The Gulfstream G450 took off from Salzburg Airport, Austria, at 10:12, bound for Bangor International Airport, Maine, USA. A few seconds after taking off from runway 33, the landing gear lever was selected to the Up position. About 20 seconds later, the crew discovered that the landing gear had not retracted. The landing gear remained in the Down & Locked position because the landing gear safety pins were set. The crew suspected that the safety pins were still set and decided to land back at Salzburg Airport, which was carried out on runway 15. After a flight time of about 5 minutes, the aircraft left the runway at about 10:17 on taxiway E. They were instructed by the control tower to taxi back to the parking area (apron). The aircraft was stopped shortly after taxiing on taxiway E, as the crew noticed an increased rolling resistance of the aircraft, whereby the crew suspected a burst tyre. The crew decided to check directly at the taxiway whether the safety pins were still on the landing gear, to remove them if necessary and to check whether a tire was defective. The control tower of Salzburg Airport was not informed about this by the pilots. The co-pilot opened the cabin door and left the aircraft to perform these checks. Meanwhile, the pilot left the cockpit to inform the passengers in the cabin of the situation. At 10:20 and 10:22, the Salzburg Tower controller tried without success to make contact with the aircraft’s crew. An aircraft passing taxiway L informed the controller about a person on the taxiway who had left the aircraft in question. The co-pilot, who had gone to the landing gear, recognized that the safety pins were installed and tried to remove them. When this attempt failed because hydraulic pressure was applied to the landing gear cylinder and the safety pin was jammed, he operated the landing gear door control valves of the nose and main landing gear several times and installed the landing gear doors control valve pins. This closed the landing gear doors, the main cylinders of the hydraulics of the main and nose landing gear were depressurized and the safety pins could be removed. After the co-pilot removed the nose landing gear door pin, the nose landing gear doors opened and the nose landing gear folded at about 10:30. The aircraft nose lowered and hit the taxiway. 5 to 10 seconds later the engines were switched off. No indication could be found that both the Landing Gear Failure to Retract and the Attempted Landing Gear Retraction with Safety Pins Installed Checklist were executed from the time the installed Safety Pins were detected until the time of the accident. Probable causes – Incomplete execution of the Preflight Checklist, thus forgetting the safety pins installed in the landing gears. – No knowledge or execution of the checklists for Landing Gear Failure to Retract or the Attempted Landing Gear Retraction with Safety Pins Installed. Thus, incorrect operation of the landing gear system and, as a result, failure to properly reset the landing gear to a defined system state. Probable Factors – Lack of “REMOVE BEFORE FLIGHT” flags on landing gear safety pins. Consequently, more difficult to detect deployed fuse pins. – Collusion between pilots on the insertion or non-insertion of the securing pins, although insertion is mandatory by the manufacturer for parking and towing. – Very extensive Exterior Preflight Inspection Checklist for daily flight operations with 136 inspection points, which tempts to simplify the checklist for daily operations to a few important inspection points, which is not permissible from a safety, operational and certification point of view. – Gradual increase in psychological stress, characterized by communication difficulties, lack of knowledge of systems and procedures, time pressure and psychophysiological stress. – Breaking the logic of landing gear system operation for pilots because despite 3 green downlock indicators and landing gear lever in DOWN position on a GIV-X (G450), the landing gear can retract on the ground due to incorrect and improper operation of the landing gear system. The selector valve of the landing gear thus remained in a position not directly visible to the pilots.