History of the Flight
On Wednesday, 11 January 2017, Hesnes Air AS flight HSG03, a Cessna Aircraft Company 560 Encore registered LN-IDB, departed from Bern Airport, Switzerland, at 1330 hours and landed at Oslo Airport Gardermoen (ENGM) at 1603 hours. The flight was operated by a crew of two: a Commander (Pilot Flying) and a First Officer (Pilot Monitoring). No passengers were on board. The planned ground stop was minimal, with only one engine shut down while the First Officer conducted an external inspection; he observed no ice or abnormalities on the inspectable surfaces.
Meteorological and Aerodrome Conditions
Icing conditions prevailed at Gardermoen and in the airspace above. The aircraft had flown for over two hours at approximately -50°C, chilling the fuselage and wings. During the approximately 15-minute ground stop, the air temperature was 0°C. The taxiways and runway were covered with 3–6 mm of slush, and snow was falling at the time of takeoff. The crew had been assigned a different runway than expected, resulting in a longer taxi and increased exposure to the weather.
Accident Sequence
Takeoff proceeded normally initially. After gear retraction, both pilots observed the airspeed rapidly approaching 200 knots, the maximum speed with flaps extended. As the flaps were retracted, the aircraft suddenly pitched nose-down violently, and the pilots reported being held by their seat belts while the aircraft banked sharply left. Flight Data Recorder (FDR) data later showed the aircraft experienced negative 2.62 G at this moment.
The Commander distrusted the instruments, but the First Officer maintained situational awareness. The First Officer quickly took control and initiated a pull-out. The aircraft descended below the cloud base; although it was dark, the pilots could glimpse the ground. Control was regained, and the aircraft levelled off at 170 feet above the ground. The pull-out subjected the aircraft to 5.99 G. The crew declared a Mayday to Air Traffic Control, which was cancelled after control was restored. The flight continued to Torp Sandefjord Airport, where an approach and landing were completed without further incident.
Investigation Findings
The investigation found no technical malfunctions in the aircraft or its control systems. The Flight Data Recorder and Cockpit Voice Recorder were downloaded by the UK Air Accidents Investigation Branch. The aircraft's anti-icing and de-icing systems on the wings and tailplane were switched on, but the AIBN assessed that these systems were not suitable for removing the type of ice and snow encountered. The AIBN determined that a probable explanation for the sudden dive was that the tailplane stalled as a result of icing caused by contamination from slush spray from the runway and/or from falling snow and sleet. Under the conditions during the ground stop, the AIBN opined that the aircraft should have been de-iced before takeoff.
Organizational and Supervisory Aspects
The investigation also revealed practices in the Civil Aviation Authority Norway's (CAA-N) approval of Air Operators Certificate (AOC) and organizational approvals that were considered unfortunate, but no correlation between those practices and this accident was established.
Conclusions
The primary conclusion of the AIBN was that the tailplane stall due to icing was the probable cause of the loss of control. The investigation resulted in two safety recommendations.