FRANKFURT, GERMANY — Revamping turbofan endurance testing: EASA proposes modernized certification requirements.
In a move to enhance modern turbofan engine safety and reliability, European regulators are planning a revamp of certification requirements pertaining to turbofan endurance testing. The modification proposal was put forth by the European Union Aviation Safety Agency (EASA), who aim to better represent contemporary engine designs and foster increased confidence in their robustness before they are introduced to the service.
EASA has pointed out incidents where certain turbofan engines have encountered unpredicted failures soon after their deployment, leading to urgent safety measures to mitigate risks associated with multiple engine shutdowns. Although the Agency did not single out any specific engines, examples of such durability concerns have been recorded with the Rolls-Royce Trent 1000, used for the Boeing 787, and more recently, the Pratt & Whitney PW1000G.
Current certification standards necessitate an "accelerated severity" test to ensure an engine's durability and operability within approved constraints. However, EASA observes that these testing procedures, conceived six decades ago for reciprocating engines, have started to become less feasible with the evolution of modern engine designs and the increasing demands of contemporary air transport.
Applicants often resort to altering the test engine's configuration and the required test sequence to reach the concurrent "redline" speed and temperature conditions, which have been traditionally viewed as a conservative and desirable safety measure. While modifications such as adding cooling circuits, thermal coatings, and grinding blade tips are permitted, they have raised doubts regarding the test's representativeness due to their growing complexity.
In response to this challenge, several aerospace entities, including the European ASD, the US-based AIA, and the Engine Harmonization Working Group, have been exploring alternatives over the past ten years, specifically concerning high-bypass, multiple-shaft turbines.
EASA believes that a more representative test would enhance the likelihood of discovering issues before an engine enters service. Therefore, they are proposing to amend the certification criteria, allowing for an optional alternate endurance test that factors in recommendations from the Engine Harmonization Working Group published in March 2021.
The Agency's proposed alternate test comprises a hybrid prescriptive- and performance-based severity test for the engine, running for more hours and cycles than the current protocol, and utilizing a simulated flight cycle. The test is designed to yield results that better represent responses to threats characteristic of revenue service.
Additionally, EASA's proposal includes the development of new certification specifications that allow applicants to exhibit an initial maintenance programme for turbine engines in a more performance-based manner.
