Flyback chronograph
A chronograph that can be reset and restarted from zero with a single press while it is still running.
Beginning a new timing on an ordinary chronograph takes three actions: stop, reset, start. For a navigator flying by dead reckoning, timing each leg of a course from the moment of the turn, those three presses cost seconds and attention at exactly the wrong moment. An error of one minute in elapsed time is worth about fifteen nautical miles of position.
In an ordinary chronograph the reset is locked out unless the mechanism is stopped, to protect the gearing. A flyback removes that interlock. One press lifts the brake, drops the hammer onto the heart cams of the seconds and minute counters, drives them to zero and lifts the hammer clear, without ever disengaging the clutch. Release the pusher and the count resumes from zero.
The cost is mechanical violence. The reset happens against a train still turning under power, so the cam, the hammer face and the pivots take a sharp blow every time. Tolerances must be tighter than in a standard chronograph and the adjustment is unforgiving: a fraction late and the hammer brakes the train, a fraction early and the hand does not settle on zero.
Longines filed the defining patent in June 1935 and built the function into calibre 13ZN. Surviving examples suggest the firm was already making flyback chronographs in the late 1920s, so the patent probably formalised an existing practice.