Magnetic Compass Turns: IFH Chapter 7

Handbook Reference

IFH Ch 7

7.compass-turns. Magnetic Compass Turns

The magnetic compass is the only direction-seeking instrument in most general aviation airplanes that requires no electrical power, vacuum, or gyroscopic spin to function. However, because it relies on a freely-pivoting magnetized float aligned with the Earth's magnetic field, it is subject to several errors during turns and acceleration. Understanding and compensating for these errors is essential when the heading indicator is inoperative or being reset.

Magnetic Dip and Its Consequences

The Earth's magnetic field is parallel to the surface only near the magnetic equator. As latitude increases, the lines of flux dip downward toward the poles. This vertical component pulls one end of the compass card down, producing two characteristic errors in the Northern Hemisphere:

Northerly turning error — the compass leads or lags when turning to or from a northerly or southerly heading.
Acceleration/deceleration error — the compass momentarily indicates a turn when the airplane accelerates or decelerates on an east-west heading.

Northerly Turning Error (UNOS)

When the airplane banks, the compass card also tilts, and the vertical component of Earth's magnetic field acts on it. The resulting indications when rolling into a turn are summarized by the mnemonic UNOS — Undershoot North, Overshoot South:

Turning toward a northerly heading, the compass lags behind the actual heading. Roll out before reaching the desired heading by approximately the latitude of your position plus half the bank angle. Example: at 30° N latitude with 15° bank, roll out at 030° + (15/2) = 037° when turning to a heading of 360°—stated another way, lead the rollout by about 37° (i.e., begin rollout at 037° when turning right to north, or at 323° when turning left to north).
Turning toward a southerly heading, the compass leads the actual heading. Roll out past the desired heading by approximately the latitude plus half the bank angle. Example: at 30° N latitude with 15° bank, roll out at 217° when turning right to 180°.

A standard rule of thumb at mid-latitudes (around 30°–40° N) is to lead or lag by approximately the latitude in degrees, using a standard 15°–18° bank.

Easterly and Westerly Turns

When turning to or from a heading of east (090°) or west (270°), there is essentially no turning error, and the compass reads approximately correctly throughout the turn. These headings make excellent reference points when resetting the heading indicator in turbulence or after gyro precession.

Acceleration/Deceleration Error (ANDS)

On easterly or westerly headings, accelerating or decelerating the airplane induces an apparent turn indication. The mnemonic ANDS — Accelerate North, Decelerate South captures the effect:

Accelerating (e.g., lowering the nose, adding power) on an east or west heading causes the compass to swing toward north.
Decelerating (e.g., raising the nose, reducing power) causes it to swing toward south.

This error is most pronounced on east-west headings and is zero on north-south headings. It occurs because the compass card's center of gravity is offset below the pivot to compensate for magnetic dip; longitudinal acceleration tilts the card, exposing it to the dip force.

Procedure for Timed Compass Turns

If turning errors are too cumbersome, a timed turn using the turn coordinator is a reliable alternative when the heading indicator is unusable:

Determine the heading change required.
At a standard rate (3°/sec), divide the heading change by 3 to obtain seconds of turn.
Establish a half-standard or standard-rate turn referenced to the turn coordinator.
Begin timing as the bank is established and roll out at the calculated time.
After wings-level and the compass has stabilized (allow 10–15 seconds), verify the heading.

Practical Considerations

Always allow the compass to stabilize in straight-and-level, unaccelerated flight before reading it.
Reset the heading indicator only when wings are level and airspeed is constant.
In turbulence, average the oscillations; do not chase momentary swings.
Compass deviation, caused by the airplane's own magnetic and electrical fields, is corrected by reference to the compass correction card mounted near the instrument.
These rules apply in the Northern Hemisphere; signs reverse in the Southern Hemisphere.

Mastery of compass turns is a partial-panel skill emphasized during instrument training and is examined on the instrument practical test under simulated heading-indicator failure.

Oral Exam Questions a DPE Might Ask

Q1What is northerly turning error and how do you compensate for it?

When turning to a northerly heading the compass lags, and when turning to a southerly heading it leads. The rule of thumb is to roll out using the latitude plus half the bank angle — undershoot north, overshoot south (UNOS).

Q2If the heading indicator fails, how would you turn to a specific heading?

I'd use a timed turn referenced to the turn coordinator. At standard rate (3° per second), I divide the desired heading change by 3 to get the time, roll into a standard-rate turn, and roll out when the time elapses, then verify with the compass after it stabilizes.

Q3On a westerly heading you lower the nose to descend and the compass swings north. Why?

That's acceleration error — ANDS, Accelerate North, Decelerate South. On east-west headings, accelerating tilts the dip-compensated compass card and produces a false northerly indication. It's not a real heading change.