Boeing is studying several options, ranging from routine checks to minor structural modifications, to reactivate the closed-out tail fuel tank on the 747-8 passenger model.

The variant was certified in December 2011 without the additional 3,300-gal. fuel capacity because Boeing could not show full compliance with FAA requirements that no structural flutter be present in the airframe after any single failure condition. The specific case concerned potential flutter in the event of a failure of the R3 under-wing, mid-spar strut-to-wing fitting, which is one of six connecting the outboard engines to the wing.

“That’s one we have not been able to show,” says the 747-8 VP and Chief Project Engineer Bruce Dickens. In the case of an R3 failure scenario, which he describes as “fairly extraordinary,” analysis indicated the aircraft could experience flutter when the fuel tanks in the horizontal stabilizer are filled over 15%.

The R3 fitting is on the outboard side of the outboard engine, “... but we can’t yet show analysis to tolerate it,” says Dickens, who adds that the scenario “is also an extraordinary impractical configuration of the aircraft.” The load condition assumes the full weight of the 3,300 gal. in the tail tank as well as a lot of weight forward. “It assumes a dumbbell loading configuration, so it’s a fairly oddball case.”

Dickens says that Boeing became aware of the situation earlier in 2011 and, because there was no likely impact foreseen to any of the initial customers until at least 2013, made the fix part of its longer-term development schedule. “It is a challenging analysis, but it wasn’t considered a priority. We’ve had a plan for months and months to lock it out, and it is possible [that to reactivate the tank] we won’t have to do anything.” However, Dickens concedes, “It is probable we’ll have to do something.”

Loads and aero-elastic characteristics of the 747-8 and 747-400 passenger versions differ because the stretched model is 18.4 ft. longer than the -400, has a 13-ft. greater span, all-new wing and is up to 100,000 lb. heavier. The 3,300-gal. tail fuel tank raises the 747-8’s total fuel capacity to 64,055 gal., giving it a nominal range of 8,000 nm. The -8 Freighter does not incorporate the tanks.

Boeing is now conducting a full analysis by combining data from load tests with structural and aero-elastic models. “We’re studying a lot of things to deal with it, both complicated and simple,” says Dickens. Although yet to be detailed, these are believed to range from routine checks to structural reinforcement. “We don’t see any risk to it not being completed in 2013,” he adds.

The aft tank is not currently required for passenger operations by launch customer Lufthansa, which is expected to receive its first aircraft around mid-March. In a nominal 467-seat configuration, with the tank full, the 747-8 exceeds weight limits if it is operated at more than a 60% load factor. However, Boeing says future operators are considering lower seating density configurations for longer-range routes on which the tail tank will be a useful option.

A company spokesman says, “Lufthansa has not identified any routes it will use the aircraft on before the tanks are reactivated, and we expect the fix to be in place before any other aircraft are delivered.” Other customers for the passenger version are Korean Air and Arik Air, as well as nine VIP versions, the first of which is due for delivery to a completion center later this month.