Boeing 787 wing structural testing was a success

ALTHOUGH THE BOEING CO.'s 787 Dreamliner has been beset by a series of embarrassing delays that have now stretched more than a year and a half, engineers accomplished a significant structural milestone when they broke a 50-foot-long section of one of the composite wings.

The wing did just what it was supposed to -- withstand, with a substantial safety margin, the highest aerodynamic forces the Dreamliner is ever expected to encounter in flight.

Now, engineers and 787 program officials must decide if they want to break the wing on one of the 787 test planes. Other than first flight, it is arguably the most dramatic moment during the testing of an all-new commercial jetliner before it can carry passengers.

For a couple of years, Boeing engineers have had a lively internal debate over whether to break the wing as part of the static ground tests. The 787 can be certified without actually breaking the wing, which could damage expensive test equipment and shower part of the Everett plant with tiny shards of composite material.

Boeing released a video of the wing box being stressed until it exploded, a jagged tear ripped across the composite structure.

Before a new commercial jetliner can be certified to carry passengers, the Federal Aviation Administration requires that the wings be able to withstand loads up to 1.5 times, or 150 percent, of the highest aerodynamic load that the jet could ever be expected to encounter during flight. After holding at that load for three seconds, the test is considered successful. There is no need to keep going until the wings break. But Boeing has done so in the past, to the breaking point, to demonstrate that the wings of its jetliners have a safety margin greater than required and to validate the analytical tools that engineers use in designing new jets.Engineers previously had loaded the wing that was destroyed Saturday up to 150 percent of the limit load, but then backed off because they did not want to actually test the wing until it broke.This time they did.

Boeing would not say how much above the 150 percent threshold the wing broke, just that it was "well in excess" of that figure."The technical team doesn't feel comfortable doing that," the Boeing spokeswoman said when asked why the break-point figure was not made public. "They said that on its own, the number is meaningless and people would try to make inferences that would not be founded with the proper context."

On the Boeing video of the test, Mark Jenks, vice president of 787 development, said the break occurred in the wing box exactly where engineers predicted.The 50-foot-long wing box weighs about 55,000 pounds. Boeing and its 787 partners, Fuji Heavy Industries and Mitsubishi Heavy Industries, which manufactures the 787 wing in Nagoya, Japan, have been doing structural testing on the 787 wing box for a couple of years at Boeing's research center on East Marginal Way South across from Boeing Field.Although it is full-scale, the 50-foot piece represents only a portion of the wing section, beginning at about the center of the 787 and stopping at about two-thirds of the span of the wing.

Boeing announced the wing-test project in July 2007 at the Farnborough air show near London and said the section of wing would be stressed until it broke. At the time, however, then-787 program boss Mike Bair said there was a "raging debate" within the 787 program about whether to actually break the wings of the static test plane as part of the certification test.

Static testing involves a one-time loading of areas of the plane to determine their ability to carry load, typically the maximum load expected in the lifetime of the airframe. There is also a fatigue-test 787. Fatigue testing helps engineers determine the plane's durability and involves cyclic loading of the structure to simulate repetitive flights.

The 787 wings are composite rather than aluminum. And that has allowed them to be very long -- 197 feet from tip to tip for the 787-8. The longer wingspan reduces drag, and that makes for a more efficient plane. But it never would have been possible without composites. The lighter and stronger composite material means more of the load can be carried on the outward part of the wing.

The 787 wings are nearly as long as the wings of the much bigger 777, which Boeing engineers broke during its static-certification testing. In January 1995, the wings were bent 24 feet above their normal position before they splintered -- at 154 percent of the design load.Airbus also broke the wings of its latest jet, the A380. But they failed at only 1.45 times the limit load.

At the time the A380 wing broke, the wingtips were deflected just over 24 feet -- much farther than they could be expected to ever bend in flight. The A380 was subsequently certified by the FAA and European regulators to carry passengers, but Airbus had to make structural modifications to the wing.The 787 wings are so long that some Boeing engineers think the tips might actually touch above the plane before they snap.

Boeing 787 changes its configuration

On January 28, 2005, the aircraft's development designation 7E7 was changed to the 787. Early released concept images depicted a radical design with highly curved surfaces. On April 26, 2005, a year after the launch of the program, the final look of the external 787 design was frozen, with a less rakish nose and a more conventional tail.
Boeing featured its first 787 in a rollout ceremony on July 8, 2007, at its Everett assembly factory, by which time it had become the fastest-selling wide body airliner in history with nearly 600 orders. Originally scheduled to enter service in May 2008, production has been delayed and it is currently scheduled to enter into service in late 2009.
When 767 sales began to go the way of the Airbus A330-200 in the late 1990s , Boeing began to consider replacement aircraft. As the 747-400 was also beginning to lose traction, the company began to consider two new projects—the Boeing Sonic Cruiser and the 747X. The Sonic Cruiser was intended to achieve higher speeds (approximately Mach 0.98) while burning fuel at the same rate as the existing 767 and A330-200 products. The 747X would stretch the 747-400 and give it a composite supercritical wing to improve efficiency

