Incident to the Cessna - 310 - R registered N443LT on 30/01/2020 in cruise flight between Toussus-le-Noble (Yvelines) and Pontoise (Val d’Oise)

Cat. 2 investigation report: simplified-format report, adapted to the circumstances of the occurrence and the investigation stakes.

The pilot receiving training and the instructor made an IFR flight between Toussus-le-Noble and Pontoise Cormeilles-en-Vexin aerodromes. After climbing to around 3,000 ft in the cloud layer, the pilot attempted to stabilize the aeroplane using the elevator trim activated with the manual control. The aeroplane fluctuated around the altitude of 3,000 ft. The pilot then decided to use the electric control of the elevator trim to stabilize the altitude. The aeroplane then descended at a calculated mean vertical speed of around - 4,800 ft/min for a period of thirty seconds. In this phase, the instructor asked the pilot to correct the aeroplane’s attitude. However, alerted by the controller and exiting the cloud layer, the instructor took over the controls and performed a pull-up manoeuvre. The aeroplane reached a height of 600 ft. The aeroplane then rapidly climbed back to 3,000 ft.

The investigation was unable to identify the reason for the significant increase in vertical speed during the descent, during which the indicated airspeed certainly increased significantly.

During this variation in altitude, a MSAW warning appeared on the controller’s radar screen. This warning led him to pay particular attention to this aeroplane given the high values of the aeroplane’s radar parameters available to him. The controller also paid particular attention to the aeroplane on its return leg when it was on a converging path towards Paris-CDG airport.