Northrop Grumman is increasing the rate at which it produces center fuselages for F-35 Lightning II fighters.

The increase comes through a shorter production interval, or PI, the average number of work days between starts or completions of a center fuselage.

A new three-day PI -- down from four days -- will enable the company to produce a center fuselage in just over 10 months compared to about 11 1/2 months.

"This increase in tempo on our F-35 Integrated Assembly Line is part of a coordinated, pre-planned effort by Northrop Grumman and its suppliers to help meet rising F-35 production requirements," said Brian Chappel, vice president and F-35 program manager, Northrop Grumman Aerospace Systems. "It also reflects our steady progress increasing the efficiency of the production line, and the size and skills of our workforce."

Northrop Grumman said the PI time has steadily dropped since its fuselage production line opened in 2011, when the interval was about eight days.

The new three-day PI came into effect last month with the start of production of BK-10, the 10th F-35B variant for Britain.

"We'll change to a three-day PI station by station as BK-10 moves down the line," said to David Tracy, director of Northrop Grumman's F-35 center fuselage integrated product team. "In parallel, our customers will be preparing to begin receiving our center fuselages on a three-day interval as well.

"When BK-10 is complete, the integrated assembly line will be fully converted to a three-day PI. At that point, we'll start delivering center fuselages at the rate of one every three work days."