# Dynamic Lateral-Directional Stability

> Interaction between roll and yaw stability. Dutch Roll vs Spiral Instability.

- **Subject:** 081 — Principles of Flight
- **Section:** 081-04 — Stability
- **Topic code:** 081-04-06
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/principles-of-flight/stability/dynamic-lateral-directional-stability/

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## The Conflict
Lateral Stability and Directional Stability are coupled. Increasing one often reduces the effects of the other.

## Dutch Roll
Occurs when **Lateral Stability is stronger than Directional Stability**.
*   Mechanism: Aircraft rolls -> Sideslips -> Strong lateral stability rolls it back aggressively -> Overshoots -> Directional stability is weak so it yaws/wags tail.
*   Result: An oscillatory motion combining rolling and yawing.
*   Solution: **Yaw Damper**. An automatic system that applies rudder opposition to yaw rates, preventing the oscillation from starting.
*   Common in swept-wing aircraft (swept wings have strong lateral stability).

## Spiral Instability
Occurs when **Directional Stability is stronger than Lateral Stability**.
*   Mechanism: Aircraft rolls -> Sideslips -> Strong directional stability turns the nose into the slip -> Weak lateral stability fails to level wings.
*   Result: The bank angle increases, the nose drops, speed increases. A tightening spiral dive.
*   Solution: Pilot intervention. Unlike Dutch Roll, this is a slow divergence, giving the pilot time to react.

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