Boeing has completed certification flight testing of its 747-8 freighter, capping a flight test programme that has brought considerable challenges, but also paved the way for major technical innovations by the US airframer.

RC523, Boeing's fifth 747-8F flight test aircraft, completed its final certification flight around 00:15 local time today, following a 17h maximum endurance functionality and reliability (F&R) flight. RC522, the third 747-8F wrapped up final certification testing on the aircraft's Honeywell-built flight management computer.

"This is such a great day for the new 747-8 and for all the employees who played a part in designing, building and testing this incredible, game-changing airplane," said Elizabeth Lund, Boeing vice president and general manager, 747 programme. "We are in the home stretch in delivering this airplane to our customers."

Powered by four General Electric GEnx-2B67 engines, the 747-8F, which is 5.6m (18ft 4in) longer than the 747-400F, completed more than 1,200 flights and 3,400h since RC001's 8 February 2010 first flight, validating 1,700 US Federal Aviation Administration certification requirements, and 300h of systems F&R resting.

With final flight testing completed, Boeing will now make final documentation submissions to the FAA. The ongoing partial shutdown of the FAA is not expected to interfere with final airframe certification, as the staff responsible are paid out of an unaffected source of funding. However, approval for the jumbo freighter's use at six major US airports remains in question with Congressional funding denied to divisions responsible for airport certification.

The hours needed to certify the new freighter was more than double the originally expected 1,600h flight test campaign. After its first flight, Boeing encountered several design challenges that combined to buckle the airframer's schedule by September 2010, pushing first delivery from the end of that year to mid-2011.

Boeing would subsequently grow the flight test fleet from three aircraft to five.

Shortly after its first flight, Boeing discovered a buffet in the aircraft's new double slotted inboard flaps at the maximum detent that was found to be caused by the outboard main landing gear doors, necessitating a redesign of the door and a readjustment of the final detent position.

Later in the programme, Boeing discovered that a slow pilot input on the jumbo jet's control wheel would cause an oscillation in the inboard aileron power control unit actuator due to high pressures in the hydraulic system.

Lastly, flutter testing revealed a 2.3Hz limit cycle oscillation (LCO) that caused the aircraft's wing to vibrate ±2.5cm (1in) under certain conditions, which spawned the programme's most notable innovation that drew its inspiration from the augmented flight control systems of fighter jets.

With the new closed-loop fly-by-wire outboard aileron, Boeing would implement an active system to dampen out the vibration. Known as OAMS or the outboard aileron modal suppression system, the computer driven aileron would engage automatically to dampen the vibration. The OAMS allowed Boeing to dampen out the aeroelastic instability without adding thousands of pounds of structural weight to the aircraft's wing.

The flight test programme also featured another first for the company, including a 454t (1,000,000lb) takeoff, well in excess of the aircraft's 442t maximum takeoff weight, as well as a milestone transatlantic biofuel flight when the aircraft made its debut at June's Paris air show.

Boeing holds 76 orders for the 747-8F, the first of which is expected to be handed over to launch customer Luxembourg-based Cargolux in September.

Source: Air Transport Intelligence news