Acceleration Error (ANDS): Why the Compass Swings on East/West Headings and How to Ignore It
Updated: Sep 26
The magnetic compass is one of the most reliable — and most misunderstood — instruments in aviation. It requires no electrical power and rarely fails, yet it's vulnerable to several predictable errors, and one of the most important is the acceleration error. Acceleration errors occur whenever an aircraft speeds up or slows down on an east or west heading, momentarily indicating a turn that isn't happening. Pilots manage it with the mnemonic ANDS — Accelerate North, Decelerate South. But the deeper insight is understanding WHY: the compass card hangs like a pendulum below its pivot, so when you accelerate, inertia swings it, and magnetic dip then rotates it toward a false heading. Once you understand that pendulum behavior, both the acceleration error and the turning error make sense as two faces of the same thing.
This post covers acceleration errors in practical depth: the dip root cause (briefly), the pendulum mechanism (why acceleration produces a false north/south swing), the ANDS rule, why it's maximal on east/west headings, the specific flight scenarios where it appears, why it's transient, the partial-panel implication, and best practices.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
The Root Cause: Magnetic Dip (Briefly)
Acceleration errors are ultimately caused by magnetic dip — Earth's magnetic field lines angling downward toward the magnetic poles.
In the Northern Hemisphere:
The field lines slope downward toward the north
The north-seeking end of the compass magnet is pulled downward
Aircraft compasses are counterweighted to reduce this, but it can't be eliminated
The dip is always present
The acceleration trigger:
In steady flight, the dip pull is balanced (by the counterweight)
When the aircraft accelerates or decelerates, INERTIA tilts the compass card
With the card tilted, the dip can now rotate it (a false heading change)
Acceleration unlocks the error
(For the full physics of dip — the horizontal/vertical field components — see our magnetic dip post.)
What an Acceleration Error Is
An acceleration error is a false compass indication that occurs when the aircraft changes speed while flying on an east or west heading.
The key points:
The aircraft is NOT actually turning
The compass APPEARS to turn anyway
The error DISAPPEARS once the aircraft stabilizes (reaches a steady speed)
A transient false indication
Where it's maximal:
Most noticeable on EAST (090°) or WEST (270°) headings
This orientation maximizes the interaction between inertia and dip
Minimal on north/south headings
An east/west phenomenon
The transient nature:
The error exists only DURING the speed change (acceleration/deceleration)
Once airspeed stabilizes, the compass returns to correct
Not a lasting error (unlike a real heading deviation)
Self-correcting
The Pendulum Mechanism: Why It Happens
Here's the deeper "why" — the compass card behaves like a pendulum, and that explains everything.
The compass card as a pendulum:
The compass card (with its magnets) is mounted so its CENTER OF GRAVITY is BELOW its pivot point
(Partly because of the counterweight added to fight dip)
This makes the card behave like a pendulum
It can swing/tilt under acceleration forces
Why the CG is below the pivot:
The counterweight (added to balance the dip's downward pull) offsets the card's center of gravity
The result is a CG below the pivot
Like a pendulum bob hanging below its pivot
The pendulum setup
What happens when you accelerate:
Acceleration is a forward force on the aircraft
Inertia makes the card's CG (below the pivot) lag BACKWARD (like a pendulum swinging back)
This TILTS the card
The tilt lets dip act on it
Why the tilt produces a north swing (accelerating):
With the card tilted (from the backward inertial swing)
The magnetic dip force (pulling the north end down) now has a rotational effect on the tilted card
On an east/west heading, this rotates the card toward a NORTH indication
Accelerate → North
Why deceleration produces a south swing:
Decelerating: the inertia swings the card FORWARD (the opposite way)
The card tilts the other direction
Dip rotates it toward a SOUTH indication
Decelerate → South
The unifying insight:
The SAME pendulum behavior (CG below pivot) causes BOTH errors:
Acceleration error: inertia (fore/aft) swings the pendulum → dip rotates it (east/west headings)
Turning error: banking tilts the card → dip rotates it (north/south headings)
Both are dip acting on a tilted/swung card
One mechanism, two triggers
ANDS: Accelerate North, Decelerate South
The rule, applied.
The mnemonic (Northern Hemisphere):
Accelerate → compass indicates a turn toward North
Decelerate → compass indicates a turn toward South
ANDS
The memory aid
The example (westbound):
Flying westbound (270°) and apply takeoff power
The aircraft accelerates straight ahead (not turning)
The compass momentarily swings toward NORTH
The airplane hasn't turned, but the compass suggests it has
The deceleration example:
Flying westbound (270°) and reduce power
The aircraft decelerates (not turning)
The compass swings toward SOUTH
Again, no actual turn
The eastbound behavior:
Same rule on east (090°)
Accelerate → compass swings north
Decelerate → compass swings south
East or west, same ANDS
The "it's lying" reality:
On east/west, during a speed change, the compass shows a turn that isn't happening
ANDS tells you which way it falsely swings
Recognize it as false
Don't chase it
Why It's Maximal on East/West (and Zero on North/South)
The heading dependence, explained.
