SEATTLE (AP) -- Boeing Co. is halting flight tests on its new 787 jet after finding that some of the planes have improperly installed parts in the tail.

"This is not a design issue or a flight testing finding. It's a workmanship issue," said the head of the 787 program, Scott Fancher, in a news conference on Friday.

The problem involves shims and fasteners, which weren't installed correctly in the horizontal tail of the plane, Boeing said. The whole assembly is built by Italian manufacturer Alenia.

Each inspection will take about a day and any work required will take up to eight days for each aircraft or tail assembly, Fancher said. The five flight-test jets will be the first inspected. The company will inspect all 25 of the tail assemblies it has in the factory, some of them not yet installed on airplanes.

The inspections began Thursday after Boeing received new testing data on the fasteners. Some of the stabilizer assemblies have the problem and some don't, the company said. A problem with the shims, which fill small gaps during assembly, was found a week ago and follow-up research on the fasteners found that issue on Thursday, Fancher said.

Fancher would not say whether any of the test planes have the problem or not. Some have not yet been inspected.

Delivery of the first 787 scheduled by the end of the year to Japan's All Nippon Airways Co. will not be impacted by the inspections and repairs, Fancher said.

The company put wiggle room into its production schedule for dealing with unanticipated problems with test flights or workmanship, Fancher said, and there's still some schedule margin available. He called the current issue typical of what Boeing deals with when building and testing a new airplane.

Fancher said Boeing has not lost confidence in Alenia, and they were working together to resolve the issue.

Boeing has relied on suppliers from around the globe to build nearly all components of the 787, but the program has been hampered by ill-fitting parts and other glitches. December's first flight was more than two years behind schedule.

In May, Boeing reported a design flaw in a bracket-like device in the plane's tail and said it would change the way the device is made. The areas affected were made by Korea Aerospace Industries Ltd., and Boeing's Charleston, S.C., plant.

In late April, Boeing told suppliers to stop delivering 787 parts to Everett for about five weeks, in part because of the tail problems that it acknowledged in May.