Boeing locks out 747-8 tail fuel tanks on flutter concerns

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In order to meet certification requirements ahead of its first delivery, Boeing will lock out the 12,500l (3,300gal) horizontal stabiliser tail fuel tanks on its 747-8 Intercontinental after a flutter condition was found to occur in a certain structural failure scenario, the airframer confirmed.

"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.

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."

747-8 horizontal tail 

 © Jon Ostrower

Boeing said the lock-out is accomplished through the pulling of a circuit breaker and the FAA has requested a physical separation in the fuel system by removing and capping certain fuel lines between fuel tail tanks.

The change to the fuel system has not affected the pacing of deliveries and only five 747-8s rolled out prior to this decision will require rework as part of the larger post-certification change incorporation operation. The balance of the 747-8s that will be assembled will have their tanks locked out inside the factory.

Reactivating the tanks and incorporating the fix will be be accomplished during normal maintenance operations as part of a service bulletin expected to be issued by the airframer once a solution is identified, said Boeing.

According to its airport planning documents, Boeing lists the 747-8's usable fuel capacity at 238,584l (63,034gal) as established during flight testing and includes the fuel stored in the horizontal stabiliser.

The horizontal tail fuel system is limited to the passenger model of the 747-8, as the recently delivered 747-8F's performance requirement prioritise cargo capacity over range.

Boeing said the absence of tail fuel tanks will reduce the range of the VIP configured 747-8 by about 550-930km (300-400nm), depending on the aircraft's configuration.

For airline operators, the impact on performance will be minimal, said Boeing, as chosen configurations of its customers will prohibit the use of the tail's fuel tanks when non-fuel payload exceeds 60% of the aircraft's maximum structural payload. The balance of the payload would be fuel stored in the aircraft's wing tanks.

"For our mission profile it's not a problem at the moment," said a Lufthansa spokesman, who said the tail fuel restriction would not restrict the aircraft's deployment on its initial routes, which have not yet been announced.

Boeing indicates the passenger capacity of the 747-8 as up to 467 seats in a three-class configuration. Lufthansa will operate the aircraft in a three-class configuration seating 386.

Lufthansa said it is "still quite positive that there will be a modification" that will restore access to the tail fuel tanks, but "of course you want an airplane that can run as long as possible" in unrestricted operation.

Lufthansa is "waiting to hear from Boeing how they will solve this problem," the airline added.

Despite not having a timeframe for a fix, the airline said it is its expectation that the restriction will not exist on deliveries in 2013.

Lufthansa launched the 747-8 in December 2006 with an order for 20 of the General Electric GEnx-2B-powered aircraft. The 747-8 will have the longest range of any aircraft in the airline's fleet, it said, exceeding that of the Airbus A380.

The 747-8's range is advertised by Boeing as being around 14,800km (8,000nm) at maximum takeoff weight of 448t (987,000lb), though the airframer is currently updating the figure for its catalog specifications.

747-8 empennage

 © Jon Ostrower

The 5.6m-shorter (18.3ft) 747-400 also features a 12,500l tail fuel system in the geometrically identical horizontal stabiliser. Despite this similarity, the 747-8 structurally varies from the -400 with an additional 1.5m (4.9ft) stretch behind the wing, along with strengthening for the higher aircraft weight and different materials from its predecessor.

Boeing declined to speculate on why the flutter condition existed on the 747-8 and not in the case of the failure of the wing-to-strut join fitting on the earlier 747-400.

Boeing notes the fitting has never failed in the type's 40 years of service.

The 747-8 programme has faced vibration concerns before, after a 2.3Hz limit cycle oscillation (LCO) in the wings prompted Boeing to develop the Outboard Aileron Modal Suppression (OAMS) system to dampen out vibration with the aircraft's fly-by-wire ailerons.

Certified by the FAA in December, the lock out of fuel tanks allows Boeing to deliver its first 747-8 to a completion centre for VIP configuration in February with airline launch customer Lufthansa to follow in March, said programme sources of the schedule.

Boeing holds firm orders for 36 747-8 aircraft, including five for Korean Air, two for Arik Air and nine for VIP customers.