A Gulfstream G150 business jet, OE-GKA, arrived at Kittil„ Airport in Finland in the afternoon of 2 January 2018. The jet carried four passengers and a three-person crew. The aircraft was parked at the north end of the apron. Once the passengers had left, the flight crew put covers on the engines and external sensors. The next planned flight was a positioning flight on 4 January 2018, to Ekaterinburg, Russia, without passengers. The crew arrived at the airport to prepare for the flight at approximately 15:00. Take-off, as per the flight plan, was to happen at 17:00. The ground handling company transported them to the aircraft by bus at approximately 15:20. The captain opened the door at which time the cabin assistant entered the cabin. The captain and the co-pilot placed their flight bags behind the cockpit and went back outside. The co-pilot placed the aircrew’s baggage into the rear baggage compartment which opens from the outside. The captain and the co-pilot removed the engine covers which they had put in place on the day of their arrival. These were put into their own storage bags and also placed in the baggage compartment. Following this, the captain went into the cockpit and started the APU, which generates electricity for aircraft systems and bleed air for heating the cabin. The co-pilot began to brush off the snow that had fallen on the aircraft. A moment later the captain came out to help the co-pilot. At first, he worked with his bare hands. Due to the extremely cold conditions (-22øC), however, he went back inside to fetch a pair of gloves. When he came back out, he closed the door. A little later the cabin assistant inside the cabin felt strange pressure in her ears and chest. She went into the cockpit and attempted to get the attention of the pilots working outside by knocking on the window. The pilots noticed the knocking and the captain went to open the door. According to the co-pilot’s observations it was unusually difficult for the captain to get the door open. Then, the captain pulled harder on the door handle at which time the door blew open with excessive force, hitting the captain who was standing underneath the door and knocking him to the ground. The pressure wave also knocked the co-pilot down, who had been standing approximately one metre from the left side of the door. The co-pilot stood up and saw the captain lying on his back on the ground. Realising that the captain was unconscious, the co-pilot turned him on his side. Then he entered the cabin and saw the cabin assistant in a semi-seated position on the floor of the cabin. The co-pilot shook the assistant’s shoulder and advised her to go outside. The captain died as a result of the serious injuries he sustained at the site of the occurrence. The co-pilot had not sustained any physical injuries. The cabin assistant had bruises on her right arm, continued to feel chest pain and was diagnosed with a mild concussion. The cabin sustained substantial damage. The cockpit’s aft left bulkhead and the cabin’s forward left bulkhead were nearly torn off. CONCLUSIONS: 1. When the aircraft is parked outside for a longer period, some pilots may close the outflow valve to prevent the ingestion of contaminants into the valve, or upstream into the cabin. 2. When the APU is being run one must check that the outflow valve is fully open. If it is not possible to ensure that the valve is open or to remove differential pressure by other means, the door must not be closed. 3. The door had no indication warning of excessive cabin pressure, nor an opening for depressurisation. The cabin was pressurised because the APU bleed air was ducted into the cabin, the outflow valve was closed and the door was also closed. Significant differential pressure existed between the cabin and the outside. 4. Cabin pressurisation on the ground also creates a hazard for several other groups of professionals, such as aircraft mechanics, ground handling staff, aircraft cargo loaders and rescue personnel.