Northerly Turning Error (UNOS): The Roll-Out Lead, Compass Turns to North, and Timed Turns
Updated: Sep 26
Despite glass cockpits and GPS navigation, the magnetic compass remains a required instrument in most aircraft — and one every pilot must understand deeply. Among its most significant limitations are northerly turning errors, a direct result of magnetic dip. Pilots manage them with a simple but powerful memory aid: UNOS — Undershoot North, Overshoot South. But knowing the rule is only half the skill. The real competence is in the numbers and the technique: exactly how much to lead the roll-out (it depends on your latitude and bank angle), how to actually execute a compass turn to a northerly heading, and when the error is big enough that you should time the turn with the clock instead of watching the compass at all. This post focuses specifically on northerly turning errors — why they occur, how much to compensate, and how to fly them.
This post covers northerly turning errors in practical depth: the dip root cause (briefly), why the compass lags turning north, the UNOS rule, the roll-out lead numbers (latitude plus half bank), why NE/NW differ from due north, executing a compass turn to a northerly heading, the timed-turn alternative, and best practices.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
The Root Cause: Magnetic Dip (Briefly)
Northerly turning errors come from magnetic dip — Earth's magnetic field lines angling downward into the planet rather than running parallel to the surface.
In the Northern Hemisphere:
The north-seeking end of the compass magnet is pulled DOWNWARD (toward the magnetic pole, which is below the horizon ahead)
The compass is counterweighted to stay usable, but dip can't be eliminated
When the aircraft banks into a turn, this downward pull acts on the tilted compass card
The result is a directional error during the turn
Why the turn triggers it:
In level flight, the card is level and the dip pull is balanced (by the counterweight)
When you BANK, the card tilts, and the dip pull now acts to rotate the card incorrectly
This is the turning error
Banking is what unlocks the error
(For the full physics of dip — the horizontal/vertical field components and the hemisphere-specific counterweight — see our magnetic dip post.)
What a Northerly Turning Error Is
A northerly turning error occurs when an aircraft turns 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 (it lags)
The indicated heading is BEHIND reality
The compass hasn't "caught up"
The lag:
As you turn toward north, the compass reads a heading that's behind where you actually are
If you're actually at 350°, the compass might still read 340°
The compass is lagging
It trails the actual turn
Why it misleads:
If you wait for the compass to show your target northerly heading, you've already passed it
The compass shows the target only after you're past it
So you overshoot (unless you anticipate)
The trap
Why the Compass Lags Turning North
The mechanism, focused.
The forces in a northerly turn:
The horizontal turning force (from the aircraft's motion)
The vertical dip force (pulling the north-seeking end down)
Gravity and inertia on the banked compass assembly
These combine to produce the lag
The lag mechanism:
Turning toward north, the dip pulls the (banked) card in a direction that OPPOSES the turn's rotation
The card rotates slower than the aircraft
The indicated heading lags behind
The compass can't keep up
The "resists rotation" picture:
The downward dip pull on the tilted card creates a rotational effect that lags the turn
Near north, this lag is maximal
The compass drags behind
The northerly lag
Why it's worst near north:
The geometry of the banked card and the dip force maximizes the lag near a north heading
Turning through north, the lag is greatest
At NE/NW, less (but still present)
North is the worst case
UNOS: Undershoot North
The rule's first half, applied.
The rule:
Undershoot North (the compass lags, so roll out BEFORE it reaches north)
Overshoot South (the other half — the compass leads turning south, roll out after)
UNOS
The northern-hemisphere memory aid
Undershoot North in practice:
When rolling out of a turn to a northerly heading
Begin the rollout BEFORE the compass shows the desired heading
Anticipate the compass "catching up" after you stop turning
Lead the roll-out
The example:
Turning from east (090°) toward north (360°)
If you wait until the compass shows 360°, you'll pass north (overshoot)
Instead, roll out EARLY — perhaps at 330° or 340° (depending on conditions)
The compass then catches up to 360° as you level
What happens if you don't undershoot:
You overshoot north
Then you S-turn to correct
Increased workload (especially in IMC)
Chasing the heading
The Roll-Out Lead: The Actual Numbers
Here's the practical depth the basic treatment gestures at but doesn't quantify: how MUCH to lead the roll-out.
