Comment Period Closes on GE CF34 Engine Corrosion AD

In comments submitted to the public docket for a proposed airworthiness directive (AD) on GE CF34 engines, Hop-A-Jet Worldwide Jet Charter urged the FAA to strengthen the AD language to call for more comprehensive inspection of the engine’s variable geometry system. 

In the NTSB final report on the fatal accident of a Hop-A-Jet Bombardier Challenger 604 on Feb. 9, 2024, in Naples, Florida, the probable cause was “corrosion of both engines’ variable geometry (VG) system components, which led to their operation in an off-schedule position and resulted in near-simultaneous sub-idle rotating compressor stalls on approach, subsequent loss of thrust in both engines, and an off-airport landing. Contributing to the accident was inadequate fault isolation guidance from the engine manufacturer, which prevented the identification of corrosion buildup in VG system components during troubleshooting of hung start events of both engines about [one] month before the accident.”

A hung start can occur due to insufficient fuel supplied to the engine during starting, or as a result of problems with airflow through the engine. During a hung start, the engine lights off normally but doesn’t accelerate to reach idle speed, and temperatures can rise rapidly if the engine isn’t quickly shut down.

In the Hop-A-Jet case, the accident Challenger had seven hung start events in the previous 10 years, and 25 days before the accident it happened again, leading to a lengthy troubleshooting and repair process at Hop-A-Jet’s South Florida Jet Center maintenance facility at Fort Lauderdale Executive Airport (KFXE). Hop-A-Jet’s maintenance team and GE technicians spent three days troubleshooting the hung start problem. Eventually, after components including the fuel control and fuel filters were replaced, successful starts eliminated the need to conduct a step in the troubleshooting procedure, according to GE’s flowchart for that process.

That step is the Maintenance Practice 68 (MP 68) pressure test. According to Hop-A-Jet, “During the GE troubleshooting procedure, according to the NTSB, the MP 68 test ‘was not performed because the engines were started and no further anomalies were noted, allowing discontinuing of troubleshooting in accordance with the flowchart. With the concurrence of the engine manufacturer, the airplane was returned to service and flew 33 uneventful flights (excluding the accident flight) over the next 25 days, accruing 57 hours of flight time until the accident.

“One way corrosion could have been identified in the engine, and specifically of the VG system components, was through the MP 68 pressure check. However, because this step was so late in the fault isolation hung start guidance, and it was not a required maintenance check, the airplane was returned to service after successful engine start and no other subsequent engine start issues. Thus, the corrosion of the VG system components continued to go undetected and eventually led to the sub-idle compressor stall during the accident flight.”

AD Requirements

The proposed AD would apply to GE CF34-1A, -3A, -3A1, -3A2, and -3B engines and would require “performing repetitive engine heat soak restart tests for a hung start,” which would then indicate next steps. The AD would require a borescope inspection (BSI) after the hung start and also at intervals from 12 to 24 months, depending on the engine model, or before further flight for engines swapped between airframes, to check for corrosion of the high-pressure compressor (HPC) case “and, depending on the results, a VG system functional check for pressure evaluation.

“This proposed AD would also require, depending on inspection results, performing a force gage test on the feedback cable for tightness and a visual inspection of the VG system for obstruction and, if necessary, removal of the engine from service. This proposed AD would also require revising the airworthiness limitations section (ALS) of the existing engine maintenance manual to incorporate the VG system functional check.”

Docket Comments

The comment period for the proposed AD ended on June 15. Fourteen comments were submitted to the public docket, including Hop-A-Jet’s.

“We believe that this AD is not only necessary to protect the lives of passengers and crew, but that it must be made even stronger to close the loopholes, gaps, and reporting issues that can allow safety to be compromised,” said Hop-A-Jet CEO Barry Ellis. “Without these measures, countless aircraft may be at a growing risk of corrosion-related safety events. No one should have to deal with this kind of preventable tragedy in the future.”

While generally supporting the AD and its HPC borescope requirements, Hop-A-Jet wants “the FAA to go further by ensuring that visual corrosion inspections are mandatory, auditable, and not subject to optional compliance pathways that could leave affected engines without direct inspection of the areas now known to be safety-critical.”

