The direct reading magnetic compass is the humblest instrument on the flight deck and one of the most heavily examined. It needs no electrical bus, no vacuum pump and no software. When every glass screen goes dark, it still points. That reliability is exactly why EASA keeps it on the ATPL Instrumentation syllabus, and why exam writers love it: the compass misbehaves in ways that are perfectly predictable once you understand the physics.
Get the logic right and the questions become free marks. Memorise blindly and you will mix up the mnemonics under pressure.
What the compass actually measures
A direct reading compass aligns itself with the horizontal component of the Earth's magnetic field, not the total field. That single fact drives almost every error you will be tested on.
The Earth's field dips into the ground at an angle called magnetic dip. Dip is zero at the magnetic equator and increases toward the poles, where the field points almost straight down. The strength that actually swings your compass, the directive force, is proportional to the horizontal component, so it fades as you fly toward high latitudes. Near the poles the compass becomes sluggish and eventually useless.
Variation vs deviation: keep them separate
Two corrections stand between a compass reading and a true heading, and EASA expects you to apply them in the right order.
- Variation is the angle between True North and Magnetic North. It depends on where you are on the planet and is shown on charts as isogonals.
- Deviation is the error caused by the aircraft's own magnetism acting on the compass. It depends on heading and is recorded on a deviation card after a compass swing.
Work from true to compass as follows: start with True, apply Variation to get Magnetic, then apply Deviation to get Compass. Reverse the sequence when you convert a compass reading back to a true direction.
Acceleration and deceleration errors
Because the magnet assembly is pendulously suspended, with its centre of gravity hanging below the pivot, any fore and aft acceleration tilts the card. Combine that tilt with magnetic dip and the compass shows a false turn.
These errors appear on east and west headings and vanish on north and south. In the Northern Hemisphere: accelerate and the compass indicates a turn toward North; decelerate and it indicates a turn toward South.
The classic memory hook is ANDS: Accelerate North, Decelerate South. Flip it for the Southern Hemisphere. On the exam, watch for a question that pairs a speed change with a north or south heading, because the correct answer there is no acceleration error at all.
Turning errors
The same pendulous mounting and dip conspire during turns, and here the worst case is turning through north or south.
In the Northern Hemisphere, when you roll into a turn through North the compass lags. It is slow to follow, so you must roll out early and undershoot. Turning through South it leads, running ahead of the aircraft, so you roll out late and overshoot.
Pilots remember this as UNOS: Undershoot North, Overshoot South. The size of the lead or lag is roughly equal to your latitude, and it is reversed south of the equator. Turning through east or west produces almost no error.
Why the fluid matters
The bowl is filled with a light damping fluid so the card settles quickly without wild oscillation, an aperiodic or dead beat response. That fluid also causes a smaller liquid swirl error in prolonged turns, and any bubble or discolouration is an unserviceability you should flag on the walk round.
How EASA tests this in 2026
The ECQB 2026 refresh reworked chart formats and regulatory items but did not fundamentally rewrite Instrumentation, so direct reading compass theory remains squarely on the EASA ATPL exam. Expect two flavours of question: a knowledge recall such as on which headings is acceleration error greatest, and an applied scenario such as Northern Hemisphere, accelerating on heading 090, what does the compass show. Answer the second with ANDS and you will not be caught out.
Drill the four triggers until they are reflex: dip, directive force, ANDS and UNOS. Everything else is a consequence of those.
Frequently asked questions
What are the main errors of a direct reading magnetic compass?
Acceleration and deceleration errors on east and west headings, turning errors worst through north and south, magnetic dip near the poles, deviation from the aircraft's own magnetism, and a small liquid swirl error during sustained turns.
What does ANDS mean for the magnetic compass?
ANDS stands for Accelerate North, Decelerate South. In the Northern Hemisphere, accelerating on an east or west heading makes the compass show an apparent turn toward north, and decelerating shows an apparent turn toward south. The rule reverses in the Southern Hemisphere.
Why does the compass lead or lag when turning through north and south?
The magnet assembly hangs below its pivot, so during a banked turn magnetic dip pulls the card off heading. Through north it lags and you undershoot, through south it leads and you overshoot. The error is about equal to your latitude and disappears when turning through east or west.
Is the magnetic compass still examined in EASA ATPL 2026?
Yes. The ECQB 2026 update did not fundamentally revise the Instrumentation syllabus, so direct reading compass errors remain a reliable source of exam questions.
Want structured EASA ATPL Instrumentation drills with worked answers? Practise thousands of exam standard questions at atpltraining.io.
Photo by Tanya Barrow on Unsplash
