An aircraft assembly comprising: a first region (14a,); and a second region spaced from the first region by a flexible region, the first region having greater positional stability than the second region such that flexing of the flexible region due to in-use loads causes the second region to deflect further from its default position than the first region deflects from its default position; characterised in comprising a deflection sensor comprising: a first gyroscope (G1, G2) rigidly mounted at, or with respect to, the first region so as to generate a first signal which represents rotation of the first region about one or more orthogonal axes (α, β, γ); a second gyroscope (G2 - G4) rigidly mounted at, or with respect to, the second region so as to generate a second signal which represents rotation of the second region about one or more orthogonal axes (α, β, γ), the second gyroscope being synchronised in time with the first; and a controller communicatively coupled to the first and second gyroscopes to receive the first and second signals, the controller being configured to subtract one of the first and second signals from the other to obtain a differential signal which represents deflection of second region relative to the first region due to flexing of the flexible region. a first region (14a,); and a second region spaced from the first region by a flexible region, the first region having greater positional stability than the second region such that flexing of the flexible region due to in-use loads causes the second region to deflect further from its default position than the first region deflects from its default position; characterised in comprising a deflection sensor comprising: a first gyroscope (G1, G2) rigidly mounted at, or with respect to, the first region so as to generate a first signal which represents rotation of the first region about one or more orthogonal axes (α, β, γ); a second gyroscope (G2 - G4) rigidly mounted at, or with respect to, the second region so as to generate a second signal which represents rotation of the second region about one or more orthogonal axes (α, β, γ), the second gyroscope being synchronised in time with the first; and a controller communicatively coupled to the first and second gyroscopes to receive the first and second signals, the controller being configured to subtract one of the first and second signals from the other to obtain a differential signal which represents deflection of second region relative to the first region due to flexing of the flexible region. a first gyroscope (G1, G2) rigidly mounted at, or with respect to, the first region so as to generate a first signal which represents rotation of the first region about one or more orthogonal axes (α, β, γ); a second gyroscope (G2 - G4) rigidly mounted at, or with respect to, the second region so as to generate a second signal which represents rotation of the second region about one or more orthogonal axes (α, β, γ), the second gyroscope being synchronised in time with the first; and a controller communicatively coupled to the first and second gyroscopes to receive the first and second signals, the controller being configured to subtract one of the first and second signals from the other to obtain a differential signal which represents deflection of second region relative to the first region due to flexing of the flexible region.