A method of measuring the bonded perimetric width of a hollow component, the hollow component comprising a first panel and a second panel, the first and second panels (110:120) being bonded to one another around their respective perimeters(112:122), the perimetric width being the width of the bonded portion joining the first panel to the second panel, the method comprising the steps of: (i) providing a hollow component; (ii) positioning an ultrasonic probe over an outwardly facing surface of the first panel; (iii) positioning an auxiliary plate against an outwardly facing surface of the second panel, the auxiliary plate being substantially normal to a beam axis of the probe; (iv) defining a probe path extending from a centre portion of the hollow component to beyond a perimetric edge of the hollow component, the probe path extending either side of the perimetric width, the probe path being substantially normal to the perimetric edge; (v) traversing the ultrasonic probe along the probe path, the ultrasonic probe directing an ultrasonic beam towards the outwardly facing surface of the first panel, and sensing a return signal from the ultrasonic beam, the return signal being indicative of a thickness of the hollow component at each of a plurality of points; (vi) determining a first end position for the perimetric width at the perimetric edge, the first end position being the position of a first step change in the return signal caused by the ultrasonic beam impinging on the auxiliary plate; (vii) determining a second end position for the perimetric width, the second end position being distal to the perimetric edge, the second end position being the position of a second step change in the return signal caused by the ultrasonic beam impinging on an interior volume of the hollow component; (viii) calculating the perimetric width being a distance between the first end position and the second end position; and (ix) repeating steps (iv) to (viii) along the perimetric edge of the hollow component. (i) providing a hollow component; (ii) positioning an ultrasonic probe over an outwardly facing surface of the first panel; (iii) positioning an auxiliary plate against an outwardly facing surface of the second panel, the auxiliary plate being substantially normal to a beam axis of the probe; (iv) defining a probe path extending from a centre portion of the hollow component to beyond a perimetric edge of the hollow component, the probe path extending either side of the perimetric width, the probe path being substantially normal to the perimetric edge; (v) traversing the ultrasonic probe along the probe path, the ultrasonic probe directing an ultrasonic beam towards the outwardly facing surface of the first panel, and sensing a return signal from the ultrasonic beam, the return signal being indicative of a thickness of the hollow component at each of a plurality of points; (vi) determining a first end position for the perimetric width at the perimetric edge, the first end position being the position of a first step change in the return signal caused by the ultrasonic beam impinging on the auxiliary plate; (vii) determining a second end position for the perimetric width, the second end position being distal to the perimetric edge, the second end position being the position of a second step change in the return signal caused by the ultrasonic beam impinging on an interior volume of the hollow component; (viii) calculating the perimetric width being a distance between the first end position and the second end position; and (ix) repeating steps (iv) to (viii) along the perimetric edge of the hollow component.