Definition
The downward aerodynamic force produced by the horizontal tail surface (horizontal stabilizer and elevator) that balances the airplane's natural nose-down pitching tendency. This force exists because the center of gravity is normally located ahead of the center of lift, which would otherwise cause the nose to drop; the tail counters this by pushing down, keeping the airplane in pitch trim.
Plain English
A downward push from the tail that holds the nose up. Because the airplane's weight sits slightly ahead of where the wings lift, the nose wants to fall. The tail pushes down to stop that from happening and keep the airplane level.
Context Anchor
Seen in glide and pitch-control discussions, especially when explaining how elevator movement affects airspeed during a glide.
Why Pilots Care
Proper tail-down force keeps the airplane trimmed and stable in a glide without constant back pressure on the controls.
Analogy
Think of a seesaw: pushing down on one end makes the other end move up. On an airplane, a downward force at the tail tends to help raise the nose.
Grounding Statement
When the tail makes more downward force, the nose tends to rise; when it makes less, the nose tends to lower.
Intuition Check
Tail-down force does not mean the tail is simply heavy or loaded down. It means the airflow over the tail is creating a downward aerodynamic force.
Example Sentence 1
As the airplane slowed in the glide, tail-down force decreased and the nose pitched gently downward.
Example Sentence 2
Trimming the airplane reduces the amount of tail-down force the pilot must hold with the elevator.