A supply shortage of clutch servo-valve filters for V-22 Osprey auxiliary power units (APU) threatened to keep a number of aircraft on the ground, until the APU group of the V-22 Fleet Support Team at Fleet Readiness Center East (FRCE) found a way to clean the disposable filters and reinstall them on the aircraft.

The APU filter is a consumable item that is replaced after 1,120 flight hours; it is ordinarily used for one cycle and disposed of after removal. With 375 aircraft in the Marine Corps fleet, the filter is a commonly used item, with a replacement rate of about 100 a month. The quarter-sized metallic filter is available from only one source and, due to supply-chain issues, the supplier couldn’t keep up with demand. With no filters to be found in the supply system, APU engineering and logistics personnel had to come up with a creative solution to fill the needs of the V-22 aircraft and the APUs undergoing overhaul at FRCE. 

“We looked around for any retail assets or hidden assets that we might have stored away somewhere, but didn’t find anything,” said Joe Carson, V-22 Power and Propulsion logistics team lead at FRCE. “The filter was a consumable item, so typically the last option would be to ask engineering to reuse something that’s a disposable item – but in this case, that was the last solution we had.”

The filter is part of a critical oil line that helps engage the APU clutch to the midwing gearbox in order to start the V-22’s main engines. Fleet maintainers are instructed to replace the filters when they are found to be clogged during inspection, but the supplier was not expecting deliveries until March 2022. That meant that several aircraft would not fly until filters could be found.

“Most engineers don’t like reusing something that’s consumable, because it’s consumable for a reason,” said Rob Wansker, FRCE V-22 Power and Propulsion APU senior engineer. “So we consider, what’s the consequence of using this for a prolonged period of time? Can we push back the timeframe of replacement? We weighed our short- and long-term options, and we decided to see if we could clean and recertify the filters we had.”

 Engineering consulted with the Materials Engineering lab at FRCE to determine the best way to clean the small mesh filter. After experimenting with a few scrap filters, FRCE chemist Megan Goold found a procedure that would allow the filters to be cleaned and reused if new filters were not available.

“I wanted to do the easiest cleaning possible, using materials the APU shop already had on hand that would be easily and readily available to the fleet as well,” Goold said. “The easiest thing would be hand cleaning, so we tried different things in the lab until we found something that worked.” Isopropyl alcohol and a commonly used degreasing solvent did the trick, and engineering created a temporary instruction for depot and fleet maintainers to follow until the filters became more readily available through supply.

Another hurdle to solving this problem was that used filters were not readily available to clean.  Disposable items like the filters are typically thrown away following maintenance procedures, which meant no stock of used filters existed. Wansker coordinated with depot and fleet mechanics working on V-22 APUs to encourage them to set aside the used filters in case they were needed for inspection and cleaning.

“We then took the filters that looked like good candidates, and cleaned and tested them according to the temporary procedure we had created,” Wansker said. “Once we were able to get a batch of clean filters that engineering had approved, we were able to send those directly to the squadrons in order for them to be installed on an APU.”

The engineering and logistics team had a temporary solution in place within a few weeks of identifying the supply problem. In that time, four aircraft had been grounded waiting for APU servo-valve filters, and the ability to clean and reissue the filters quickly returned those V-22 Ospreys to duty.

V-22 team members say the incident reminds them that even projects that seem routine can have profound impacts on the warfighters they serve.

“The V-22 Osprey is a massive aircraft, and to think that our team’s assistance with this project was able to get these aircraft back in the air helps put the importance of what we do in perspective,” said Wansker.

FRCE is North Carolina's largest maintenance, repair, overhaul and technical services provider, with more than 4,000 civilian, military and contract workers. Its annual revenue exceeds $1 billion. The depot provides service to the fleet while functioning as an integral part of the greater U.S. Navy; Naval Air Systems Command; and Commander, Fleet Readiness Centers.