EVERETT — Untimely work and substandard supplier components continue to be a drag on Boeing’s 787 Dreamliner program, say factory workers and others familiar with the production process.

The problems prompted the aerospace giant last month to slow the two 787 assembly lines at Paine Field. From mid-August until early September, no new airplanes rolled out of the factory. Meanwhile, an unusual number of 787s were parked on the Boeing flight line and in other areas of the airport.

The slowdown was part of a production-rate plan drafted two years ago, and that production is getting smoother, said Debbie Heathers, a spokeswoman for the Dreamliner program. “We know that changing a program’s rate can result in disruption, and we scheduled time to address this.”

Boeing increased 787 production to 10 planes a month at the beginning of the year. The monthly workload is roughly split with seven planes rolling out of the factory here and three produced by Boeing’s plant in North Charleston, South Carolina.

Most months this year, eight 787s were started on assembly lines in Everett. In North Charleston, work was begun on an average of 2.6 planes per month.

But in August, the two plants “loaded” a total of seven planes on the assembly lines — three in South Carolina and only four in Everett.

No airplanes were loaded on the Everett assembly lines during the second half of August, according to several sources in the factory. The production rate appears to have returned to normal in September.

During August, 787 workers had to work 10- and 12-hour days and on Saturdays, several workers said, to fix problems. Overtime in Boeing factories is not uncommon, but workers say their workload this time was greater than usual.

The workers asked not to be identified because they are not authorized to talk publicly about the program.

Boeing Commercial Airplanes President Ray Conner on Tuesday noted that the company has doubled the 787 production rate in the past year — no small accomplishment.

“We’ve had a pretty hefty incline here in recent years,” including adding the 787-9, a new version of the plane, to the line this year. “We want to give ourselves a nice long run where we can get all the production processes down,” Conner told investment analysts in Laguna Niguel, California.

Boeing called the August period of the reduced rate “productivity days,” said one worker. “It’s saying we’re stopping for two weeks, but we’re not admitting it,” the worker said.

Workers had time to catch up on “traveled” work, but the program is still “thousands of jobs behind,” said another shopfloor worker. Traveled work consists of tasks that are not completed in the proper order and are passed downstream.

As recently as August, airplanes have come out of both factories regularly requiring extensive additional work, the Boeing workers say. A few days ago, 34 Dreamliners were parked at various locations on Paine Field. That is about 10 more than usual, said Matt Cawby, a plane-spotter and photographer who is at the airport most days.

Last month, the Chicago-based company told Snohomish County, which owns the airport, that it would exercise an option to occupy more pavement at Paine Field, said Dave Waggoner, the outgoing airport director.

Boeing spokeswoman Heathers said the company routinely adjusts the amount of parking space it requires. Workers, however, say the increase in the number of parked planes is noteworthy.

“Any plane that hasn’t been delivered has work that needs to be finished,” said a worker on the 787 flight line, where planes are prepared for flight test and delivery. Aircraft in established programs typically spend only a short period on the flight line before a customer gets the keys and flies home.

Dreamliners certainly require less post-assembly work than before. But the program has plenty of room for improvement.

“We’ve been building them for six years. It shouldn’t take this long,” the flight line worker said.

Boeing says smoothing out the production process is something every program goes through.

“Traveled work has occurred on every modern Boeing airplane program and results from a number of factors, like part shipment delays, supplier and internal quality control, and rate increases, among others,” Heathers said.

Workers in Everett say many of their headaches start in South Carolina and with suppliers such as Alenia Aermacchi, an Italian company that makes composite fuselage sections for the 787, among other things.

Everett work is not flawless, either.

“It’s the whole program,” the flight line worker said. But “it’s worse in South Carolina.”

Boeing acknowledged as much earlier this year when it hired hundreds of contract workers in North Charleston to compensate for an inexperienced workforce.

“We have seen a reduction in traveled work over the past six months,” including in mid- and aft-body assemblies from South Carolina, Heathers said.

She declined to comment on recent unfinished jobs.

Regardless of where a 787-8 is assembled, delivered airplanes have performed equally well in service, based on in-service fleet data, Heathers said.

But some airplanes assembled in North Charleston still are flown to Everett for rework. The reverse is not the case.

And contrary to what Boeing says, a shopfloor worker said that mid- and aft-body fuselage sections from South Carolina continue to arrive in Everett with problems.

Some of the issues trace back to Alenia, he said. “They are not building the structures of the aircraft per our specs the first time.”

For example, passenger and cargo doors are not properly sealing, requiring Everett mechanics to substantially rework the structures, the worker said. That delays an airframe pressurization test, which causes further delays down the line.

Dan Catchpole: 425-339-3454; dcatchpole@heraldnet.com; Twitter: @dcatchpole.