As Boeing prepares to begin the 747-8's flutter trials, it has inspected the test fleet for defective machined stringers that were found to be prone to cracking under certain loads. Meanwhile, the airframer is evaluating whether to bolster the flight-test effort through the addition of a fourth aircraft.

The area of concern centers on the upper part of the 747-8F's fuselage known as the "bonnet". The stringers, which run longitudinally along the aircraft, were inspected for thickness of the flange, which as a result over-machining can crack causing damage to the aircraft's primary structure.

Boeing has confirmed the quality issue, which originated with a sub-tier supplier to Vought Aircraft Industries. Vought builds panels for the majority of the 747-8's fuselage and empennage.

While Boeing maintains that "all three flight-test airplanes have been inspected and cleared for flight," two programme sources suggest that while the aircraft remain airworthy, constraints on g loading could impact the operating envelope of the aircraft until the issue is fully resolved.

A Boeing spokesman says he is unaware of any restrictions on the aircraft, but adds that early flight tests are still working to expand the operating envelope of the fleet.

747-8F RC522 

 © Joe Walker

The same sources add that flaps 30 buffet investigation would still continue as a final fix for the landing gear door induced vibration is being resolved.

Though another source close to the manufacturing of the aircraft says that inspections will continue on aircraft in production and could create a time consuming challenge to find, remove and replace the defective stringers.

With just over two months have passed since its 8 February first flight, Boeing's new jumbo is moving into the next phase of its 1,600h flight test campaign as aeroelastic flutter tests are set get underway.

Boeing says it is "making steady progress" on the flight test campaign and the aircraft are "performing as expected" is still targeting late 2010 for handover of the first 747-8F to Cargolux.

Following flutter testing, Boeing's three 747-8F flight test aircraft will transition to Palmdale, California for the remainder of the certification campaign that will encompass a total of 3,700h of testing, including 2,100h of ground testing.

While maintaining the pace of the programme, Boeing is considering "potential opportunities to gain efficiencies", including adding a fourth flight test aircraft.

Programme sources say that the second production aircraft (RC503), which is the fifth 747-8F off the line, is the likely candidate and would be used for engineering flight tests. This aircraft's role would be supplemental, as it lacks an instrumentation suite.

Boeing says that while no decision has been made about expanding the test fleet, the addition of a production aircraft for limited testing is a normal part of certification. It adds that the move is being studied as a potential opportunity to bolster the test programme.

Source: Air Transport Intelligence news