As an aircraft accelerates through the transonic range, the centre of pressure moves rearwards in conjunction with the aft-moving spread of supersonic flow. From a starting position of approximately 15% of the chord, the C of P may end up at 20% chord, well behind the point of maximum camber (depending on the wing). This rearward shift of the C of P increases the moment arm between the centre of pressure and the centre of gravity, causing a nose-down pitching moment.