APU fuel line chafing prompts Service Letter for Citation X

 - October 16, 2006, 12:07 PM

Ed Duracka, director of aircraft maintenance for Safeway Insurance, made a chilling discovery while complying with Cessna Alert Service Letter (ASL) 750-29-09. The Romeoville, Ill., Lewis University Airport-based flight department operates a Cessna Citation X.

“I had just finished doing the required hydraulic tube/air duct interference inspection and was just looking things over to be sure there were no other visible problems in the area,” he said. “It was obvious the elevator control cable was sawing through the APU fuel feed line.” Duracka said he consulted the maintenance manual, which noted that chafing cannot exceed 10 percent of the wall thickness. “Our APU fuel line was 50 to 60 percent chafed through,” he said.

After replacing the fuel line, Duracka rechecked its proximity to the elevator control cable. Once again the two came in contact. “I just put a little hand pressure on the (fuel) line, causing it to relocate away from the cable, added a chafe strip and that solved the problem,” he said.

Bill Konn, Safeway’s chief pilot, was upset. “The aircraft has only 1,180 hours on it,” he told AIN. “Ed said he’d called Cessna but it sounded to me as if Cessna wasn’t overly concerned, so I called them myself; they told me they were looking into it. I felt disappointed with the conversation when I was told it’s really not a problem if the APU isn’t running!”

Keeping the APU Running In Flight

Konn said that piece of advice offered little consolation because it’s common for operators to keep the APU running through takeoff and initial climb. “We will also run the APU when we reach 50 miles out from the destination or whenever we’re below 10,000 feet,” he said. “It’s a backup for the generators and it can facilitate an in-flight restart should a problem occur. You know, a lot of operators do that as both a convenience and a safety issue, but the point is the APU is used a lot more than just on the ground.”

Konn explained his concern. “The APU line is part of the left fuel system. If the line ruptures in flight while the APU is running, the motive flow pressure sensor turns on the left boost pump to boost the lost pressure, and it’d be like an atomizer. Imagine what would happen if there were any kind of ignition to touch off the vapor. It would be like a surface-to-air missile hitting the aircraft!”

Konn called a few other Citation X operators. “In no time at all I found two other aircraft that had the same problem,” he said. “So I posted the problem on the NBAA Web site and received a whole bunch of e-mails from people I didn’t even know thanking me. That’s when I called Cessna back.”

During his second telephone conversation with Cessna he was told they had already identified another operator in California with the same problem. Konn asked who it was and when he was told he said, “Right, they called you because I called them and alerted them to the problem.”

Hector Fuentes, systems engineer for Cessna’s Citation Team X, told AIN that after receiving Duracka’s initial phone call the company began looking into the problem and as a result issued Citation ASL750-49-09 (Feb. 25, 2005) Auxiliary Power–APU Fuel Line Inspection. The letter pertains to model 750s, serial numbers -0001 through -0240. Fuentes said the company has already incorporated the equivalent of the Service Letter on production airplanes-0241 and after.

Required Inspection of APU Fuel Line

The Service Letter details a mandatory inspection of the APU fuel tube assembly. Operators must make the inspection within 150 flight hours or 60 days, whichever occurs first, from the date of receipt of the letter. The letter states that the suspected elevator control cables have aluminum cladding over the cables and requires that the fuel tube assembly be replaced if any of the following exist: nicks or scratches exceeding 0.002 inch deep; any scratch that runs lengthwise; chafing that exceeds 10 percent of the wall thickness (wall thickness is 0.28 inch); or the tube is dented beyond 5 percent of the 0.375-inch outside diameter).

The letter also requires operators to look for chafing damage on the elevator control cables and to be sure that the fuel tube assembly has sufficient clearance. It needs a minimum of half an inch clearance from the elevator control cable and at least 0.13-inch clearance with other fluid lines, electrical wiring and the airframe structure.

Authorized Citation service facilities, individual operators or other maintenance faculties can submit a credit claim form for the parts kits required to comply with the Service Letter. The offer expires July 1, 2005.