The US Air Force has delayed a full-rate production decision for the KC-46A Pegasus until August and deliveries to the first operational unit will come about three to six months late, and even then will lack one critical system for at least another 12 months.

The announcement on 27 May on the eve of a USA federal holiday confirms months of findings by operational testers and government auditors that the five-year-old KC-46A development programme was unlikely to stay on original schedule, despite repeated vows by USAF and Boeing officials to the contrary.

The Milestone C decision, which determines the start of full-rate production, had already slipped from April to June and is now in August, the USAF says. Service officials blame a nagging hardware problem with unexpected loads on the fly-by-wire refueling boom for the move to delay the Milestone C decision another two months.

Boeing also is struggling to integrate a long list of engineering changes into a production system already supporting 20 aircraft derived from the 767-2C freighter in various stages of assembly, the USAF says.

The Cobham-made wing aerial refueling pods (WARPs) continues to face qualification challenges and are now expected for delivery to the USAF in October 2018, Boeing says. The first 18 aircraft delivered in the summer or fall of 2017 will come equipped with refueling booms and centerline drogue refueling systems, but the WARPs will not be available for another year.

“Throughout KC-46 development, the air force remained cautiously optimistic that Boeing would quickly address these issues and meet the original goal,” USAF programme executive officer Brig Gen Duke Richardson says in a statement. “However, we understand that no major procurement programme is without challenges and the Air Force remains committed to ensuring all aircraft are delivered as technically required.”

Since the $4.4 billion development contract for 179 tankers was awarded in 2011, the USAF has reimbursed an extra $500 million in contractually-allowed overruns and Boeing has recorded a further, combined $1.5 billion loss over several write-downs, including most recently a $243 million announced for the first quarter. Boeing won the tanker contract by submitting a bid priced $4 billion less than EADS North America (now called Airbus Group Inc).

Boeing says it continues to demonstrate its commitment to the programme, despite the losses and new delays.

“The KC-46 tanker will play a vital role in America’s air mobility forces providing both global power projection and unrivaled global reach for decades to come,” the USAF says in a statement.