Preventing Tail Stall
When the -H4 model was certified, the nose down limit of trim travel for the Horizontal Stabilizer was reduced from 10º to 8º while the Elevator range of movement stayed constant [TCDSr13]. Furthermore, the Dual Motor Trim motor attached to the Horizontal was made standard equipment on the -H4 [TCDSr13].A reduction in nose-down movement of the Horizontal equates to a reduction in angle-of-attack for the Horizontal. This in turn equates to a reduced potential down force by the Horizontal to be used for aircraft nose-up trim. While the MTOW was increased on the -H4, the reason for this limitation probably has more to do with preventing tail stall than it does with the reduced need for aircraft nose-up trim (with a more aft CG).
The AFM warns that a tail stall can be caused by dirt or ice buildup on the Horizontal [AFMr7, 2-10]. It further recommends that a flapless landing be made. The deployment of flaps changes the downwash from the wings likely increases the AOA on the Horizontal (bringing it closer to stall). With buildup on the Horizontal, the stability of the airflow over the Horizontal decreases and aggravates the condition potentially resulting in a tail stall.
Therefore, the pilots responsibility is to be mindful of the presence of dirt/ice buildup after takeoff and during flight and to respond appropriately should a buildup be suspected.
