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Magnetic Compass Northerly Turning Errors in Aviation (UNOS Explained)

Despite glass cockpits and GPS navigation, the magnetic compass remains a required instrument in most aircraft—and one that every pilot must understand deeply. Among its most significant limitations are northerly turning errors, a direct result of magnetic dip.

To manage these errors, pilots rely on a simple but powerful memory aid:

  • UNOS — Undershoot North, Overshoot South


This article focuses specifically on northerly turning errors, explaining why they occur, when they are most pronounced, and how pilots use UNOS to compensate during flight.



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The Root Cause: Magnetic Dip

Earth’s magnetic field does not run parallel to the surface of the planet. Instead, magnetic field lines angle downward toward the magnetic poles. This downward component is known as magnetic dip.


In the Northern Hemisphere:

  • The north-seeking end of a compass magnet is pulled downward

  • The compass is counterweighted to remain usable, but dip cannot be eliminated

  • When the aircraft turns, this imbalance causes directional errors


Northerly turning errors are one of the most visible results of this phenomenon.


What Is a Northerly Turning Error?

A northerly turning error occurs when an aircraft is turning toward a northerly heading (north, northeast, or northwest), and the magnetic compass lags behind the aircraft’s actual heading.


In simple terms:

  • The aircraft turns

  • The compass turns more slowly

  • The indicated heading is behind reality


This lag can mislead pilots unless they anticipate it correctly.


Why the Compass Lags When Turning North

When an aircraft banks into a turn, several forces act on the magnetic compass simultaneously:

  1. Horizontal turning force from the aircraft’s motion

  2. Vertical magnetic dip force pulling the magnet downward

  3. Gravity and inertia acting on the compass assembly


In a turn toward north:

  • Magnetic dip pulls the north-seeking end of the magnet downward

  • This downward pull resists the compass’s ability to rotate freely

  • The result is a delayed (lagging) compass indication


The compass simply cannot keep up with the aircraft’s turn.


UNOS: Undershoot North

This behavior leads directly to the first half of the UNOS acronym:

  • Undershoot North


What This Means in Practice

When rolling out of a turn to a northerly heading, the pilot must:

  • Begin rollout before the compass indicates the desired heading

  • Anticipate that the compass will “catch up” after the turn stops


Example

You are turning from east (090°) to north (360°):

  • If you wait until the compass shows 360°, the aircraft will already have passed north

  • Instead, you roll out early—perhaps at 330° or 340°, depending on conditions


Failing to undershoot can result in:

  • Overshooting the desired heading

  • S-turning to correct

  • Increased workload, especially in IMC


Factors That Increase Northerly Turning Errors

Northerly turning errors are not constant; they vary in magnitude depending on several factors.


1. Latitude

  • The farther north you fly, the stronger magnetic dip becomes

  • Errors are minimal near the magnetic equator

  • Errors increase significantly in higher latitudes


2. Bank Angle

  • Steeper turns exaggerate the forces acting on the compass

  • Standard-rate turns produce smaller errors than steep maneuvering turns


3. Rate of Turn

  • Rapid heading changes increase lag

  • Smooth, coordinated turns reduce—but do not eliminate—error


Why Northerly Turning Errors Matter in Training and Real Flight

Student pilots often first notice northerly turning errors during:

  • Basic maneuvers

  • Compass turns

  • Ground reference training


But these errors matter just as much for experienced pilots, especially when:

  • Flying partial panel

  • Experiencing gyro or electrical failures

  • Using the compass as a backup heading reference


In instrument conditions, misunderstanding compass behavior can lead to:

  • Heading deviations

  • Poor intercepts

  • Loss of situational awareness


Best Practices for Pilots

To manage northerly turning errors effectively, pilots should:

  • Use the heading indicator for all turns when available

  • Cross-check the magnetic compass only in straight-and-level flight

  • Apply UNOS automatically during compass turns

  • Avoid abrupt maneuvers when referencing the compass

  • Practice compass turns regularly to build intuition


Remember: the magnetic compass is most reliable when the aircraft is stable, wings level, and unaccelerated.


A Note on the Southern Hemisphere

In the Southern Hemisphere, the behavior is reversed:

  • Turning toward south causes lag

  • Turning toward north causes lead


However, the UNOS mnemonic applies specifically to the Northern Hemisphere, where most primary flight training occurs.


Conclusion

Northerly turning errors are a classic—and critical—example of how physics affects flight instruments. Caused by magnetic dip, these errors make the compass lag during turns toward north, requiring pilots to undershoot northerly headings.


The UNOS acronym distills a complex physical phenomenon into an easy-to-remember rule that improves accuracy, safety, and confidence:

  • UNOS — Undershoot North, Overshoot South


Understanding why UNOS works—not just memorizing it—makes pilots better instrument interpreters and more precise aviators.



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