The maingear uplock assembly had just been fitted with a new seal and the aircraft was flown to ensure the correct operation of the system. The gear retracted normally but could not be lowered and a gear up landing was made. Inspection revealed that the unit had been incorrectly reassembled after the seal was changed. The maintenance had been performed by an unlicenced engineer who had not previously completed a similar task. When carrying out the work he did not refer to either the aircraft maintenance manual or a licenced engineer. On completion of the job no retraction tests were carried out. The engineer believed that the work he had performed would only effect the operation of the gear doors, and not the retraction and extension of the landing gear.
