Cobham RJ1H at Perth on Apr 29th 2014, uncontained engine failure

Last Update: May 2, 2016 / 13:51:23 GMT/Zulu time

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Incident Facts

Date of incident
Apr 29, 2014

Classification
Incident

Airline
Cobham Aviation Services

Aircraft Registration
VH-NJI

Aircraft Type
AVRO RJ-100 Avroliner

ICAO Type Designator
RJ1H

On May 2nd 2016 the ATSB released their final report concluding the probable cause of the serious incident was:

Contributing factors

- A repair to the two o’clock combustion liner retention boss of the No. 2 engine combustor turbine module housing was not performed in accordance with the manufacturer’s repair specification, resulting in a thin-walled housing that increased local stresses in that location.

- As a result of fatigue, the No. 2 engine combustor turbine module housing cracked, then fractured adjacent to the two o’clock combustion liner retention boss weld, propagating at an unpredictable rate as a result of the non-approved repair.

- High-temperature combusting fuel and gases escaped radially from the fracture in the No. 2 engine combustor turbine module housing, leading to an in-flight engine fire.

Other factors that increased risk

- The Honeywell International Inc documentation for blending did not limit the amount of material that could be removed from the combustor housing.

Other findings

- The Honeywell International Inc LF507-1F heavy maintenance schedule was adequate to identify and repair cracks in the combustor turbine module housing combustion liner retention boss weld.

The ATSB describe maintenance actions:

A non-penetrating crack, a crack that did not penetrate through the full thickness of the housing sheet metal, had propagated over a period of about 2,680 load cycles. Each load cycle on the affected components was considered by Honeywell to be associated with pressure variations within the engine. Pressure variations can be caused by engine power level changes, compressor stalls, combustion rumble and temperature variations that lead to expansion and contraction of the housing. Numerous load cycles can occur during normal flight.

Engine maintenance is based on engine hours, cycles4 or calendar days, depending on the type of component. Engine maintenance is not based on load cycles as each flight can produce a significant variation due to conditions at that time. The examination of the fractured welded boss identified three separate weld repairs. These were identified on cross-sectional samples taken through the joints surrounding the recovered welded boss (figure 5). Honeywell concluded that the fatigue cracking was not directly associated with the weld. Hardness measurements taken from the housing sheet metal, boss casting, and weld indicated that heat treatment was performed on the assembly subsequent to weld repair, as specified by Honeywell.

Further examination of the combustor housing in the location of the fractured welded boss found that a section of the combustor housing sheet metal had been thinned at the inside and outside surfaces by grinding (Figure 6). This grinding was associated with a weld repair where the boss was weld fused to the sheet metal. Thicknesses of 0.035 inches–0.040 inches were measured at the CTM housing adjacent to the two o’clock welded boss.

The thickness of the combustor housing sheet metal was specified by Honeywell as nominally 0.049–0.050 inches thick. This indicated that during a repair, about 20–30 per cent of the housing thickness had been removed. In this regard, the combustor housing is a pressure vessel. Honeywell does not approve the removal of material leading to a reduction of housing wall thickness.

The ATSB analysed: "At a high engine power setting during the climb, the welded boss at the two o’clock position of the No. 2 Honeywell International Inc (Honeywell) LF507-1F engine fractured and separated from the combustor turbine module (CTM) combustor housing (housing). This led to the fracture of the combustion lining and allowed high-pressure combusting fuel and gases to escape radially from the engine. The engine cowling was weakened and melted from the resulting in-flight engine fire. The engine fire detection and suppression system was effective in alerting the crew to the situation. The crew extinguished the fire using normal operating procedures and returned the aircraft to Perth Airport for landing."

The ATSB analysed with respect to the non-approved repair: "Metallurgical examination of the fractured welded boss found that it had been weld-repaired on three separate occasions. The only recorded weld repair to the CTM housing was in 2005 although, as the CTM housing was an exchanged unit, its history could not be established. Therefore, either the CTM housing was repaired at or prior to 2005, or the housing was repaired after 2005 and the repair was not recorded. From the evidence available, the ATSB could not determine which was the case. In any event, the housing had been ground adjacent to the welded boss, reducing the housing wall thickness by 20–30 per cent. Grinding of the housing was not in accordance with Honeywell’s approved repair scheme, and the non-approved repair was not identified in any of the subsequent heavy maintenance inspections. The reduction in the housing wall thickness increased the operational stresses at that location. This would have affected the initiation of the fatigue crack and increased its rate of propagation during normal engine operation."

The final report references another engine fire on Mar 10th 2016, see Incident: Swiss RJ1H at Zurich on Mar 10th 2016, rejected takeoff due to sparking engine as "subsequent occurrence".

Figure 5: Metallographic image of a cross section of the fractured
welded boss. The red dots likely indicate the original weld. The
white, yellow and black dots likely indicate subsequent weld
repairs. The red arrows indicate a crack emanating from the forward
side of the weld at the housing/casting interface. The white arrow
is the fracture surface, where the boss separated from the combustor
housing. The fracture initiated at the outside diameter of the