An aircraft assembly having a first region, a second region spaced from the first region by a flexible region, and a deflection sensor. The first region has greater positional stability than the second region such that the second region deflects further from its default position than the first region during in-use loads. The deflection sensor includes a first gyroscope at the first region to generate a first signal representing rotation of the first region about one or more orthogonal axes, and a second gyroscope at the second region to generate a second signal representing rotation of the second region about the one or more axes. The second gyroscope is synchronised in time with the first. A controller subtracts one of the first and second signals from the other to obtain a differential signal representing deflection of the second region relative to the first due to flexing of the flexible region.