About 5 minutes after take-off for a sight-seeing flight, the latches on the cabin door released and the door partially opened. The pilot was distracted by the resulting airflow noise and was concerned that the door may have opened further. An immediate return was made to the departure point, but the pilot then forgot to lower the landing gear before touchdown. The subsequent investigation revealed that the top door latch had probably not been correctly secured before departure. The pilot had not checked the security of the door, but had asked his passenger to make sure the door was closed. The main latch was found to be out of adjustment, such that a firm push could cause the door to spring open. The pilot had not previously experienced a door open in flight situation. He had been unduly anxious to land, and had not completed the pre-landing checks. The noise of the airflow past the door had masked the sound of the gear warning horn. The aircraft is equipped with an automatic gear lowering system, however the handle for this system was in the manual over-ride position.
