Definition
A tendency of an airplane on the ground to swerve or yaw away from a straight path rather than naturally tracking forward. In tailwheel airplanes, this occurs because the center of gravity is located behind the main wheels, so any sideways motion at the tail tends to amplify rather than self-correct, which can lead to a ground loop if the pilot does not actively counter it with rudder input.
Plain English
The airplane does not want to roll straight on the ground on its own. If the tail starts to swing one way, it tends to keep swinging that way and get worse, so the pilot has to actively steer to keep it tracking straight.
Context Anchor
Seen in landing gear discussions, especially during takeoff rolls, landing rolls, and tailwheel airplane handling.
Derivation
From 'directional' (relating to the direction of travel) and 'instability' (not staying steady on its own). Together: the airplane does not stay pointed where it's going without help.
Why Pilots Care
Uncorrected directional instability can cause a runway excursion or loss of control on the ground.
Analogy
Pushing a shopping cart forward by pulling it from behind. Any small wobble grows quickly and the cart wants to spin around. Pushing it from the front, like a nosewheel airplane, naturally tracks straight.
Grounding Statement
Picture the airplane beginning to swing left after touchdown and, instead of settling back straight, continuing to swing farther left unless you stop it.
Intuition Check
Directional instability does not mean the airplane is impossible to steer. It means that, after a small swing from straight ahead, the airplane tends to swing farther unless the pilot actively corrects it.
Example Sentence 1
The instructor reminded the student that directional instability in the tailwheel trainer means the rudder pedals must stay active all the way through the landing rollout.
Example Sentence 2
Tricycle-gear airplanes are usually designed to reduce directional instability by placing the main wheels slightly forward of the center of gravity.