The KC-46 aerial-refueling tanker Boeing is building for the Air Force finally made its long-delayed first flight Friday. Critical flight tests ahead will test its ability to dispense fuel in the air to six different military jets.

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Boeing’s KC-46 aerial-refueling tanker for the Air Force finally made its first flight Friday, seven to eight months later than planned.

Chief KC-46 test pilot Ron Johnston took off from Paine Field at 1:24 p.m. and the plane landed four hours later at Boeing Field.

The flight, originally scheduled for early in 2015, marks the beginning of months of critical flight tests. Boeing has no more breathing room in the schedule if new issues arise during those tests.

On Friday’s flight, the pilots performed operational checks on engines, flight controls and environmental systems and took the tanker to a maximum altitude of 35,000 feet

On the next test flight, they will likely deploy for the first time the advanced refueling boom, a rigid tube that telescopes out the aircraft’s rear underbelly to feed jet fuel to other aircraft.

On subsequent flight tests, the crew will deploy the wing aerial-refueling pods, an alternative way to dispense fuel through flexible tubes that extend from the wings.

The first major testing task will be to show that the tanker’s flight is stable with these refueling devices extended. At some later point, the tanker will have to connect with six different types of aircraft and show that it can dispense fuel to each.

Next April, the Air Force is scheduled to assess the tanker program’s status and give Boeing the go-ahead for production.

Flight tests will continue through the end of next year. If all goes to plan, Boeing is to deliver the first 18 operational tankers to the Air Force by the following August.

The Air Force plans to buy 179 tankers under a contract worth about $50 billion.

Friday’s KC-46 first flight had been pushed back by a series of issues, ranging from wiring design flaws to redesigning the plane’s complex fuel system.

To keep to the 2017 initial delivery schedule, Boeing has had to re-sequence many of the flight tests required for certification.