Despite Boeing's strenuous efforts to reduce the 787 Dreamliner's weight, the plane weighed more than expected when it first rolled out two years ago.
Days before the plane's maiden flight last week, Boeing published a document for airlines that suggests to some weight-watching industry analysts that the 787 still exceeds its original target weight by a few tons.
Airlines have ordered 840 of the pioneering composite-plastic planes based on Boeing's projections for its range, payload and fuel efficiency -- all reduced by added weight.
In an interview, 787 chief project engineer Mike Delaney insisted that the weight has been stable for the last two years. He said the Dreamliner would meet its targets for range and payload and still deliver on the original promise of being 20% more fuel-efficient.
Excess weight is a constant worry on any new airplane program. On the composite-plastic 787, the concern was amplified this year when Boeing discovered a structural flaw in the design and had to reinforce sections of the wing-body joint with titanium fittings.
One number Boeing won't disclose is the basic weight of the empty plane -- it never does during development. That fed speculation as the company made the modifications this fall.
"The 787-8 appears to have evolved from a once-elegant composite design to one saddled with carbuncles of heavy titanium added throughout for strengthening," Morgan Stanley financial analyst Heidi Wood wrote in an October research note.
Wood downgraded Boeing stock to the equivalent of a "sell," citing concern about the weight of the airplane, among other issues.
Just before last week's first flight, airlines received a briefing document that listed the maximum allowed takeoff weight of the jet as 9.25 tons heavier than in the version published two years ago.
Delaney said the document doesn't mean many tons of weight have been added. Rather, he said, it describes the plane's allowed operational weight, which Boeing has bumped up after its modeling and analysis showed the airplane structure to be strong enough to carry extra loads.
There is an economic penalty from carrying more weight. A heavier plane burns more fuel per trip, increasing fuel costs to the airline.
Yet Delaney insisted that Boeing still will reach the Dreamliner's fuel-efficiency target: an average 20% improvement over today's airplanes.
He said 787 engine makers Rolls-Royce and General Electric are working on improving fuel consumption. That, along with Boeing's weight and drag reduction and other improvements, will make up for the fuel-burn penalty that comes from the added weight, he said.
Nelson Klug, an engineer who worked for both Boeing and Douglas Aircraft and who now is senior director of consulting at aviation firm Avitas, said the various weights listed in the document -- takeoff weight, landing weight, zero-fuel weight and others -- suggest to experts the size of the weight growth in the basic empty airplane.
"It's not unreasonable to assume that the operating empty weight is about 10,000 pounds [5 tons] heavier than what was originally on the drawing board," Klug said.
Gates writes for the Seattle Times.