LOS ANGELES—The already delayed first flight of the Boeing 777-9, currently targeted for the end of June, looks set to be pushed back again after an ‘anomaly’ was detected in a General Electric GE9X engine undergoing pre-delivery factory tests.

The first shipset of GE9Xs have already been installed on the initial 777-9, WH001, and were started up for the first time on May 29. However, as a result of the incident, Boeing and GE Aviation are now expected to modify the engines to a finalized, certifiable configuration standard before attempting the first flight.

The 777-9 was originally due to start its flight test campaign earlier in 2019, but according to Tim Clark, president of launch operator Emirates Airlines, it was most recently expected to take to the sky for the first time on June 26.

Revealing the engine issue June 5 at the UBS Global Industrials and Transportation Conference in New York, Boeing CFO Greg Smith said the “long pole in the tent right now is the GE engine. There’s some challenges they are working through there on testing.” Despite uncertainty over what this potentially implies for further delays, Smith added that entry-into-service in 2020 is “still the current assumption.”

Commenting to Aviation Week on the incident, GE Aviation GE9X general manager Ted Ingling said investigations are underway but it is “too early to say how it will impact” the flight test schedule. “During a factory test an engine gave us a signal about its health. We had an anomaly in the compressor and we halted the run.”

The problem area is in the front of the engine’s 11-stage high-pressure compressor. “It’s a mechanical issue and nothing to do with the overall performance of the engine or the way it is set up,” Ingling said. “It is not an aerodynamic issue whatsoever.” Various solutions are being studied, although he declined to provide more detail until the fix is finalized. “We have the opportunity to put more durability into it, so we are in the midst of investigating that,” Ingling said.

GE has previously encountered issues related to this part of the engine. In February 2018, the GE9X’s first flight was delayed by durability problems discovered in the high-pressure compressor variable stator vanes (VSV) lever arms.

Other modifications have also recently been made as a result of the fan blade out certification test which was conducted at the end of January. Although the fan case, which itself was given additional strengthening prior to the test, successfully contained the released blade, the case and strut by the turbine rear frame sustained damage. Changes included improvements to the turbine rear frame, revised under-cowl components and upgrades to the mounting system for the FADEC which, after the fan blade out test, had become loose.

“We have a handful of tests still to go to certify the engine,” Ingling said. A key test is a full durability block test which replaces the conventional ‘triple redline’ test in which the engine runs at simultaneously very high temperatures, pressures and speeds for 150 hours to represent worst case conditions. “We have a mission that emulates the worst conditions and we [are] no longer going to be running three red lines at the same time.”

The new block test, which succeeds the baseline standard established for jet engine certification in the late 1950s, is more suited to modern high-bypass ratio engines that in most cases cannot achieve all maximum operating conditions near sea level. In contrast to early engines, which were designed to operate at maximum conditions at take-off, modern engines independently reach maximum physical speeds and temperatures at different conditions. “We have to finish it this year,” Ingling said.

Final flight tests of the engine were completed on the company’s 747-400 flying testbed in early May after a campaign which stretched over 53 flights and about 320 flight hours. The second phase, which followed an earlier preliminary test program between March and May 2018 and focused on the engine’s high-altitude cruise fuel burn performance, “was by all measures a huge success,” Ingling noted.