Fallout from the October uncontained failure of a GE90-115B engine on a Thai Airways Boeing 777-300ER continues.

On May 12, the FAA issued another airworthiness directive (AD) related to the event, this one requiring initial and repetitive ultrasonic inspections of certain interstage high-pressure turbine (HPT) rotor seals installed on GE-90-110B1 and -115B engines. Depending on the inspection results, the AD also calls for removal and replacement of the HPT rotor seal.

The latest AD, effective May 27, also requires the removal of the interstage HPT rotor seal at the next engine shop visit.

The ADs are targeted at a group of 16 engines that are operated by just two carriers. None of the engines is installed on U.S.-registered aircraft, according to the FAA.

A spokesman for GE said the company is limited in what it can say because the AD is part of an ongoing investigation but that the engine-maker is working closely with its customers and regulatory authorities to inspect or replace HPT interstage seals on a “limited population” of -115Bs.

On Oct. 20, the Thai Airways 777 was taking off from Bangkok on a scheduled flight to Zurich when the HPT failed, and the crew conducted a low-speed rejected takeoff. Debris from the uncontained failure damaged the aircraft’s fuselage and the other engine. The root cause of the failure is still under investigation.

In the days after the failure, the FAA issued an emergency AD and later followed up with a final ruling requiring removal of the interstage seal from eight specific -115B engines. In January, the FAA issued a second emergency AD (2020-01-55) extending the seal removal requirement to select GE90-110B1 engines, bringing the number of engines involved to 16.

In January, GE issued a service bulletin to -115B and -110B1 operators on how to do an on-wing ultrasonic inspection at the air holes of the interstage HPT rotor seal. The FAA cited the GE service bulletin in the latest AD.

The first GE90-powered 777 completed its initial flight in February 1995. Since entering service, the engine family has accumulated more than 95 million flight hours and 13 million cycles, according to the manufacturer’s data. The -115B has a dispatch reliability rate of 99.97% and an engine-caused inflight shutdown rate of 0.0004, the OEM says.

The Aviation Week Network’s Fleet Discovery database shows that about 200 of the 1,200 Boeing 777 aircraft fleet are in long-term storage, with about 340 in parked reserve (idled) or parked status.

In 2013, two GE90 engines suffered inflight shutdowns in just three months, resulting in an emergency AD in May 2013 for -110B1s and -115Bs equipped with particular transfer gearbox assembly (TGB) radial gearshafts. The assemblies targeted had been manufactured by a GE supplier during a six-month period between September 2012 and March 2013.

The AD prohibited an aircraft from being operated if more than one installed engine had an affected TGB radial gearshaft. The final AD revised the applicability of the directive to include -76B, -77B, -85B, -90B, -94B and -113B engine models. The AD also required installing a new TGB radial gearshaft.

Another series of ADs was issued following the September 2015 uncontained failure of a GE90-85B engine on a British Airways 777-200ER. The engine failed and caught fire during the takeoff roll on a flight from Las Vegas McCarran International Airport. The crew rejected the takeoff, and the airplane was evacuated on the runway.

A subsequent investigation found the engine had failed because of a fatigue crack in the high-pressure compressor (HPC) stage 8 disk, according to the NTSB. The board said the HPC stage 8-10 spool failed due to a sustained-peak low-cycle fatigue crack, but the cause of the crack’s initiation could not be determined.

In January 2016, the FAA issued an AD requiring eddy current or ultrasonic inspections of the HPC stage 8-10 spool and the removal of failed parts. In April 2017, the FAA issued an AD requiring eddy current inspection of the spool at each shop visit for all of the affected engines and repetitive on-wing eddy current or ultrasonic inspections for certain affected engines prior to shop visits.

A product quality escape at a GE supplier prompted the FAA to issue AD 2018-20-22 in October 2018 requiring the removal and replacement of certain combustion cases from GE90-110B1, -113B and -115B engines. According to the AD, AECC Aero Science and Technology Co. Ltd. had performed welds on newly manufactured components to correct errors introduced during the manufacturing process, but the welds were not approved by GE or the FAA. The combustion cases are life-limited parts, and the unapproved welds reduced the material capability of the cases, the FAA said.

Since some airlines, including Delta Air Lines, have recently announced plans to retire their 777s, aftermarket demand for the GE90 will most likely decrease because of the COVID-19 pandemic unless those aircraft are picked up by other carriers or converted into freighters.