The latitude rule of thumb:
The lead for a northerly roll-out is approximately your LATITUDE
Plus the normal roll-out lead of about half the bank angle
Lead ≈ latitude + (½ × bank angle)
The working formula
Why latitude:
The turning error magnitude is roughly equal to the latitude (dip increases with latitude)
At 30°N, the error is about 30°
At 45°N, about 45°
Latitude-driven
Why half the bank angle:
Even without the compass error, you lead any roll-out by about half your bank angle (the normal rollout lead)
Add this to the latitude-based lead
The combined lead
Standard technique plus the error
The worked example (due north):
Flying at 40°N latitude, turning to north with a 20° bank
Lead = latitude (40°) + half bank (10°) = 50°
Roll out about 50° before north
Turning to 360°, begin rollout at about 310°
(The compass catches up to 360° as you level)
Another example (30°N, 15° bank):
Lead = 30° + 7.5° ≈ 37°
Turning to 360°, roll out at about 323°
The compass then settles on 360°
Latitude plus half bank
The practical reality:
These are rules of thumb (the exact error varies)
With practice, you develop a feel
Start with latitude + half bank, adjust from experience
A starting point
The high-latitude caution:
At high latitudes, the lead is large (e.g., 60° at 60°N)
The compass turn becomes difficult (huge lead)
Timed turns may be better (below)
The error grows with latitude
Why NE and NW Differ from Due North
A nuance the basic treatment mentions but doesn't develop: the error varies across the northerly headings.
The error is largest at due north:
Turning to DUE NORTH (360°): maximum lag, maximum lead needed
The worst case
Full latitude + half bank
The error is smaller at NE/NW:
Turning to NORTHEAST (045°) or NORTHWEST (315°): less lag than due north
The lead needed is less than for due north
Partial error
Between north (max) and east/west (min)
The error is zero at east/west:
Turning to DUE EAST (090°) or DUE WEST (270°): NO turning error
The compass is accurate on east/west (for turning)
(East/west is where the ACCELERATION error lives instead — ANDS)
The turning error vanishes at east/west
The gradient:
Maximum lag/lead at north (and south)
Decreasing toward east/west
Zero at east/west
A smooth variation
The practical application:
Turning to due north: use the full lead (latitude + half bank)
Turning to NE/NW: use LESS lead (a fraction)
Turning to east/west: no lead for the turning error (but watch acceleration error)
Adjust the lead by how "northerly" the target is
Executing a Compass Turn to a Northerly Heading
The step-by-step technique.
When you need it:
Partial panel (heading indicator failed)
Turning to a northerly heading using only the compass
The compass is your heading reference
A practiced skill
The steps (turning to north):
Note your current heading and the target (north)
Determine the lead (latitude + half bank)
Start a standard-rate (or shallow) turn toward north
Roll out when the compass reads the target MINUS the lead (i.e., before north)
Let the compass settle (it catches up to north)
Verify the compass reads north in steady flight
Adjust if needed (small correction in steady flight)
The steady-flight verification:
After rolling out and leveling, let the compass stabilize
Read it in steady, level flight (where it's accurate)
Make a small correction if you're off
The compass is trustworthy once steady
Using standard-rate turns:
Standard-rate (3°/sec) or shallow turns reduce the error magnitude
Easier to manage than steep turns
Smoother, more predictable
Recommended for compass turns
The small-correction approach:
Rather than one big turn, some pilots turn close, level off, read the compass, then make a small final correction
The final correction is made in steady flight (accurate)
Reduces the compensation guesswork
A practical method
The Timed-Turn Alternative
An important technique the basic treatment doesn't cover: when the error is large, time the turn instead.
