With reference to a FSTD, G – Break refers to an uncontrolled abrupt nose down movement of an aeroplane at or after an aerodynamic stall from level flight, where the pilot is maintaining level flight up to that point with idle thrust.
G – Break is likely the most widely used definition for the point of aerodynamic stall of an aircraft at a 1g loading. However, the aerodynamic data provided by an aircraft manufacturer frequently does not provide such a manoeuvre so aerodynamic stall is normally tested with the aeroplane slowly descending but maintaining a deceleration of 1kt/sec with idle thrust until the point of stall is reached.
If the aeroplane does demonstrate an abrupt nose down tendency, or “g-break”, after full stall this characteristic should be represented when testing the FSTD.
Further information on this can be found in the excellent research article “Stalling Transport Aircraft” by P. J. Bolds-Moorehead, V. G. Chaney, T. Lutz and S. Vaux in The Aeronautical Journal , Volume 117 , Issue 1198 , December 2013 , pp. 1183 – 1206. (https://tinyurl.com/53fhxw54)