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FAA and Boeing agree on 747-8 OAMS special condition

Boeing and the US FAA have come to a final agreement on the regulatory special condition required for the 747-8's outboard aileron modal suppression (OAMS) system designed to dampen out a structural vibration in the wing.

The publication of the 9 March Special Condition is a significant step forward for the delayed 747-8 Freighter programme, which is set to now enter service mid-year with Luxembourg-based Cargolux, about two years after first intended. The 747-8 Intercontinental, the passenger model, will deliver in the fourth quarter to a completion centre for conversion to a Boeing Business Jet, over a year later than planned.

Todd Zarfos, 747-8 vice president of engineering, said in February at the time of the 747-8I's unveiling: "We know [OAMS] will work and now we just have to go through the last aspects of certification associated to that."

"We had long conversations with the FAA on whether existing [Federal Aviation Regulations] covered what we were already doing. I'll just be frank, we thought they did, they characterised it as something "new and novel" and that resulted in the need for a special condition," he says.

The FAA in its special condition addresses this debate saying "the regulations do not anticipate the use of systems to control flutter modes that do not completely suppress them," adding it requires "the airplane to remain flutter free after certain failures".

The vibration - also known as a limit cycle oscillation (LCO) - was observed during flutter testing and saw the aircraft wing tip deflecting ±2.5cm (1in) at frequency of 2.3Hz.

While the FAA says "the sustained oscillation is caused by an unstable aeroelastic mode" the vibration does not intensify and is considered ""stable" if it maintains the same frequency and amplitude for a given excitation input and flight condition", nonetheless "the FAA considers it to be an aeroelastic instability".

The OAMS system, which is built out of the 747-8's roll-axis fly-by-wire flight control system, reduces, but does not eliminate, the "amplitude of the sustained oscillation and control the aeroelastic instability", says the FAA.

When the aircraft enters the area of the observed oscillation, OAMS activates automatically, dampening out the LCO by offering counter loads generated by the outboard aileron.

The FAA has added OAMS to the master minimum equipment list, prohibiting the aircraft from being dispatched with the system inoperative.

In addition to validating the OAMS system, which has been in flight testing for months on the 747-8F test fleet, Boeing must show by test and demonstrate through analysis that the LCO is "stable throughout the nominal aeroelastic stability envelope" when OAMS is inoperative and "must be shown to have negligible impact on structure and system, including wear, fatigue and damage tolerance".

Boeing will be required to demonstrate through flight flutter test that the OAMS system can provide a "proper margin of dampening disturbances above the sustained amplitude of oscillation of all speeds up to" the demonstrated flight diving velocity, and there is no "large and rapid reduction in dampening as [diving velocity] is approached".

Boeing says the OAMS system was not installed on the 20 March first flight of RC001, the first 747-8I, as the aircraft is structurally different enough from the freighter, with is elongated upper deck and tail fuel tanks. The airframer will first conduct flutter evaluations to see if the LCO is present on the -8I as well.

The LCO was one of two primary reasons the delivery of the jumbo freighter slipped from the end of 2010 to mid-2011, along with 3000psi hydraulic pressure spikes that caused the aircraft's non-fly-by-wire inboard aileron actuator to oscillate at certain control inputs.

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