Boeing has cautioned that 747-8F entry into service "could slip in 2011" as the company works through two significant technical issues that have made the airframer's quest for flight-testing momentum elusive.

Speaking at the air show, ­Boeing Commercial Airplanes chief executive Jim Albaugh mirrored the previously issued "­cautionary note" on the 787 schedule for its jumbo freighter, which has completed roughly 550h of 1,600h in its flight-test ­programme.

The first of two issues centred on the aircraft's inboard ailerons and the Nabtesco-built power control unit that drives them. Boeing found that when a pilot makes a slow input on the jumbo jet's controls the high pressures in the hydraulic system cause a limit cycle oscillation, requiring a fix in the actuator, according to two 747-8 programme sources.

Cargolux 747-8F
 © Boeing

In the second, the freighter was found to encounter a structural flutter at mid-weight near cruise speed, requiring engineers to dampen out the aeroelastic vibration. Options for resolution include adding structure to the wing or potentially developing a new control law.

The sum of these two issues, plus a now-resolved landing gear door flap vibration issue, has placed significant pressure on the certification campaign, virtually eliminating any remaining margin left before sliding into 2011.

Boeing declined to address the specific issues encountered by the programme, although the company continues to caution that while first delivery to Cargolux remains "at risk", certification is still targeted for year end.

The airframer emphasises that the 747-8F has "completed substantial airworthiness, aerodynamic performance and systems testing" to date.

The approximately 10-month flight-test programme is on par with past Boeing programmes, although the pressures of simultaneous flight testing of both the 747-8 and 787 side-by-side made its schedule aggressive.

RC501, the lead flight-test aircraft, has for the last month and a half been dedicated to troubleshooting both issues, with an eye to resolving the flutter issue by end-July.

With five months to go before first delivery to launch customer Cargolux, Boeing is exploring ways to find contingency in the flight-test programme. The company has already opted to grow its flight-test fleet from three to four, with the addition of RC503, expected to head immediately into high-intensity radiated field testing in Arizona.

Boeing holds orders for 108 747-8s, including 76 for the freighter variant.

Source: Flight International