Servicing fuel systems, a series of pumps and valves that feed fuel from a tank to an aircraft’s engines and auxiliary power unit (APU), is among the broader repair areas in an aircraft’s maintenance cycle. Fuel systems typically encompass many variable parts differing in scale, depending on the design of the aircraft.

On commercial aircraft, fuel tanks are usually situated in the fuselage and across both wings, designed to provide a counter-stress to the wings once airborne while reducing the amount of weight off-center from an aircraft’s center of gravity. Despite this careful design, wings still undergo high stress levels during flight, and it is in the highly stressed areas of the wings where fuel leakages can occur, says Craig Rose, senior vice president and general manager of Tank Tigers, which supports all types of fuel tank component removals and replacements. “The majority of leak repairs can be accomplished by isolating the leak path and replacing deteriorated sealant,” he says. “Corrosion or structural cracks can also cause fuel leaks and require more extensive repair processes.”

For its repairs, Rose says Tank Tigers typically sends specialist teams for a single aircraft-on-ground (AOG) event or if necessary, a mobile repair team to support fuel system work associated with a complete heavy maintenance visit. “An AOG inspection and repair can be completed as quick as the same day, depending on the work scope,” he says. “These inspections are incorporated into a heavy maintenance visit and require the removal of fuel tank sealant in order to accomplish non-destructive testing inspections. . . . Aircraft undergoing heavy maintenance typically include a fuel-leak inspection, full access of the center and wing fuel tanks, and an inspection of the complete internal wing structure and components. Any discrepancies are documented, and repairs are accomplished.”

However, engine fuel pumps are usually repaired at different intervals than most fuel systems components. Explaining the repair when a main engine fuel pump fails, Lyon says the process will begin with preliminary testing, followed by disassembly, cleaning and inspection, kitting and the replacement of bearings and gearset replenishment in cases of excessive wear. Following this, the parts are reassembled, tested and certified to be airworthy by a final inspection verification process. “For main engine fuel pumps, about 80% or more are driven by time removals, and 20% are on-condition,” Lyon estimates. “Our average turnaround time for that product line is 14 days,” he adds.

While fuel leakages and corrosion are common challenges, the flammability of the fuel itself is an obvious hazard to the tank. The FAA’s Fuel Tank Flammability Reduction (FTFR) ruling was enacted after the 1996 crash of a Trans World Airlines Boeing 747-100, which investigators concluded was likely caused by exploding flammable fuel vapors located in the aircraft’s center fuel tank. The FTFR rule went into effect in July 2008 and requires ignition sources and flammability exposure to be reduced in fuel tanks that are most at risk. This applies to older aircraft types, with new-generation models being compliant at the production stage.