The European Aviation Safety Agency (EASA) moved quickly on an August Rolls-Royce recommendation to mandate inspections of Trent 900 engines undergoing overhaul work to ensure that routine maintenance is not leaving them susceptible to a problem that has caused a rubbing issue tied to at least two in-service incidents.
The airworthiness directive calls for inspection of low-pressure turbine (LPT) disk seal fins and interstage seals. The checks are required following so-called pass-off tests when the engines, which power Airbus A380s, are in shops.
The inspections are the latest steps to keep LPT disk seal fins from rubbing against interstage seals (ISS), which can cause the honeycomb seals to crack. One A380 operator discovered pieces of honeycomb in a Trent 900 tailpipe after a vibration that was within limits. A closer inspection revealed damage in downstream LPT stages and “material loss” from part of the Stage 2 rear disk seal fin area. Rolls-Royce responded by recommending on-wing inspections each time engine health monitoring of vibratory levels takes place, and EASA mandated the procedure.
Following the mandates, another A380 experienced an LPT vibration during climb. The captain shut the affected engine down and turned back. The postincident inspection found separation between the LPT Stage 1 and 2 disks and also Stages 2 and 3, “as well as missing LPT blades and vanes,” EASA says. Investigators concluded that rubbing was the cause.
“At this time, no clear cause has been identified for the ISS rub which resulted in the material release or disk separation,” EASA states. “Nevertheless, a review of all the potential causes of an ISS rub did identify that some of these could be linked to activities [that] take place during an overhaul shop visit.”
Rolls-Royce “has introduced additional criteria for inspection of the Stage 2, 3 and 4 LPT disk seal fins and interstage seals, and removed the requirement to inspect Stage 5 LPT disc seal fins and interstage seal,” EASA says.