Fuze Keeping Clock

The Fuze Keeping Clock (FKC) was a simplified version of the Royal Navy's High Angle Control System analogue fire control computer. It first appeared as the FKC MkII in destroyers of the 1938 Tribal class, while later variants were used on sloops, frigates, destroyers, aircraft carriers and several cruisers. The FKC MkII was a non-tachymetric anti-aircraft fire control computer. It could accurately engage targets with a maximum speed of 250 knots (460 km/h; 290 mph).

The FKC received vertical reference information from a Gyro Level Corrector and aircraft altitude, range, direction, and speed input information from the Rangefinder-Director, and output to the guns the elevation and deflection data needed to hit the target, along with the correct fuze timing information, so that the shells fired would explode in the vicinity of the target aircraft. Most guns controlled by the FKC had Fuze Setting Pedestals or Fuze Setting Trays where the correct fuze timing was set on a clockwork mechanism within the AA shell warhead.

Type 285 radar was an early addition to the FKC system, being fitted on new destroyers from mid-1941 onward, and retrofitted to existing destroyers as time and opportunity permitted. Later variants increased the maximum target speed to 500 knots (930 km/h; 580 mph), and were combined with Gyro Rate Units (GRU) which gave tachometric capabilities to the system, and radar which greatly improved ranging and rate keeping accuracy.

The FKC saw extensive use during the war on British Commonwealth naval ships, typically on destroyers and sloops. Prior to the widespread use of radar, optical detection and ranging on high altitude aerial targets was a daunting task, as shown by HMAS Parramatta's Report of Proceedings, for 3 September 1940:

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