an un-contained rotor burst. The working group came to this conclusion after reviewing the performance of aircraft designed both before the circular was released and those designed after it was released (and therefore were designed to meet it’s criteria). They found that during rotor burst events earlier aircraft had experienced systems damage affecting controllability. In comparison aircraft designed to meet the criteria of AC 20-128A fared much better with minimal system level damage effects.

So far, airplanes designed using that criterion and the associated mitigating design features have shown sufficient system robustness for continued safe flight after disk burst. In contrast, first generation high-bypass turbofan airplanes, which were designed before the criterion was published, have experienced systems damage affecting controllability on multiple occasions.

AIA Working Group 2010

The working group also found that for rotor bursts resulting in major fragments of the rotor striking the aircraft:

  1. for early generation wide bodies, designed before AC 20-128A, system damage affecting controllability occurred on the wing leading edge inboard of the engine within one or two nacelle diameters of engine centerline (See Figures 1 & 2).
  2. where large rotor fragments were thrown in most cases they did not pass through the wing and the authors concluded that the wing provided a significant degree of shielding, and
  3. in flight fires involved the engine nacelles and pylons (in one case the wing leading edge), these were fueled by engine fuel or oil services within the nacelle/pylon, probably ignited by hot engine surfaces and subsequently controllable by the use of engine fuel shutoff valves.

Overall they concluded that the data supported the premise that the (then) current generation aircraft designs met the design intent of circular AC 20-128A and the associated probability criteria of no more than a 1 in 20 chance of a catastrophic event per impact of a 1/3 disc fragment. Importantly the working group also concluded that system damage was most likely to occur where systems are closely grouped, within 2 nacelle diameters of the engine centreline (termed the near field zone), and in the plane of the disc. In each case this damage was either caused by large fragments or where the shielding structure was lightweight.

In contrast to the conclusions reached by the AIAA when we look at the reported damage to QF32 we find it included:

  1. Multiple large sized rotor fragments (three) hit the aircraft,
  2. Two high speed fragments penetrating the wing completely one penetrating the forward spar and fuel tank,
  3. The two wing penetrations caused significant localised system damage with significant knock on effects,
  4. The third (smaller) fragment penetrated the left wing to fuselage fairing causing significant system damage, and
  5. Significant degradation of aircraft functions.

So why is there such a difference between the conclusions of the working group and the actual severity of QF32 experienced damage? In part this may be because of the limited set of data available for the AIA working group, in total the looked at 58 events. For example, the working group considered fuel fires only in the context of nacelle/pylon location because those were the only fires they had in the data set. However this conclusion is based on a very small number of aircraft rotor burst incidents where such fires subsequently occurred (1). So the working group did not consider a scenario of an aircraft wing tank and leading edge void being penetrated with subsequent electrical arcing and ignition as occurred with QF32. The group implicitly assumed that the fires experienced were the only way that a fire could occur and of course QF32 proved them wrong (2).

This is of course the problem with historical data, in effect we’re trying to guide ourselves by shining a lamp over the stern of the ship and looking where we’ve been. While useful such data can never predict with absolute certainty future events. More subtly there’s also a problem in the AIA’s affirmative approach to the analysis that is the working group set out to corroborate the extant standard rather than trying to expose it’s flaws, in this case that affirming approach resulted in their failure to identify the potential for a combined void, tank wiring damage fire scenario (3).

Another major problem with the AIA’s conclusions is the evolution of aircraft design and how close the design of the A380 aircraft lies to the aircraft that the AIA studies. The A380 is designed to meet strict and challenging weight constraints and to meet this requirement alloys with a higher strength to weight ratios were used (4) along with a higher proportion of composites (5) than traditional wide body jets, as a result there much less mass in the aircraft structure when compared to earlier aircraft. All this means that the ballistic protection provided by the A380 structure will be different to earlier aircraft. This difference in ballistic protection may in fact invalidate many of the assumptions made in the AIA report as to damage tolerance and make questionable the efficacy of the application of AC 20-128A to the low mass, high composite construction techniques for current generation of aircraft. Further vulnerabilities unanticipated by the authors of the AIA report may also have been introduced by changes in the geometry of the A380 aircraft wing and engine nacelles relative to the set of aircraft types reviewed by the AIA (6).

My conclusion from re-reading the AIA report in light of the damage sustained by QF32 is that the conclusions that they drew may well be unsound and should be carefully reviewed in light of QF 32.

Notes

1. 46 involved 1st generation engines, 12 involved 2nd generation and (at the time of the report) none involved 3rd generation engines. The six instances of system level damage (beyond the affected engine) occurred on early generation wide-body aircraft designed prior to AC 20-128A.

2. There’s also the problem of arguing from a cohort of survivors, for example if an aircraft was lost without trace over the ocean due to a catastrophic engine failure it would not be included into the statistics.

3. For example, the report notes that in the case of an on-ground rotor burst of a CF6-80 engine HPT rotor the damage done to the aircraft’s keel beam by the 20% fragment of the rotor caused significant damage to the aircraft’s fuselage, well outside the near field zone (Figure 3). The AIA report also indicated the fragment may well have passed completely through the wing if the trajectory was different. However even though this event violated the ‘near field’ hypothesis it was not investigated further nor was it considered as a potential wing penetrating event.

4. For example a new aluminium alloy 7040/T7651 was used in the A380 as it provided improved fracture toughness and static strength when compared to traditional 7010 /50-T7651 aluminium alloy.

5. Thermoplastic composites made up two-thirds of the fixed leading edge from each wing’s inboard engine to wingtips. The A380 also makes extensive use of GLARE (glass-fibre-reinforced aluminium alloy) in the fore and aft fueselage crown panels (but not in the center section). Bottom fuselage skin panels, engine naccelles and engine shrouds use CFRP while the belly fairing uses extensive composite honeycomb panels.

6. For example due to the relatively short height of landing gear (in part as a weight saving measure) the A380’s engines are placed closer to the wing when compared to previous generation aircraft such as the 747.

Figures

Figure 1. 1981 Engine disc burst event

Figure 2. 2006 Engine disc burst

GE CF6-80 HPT1 rotor burst

Figure 3. GE CF6-80 HPT1 rotor burst induced damage to US Airways B767 aircraft keel (Note the severe damage occurring well outside the 2 nacelle diameter near zone)

References

1. AIA Working Group, AIA Report On High Bypass Ratio Turbine Engine Uncontained Rotor Events And Small Fragment Threat Characterization Volume 1, January 2010.

2. ATSB, Preliminary Report, AO-2010-089, In-flight un-contained engine failure overhead Batam Island, Indonesia 4 November 2010, VH-OQA Airbus A380-842, Commonwealth of Australia, Dec 2010.

3. Federal Aviation Administration (FAA) AC 20-128A Design Considerations for Minimizing Hazards Caused by Un-contained Turbine Engine and Auxiliary Power Unit Rotor Failure, 25 March 1997.

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