The Boeing KC-46 test programme has completed a series of flutter tests of the 767-2C tanker type, certifying it to operate with wing aerial refuelling pods and its tanker tail boom stowed.

Boeing released a video this week that shows the EMD-1 aircraft, used to certify the 767-2C airframe, passing through basic manoeuvres.

A spokesman for the company says completion of the flutter tests certify the aircraft to fly in the tanker configuration with the boom stowed.

The expanded flight envelope comes amid growing concern that the first fully equipped KC-46A won’t be ready to fly this summer.

The programme has been hit by repeated delays and cost overruns because of wiring and supplier issues, but more recently a chemical mix-up during fuel system testing and other design issues have caused more problems.

The spokesman says there is currently no firm date for the first flight.

“Right now it looks to be roughly a month beyond our previous plan to fly EMD-2 in the late August to early September timeframe,” he says.

Despite missing several targets already, Boeing says it still plans to deliver the first 18 operational aircraft by August 2017 as required by the contract. The company recently recorded a $536 million after-tax charge (on top of the $272 million charge announced in 2014) due to tanker cost overruns, since the aircraft is being developed under a fix-price contract.