Reference: AIM AIR 2.12.3.4
As the rate at which ice accumulates on an airfoil is related to the shape of the airfoil, with thinner airfoils having a higher collection efficiency than thicker ones, ice may accumulate on the horizontal stabilizer at a higher rate than on the wings. A tail plane stall occurs when its critical angle of attack is exceeded. Because the horizontal stabilizer produces a downward force to counter the nose-down tendency caused by the centre of lift on the wing, stall of the tail plane will lead to a rapid pitch down. Application of flaps, which may reduce or increase downwash on the tail plane depending on the configuration of the empennage (i.e. low set horizontal stabilizer, mid-set, or T-tail), can aggravate or initiate the stall. Therefore, pilots should be very cautious in lowering flaps if tail plane icing is suspected. Abrupt nose-down pitching movements should also be avoided, since these increase the tail plane angle of attack and may cause a contaminated tail plane to stall.
A tail plane stall can occur at relatively high speeds, well above the normal 1G stallspeed. The pitch down may occur without warning and be uncontrollable. It is more likely to occur when the flaps are selected to the landing position, after a nose-down pitching manoeuvre, during airspeed changes following flap extension, or during flight through wind gusts.
Symptoms of incipient tail plane stall may include:
(a) abnormal elevator control forces, pulsing, oscillation, or vibration;
(b) an abnormal nose-down trim change (may not be detected if autopilot engaged);
(c) any other abnormal or unusual pitch anomalies (possibly leading to pilot induced oscillations);
(d) reduction or loss of elevator effectiveness (may not be detected if the autopilot is engaged);
(e) sudden change in elevator force (control would move down if not restrained); and/or
(f) a sudden, uncommanded nose-down pitch.