The aircraft was being flown on a freight charter between Canberra and Goulburn, with a company approved check pilot occupying the right control seat. He had earlier conducted a route check on the left seat (handling) pilot who had been acting as pilot in command under supervision (ICUS). At about 10 NM from Goulburn, the crew heard a radio transmission from an aircraft at Goulburn reporting a flock of birds on the approach to runway 04. The check pilot stated that as the aircraft joined the circuit, his attention was primarily focussed on searching for the reported birds. As the aircraft entered the downwind leg of the circuit the check pilot advised the handling pilot to tighten the approach and look for the birds. The handling pilot did not lower the landing gear prior to landing, although both pilots reported having seen the gear down indicators illuminated. They later stated that the indicator lights may have been illuminated by the late afternoon sun. The aircraft subsequently landed with the landing gear retracted, scraping the lower fuselage and damaging both propellors. Whilst the check pilot had intended to spend the day route checking the handling pilot, he noted that the handling pilot was already qualified on the Canberra to Goulburn route. The check pilot asked the handling pilot if he was happy to fly the sector to Goulburn, to which the handling pilot replied that he was. The check pilot assumed that he had passed the pilot in command (PIC) responsibility to the handling pilot and therefore had no formal role in the operation of the aircraft, other than that of cursory supervision. The PIC name on the flight plan was not amended, and with no formal handover of the PIC responsibility, the handling pilot continued to consider himself ICUS. Company operations in the aircraft were normally single pilot, however the conduct of a route check involved two pilots, a PIC and a pilot acting ICUS. The cockpit during the route check was in effect a multi crew environment, although neither pilot had received any specific training in multi crew operations or crew resource management. The check pilot had not received training in conducting route checks. The pilots were of similar age and experience, and the check pilot had become confident in the handling pilot's ability during the previous route sectors. This had the effect of him relaxing his supervision of the handling pilot. As the aircraft approached Goulburn the check pilot's attention became focussed on activities outside the cockpit, rather than supervising the performance of the handling pilot prior to landing. The handling pilot normally conducted the prelanding checks on early downwind. The check pilot's advice to tighten the circuit and look for birds was made early on downwind, probably interrupting the handling pilot's habit pattern, resulting in landing gear extension being omitted from the pre-landing actions. SAFETY ACTIONS The Bureau is considering the safety issues revealed in the investigation. Any safety output will be published in the Quarterly Safety Deficiency Report.