On 21 December 2015, a Boeing 787-8 at FL400 in the vicinity of convective weather conducive to ice crystal icing penetrated an area which included maximum intensity weather radar returns. A very short period of erratic airspeed indications followed and the FCS reverted to Secondary Mode requiring manual flying. Since this Mode remained 'latched' and could therefore only be reset on the ground, it was decided that an en route diversion was appropriate and this was accomplished without further event. Boeing subsequently modified the FCS software to reduce the chances of reversion to Secondary Mode in short-duration unreliable airspeed events.