Boeing Begins Analysis Of 787 Fatigue Tests
AeroBD | The AERO news Company…January 25, 2016, LOS ANGELES : Boeing has completed a five-year fatigue test of the 787 airframe, validating the basic strength of the predominantly composite primary structure and helping to define maintenance and inspections to cover the aircraft’s full projected-service lifetime. The tests, which began in September 2010 on ZY998, the third 787 airframe built, simulated entire flights, from taxi through ascent, cruise and descent back to taxi and were aimed at creating a data set for the airframe’s durability. The tests subjected the structure to loads which simulated more than 160,000 cycles, or more than 3.6 times the design life of 44,000 flight cycles. “Test results were right in line with our expectations, with no significant findings,” says Bob Whittington, Boeing 787 vice president and chief project engineer. “We validated the robustness of the 787 design as well as our modeling, and the airplane performed so well that we extended the testing.”
The fatigue tests were originally scheduled to begin in 2009, but were delayed by more than 18 months after the company was forced to reinforce the side-of-body join area where the wing and center wing box meet. The redesign, which also pushed back first flight of the Boeing 787 to December 2009, was validated in static tests the previous month and later incorporated into ZY998, which allowed fatigue tests to begin in 2010.
Although Boeing originally selected lower-density composites for lower structural weight, other key attributes included improved fatigue and corrosion resistance. This expectancy is reflected in the extent of the 787 tests compared to previous fatigue tests on conventional aluminum-built airframes such as the 777 which completed the then record number of 120,000 simulated cycles in 1997. This represented the equivalent of 60 years in operation or twice the 777 design service objective (DSO) of 60,000 flights. The DSO of the 787 and 777 have both been significantly extended beyond previous generations such as the 757 and 767 which underwent fatigue tests simulating 100,000 flights, or twice a 20-year DSO of 50,000 flights.
Boeing aims to translate the improved fatigue life into reduced maintenance, and the results of the tests will play a key part in validating inspection intervals targeted for the program. These include a first external visual inspection of the structure at six years and first heavy-check internal inspection at 12 years. It also expects the fatigue resistance to be reflected in total scheduled labor hours which will be reduced by around 60% over the life of the aircraft compared to the 767. Projections were partly based on scheduled maintenance experienced with the composite tail of the 777 which, despite being 25% larger than the 767’s conventional aluminum tail, requires 35% fewer scheduled maintenance labor hours.
Testing on the engineless airframe was conducted in a steel rig weighing more than 1 million lb. on the northwest corner of the manufacturing plant in Everett, Washington. The rig included more than 100 mechanical connections to push, pull and twist the 182-ft.-long fuselage, wing forward leading edge and vertical stabilizer. The airframe was festooned with over 3,000 sensors and evaluated more than 40 million discrete load conditions.