EVERETT — The Boeing Co. added a sixth test plane to its KC-46 aerial refueling tanker program.

The aerospace giant expects the additional tanker will help it complete remaining ground and flight tests. Boeing is slated to deliver the first KC-46 Pegasus tanker to the U.S. Air Force later this year, later than the contract’s deadline to hand over the first 18 aircraft.

The new test plane will verify that electromagnetic fields produced by radars and other systems will not affect the KC-46s, which are based on Boeing’s 767 and assembled in Everett.

So far, the KC-46 test aircraft have flown more than 1,600 flight hours.

“Adding another tanker will help us to become even more efficient and significantly improve our ability to complete test points going forward,” said Jeanette Croppi, Boeing KC-46 tanker test team director.