On 13 February 2023 a Boeing 777-300 encountered unforecast severe mountain wave turbulence at FL350. The PF was a very inexperienced First Officer undergoing line training whilst the Training Captain-in-command was taking crew rest. Although the Line Captain responded with control inputs, he did not announce this and the trainee continued his own inputs. As a consequence of competing pitch commands, the aircraft lost 8,000 feet of altitude in approximately one minute at up to 17,100 fpm before stabilised flight was regained and a recovery climb initiated. The flight continued to Istanbul where the injured crew and passengers were hospitalised.