The twin-wheel nose undercarriage retracts forwards and its tyres are inflated to a pressure of 191 psi (1,320 kPa), and the wheel assembly carries a spray deflector to prevent standing water being thrown up into the engine intakes. The tyres are rated to a maximum speed on the runway of 250 mph (400 km/h).[147] The starboard nose wheel carries a single disc brake to halt wheel rotation during retraction of the undercarriage. The port nose wheel carries speed generators for the anti-skid braking system which prevents brake activation until nose and main wheels rotate at the same rate.