Abstract: A detection means to detect when a relative angular displacement between a first shaft and a second shaft of a gas turbine engine reaches a predetermined value comprises an aperture in the first shaft and a cooperating ratchet device attached to the second shaft. An arm of the ratchet device abuts the circumference of the first shaft, and the aperture in the first shaft is initially positioned a predetermined angle in the direction of rotation of the shafts from the arm. In operation when the relative angular displacement between the shafts reaches the predetermined value the arm of the ratchet device aligns with and moves into the aperture in the first shaft.The ratchet device has a pawl which moves radially outwards from the shaft and engages with and displaces a lever when the arm moves into the aperture. The displacement of the lever may be used to shut off the supply of fuel to the gas turbine engine or to operate an indication means.