ATR-72 flight crew reported a loss of pitch control during departure and an accompanying alert from the ice detection system. Crew regained control and returned to the departure airport where significant ice accumulation was observed.

Narrative

Captain briefed de-ice crew on ATR de-ice/anti-ice procedures prior to block-out. Aircraft received Type 1 and Type 4 fluid. Runway XXC was assigned and the taxi time was approximately 12 minutes. Aircraft level 2 ice protection was set during before takeoff procedures. On departure; aircraft rotated at Vr and immediately pitched up to high pitch attitude. Captain was the PF (Pilot Flying) and he commanded full nose down pitch trim to no effect. At approximately 1;500 ft MSL; the aircraft ice detection warning was triggered and IMC conditions were entered. PM (Pilot Monitoring) engaged level 3 ice protection. Crew leveled at 3;000 ft AGL; advised ATC; and requested return to [Runway] XXC. PF was using full strength and both hands to maintain a non-stall attitude. PM pressed against the flight controls with his leg to provide additional bracing to aid the PF in leveling the aircraft. Crew executed ILS XXC to land. Upon return to the ramp; significant ice accumulation was found on the aircraft during post flight. Uncommanded pitch-up after takeoff without PF input. Suspected ineffective de-ice procedure on T-tail horizontal stabilizer and elevators; likely accumulation inhibited pitch trim functionality. Hand flown return to land on ILS XXCDespite the Captain briefing the de-ice crew on procedures and having an outside observer to confirm adequate deice and anti-ice procedures were performed; the T-tail stabilizer is not able to be checked due to the height above ground. This results in flight crews entrusting that de-ice crews performed to standard on a critical flight control surface. Consistent and vigilant de-ice/anti-ice training and certification must take place for all ground crews on all fleet aircraft.