The US Government Accountability Office (GAO) decided this week to review in private Boeing's protest against the KC-X contract award, while all parties continued to squabble in public over details of the US Air Force selection process.

One month after the USAF selected the Northrop Grumman/EADS North America KC-45 for the $12.1 billion tanker award, the GAO rejected separate motions by the air force and the winning bidder to throw out Boeing's protest claim.

The GAO, an investigative agency for the US Congress, now has until mid-June to judge Boeing's case for overturning the USAF's contract award.

The next step requires the USAF to submit a detailed response to Boeing's evolving protest claims to the GAO by 16 April.

Meanwhile, Boeing and Northrop both have teams of lawyers sequestered at GAO's offices reviewing thousands of pages of official USAF documents, seeking to dig out new details to support their respective cases.

Mark McGraw, Boeing vice-president for tanker programmes, says Boeing's lawyers have made several findings buried in the USAF documents.

Those discoveries have prompted Boeing to submit three updates to its original protest document, submitted in mid-March. The two latest updates remain undisclosed, but - in an unusual move - Boeing has publicly released redacted versions of the introductory sections for its first two protest documents.

Boeing's original protest focused on two points - that the USAF selection team "unfairly" rejected some of Boeing's cost estimates, and that "last-minute" changes to a software-based evaluation model biased the outcome of the competition in favour of Northrop's offering.

One new argument appears in Boeing's first updated protest, which is based on USAF documents unearthed by Boeing's lawyers, says McGraw.

The documents show the USAF selection team had "grave" concerns about the safety and design of the Northrop team's EADS CASA-supplied air refuelling boom, he says.

McGraw adds that the documents showed the Northrop team may need to redesign the boom to satisfy the USAF's concerns, but he did not elaborate.

Asked about McGraw's statement, a Northrop spokesman said: "There is no new boom design."

Northrop also released a statement immediately after McGraw spoke to reporters on 3 April.

"Northrop Grumman and its partners have built, tested and flown the KC-45 and we have built and successfully passed fuel through the boom that will be used," the spokesman said. "No amount of insistence on the part of Boeing officials that its scores should be higher will make it so."

 KC-45 B-2
 Northrop Grumman/EADS North America KC-45

In a congressional hearing on 1 April, USAF officials also defended changes made to the air refuelling assessment model, which is one of five areas evaluated for each bid.

The USAF updated the model to include another runway at Bahrain because that landing site would be used in a wartime scenario, officials said. The model also slashed the required wingtip clearance on the ramp to reflect proper wartime conditions.