A report by the UK’s National Audit Office (NAO) has listed some risks arising from the decision by the UK Ministry of Defence (MoD) to switch its Joint Strike Fighter selection from the F-35B STOVL (short takeoff and vertical landing) version to the F-35C carrier version.

The decision was part of a defense review that delayed until 2018 the in-service date of one of two new aircraft carriers to which the MoD was already fully committed. The other new carrier will be kept in reserve or sold. The UK provided much STOVL expertise and nearly $4 billion to help develop the F-35B.

No firm cost to convert the carriers for “cat and trap” operations has yet been specified, the NAO report notes. The industrial consortium led by BAE Systems that is building the carriers, at a cost of more than $8 billion, has tentatively estimated $800 million to add steam catapults to one carrier, or $1.280 million to add the new electromagnetic aircraft launch system (EMALS). The U.S. Navy is developing EMALS for service on its new carriers beginning in 2016, but there are technical risks and safety issues, according to the NAO.

Moreover, the F-35C design has been optimized for Nimitz-class carriers. In adapting the type to the new British carriers, there could be issues with recovery speeds, fatigue strength and airframe life, the report claims. And because the F-35C (unlike the F-35B) needs a fully cleared carrier deck to land, a buddy-type air refueling system must be developed and added to British F-35Cs in case of a blocked deck. (The U.S. Navy plans to use F-18s to air-refuel its carrier-based F-35Cs.)

The NAO report fails to note that the unit cost of the F-35C might actually be higher than that of the F-35B. The F-35Cs, which are being procured under the latest low-rate production contract (LRIP4), will each cost $16.5 million more than the F-35Bs. Before the defense review, the UK requirement was 132 aircraft. But with only one carrier in service, and this with a reduced number of F-35s embarked (12 versus the previous plan of 36), the UK total order could be cut by half, AIN estimates. The final total will be decided after the next defense review in 2015, the NAO confirms.

The UK Royal Air Force is also planning to operate the F-35 as a land-based strike aircraft. The report notes that one advantage of switching to the carrier version is greater combat radius, 650 nm versus 480 nm for the STOVL version. (The F-35 program office has recently revised these values downwards to 615 nm and 469 nm, respectively.) The more spacious internal weapons bay is another plus for the F-35C.

The MoD described the F-35C as “more capable and cost-effective” and noted that the defense review had yielded savings of nearly $5.5 billion to the UK’s carrier-strike program.