Tailplane or main wing stall? LOC-I event due to suspected icing

In January 2017, a business jet was being flown in Norway on a short repositioning flight with two pilots onboard, no passengers or cargo. Initially, the take-off proceeded as normal but as the landing gear was retracted both pilots observed that the airspeed was rapidly approaching the flap limiting speed of 200 kts. When the flaps were fully retracted at a height above ground level of approximately 2000, the crew experienced a violent nose-down pitch motion restrained by their seat belts, as the aircraft started banking sharply to the left. It is likely that the commander (PF) and first officer (PM) experienced different levels of startle and/or surprise during the upset. Control was regained at a height of approximately 170 feet above ground level and following the accident. Data from the Flight Data Recorder showed that the aircraft experienced -2.62 G during the pitch down upset and +5.99 G during the pull-out. A tailplane stall due to icing was suspected, however the flight data recorder being limited to 36 parameters was not available to confirm this.

A review of loss of control in flight accidents where icing was contributory/causal factor was conducted. Theory related to the effects of icing on main wing and tailplane aerodynamics was undertaken and the relationship to stability and control investigated. Due to the limited flight data parameter set, it was decided to use modelling to simulate the effects of tailplane icing and compare with available, known data. Several modelling methods were considered including computational fluid dynamics, scale model wind tunnel testing, scale model flight testing etc. but due to time and cost constraints associated with these methods it was decided to develop a representative (similar but not exact) model using available aircraft design software and desktop mathematical modelling and simulation software.

A generic business jet linear flight dynamics model was developed using Matlab/Simulink, aircraft geometry, mass and balance, initial flight conditions from the Flight Data and estimated stability and control derivatives. Aircraft static and dynamic stability of the generic business jet was assessed for a range of tailplane efficiency factors to simulate the effects of tailplane icing. The results showed that static stability decreases as tailplane efficiency decreases, simulating the onset of tailplane icing and that increasing UP elevator (-VE) is required to compensate. At low tailplane efficiency the aircraft pitch response to elevator commands becomes unstable. Flap retraction initially helps, but not sufficiently to stabilise the response with low tailplane efficiency and a tailplane stall destabilises the system response. The safety investigation authority issued a safety recommendation to the aircraft manufacturer, requesting they inform the customers about the nature of the accident and the risk of tailplane stalling.