Deliveries of Boeing’s troubled KC-46 tanker were halted for about a month earlier this year after Air Force and company inspectors found a red plastic cap lodged in a fuel valve that caused the uncontrolled flow of fuel from a one tank into another.

Tanker deliveries resumed after the previously unreported halt once Boeing confirmed that the debris “was missed on the aircraft” before an April 30 delivery flight, according to Captain Samantha Morrison, an Air Force spokeswoman. She said “a shortfall was identified in the previous inspection process, and additional layers of inspections have been added.” The company gave assurances that “future aircraft deliveries” would be debris-free, Morrison said in a statement.

Although deliveries resumed in June, seven serious “Category 1” deficiencies with the refueling tanker are unresolved. “Current plans show delivery of a fully mission-capable KC-46 by fiscal 2024,” she said. That’s 13 years after Boeing won the contract for the program.

In addition to repeated problems with debris, the unresolved problems include flaws in the “Remote Vision System” used to connect the tanker with planes in flight for refueling, a stiff refueling boom, fuel system leaks, issues with the flight management system, cracks in the air refueling drain tube and cracks in the auxiliary power unit drain mast, Morrison said.

“Boeing is making progress on resolving all seven deficiencies,” Morrison said.  

Forty-seven of an eventual 175 tankers have been delivered since Boeing won the contract in February 2011. 

In the latest incident, a tanker was being flown to Seymour Johnson Air Force Base in North Carolina for delivery when the aircrew discovered an “uncommanded fuel transfer” from one tank to another, according to Morrison. The crew “was not able to stop the transfer of fuel as desired,” she said. The problem “was found to be caused by foreign object debris — a red plastic cap — that had become lodged in the right main tank shut-off valve,” she said.

Boeing said in a statement that the stray cap had been used to protect aircraft components during manufacturing and installation. Across company operations, “we have instituted significant measures, establishing internal controls such as additional training for mechanics and inspectors and ensuring process discipline” to prevent debris incidents, the contractor said. “We have more work to do, and are taking additional actions.”