When the flight deck becomes aware of upcoming turbulence, it is customary to give passengers an early warning. At Roll-Royce, the captains let investors sit back and relax. The world’s second-largest engine maker must have thought shareholders were well aware that falling military spending was a headwind. But a forecast that shrinking defence orders will wipe out profit growth this year has sent Rolls-Royce shares down 18 per cent. This rapid descent may have been avoidable.