Hop-A-Jet: NTSB Final Report Is ‘Accurate Assessment’

The NTSB released the final report on the crash of a Hop-A-Jet Bombardier Challenger 604 on Feb. 9, 2024, citing corrosion in the jet’s GE CF34-3B engines’ variable geometry (VG) system. Hop-A-Jet president Barry Ellis told AIN, “We’re very happy, the NTSB did an accurate assessment, and the way that it was written is understandable to a layman.”

Both pilots died in the accident after the Challenger’s engines lost power on approach to Naples Municipal Airport (KAPF), and they landed on the southbound lane of I-75. The flight attendant and two passengers were able to escape through the baggage door.

One element of the NTSB final report surprised Ellis: that there were no recommendations issued by the agency. While Ellis appreciates that GE Aerospace has revised service bulletin and maintenance manual procedures to emphasize corrosion-detection procedures, he also believes that the service bulletin should be made mandatory. This could be done by an FAA airworthiness directive that references a service bulletin, and he expects some action on this by the FAA.

“We believe that in the interest of safety, there needs to be an FAA mandate to adhere to these service bulletin procedures, and even go further,” he said. That could mean requiring a baseline borescope inspection and possibly more frequent borescope inspections than the current 3,200-hour interval, especially for airplanes that don’t fly as much. Another helpful maintenance procedure would be a mandatory pressure test of the VG system at certain intervals, which can detect VG system corrosion.

The probable cause of the accident 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.”

According to the NTSB, after both engines were sent to GE for post-crash evaluation, VG tests revealed “corrosion was observed in the high-pressure compressor (HPC) case flow path area, with the most significant corrosion found in the VG stage 5 area. Extensive corrosion was observed in the HPC case VG stage 5 stator vane spindle bores.”

Additionally, the VG stage 5 stator vanes were unable to travel fully (that is, the distance from fully opened to fully closed) when tested using the specified maintenance procedures, and higher than normal actuation pressures were required to move the VG hardware through its full range when compared to other engines without corrosion on the HPC spindle bores, with a slower than normal VG system response when tested with pressurized air.

“At low power conditions, as was the case at the time of the accident, it can lead to sub-idle rotating stalls,” according to the NTSB. “It is likely the corrosion limited the VG hardware travel as the flight crew reduced the power for landing, resulting in near-simultaneous, sub-idle rotating compressor stalls and a subsequent loss of thrust in both engines, which was unrecoverable at the low altitude.”

A Maintenance Practice 68 (MP 68) pressure test is part of the troubleshooting procedure for a hung start problem, which the accident airplane experienced 25 days before the accident. With GE’s assistance, Hop-A-Jet’s maintenance team spent three days troubleshooting the Challenger. 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.

“According to the engine manufacturer, a hung start may be an indicator of corrosion buildup in the engine and will result in poor engine starting and operating performance. (In addition to the hung starts twenty-five days before the accident, the operator experienced 7 additional hung start events in the previous 10 years.) 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.”

After the accident, Hop-A-Jet grounded its Challenger fleet—all the jets powered by the CF34-3B engines—and GE technicians traveled to Hop-A-Jet’s Fort Lauderdale Executive Airport (KFXE) headquarters to examine those engines. Since then, Ellis said, “We’ve maintained strict adherence to the service bulletins that they have issued. We are super cognizant of anything that would indicate that there could be a guide vane issue.” Hop-A-Jet’s maintenance crew also built its own MP 68 pressure test tool so it didn’t have to rely on borrowing the two sets that GE owns. “We have preemptively used [that tool] in testing routinely,” he said.

Ellis wants to make sure other operators understand VG corrosion issues and thinks the NTSB report will help with that. “We’ve tried to be very transparent about this from the beginning,” he said. “We haven’t tried to hide behind anything, but we’ve definitely learned a lot that has made us a safer company, just through things that we’ve learned, and we’d like for everybody to have that knowledge.”

While Ellis also thinks that there is more to the story that will eventually be revealed, he concluded, “We believe the NTSB did a good job…of making this [final report] understandable to everybody. It’s a very complex issue with internal components of engines, but the way that they finally wrote the final report, [people] should be able to understand exactly what happened.”