The Whiskey Compass, ANDS and UNOS, and Why the One Instrument With No Moving Parts Lies to You in Every Turn

The magnetic compass is the most reliable direction-finder in your airplane - no gyro, no vacuum pump, no electrical power, no software - yet it gives false readings in two predictable situations: while you turn and while you change airspeed. Two memory aids explain those errors: UNOS (Undershoot North, Overshoot South) covers turning error, and ANDS (Accelerate North, Decelerate South) covers acceleration error. The compass tells the truth only in steady-state flight: wings level, constant airspeed, no turn.

Why Does the Magnetic Compass Give False Readings?

The root cause of nearly every compass error is magnetic dip. The Earth’s magnetic field doesn’t run flat along the ground the way heading lines run across your sectional. Except right at the magnetic equator, the field angles down into the Earth.

In the northern latitudes where most of us fly, the field pulls the compass card not just north, but downward, into the ground. The card is built to float as level as it can, but that downward tug is always working on it.

Hold onto that single idea: the field pulls down, not just sideways. Everything else about compass behavior follows from it.

How Do I Roll Out on Heading Using the Compass? (Turning Error and UNOS)

Turning error is the one that bites pilots on a checkride, when the examiner wants you to time a compass turn or roll out on a heading using the compass alone. The memory aid is UNOS: Undershoot North, Overshoot South.

Turning toward north: As you turn toward a northerly heading, the compass lags - it moves slower than the airplane actually turns, because dip is dragging the card the wrong way. If you wait until the compass reads 360 to roll out, you’ve already blown past north.

That’s the undershoot. To roll out on north, stop the turn early - lead the rollout. A rough rule of thumb is to lead by roughly your latitude. Near the 30-degree latitude line, lead by about 30 degrees: start rolling out when the compass shows about 330 if you want north. The card catches up and settles on the number.

Turning toward south: The compass does the opposite - it runs ahead of the airplane and over-indicates. If you wait for it to read 180, you haven’t turned far enough. Roll out late - let the compass pass the number and keep turning, again by roughly your latitude.

Turning through east or west: Turning error is at its minimum. The compass is most trustworthy pointed east or west and most treacherous turning through north or south - which is exactly why the heading everyone instinctively wants, straight north, is the one the compass fights hardest.

Real Scenario: Failed Heading Indicator

Picture a training flight under the hood. Your instructor fails and covers your heading indicator, and now you’re navigating on the compass alone. Approach says, “fly heading 360.”

If you crank into a standard-rate turn and wait for the compass to show 360, you’ll swing well past it, correct back, overshoot again, and end up in a slow, ugly wobble around the sky. The pilot who understands UNOS knows north undershoots, rolls out early, lets it settle, and nails it on the first try. That anticipation - flying ahead of the error - is what the examiner is actually looking for.

Why Does My Compass Show a Turn When I Speed Up? (Acceleration Error and ANDS)

Acceleration error shows up on a totally normal day in smooth air. The memory aid is ANDS: Accelerate North, Decelerate South. Critically, this error appears when you change airspeed while flying on an easterly or westerly heading - not north or south.

Accelerating on east or west: Cruising along heading east (090), push the throttle up or lower the nose to let the airplane accelerate. The compass swings toward the north, toward a lower number, even though your wings are level and you never touched the controls. Accelerate, shows North.

Decelerating on east or west: Pull the power back or raise the nose to slow down on the same easterly heading, and the compass swings toward the south. Decelerate, shows South.

This happens because dip acts on an off-center weight built into the card. When the airplane speeds up or slows down, that weight is thrown forward or back, tipping the card into a false turn indication. The physics matter less than the pattern: Accelerate North, Decelerate South, on east and west headings.

Real Scenario: The Descent That Isn’t a Turn

You’re in the clouds and you lower the nose to start a descent. Your airspeed increases - you’ve accelerated. If you happen to be heading roughly east or west, the compass now shows you drifting north.

Chase that false indication with a “correction,” and you’ve rolled into a real turn in the clouds, based on a lie. That is how a healthy pilot ends up in a graveyard spiral - not because the compass broke, but because it did exactly what it always does and the pilot didn’t know the rule.

When Can I Actually Trust the Magnetic Compass?

Write this one on your kneeboard. The compass tells the truth in only one condition: straight and level, wings level, constant airspeed, no acceleration, no turn. Steady state. That is the only time you read a heading straight off the compass and believe it. Any other time - in a turn or changing speed - treat it as suspect and cross-check.

How Do I Reset My Heading Indicator Off the Compass?

Your directional gyro precesses - it wanders off over time, sometimes several degrees every 15 minutes. So you reset it to match the compass, but timing is everything.

Reset it only when the compass is telling the truth: straight and level, at constant airspeed. Not in a turn, not while accelerating out of a climb. Get stable, let the compass settle, then set the heading indicator to match. Build the habit of doing this every 10 to 15 minutes, like scanning for traffic - and set it off the compass before you take the runway, every time.

The Airman Certification Standards (ACS) expect this on both the private and instrument checkrides: you should understand your instruments’ errors and limitations and be able to fly compass turns and timed turns when the gyro isn’t available. It’s the examiner making sure that when your vacuum pump dies in the clouds, you can still point the airplane home.

How Do I Keep ANDS and UNOS Straight?

The two acronyms blur together easily, so anchor them to opposite lines of the compass rose:

• ANDS is about speed. Accelerate, Decelerate. It happens on east and west headings. Hear the word accelerate, reach for ANDS.
• UNOS is about turning. Undershoot, Overshoot. It happens on north and south headings. Hear the word turn, reach for UNOS.

Speed errors live on the east-west line; turning errors live on the north-south line. Between them, they cover the whole rose.

Real Scenario: The Uncorrected Gyro

You’re on a cross-country and forgot to reset your heading indicator off the runway. It has quietly precessed about 15 degrees. Fifteen degrees doesn’t sound like much, but over 50 miles it walks you a long way off course - your landmarks stop matching the chart and your confidence drains away.

The fix was 30 seconds of discipline an hour ago: wings level, constant speed, glance at the compass, set the gyro to match. The compass was telling the truth the whole time. You just have to know when to ask it and when not to trust it.

Key Takeaways

• Magnetic dip is the root of compass errors - the field pulls the card downward, not just sideways.
• UNOS (Undershoot North, Overshoot South) governs turning error on north-south headings; lead north rollouts, lag south rollouts by roughly your latitude.
• ANDS (Accelerate North, Decelerate South) governs acceleration error on east-west headings; a speed change shows a false turn.
• The compass is truthful only in steady-state flight - wings level, constant airspeed, no turn.
• Reset your heading indicator off the compass straight and level every 10–15 minutes, and before every takeoff.