Why east/west maximizes it:
On an east/west heading, the compass magnet is oriented east-west (perpendicular to the north-south field)
The fore/aft acceleration (along the east-west heading) acts perpendicular to the magnet
This maximizes the tilting/rotating effect
The geometry is ideal for the error
Why north/south minimizes it:
On a north/south heading, the magnet is aligned north-south (along the field)
The fore/aft acceleration is along the magnet's axis
This produces little rotational effect
The geometry doesn't cause the error
The gradient:
Maximum error on due east/west
Decreasing toward north/south
Zero on due north/south
Opposite of the turning error (which is max at north/south)
The complementary pattern:
Turning error (UNOS): maximal at NORTH/SOUTH, zero at east/west
Acceleration error (ANDS): maximal at EAST/WEST, zero at north/south
They're complementary (each maximal where the other is zero)
A neat symmetry
Why this matters:
On east/west, watch for acceleration error (during speed changes)
On north/south, watch for turning error (during turns)
Different errors on different headings
Know which applies
The Specific Flight Scenarios Where It Bites
Where you'll actually encounter it, expanded.
The east/west takeoff:
Taking off on an east/west runway (090° or 270°)
Full power → strong acceleration
The compass swings toward north
Can confuse a student who's watching the compass during the takeoff roll/climb
Leveling off from a climb:
Leveling off (on an east/west heading) and accelerating to cruise speed
The acceleration swings the compass north
A transient error during the level-off acceleration
Wait for stable cruise speed
Airspeed changes on an east/west leg:
Speeding up or slowing down while flying east/west
The compass swings (north accelerating, south decelerating)
During the speed change
Ignore it until stable
Go-arounds:
A go-around (on an east/west heading) with full power added
Strong acceleration → north swing
During a high-workload moment
Don't be fooled
Turbulence:
Rough air causes frequent acceleration/deceleration
The compass oscillates (swinging north and south)
Unreliable for short-term heading in turbulence
Average or ignore it
The pattern:
Any significant speed change on an east/west heading
Takeoff, level-off, go-around, airspeed adjustments, turbulence
The compass momentarily lies
Recognize the scenario
Why It's Transient (and Why That Matters)
A point worth emphasizing: the error is temporary.
The self-correcting nature:
The acceleration error exists only DURING the speed change
Once airspeed stabilizes (constant speed), the compass returns to correct
It self-corrects
Not a lasting deviation
Why this matters:
Unlike a real heading error (which persists), the acceleration error goes away on its own
If you make a heading correction based on it, you'll actually be wrong once it self-corrects
So DON'T correct for it — wait it out
Chasing it creates a real error
The "don't chase it" principle:
The worst response is to "correct" the false turn
If the compass swings north (accelerating) and you turn south to "fix" it, you've created a real southerly error
When the acceleration error self-corrects, you're now actually off-heading
Ignore the swing; don't chase
The contrast with a real turn:
A real turn: the compass change persists (you're actually turning)
An acceleration error: the compass change is transient (you're not turning)
Tell them apart by the context (are you changing speed? on east/west?)
Recognize the false one
The Partial-Panel Implication
Where the error becomes operationally critical.
In partial panel (no heading indicator):
The compass is your heading reference
During speed changes on east/west, it lies (ANDS)
You must recognize the false swing and not correct for it
A real skill
The false-correction danger:
In partial-panel IMC, chasing the acceleration error creates real heading errors
Compounds the workload and disorientation
Recognize ANDS, ignore the transient swing
Wait for stable airspeed to read the compass
The technique:
Make speed changes deliberately, then let the airspeed stabilize
Read the compass only after it settles (stable speed)
Don't make heading corrections during the acceleration
Stable airspeed = trustworthy compass
The connection to timed turns:
For heading changes in partial panel, timed turns (clock + turn coordinator) avoid the compass errors entirely
(Covered in the northerly turning error post)
The compass is verified in steady, unaccelerated flight
Bypass the errors
When Acceleration Errors Are Most Noticeable
The factors, expanded.
East or west headings:
Maximum effect on east/west
Minimal on north/south
The heading matters most
Rapid speed changes:
Takeoff (strong acceleration)
Go-arounds (full power)
Power reductions (deceleration)
Turbulence (repeated accel/decel)
Faster changes = bigger swing
Higher latitudes:
Dip increases with latitude
So the acceleration error increases with latitude too
Minimal near the magnetic equator
Larger at high latitudes
The compounding:
East/west heading + rapid speed change + high latitude = maximum error
North/south + steady speed + low latitude = no error
The factors combine
Manage what you can (recognize the scenario)
Best Practices for Pilots
The technique, expanded.
