Its first delivery of the 747-8 Intercontinental internally slated for February, Boeing is locking out the tail fuel tanks of its new jumbo after a structural flutter was found to occur in the event of a certain structural fitting failure. FULL STORY
"Boeing certified the 747-8 Intercontinental with the tail fuel tanks locked out because during design review of flight test data...it was discovered that, under a certain regulatory-required structural failure scenario, the airplane can experience flutter events when the fuel tanks in the horizontal stabiliser are filled over 15% of their capacity," said Boeing.The lock-out will be achieved through the pulling of a circuit breaker and the physical disconnection and capping of fuel lines running to the horizontal stabilizers' 3,300gal tanks.
To comply with US Federal Aviation Administration regulations, Boeing will deactivate the tail fuel system to satisfy the requirement that no structural flutter be present in the airframe after any single failure condition.
"These conditions do not present themselves when the tanks are empty," Boeing said of the structural failure evaluations, which were only found to occur if the aircraft's wing-to-strut join fitting had failed.
The "requirement for all key structural fittings...need to have a design tolerant of 'any single failure'", said Boeing. "We're actively working on ways to activate the fuel tanks for the long term."
Boeing says the restriction will shave 300-400nm off the range of the VIP configured 747-8 and will have little impact to the airline configured aircraft, as tail fuel usage is precluded if the non-fuel weight exceeds 60% of the aircraft's maximum structural payload. In such a situation, the fuel payload would be entirely carried by the wing tanks.
Boeing recently updated, then removed - pending review - its 747-8 and -8F maxiumum structural payload and operating empty weight (OEW) weights:
The document listed the maximum structural payload for the passenger configured model as 82.1t (181,000lb) and the pre-service bulletin for the 747-8F at 134t (295,200lb), which increased after April's update to 139t (306,000lb).
47-8 airline launch customer Lufthansa says, "For our mission profile it's not a problem at the moment," adding the tail fuel restriction would not restrict the aircraft's deployment on its initial routes, which have not yet been announced.
"Of course you want an airplane that can run as long as possible" in unrestricted operation, it added.