Francis Tusa London

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Britain's decision to change the basic design of its new aircraft carriers, centerpiece of the nation's biggest arms program, for the second time in as many years is raising doubts about the country's defense decision-making at the highest level.

In early May, the British Defense Ministry dumped its late-2010 plan to convert one and possibly both in-build Queen Elizabeth II-class aircraft carriers to operate the F-35C catapult-arrest version (known as the CV) of the Joint Strike Fighter. The U.K. opted instead to revert to the short-takeoff-and-vertical-landing (Stovl) F-35B.

The first decision to switch to the F-35C was based on simple arguments, put forward by Prime Minister David Cameron in late 2010. The F-35C has a larger weapons load than the F-35B, thanks to a larger bomb bay, and longer range than the F-35B because it carries fuel where the latter has its lift fan. The F-35C was said to be less costly to buy and operate than the mechanically complicated F-35B. The F-35B was on probation at the time, so was deemed to be risky by the U.K. The more capable F-35C would also make up for the fact that the U.K. would not be buying 138 JSFs, as originally envisaged.

The switch to the C was contingent on using the U.S.-developed Electromagnetic Aircraft Launch System (Emals). The idea of fitting dedicated steam generators to the turbine-electric carriers never looked attractive, and by late 2010, the once-problematic Emals had started showing success in U.S. testing.

It all seemed logical, which made the decision to revert to the Stovl approach more startling.

What became apparent at a briefing held by the Defense Ministry in early May was that the cost estimates for fitting the ships with Emals and arrester gear had been either slapdash or wildly optimistic. The estimated cost of converting the second-in-class ship, HMS Prince of Wales, had more than doubled, from just under £1 billion ($1.5 billion) to £2 billion. The first-of-class ship, Queen Elizabeth II, which was more advanced in construction, would need £3 billion in modification costs. Modifications for both ships would cost £5 billion, close to what they had been expected to cost in total without them.