Originally Posted by
Rainboe
so that debris is not thrown around inside the undercarriage bay.
Erm.... not quite.
Each wheel on the 747 weighs 160kgs. So 16 wheels weighing 160kgs doing 180knots is a serious gyroscopic force - 2.56 tonnes of gyro (half that actually as it is only the wing gear that retracts sideways, body and nose retract forward along the direction of gyro rotation) - which makes it extremely difficult to move the legs, so stopping the rotation removes the force and the legs move with 'ease'....
applying the brakes prevents a very slow deceleration of rotation speed with added vibration as they will all slow at different speeds.
Certain MEL brake deficiencies require the gear to be left down for up to 5 minutes after takeoff to allow the wheels to stop rotating prior to retraction.