The pilot extended the landing gear but failed to get a down and locked indication from the single green indicator light on the instrument panel. The pilot recycled the gear twice, checked the indicator light was in the day mode but still did not get a down and locked green light. The pilot raised the gear, proceeded to the west of the airfield and reduced speed, and then carried out a manual extension of the gear. He still did not obtain a green light. The pilot advised that during the manual extension he was unable to push the extension lever to the stop and the handle felt slightly spongy. The pilot was advised that a visual inspection of the landing gear from the control tower showed that the gear appeared to be fully down, however, during the subsequent landing the gear collapsed. Later investigation showed that the green light bulb was loose in its socket. After it was properly secured the green light illuminated satisfactorily. The manual landing gear extension system was found to have been not correctly manipulated. For the gear to be successfully manually lowered, two actions are required; the electric motor must first be disconnected and then the gear must be manually lowered. It was found that the motor release arm used to disconnect the electric motor had not been pushed fully forward and the motor was still partially engaged. Consequently the emergency gear handle was unable to be pushed fully forward to its stop. This meant that the gear was not able to be locked down, and the restriction caused by the partially disconnected electric motor led to the spongy feel reported by the pilot. Significant Factors The following factors were considered relevant to the development of the accident. 1. The landing gear down and locked globe was loose in its socket. 2. The pilot did not get a safe indication when he extended the landing gear normally, even though the gear was most probably locked down. 3. The pilot retracted the landing gear and then attempted to manually lower it. 4. The manual extension system was not able to lock the gear down because the electric motor was still connected.