SEATTLE - Despite Boeing Co.'s strenuous efforts to reduce the 787 Dreamliner's weight, the plane weighed more than expected when it first rolled out two years ago.
And days before the new plane's maiden flight this month, Boeing published a document for airlines that suggests to some 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 the weight has been stable for the last two years.
And he said the Dreamliner would meet its targets for range and payload, and still deliver on the original promise of being 20 percent more fuel efficient.
Excess weight is a constant worry on any 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 so 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.
Just before this month'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 did not mean many tons of weight had 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.
That means airlines can load the planes with more fuel and thereby meet payload and range requirements, despite the heaviness lingering from two years back.
Delaney conceded that when the first large sections of the Dreamliner were built, the 787 was heavier than projected.
In 2006, the target empty weight of the plane was 108 tons, according to internal Boeing documents from the time.
Because of "the initial weight issues . . . in the 2005, 2006 time frame," Boeing either had to take the extra weight out or increase weight allowances to carry more fuel, Delaney said.
Since then, however, Boeing's weight-control programs have kept the basic empty weight of the plane in check, Delaney said.
Allowed now to carry more fuel, the jet will be able to fly fully loaded as far as advertised.
But there is an economic penalty from carrying more weight. A heavier plane burns more fuel per trip, increasing fuel costs to the airline.
Delaney insisted Boeing still will reach the Dreamliner's fuel-efficiency target: an average 20 percent 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 bumping up the weights, 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 [five tons] heavier than what was originally on the drawing board," Klug said.
Making a rough calculation and feeding estimates into his computer models, he said that on a flight of 5,000 nautical miles, with all else the same, a weight increase of 10,000 pounds burns about 450 extra gallons of fuel. That would cost the airline an extra $1,000 or so.