The Cessna Citation Encore had flown a passenger from Bern in Switzerland to Oslo-Gardermoen Airport in Norway. The crew had planned a shortest possible ground stop before flying to Torp Sandefjord Airport. The captain flew the aircraft (PF), while the First Officer monitored the flying (PM). During the ground stop at Gardermoen, only one engine was stopped while the First Officer completed an external inspection of the aircraft He did not observe any ice or anything out of the ordinary on the areas of the aircraft that could be inspected. There were icing conditions at Gardermoen and in the airspace above the airport. After flying from Switzerland for more than two hours in approximately minus 50øC, the aircraft’s surfaces (fuselage and wings) were more than likely chilled. When the crew requested taxi clearance, they were assigned a different runway than expected. This entailed a longer taxi time and thus longer exposure to the prevailing weather conditions. The aircraft’s ground stop lasted approximately 15 minutes at an air temperature of 0øC. The taxiways and runway were covered with 3-6 mm of slush and it was snowing when the aircraft took off. Initially, the take-off proceeded as normal. The landing gear was retracted and both pilots observed that the speed was rapidly approaching 200 kt, which is the maximum speed with flaps deployed. As the flaps were retracted, the crew experienced a violent nose-down movement and the pilots were “hanging by their seat belts”, while the aircraft started sharply banking to the left. The aircraft at this moment experienced negative 2.62 G. The captain did not trust the instruments while the First Officer had better situational awareness. The First Officer quickly took control and started a pull-out from the dive. The aircraft descended below the cloud base, and even though it was dark, the pilots could glimpse the ground. Control was regained and the aircraft levelled off 170 ft above the ground. The aircraft was overstressed to 5.99 G during the pull-out. The crew called “MAYDAY” to the Air Traffic Control. Once control was regained, the “MAYDAY” was cancelled and the flight continued towards Torp where an approach and landing took place without further problems. The aircraft was overstressed well above Ultimate Load Factor (-2.16 to +5.40 G) and considered not to be economically viable to repair. Primary conclusion: 1. The probable explanation for the aircraft suddenly diving, is that the tailplane stalled. AIBN has not found other explanations for this than slush spray from the runway and falling snow and sleet settled on the tailplane’s leading edge and underside during taxi and take-off. This contamination is presumed to have frozen to ice. 2. The aircraft’s anti and de-icing systems on the wings and tailplane were switched on, but the tailplane de-icing system had completed a “cycle” before take-off and was in rest mode during take-off. In the assessment by AIBN, the aircraft’s anti- and de-icing systems were not suitable to remove the type of ice and snow that had most likely settled on the aircraft’s tailplane. The aircraft should have been de-iced before take-off, in line with the company’s de-icing procedure, to avoid potential consequences of contamination on the tailplane. 3. This accident shows the significance of good crew resource management (CRM) in the cockpit when an unexpected and extreme flight situation occurs. In this instance, the First Officer’s situational awareness and initial pull-out contributed to the aircraft not crashing.