A type of bicycle under-carriage in which two pairs of main wheels are located fore and aft on the fuselage centerline.
This arrangement is normally not used in civil aircraft. The tandem under-carriage arrangement is found in aircraft that may not have sufficient space to enclose the wheels. (Small auxiliary outrigger wheels that are not very thick are attached to the wing, which can be easily retracted into the wing.) Landing aircraft in this undercarriage configuration is more difficult than with a tricycle undercarriage.