Both the FAA and the EASA released an Airworthiness Directive regarding the new, Trent 1000 engine, specially designed for Boeing 787 family.
The engine is a Rolls-Royce, 3-shaft, high bypass ratio turbofan engine, capable of maximum thrust: 265–360 kN. When experiencing an inflight shut down, the other engine is operated on a higher load, at its maximum continuous thrust, during which the intermediate compressor blades may be exposed to severe vibrations. This, in an ETOPS operation could fail, resulting in an intermediate pressure turbine blade (IPTB) cracking/fracture.
According to the FAA Directive:
"Over the past year, we have been aware of several engine failures of Trent 1000 Package C
engines due to failed compressor and turbine blades and seals. Package C engines are Rolls-Royce
plc (RR) Trent 1000-A2, Trent 1000-AE2, Trent 1000-C2, Trent 1000-CE2, Trent 1000-D2, Trent
1000-E2, Trent 1000-G2, Trent 1000-H2, Trent 1000-J2, Trent 1000-K2, and Trent 1000-L2 turbofan
engines. During that same period, under the management programs for those engine issues, we have
been aware of numerous reports of engine inspection findings of cracked blades resulting in
unscheduled engine removals. Boeing reported to the FAA that the engine manufacturer recently
determined that intermediate pressure compressor (IPC) stage 2 blades have a resonant frequency that
is excited by the airflow conditions existing in the engine during operation at high thrust settings
under certain temperature and altitude conditions. The resultant blade vibration can result in
cumulative fatigue damage that can cause blade failure and consequent engine shutdown. In the event
of a single engine in-flight shutdown during the cruise phase of flight, thrust on the remaining engine
is normally increased to maximum continuous thrust (MCT). During a diversion following a single
engine shutdown under an ETOPS flight, the remaining engine may operate at MCT for a prolonged
period, under which the IPC stage 2 blades would be exposed to the resonant frequency condition.
Therefore, an ETOPS diversion will put the remaining engine at an operating condition that would
significantly increase the likelihood of failure of the remaining engine. In addition, if the remaining
engine already had cracked IPC stage 2 blades, the likelihood of the remaining engine failing will
further increase before a diversion can be safely completed."
In the EASA's Directive:
"An occurrence was reported where, following N2 vibration and multiple messages, the flight crew performed an engine in-flight shut-down (IFSD) and returned to the departure airport, landing uneventfully. The post-flight borescope inspection of the engine revealed an intermediate pressure turbine blade (IPTB) missing at the shank. Analysis shows that this kind of failure is due to sulphidation corrosion cracking.
This condition, if not detected and corrected, could lead to IPTB shank release, possibly resulting in an IFSD and consequent reduced control of the aeroplane.
To address this potential unsafe condition, RR issued Alert NMSB Trent 1000 72-AJ575 to provide instructions for engine removal from service when any IPTB with a high level of sulphidation exposure is identified by corrosion fatigue life (CFL) model. Consequently, EASA issued AD 2017-0056 to require removal from service of certain engines, to be corrected in shop.
Since that AD was issued, prompted by further occurrences and analyses, it has been decided that, to reduce the risk of dual IFSD, a new cyclic life limit must be applied to certain engines, which determines when an engine can no longer be installed on an aeroplane in combination with certain other engines.
For the reason described above, this AD requires de-pairing of the affected engines. This AD is considered an interim action and further AD action may follow."
What does ETOPS stand for?
It is an extended range operations with two-engined aeroplanes.
"In commercial air transport operations, two-engined aeroplanes shall only be operated over routes that contain a position further from an adequate aerodrome that is greater than the threshold distance determined in accordance with CAT.OP.AH.140, if the operator has been granted an ETOPS approval by the competent authority."
For an airline in order to..."obtain an ETOPS operational approval from the competent authority, the operator shall provide evidence that:
(a) the aeroplane / engine combination holds an ETOPS type design and reliability approval for the intended operation;
(b) a training programme for the flight crew and all other operations personnel involved in these operations has been established and the flight crew and all other operations personnel involved are suitably qualified to conduct the intended operation;
(c) the operator‟s organisation and experience are appropriate to support the intended operation; and
(d) operating procedures have been established."
There is also a minima requirement when planning the alternate aerodrome.
In case of performance class A aeroplanes (seating configuration of 20 or more, MTOM of 45360 kg or more), this distance flown is 60 minutes at one-engine-inoperative (OEI),
class A aeroplanes (seating configuration of 19 or less, MTOM less than 45360 kg), this distance flown is 120 minutes, or subject to approval by the competent authority, up to 180 minutes for turbo-jet airplanes, at OEI.
Summarizing it, the FAA AD requires revising the AFM to limit ETOPS operation and the EASA's requirements are de-pairing the affected engines.
Sources:
- FAA AD: FAA AD 2018-0229
- EASA AD: EASA 2017-0253
- EASA OPS: CRD c.8 Part-SPA
