Definition
An uncommanded pitch movement in which the aircraft's nose drops on its own, often felt as a sudden loss of elevator effectiveness or a forward pull on the control column. In the icing context, it is a key warning sign of an impending or actual tailplane stall, caused when ice on the horizontal stabilizer disrupts the downward lift the tail normally produces.
Plain English
The nose pitches down by itself, without the pilot pulling forward on the controls. In an icing situation, this is a warning that the tail -- not the wing -- is stalling.
Context Anchor
Seen in icing discussions, especially as a possible symptom of a tailplane stall during approach or after a flap change.
Derivation
“Trim” originally means to put something in proper order or balance. In aviation, trim is the adjustment that helps the airplane hold an attitude without the pilot having to keep a steady push or pull on the controls. That background helps here because a “trim change” means the airplane’s balance has changed.
Why Pilots Care
This symptom warns of impending tailplane stall; recognizing it early lets the pilot reduce flap angle or apply nose-up trim to regain control before the situation becomes unrecoverable.
Grounding Statement
Picture flying level and suddenly needing much more back pressure to keep the nose from dropping; that is the kind of change this term is pointing to.
Intuition Check
Do not assume this means the pilot deliberately trimmed the airplane nose-down. Here, it means the airplane’s balance has changed so it acts as if nose-down trim has been added.
Example Sentence 1
After lowering the flaps on approach in icing conditions, the crew felt a sudden nose-down trim change and immediately suspected a tailplane stall.
Example Sentence 2
Applying nose-up trim and retracting flaps helped the pilot counteract the nose-down trim change before a full tailplane stall developed.