According to Hop-A-Jet (and it made a submission to the NTSB to highlight this issue), after the accident, it learned that prior borescope inspection imagery from GE or vendor inspections “showed visible corrosion around variable geometry vane bore holes that was not reported to the operator…That distinction is central to the safety issue: if the manufacturer’s inspection program did not require corrosion to be identified, reported, and escalated, the operator had no practical way to know the condition existed.”

“This accident raises a question every operator, regulator, and manufacturer should be asking: If corrosion was visible in the engine’s internal compressor case area, why was it not identified and reported before the accident?” asked Tim Rounds, Hop-A-Jet’s director of maintenance. “If the prior borescope inspection was not intended to detect corrosion, then what corrosion-detection program was in place to protect crews, passengers, and the public?”

Hop-A-Jet does support the AD’s requirement for reinspection of engines that were inspected previously per GE service bulletin revisions that didn’t require a borescope inspection of the relevant compressor case areas, the company explained. Further comments “urge the FAA to account for sea/salt operating environments, require reporting of corrosion findings, shorten follow-up variable geometry system functional check intervals when active corrosion is found, and require maintenance manual and flight manual revisions that clearly distinguish a hung start from a slow start.”

“The industry cannot rely on operators to detect a condition they were not clearly told existed and were not positioned to inspect themselves,” Rounds said. “These specialized engine borescope inspections are performed by the manufacturer or manufacturer-authorized vendors under manufacturer instructions. A maintenance program must tell those inspectors where to look, what condition to identify, when to report it, and what action is required when corrosion is found. Anything less leaves too much uncertainty in a system where uncertainty can have catastrophic consequences.”

Other Commenters

Generally, GE agrees with most of the AD’s requirements, but suggests some clarifications to ensure that the proper areas are being examined, that correct inspection techniques are employed, and that more accurate descriptions of the VG corrosion are used.

Engine overhauler StandardAero recommended a change to the AD that would allow a VG system functional check to be done, without the required BSI. According to StandardAero, “The BSI prescribed by Accomplishment Instructions, paragraph 3.C.(2), of GE SB CF34-BJ 72-0347 R02 is difficult to perform, risking misses during inspection and damage to BSI equipment. With the result of a failing BSI (i.e. corrosion identified) being to perform a VG system functional check, the inspector should have the option to skip the BSI and go directly to a VG system functional check. CF34-3 BJ SB 72-0347 R02 Para 1.B. provides specific verbiage that the inspector may skip the BSI and go directly to the VG system functional check. This clarity should also be present in the proposed AD.”

Commenter Arthur Coffman of Aero Services Unlimited, an A&P/IA with decades of turbine engine experience who attended the inspection of the Hop-A-Jet Challenger wreckage, disagrees with StandardAero’s conclusion that the BSI could be skipped if the VG system functional check is performed.

“[StandardAero] not wanting to perform the borescope inspection is contrary to the aircraft industry standards for safety and airworthiness,” Coffman wrote. “This inspection is not difficult to perform by a trained technician…If this inspection is too difficult to perform, these engines must, at a minimum, be on an overhaul schedule. The NTSB airworthiness chairman’s final report noted that if these engines had been on a TBO [time between overhaul instead of on-condition schedule], this accident would not have occurred.” 

Coffman also recommended that technicians conducting the BSI be trained, because “an untrained technician may miss the VGV corrosion and wear.” Further, he added, “The cost of damaging equipment over an inspection that could save lives must never be a consideration for not performing any inspection for airworthiness. Properly trained technicians rarely damage equipment.”

With regard to the proposed AD, Coffman believes that the heat soak restart test does not provide sufficient warning of an imminent failure and should be removed. The AD should then require a BSI for corrosion and wear every 1,600 hours for all CF34 engines with more than 3,200 hours, with no terminating action, he explained.

Boeing Executive Flight Operations commented on the proposed AD, recommending that if the MP 68 VG system functional check had been done “within the previous 48 months of the effective date of this AD [then it] constitutes terminating action for the repetitive heat soak restart required by paragraph (h)(1) and borescope inspection (BSI) of the high pressure compressor (HPC) case” for affected engines other than those that are swapped between airframes.

Coffman concluded his comments as follows: “I support decisive action of the FAA to ensure we do not have more in-flight failures of the GE CF34 fleet that result in the loss of life. The inspection and maintenance of the VG system for corrosion and wear is an important part of the engine operation. It is a necessary and standard part of any maintenance program.”