Boeing testing modal suppression for 747-8I stabilizer

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Boeing is using a flight test 747-8 Freighter to investigate whether a software-based fix, similar to the outboard aileron modal suppression (OAMS) system used on the cargo aircraft, will solve a concern about possible flutter in fuel-laden 747-8I horizontal stabilisers in the event of a structural failure.

"The desired path is software," Boeing 747 chief pilot, Mark Feuerstein, told Flightglobal on 1 May at Paine Field during the fly-away ceremony for Lufthansa's inaugural 747-8 Intercontinental, the first of 20 deliveries to the carrier over the next four years.

Boeing deactivated the 747-8I's horizontal stabiliser fuel tanks prior to certification last year after analysis showed that a failure of the wing-to-strut join fitting could allow flutter in the horizontal stabiliser when the fuel tanks on either side are filled to more than 15% of their capacity. US Federal Aviation Administration (FAA) regulations stipulate there can be no structural flutter present in the airframe after any single failure condition.

The stabiliser tanks are only available on the Intercontinental, which carries a total fuel load of 242,470l (66,500gal) compared with 229,980l for the freighter, with the 12,500l difference held in the 747-8I's stabiliser tanks. The greatest impact is likely to be felt by operators of VIP variants of the 747-8, who could see range trimmed by 300-400nm (550-930km), depending on configuration, says Boeing.

To date the airframer has delivered two 747-8I aircraft to VIP customers, one 747-8I to Lufthansa, and 14 747-8F aircraft to various operators.

Boeing believes the impact of the horizontal stabiliser fuel tank shut-off is minimal for airline operators, though launch customer Lufthansa says that "an aircraft that can run as long as possible" is desirable.

Assuming the tests are successful and regulators approve the solution, Boeing plans to have the fix available for the 747-8I fleet by late 2013, allowing operators to begin carrying the additional fuel.

For the 747-8F, which exhibited a 2.3Hz limit cycle small amplitude oscillation in the wings during flight testing, Boeing proposed and gained FAA approval to implement an OAMS that uses the aircraft's roll axis fly-by-wire system, which includes the outboard ailerons, to reduce the amplitude of the oscillation.

The 747-8's elevator, likely to be the actuator for a horizontal stabiliser modal suppression system, uses a hybrid mix of fly-by-wire and electromechanical actuation for control.

Feuerstein says the Boeing 747-8F performing the tests is the only flight test aircraft remaining in what had been a seven-aircraft fleet for the 4,500h Freighter and Intercontinental flight test programme. The aircraft is operating out of Moses Lake in Washington State, testing what he calls "early engineering work" for a software-based horizontal stabiliser fix.