Boeing 787 uses extensive composites

When Boeing first considered extensive use of structural composites on the 787 Dreamliner, its engineers knew intuitively the epoxy/carbon fiber matrices would reduce weight significantly, allowing fuel savings and extended flying range. But after an intensive early look at composites, they realized fundamental design changes were possible that would allow functional systems integration, as well as changes in lamellar flow that would improve aerodynamics.
From a materials’ point of view, the 787 Dreamliner is one of the most revolutionary leaps in the history of manufacturing.
But in order to meet an ambitious delivery schedule – the first delivery is scheduled for May 2008 – there were tremendous hurdles to jump:
No one ever attempted to mass produce very large carbon-reinforced plastic structures, which are thermoset materials with significantly slower processing times than thermoplastics,
The critical tooling for such large sections was still very much in the development stage and, in fact, represented one of the few, small stumbles in the development program,
New coatings had to be developed to deal with the crack propagation issues, which are not a factor with aluminum. Engineers had to devise different systems to deal with electrical shorts because composites are not electrically conductive.

The Boeing 777 is 9 percent composites by weight, compared to 50 percent for the Boeing 787. Throughout the life of the 777, the Carbon-reinforced plastic materials (composites) were enhanced in terms of their properties, manufacturing and cost structure.
There are several different types of composites used on the 787, including bismaleimide, depending on specific applications requirements. There are several smaller parts made from discontinuous fibers that can be molded into odd shapes. There is also extensive use of thermoplastics in the interior of the aircraft, but that’s not a departure from previous designs.
All of the composites are supplied by Toray Industries, the world’s largest producer of carbon fiber. Since 2004, Boeing has placed composites orders with Toray estimated at more than $6 billion, creating pressure on prices and supplies for other users. The estimate was based on projected production as of 2006, numbers which are already out-of-date because of the spectacular success of the 787.

Composites aren’t the only materials’ news in the 787 Dreamliner. While composites represent 50 percent by weight (80 percent by volume) of the Dreamliner structure, other materials represented are aluminum, 20 percent; titanium, 15 percent; steel, 10 percent and 5 percent, other. Most notable among the “other” is the first-time widespread use in aircraft structures of plastic heat sinks. That’s right – plastic heat sinks. Plastics that are highly loaded with heat-removing materials such as carbon or ceramics have been around for a while, but have not yet penetrated the aircraft market. Their great advantage is their ability to be molded into net shapes. The economics for plastics can be favorable depending on total tooling and finishing costs. They can be designed with additional surface areas as fins and ribs to improve convective heat transfer. Costs and properties can be balanced depending on which engineering thermoplastics are used. For example, nylon can improve economics while liquid crystal polymer can improve properties. They are typically loaded 30 to 40 percent with thermally conductive materials.

Airbus a380 wing Static testing

Airbus is downplaying test results in which an A380 wing undergoing static testing failed slightly before the required design limit.
The wings are supposed to take 1.5 times the design load limit but this one failed at 1.45 times, about 3.3 percent shy of the certification requirement.
Airbus spokeswoman Barbara Kracht said the wing will need some “refinements” but the aircraft is on schedule for certification and first deliveries late this year. “We will need to find out from the data what is really needed but it’s certainly not a redesign of the wing,” Kracht told Associated Press.
In order for an aircraft to be certified here in the U.S., it must withstand the maximum “g” loading specified for that category, plus a 50% overload factor. For example, in the Normal category, the aircraft must withstand a positive load of 3.8g and a negative load of -1.52g at maximum gross weight. It must also be able to withstand an additional 50% of those loads without failing.
These “static” tests, as they’re called, are accomplished on the ground by mechanically loading up the wings. Sometimes this is done by simply placing sandbags on the wings to simulate a load. Large manufacturers like Boeing and Airbus use slightly more expensive methods. I’ve seen video of a 777 wing being tested to failure — the wings bent up to the point where they almost touched. In other words, it handled far more than the required loading.
The requirements for the Transport category are set out in 14 CFR 25.337(b)-(d):
(b) The positive limit maneuvering load factor n for any speed up to Vn may not be less than 2.1+24,000/ (W +10,000) except that n may not be less than 2.5 and need not be greater than 3.8 — where W is the design maximum takeoff weight.
(c) The negative limit maneuvering load factor –
(1) May not be less than −1.0 at speeds up to VC; and
(2) Must vary linearly with speed from the value at VC to zero at VD.
(d) Maneuvering load factors lower than those specified in this section may be used if the airplane has design features that make it impossible to exceed these values in flight.
So if the plane is going to be certified in the Transport category, it will have to handle somewhere between 2.5 and 3.8 positive G — plus 50% — depending on the maximum takeoff weight.
I’ve never heard of an aircraft failing to withstand the 1.5x test. That’s not to say it’s never happened, just that I’m not familar with such an ocurrance.
However you slice it, this has got to be a huge embarrassment for Airbus. Even if the flaw was simply a construction defect in the prototype, it will bring into question every other aspect of the A380’s design and construction in the minds of potential customers, not to mention the flying public.