The concept:
Instead of watching the (lagging) compass during the turn, TIME the turn using the clock
A standard-rate turn is 3° per second
Calculate the time needed for the heading change, and turn for that time
Ignore the compass during the turn (it's unreliable while turning)
Why timed turns:
The compass is unreliable DURING the turn (lag/lead)
The turn coordinator (rate) and clock ARE reliable
Time the turn precisely, then verify with the compass in steady flight
Bypass the compass error entirely
How to do a timed turn:
Determine the heading change needed (e.g., 90°)
At standard rate (3°/sec), 90° takes 30 seconds
Roll into a standard-rate turn (using the turn coordinator)
Turn for the calculated time (30 seconds)
Roll out
Verify the heading with the compass in steady flight
The standard-rate timing:
3° per second (standard rate)
360° = 2 minutes; 180° = 1 minute; 90° = 30 seconds; 30° = 10 seconds
Calculate: time = heading change ÷ 3
Precise timing
Why timed turns beat compass turns at high latitude:
At high latitude, the compass turning error is huge (big lead needed, hard to judge)
A timed turn avoids the compass error entirely (you use rate and time)
More reliable when the error is large
The better tool sometimes
The partial-panel connection:
In partial panel (no heading indicator), timed turns are a key technique
Combined with the turn coordinator and clock
More reliable than fighting the compass lag
A core instrument skill
The verification:
After any timed turn, verify with the compass in STEADY flight
The compass is accurate once level and unaccelerated
Confirm and correct
Trust it when steady
Factors That Increase Northerly Turning Errors
The variables, expanded.
Latitude:
The farther north (higher magnetic latitude), the stronger the dip
Minimal near the magnetic equator
Large at high latitudes
The dominant factor (the lead ≈ latitude)
Bank angle:
Steeper turns exaggerate the forces on the compass
Standard-rate turns produce smaller errors
Steep turns: bigger error
Keep turns shallow for compass work
Rate of turn:
Rapid heading changes increase the lag
Smooth, coordinated turns reduce (but don't eliminate) it
Slow and smooth
Rate matters
The compounding:
High latitude + steep bank + fast turn = large error
Low latitude + shallow bank + slow turn = small error
Manage the factors you can (bank, rate)
Latitude you can't change
Why These Errors Matter
The stakes, expanded.
When they're first noticed:
Student pilots notice them during basic maneuvers, compass turns, ground reference training
An early lesson
Foundational
When they matter most:
Flying PARTIAL PANEL (heading indicator failed)
Gyro or electrical failures
Using the compass as the backup heading reference
The compass becomes primary
The IMC danger:
In instrument conditions, misunderstanding compass behavior leads to:
Heading deviations
Poor intercepts (of courses, radials)
Loss of situational awareness
High workload
Critical to get right
The backup scenario:
If the heading indicator fails in IMC, you're on the compass
Applying UNOS (and timed turns) correctly is essential
A real emergency skill
Why it's trained
Best Practices
The technique summary, expanded.
Use the heading indicator for all turns when available (no dip error)
Cross-check the compass only in straight-and-level, steady flight
Apply UNOS automatically during compass turns (undershoot north)
Use the lead: latitude + half bank angle (for due north)
Use less lead for NE/NW (partial error)
Consider timed turns (especially at high latitude or for large heading changes)
Avoid abrupt maneuvers when referencing the compass
Practice compass turns and timed turns regularly (build intuition)
Always verify the heading with the compass in steady flight
The reliability reminder:
The compass is most reliable when the aircraft is stable, wings level, and unaccelerated
That's when to trust and read it
During turns, anticipate the error (or use timed turns)
Steady = trustworthy
A Note on the Southern Hemisphere
The reversal, clarified.
