The U.S. Federal Aviation Administration (FAA) has granted the Supplemental Type Certificate (STC) to Boeing's KC-46 mid-air refueling tanker to complete the aircraft's FAA certification process, said U.S. top aircraft manufacturer Boeing Company on Wednesday.

The STC is one of two required FAA airworthiness certifications that the Boeing KC-46 Pegasus tanker has been sought for over the past three years, when a Boeing team started a series of lab, ground and flight tests in 2015.

"This milestone is important in that it is one of the last major hurdles in advance of first delivery to the U.S. Air Force," said Mike Gibbons, Boeing KC-46A tanker vice president and program manager.

The KC-46 is a multirole tanker derived from Boeing's commercial 767 airframe to provide refueling support for U.S. military aircraft.

Boeing received an Amended Type Certificate for its core 767-2C aircraft configuration in December 2017.

Boeing engineers completed flight testing including the jet's aerial refueling, defensive and other military-specific systems in early July to qualify for the Military Type Certificate to be granted in the next few months.

Boeing said six KC-46 tankers have completed a total of 3,500 flights hours and offloaded more than 1.35 million kg of fuel during refueling flights with F-16, F/A-18 and other military aircraft.

Boeing is currently on contract for the first 34 of an expected 179 tankers for the U.S. Air Force, and the jets are being built in the company's Everett facility in Washington state on the U.S. west coast.