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Understanding the Ultimate Load-Wing test: A350

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The Airbus A350 program achieved another milestone with the successful completion of the ultimate load wing test in December 2013. The ultimate load wing test is a test in which the wing is deflected to simulate the “ultimate” load, beyond or at which the wing is expected to fail.

The ultimate load is calculated as 2.5 times the maximum expected G load that the aircraft would ever encounter in its service life. For the Airbus A350, which is limited in the G loads that it may experience, by the Fly By Wire system to +2.5G, or with the FBW system deactivated, as is the case with a reversion to direct law, approximately between 3-3.5G with the aerodynamic limitations of the flight control surfaces. The ultimate load is then possibly between 7.5 – 8.75G.

Based on this G force, the expected wing flex due to aerodynamic loading is computed, and the wing of a static test airframe flexed (loaded) to the corresponding load. The wing is expected not to fail at this “ultimate” load equivalent flex. At this loading, the A350’s wings flexed in excess of 5 meters, while at a similarly scaled G loading, the A380’s wings flexed to close to 7.5 meters. The 787’s wing flexed up to 7.6 meters in a similar test, mandatory for certification.

In February 2006, the A380’s wing gave way just before the 1.5 times greater G load limit was reached.

Unlike in the past, aircraft manufacturers don’t seem to be stressing the wing beyond 1.5 times greater load, to the point of wing failure. The actual failure load may not be known.

According to Airbus, “This test was performed on the A350 XWB static test airframe that was built specifically to demonstrate the structural integrity of the airframe. The strains induced into the airframe were measured and monitored in real time using more than ten thousand measurement channels. The huge volume of data recorded was analysed and correlated to the structural computer models which have been used to design the airframe.”

With the comforting thought of a safe-enough wing, the first A350 airframe intended for commercial service, MSN6, is being assembled for launch customer Qatar Airways.

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