Investigators have not pinpointed the cause of four Rolls-Royce Trent 700 engine inlet cowling panel failures, but regulators and Rolls are confident that changes, including a new design and reduced in-service inspection intervals, have sufficiently mitigated risks.
The most recent incident occurred in June 2017 on a China Eastern Airlines A330-200 departing from Sydney Kingsford Smith Airport. “During take-off, one of the three structural acoustic panels of the aircraft’s left engine inlet cowling, and the inboard outer skin, failed,” the Australian Transport Safety Bureau (ATSB) said in its final report on the incident. The crew returned to Sydney after 42 min. and made an uneventful landing.
Most of the failed panel and other cowling debris was ingested into the engine, leaving little physical evidence. Investigators, aided by cowling supplier Bombardier and Rolls, examined the debris.
“As the section of the No. 1 inlet cowling acoustic panel that likely initiated the failure sequence was not recovered, the findings were inconclusive,” ATSB said. Rolls advised that “disbond between” the panel’s exterior layer, or facing sheet, and honeycomb core “was the most likely cause, but this is impossible to confirm with most of the panel destroyed.”
Manufacturing records showed that the No. 3 panel was constructed and fitted to the cowling at “about the same time” as the panel that failed, ATSB said. “Therefore, it was likely that both would be a similar product in terms of materials and workmanship standards. Further, both panels would have likely been exposed to the same operating loads and environmental conditions during the inlet cowlings service life.”
Investigators and Rolls tested the No. 3 panel as a representative of the failed panel. No anomalies were found.
“Therefore, it was reasonable to conclude that the manufacturing or design process likely did not contribute to the failure,” the ATSB said.
“The reason for the local disbonding could not be established,” ATSB added. “Nevertheless, contributing factors such as maintenance activities, ground handling and foreign object debris damage could not be discounted as a possible initiation source.”
The cowlings have been subjected to mandatory repetitive checks since 2011, when EASA issued an airworthiness directive (AD) based on Rolls-Royce service instructions. Rolls, acting based on information gleaned from Trent 700 panel failures in October 2006 and August 2009, recommended inspections every 24 months. The China Eastern panel had undergone a routine inspection three months before it failed. No findings were recorded. Examination of maintenance records showed that other panel repairs were made, suggesting that the inspection procedure was working, ATSB found.
Following the second incident, Rolls modified the panel design, reducing the honeycomb core’s cell size, doubling its density, and increasing the bond area to the outer surfaces. The new cowls were introduced on the production line in 2014. As of August 2019, about 900 engines with the older cowlings remained in service.
In May 2017, a third cowling failure occurred. Like the first two and the China Eastern incident, the cowling was a pre-modification version. Probes into the first three failures were inconclusive, but investigators suspect disbonding.
Following the fourth incident, Rolls and Airbus recommended halving the inspection interval, to 12 months. EASA mandated the change in February 2019 and the FAA followed suit with its own AD on Nov. 13.