In the Southern Hemisphere:
The behavior is REVERSED
Turning toward SOUTH causes lag
Turning toward NORTH causes lead
Opposite of the Northern Hemisphere
Why it reverses:
In the Southern Hemisphere, the dip pulls the SOUTH-seeking end of the magnet down
The errors flip accordingly
The counterweight is also balanced for the other hemisphere
Mirror image
The UNOS caveat:
UNOS applies specifically to the NORTHERN Hemisphere
Where most primary flight training occurs
In the Southern Hemisphere, the logic reverses
Know where the rule applies
Conclusion
Northerly turning errors are a classic — and critical — example of how physics affects flight instruments. Caused by magnetic dip, they make the compass lag during turns toward north, requiring pilots to undershoot northerly headings. But the real skill goes beyond the UNOS rule: knowing the roll-out lead (approximately your latitude plus half the bank angle for due north, less for NE/NW), executing a smooth compass turn, and recognizing when a timed turn (using the clock and turn coordinator) is the better tool — especially at high latitude or in partial-panel IMC.
The UNOS acronym distills a complex phenomenon into an easy rule, but understanding why it works — and how much to compensate — makes you a better instrument interpreter and a more precise aviator. When the heading indicator fails and you're down to the compass, that understanding becomes not just academic, but essential.
On the Written Test and Checkride
Northerly turning errors appear on tests and checkride orals (instrument especially). The most commonly tested topics:
What causes northerly turning errors (magnetic dip)
UNOS (Undershoot North, Overshoot South)
The roll-out lead (latitude + half bank)
That the error is maximal at north, zero at east/west
Timed turns as an alternative
The Southern Hemisphere reversal
Quick Reference
Northerly Turning Error:
The compass LAGS when turning toward north (Northern Hemisphere)
Caused by magnetic dip acting on the banked card
Indicated heading is behind the actual heading
Requires undershooting north
UNOS:
Undershoot North (roll out BEFORE the compass reaches north)
Overshoot South (roll out AFTER, turning to south — the compass leads)
Northern Hemisphere
The memory aid
The Roll-Out Lead (due north):
Lead ≈ latitude + (½ × bank angle)
Example: 40°N, 20° bank → 40 + 10 = 50° lead
Example: 30°N, 15° bank → 30 + 7.5 ≈ 37° lead
Roll out that many degrees before north
Error by Heading:
Due north: MAXIMUM error (full lead)
NE/NW: less error (partial lead)
East/west: NO turning error (but acceleration error lives here)
Decreases from north toward east/west
Compass Turn to North:
Determine the lead (latitude + half bank)
Turn at standard rate
Roll out at (target − lead) — before north
Let the compass settle (it catches up)
Verify in steady, level flight
Timed Turn (alternative):
Time the turn using the clock (ignore the compass during the turn)
Standard rate = 3°/sec (360° = 2 min, 90° = 30 sec, 30° = 10 sec)
Time = heading change ÷ 3
Verify with the compass in steady flight
Best at high latitude or for large changes / partial panel
Factors Increasing the Error:
Higher latitude (the dominant factor)
Steeper bank
Faster turn rate
When It Matters:
Partial panel (heading indicator failed)
Gyro/electrical failures
Compass as backup in IMC
Southern Hemisphere:
Reversed (lag turning south, lead turning north)
UNOS is for the Northern Hemisphere
Compass Reliability:
Accurate: steady, level, unaccelerated flight
Unreliable: during turns (lag/lead)
Read and trust it when steady
Key Principle:
Northerly turning error is the compass lagging when you turn toward north (from magnetic dip on the banked card) — so you Undershoot North, Overshoot South (UNOS). Lead the roll-out by approximately your latitude plus half the bank angle for due north (less for NE/NW, none at east/west). When the error is large — high latitude or partial-panel IMC — a timed turn (clock plus turn coordinator) is often the better tool. Always verify the heading with the compass in steady, level flight.
Study Full Aviation Courses:
wifiCFI's full suite of aviation courses has everything you need to go from brand new to flight instructor and airline pilot! Check out any of the courses below for free:
Study Courses:
Checkride Lesson Plans:
Teaching Courses:
Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.