IGNORE compass movement during acceleration or deceleration (it's false)
Use the heading indicator whenever available (no acceleration error)
Reference the compass only in steady, level, CONSTANT-SPEED flight
Memorize and apply ANDS automatically
Do NOT make heading corrections based on compass movement during power/speed changes
Make speed changes, then let airspeed stabilize before reading the compass
In partial panel, use timed turns for heading changes (verify with the compass when stable)
The compass-accuracy conditions:
The compass is most accurate when the aircraft is:
Wings level
At constant airspeed
Unaccelerated
That's when to read and trust it
Steady, level, constant-speed
The "know when not to trust it" lesson:
Knowing WHEN NOT to trust the compass (during speed changes on east/west) is as important as knowing how to use it
The acceleration error is a reminder
Recognize the false indication
Trust it only when steady
Northern vs. Southern Hemisphere
The reversal, clarified.
In the Southern Hemisphere:
Acceleration errors are REVERSED
Accelerate → compass indicates SOUTH
Decelerate → compass indicates NORTH
Opposite of ANDS
Why it reverses:
In the Southern Hemisphere, dip pulls the SOUTH-seeking end down
The errors flip
The counterweight is balanced for the other hemisphere
Mirror image
The ANDS caveat:
ANDS applies specifically to the NORTHERN Hemisphere
Where most primary flight training occurs
The logic reverses in the Southern Hemisphere
Know where the rule applies
Why Acceleration Errors Still Matter Today
The enduring relevance, expanded.
Even with glass cockpits, AHRS, and GPS:
The compass is required as a backup instrument (91.205)
Electrical failures can disable modern heading systems
Fundamental knowledge improves situational awareness
The compass remains the last-resort reference
The backup scenario:
If the heading systems fail, you're on the compass
During speed changes on east/west, it lies (ANDS)
Recognizing this is essential
The backup skill
The "technically correct yet misleading" lesson:
Acceleration errors show how an instrument can be technically working yet operationally misleading
The compass isn't broken — it's responding to physics
Understanding the physics lets you interpret it correctly
A key lesson
Conclusion
Acceleration errors are an unavoidable consequence of magnetic dip and aircraft inertia acting on a compass card that hangs like a pendulum below its pivot. When flying on east or west headings, changes in speed swing the card, and dip rotates it toward a false north (accelerating) or south (decelerating) indication. The ANDS rule — Accelerate North, Decelerate South — tells you which way the compass falsely swings, so you can recognize it as false and NOT chase it.
Understanding the pendulum mechanism ties it all together: the same card behavior causes both the acceleration error (on east/west, from fore-aft inertia) and the turning error (on north/south, from banking). In aviation, knowing when NOT to trust an instrument is just as important as knowing how to use it — and the magnetic compass, with its transient acceleration errors on east/west headings, is a perfect reminder of that lesson.
On the Written Test and Checkride
Acceleration errors appear on tests and checkride orals (instrument especially). The most commonly tested topics:
What causes acceleration errors (magnetic dip + inertia)
ANDS (Accelerate North, Decelerate South)
That they're maximal on east/west headings (zero on north/south)
That they're transient (self-correct with stable speed)
Not chasing the false indication
The Southern Hemisphere reversal
Quick Reference
Acceleration Error:
False compass turn indication during a SPEED change on an east/west heading
The aircraft is NOT turning
Transient (self-corrects when speed stabilizes)
Caused by dip + inertia
ANDS:
Accelerate → compass swings toward North
Decelerate → compass swings toward South
Northern Hemisphere
Which way the compass falsely swings
The Pendulum Mechanism:
The compass card's CG is BELOW its pivot (from the counterweight) → behaves like a pendulum
Accelerate: inertia swings the card back → dip rotates it toward NORTH
Decelerate: inertia swings it forward → dip rotates it toward SOUTH
Same mechanism as the turning error (dip on a tilted card)
Heading Dependence:
MAXIMAL on east/west (090°/270°)
ZERO on north/south
Complementary to the turning error (max at north/south, zero at east/west)
Where It Bites:
East/west takeoff
Leveling off (accelerating to cruise)
Airspeed changes on an east/west leg
Go-arounds
Turbulence (oscillation)
Why Transient:
Exists only DURING the speed change
Self-corrects when airspeed stabilizes
DON'T chase it (chasing creates a real error)
Wait for stable speed
Partial Panel:
Recognize ANDS, don't correct for the false swing
Read the compass only when airspeed is stable
Use timed turns for heading changes
Factors:
East/west heading (maximal)
Rapid speed changes (takeoff, go-around, turbulence)
Higher latitude (more dip)
Southern Hemisphere:
Reversed (accelerate → south, decelerate → north)
ANDS is for the Northern Hemisphere
Compass Accuracy:
Accurate: wings level, CONSTANT airspeed, unaccelerated
Unreliable: during speed changes (on east/west)
Read and trust it when steady
Key Principle:
Acceleration error is a transient false turn indication when you change speed on an east/west heading — Accelerate North, Decelerate South (ANDS). It happens because the compass card hangs like a pendulum below its pivot (from the dip counterweight), so fore-aft inertia swings it and dip rotates it. It's maximal on east/west (zero on north/south — complementary to the turning error), and it self-corrects when speed stabilizes, so recognize it and don't chase it.
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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.