1. A method of characterising mechanical and thermal growth of a rotor tip clearance arrangement ( 78 ), comprising steps to: a. construct a finite element model ( 48 ) of components in the rotor tip clearance arrangement ( 78 ); b. define reference points (A, B, C, D) on components in the model ( 48 ); c. define calibration cases ( 60 ) for the model ( 48 ); d. run the model ( 48 ) for each calibration case ( 60 ) and record displaced position (d A , d B , d C , d D ) of each reference point; e. calculate component growth from the reference points (A, B, C, D) and displaced positions (d A , d B , d C , d D ); f. characterise the component growth as comprising mechanical growth and thermal growth; g. interpolate mechanical growth and thermal growth between calibration cases ( 60 ); and h. output a mechanical growth and thermal growth characterisation map.