9.2 Compass Errors & Magnetic Interference

Why the Compass Matters for Near People Flying

Section 9.1 covered where the aircraft is (GNSS). This section covers which way it points. The flight controller needs a reliable heading so that "forward" stick input moves the aircraft forward in the pilot’s intended direction and so that automatic systems (orbit, course lock, some RTH behaviours, and yaw stabilisation) command the correct motors.

That heading usually comes from a magnetometer—a sensor that measures the Earth’s magnetic field, commonly called the compass in pilot apps. When the local field is the clean Earth field, heading is good. When steel, current-carrying cables, magnets, or vehicles distort the field, the controller believes a false north. It then fights the wrong error: yawing, circling, or accelerating in a direction you never commanded. Near uninvolved people, that is a ground-risk and flight-performance failure—exactly the A2 syllabus territory.

Compass Calibration — Purpose and Discipline

Manufacturers require compass calibration (sometimes called IMU/compass combo steps, depending on model) so the aircraft maps sensor readings to real headings in the current magnetic environment. Calibration is not a superstition and not a daily fashion statement—it is a performance configuration step:

• Perform it when the app or manual instructs (new location, after transport, after magnetic interference warnings, after major component changes, or on a schedule the manufacturer sets).
• Perform it in an open area with a relatively clean magnetic field.
• Follow the exact manufacturer motions (horizontal rotation, vertical flip patterns, etc.). Incomplete calibration is often worse than a deferred flight.
• After calibration, do a short hover test far from people before any Near People profile.

Where not to calibrate (exam-critical)

Never calibrate on metal surfaces. That single rule appears in manufacturer manuals and in exam distractors. A "perfect" calibration dance on a steel plate teaches the aircraft a wrong world, so later flight in a normal field looks like a sudden compass failure—or produces an immediate flyaway after take-off.

Also never calibrate near speakers (stage monitors, car audio, PA stacks) or other strong magnets. The sensor may report a strong, stable field that is not Earth north.

Sources of Magnetic Interference in A2 Work

Near People sites are full of interference sources:

• Vehicles — cars, vans, buses, trains, and construction plant (the most common launch-site mistake is calibrating beside the work van).
• Steel structures — bridges, cranes, scaffolding, shipping containers, industrial frames, metal cladding.
• Reinforced concrete — high-rise cores, multi-storey parks, some modern façades.
• Power lines and substations — AC fields and ferromagnetic hardware.
• Magnets — magnetic camera filters, mounts, tool trays, speaker magnets, phone holders on the controller or aircraft bag.
• Cameras and payloads — some gimbals and lens assemblies introduce hard-iron offsets; follow the manual for re-calibration after payload changes.
• Underground utilities and metal street furniture — less obvious but real in city centres.

Question-bank style fact: magnetic interference near large metal structures can disturb the compass, causing heading errors or fly-off—it does not improve accuracy, charge the battery, or increase motor torque.

Symptoms of Compass Errors

Recognise these so you intervene before people are at risk:

1. Toilet-bowling — the aircraft circles or spirals around the hover point instead of sitting still, as if flushing around a drain. Often worse as speed or radius grows.
2. Unexpected yaw — the nose swings without stick input, or yaw response feels inverted relative to what you see.
3. Fighting the sticks — left stick yaw or right stick roll produces surprising ground track; the aircraft "wants" to go sideways.
4. Immediate post-take-off aggression — lifts off and accelerates in a random direction; classic bad calibration or strong local field at the pad.
5. App warnings — "compass error," "magnetic interference," "move away from metal," inconsistent with a green GNSS icon (GNSS and compass are related but not the same sensor).
6. RTH or course modes misbehaving — aircraft turns the wrong way to "home" because heading is wrong even if the map pin looks sensible.

Toilet-bowling is high-yield exam vocabulary: if a stem describes circling hover behaviour after take-off near metal, think compass, not "low battery" or "prop guards missing."

Recalibrate Away from Metal — Operational Response

When symptoms appear:

1. Do not continue a tight A2 profile. Performance evaluation has failed for 5 m (and often for any Near People closeness).
2. If airborne and controllable, move to a clear area and land with calm inputs; avoid heroic cinematic finishes.
3. Power down if the manual recommends it after a severe compass fault.
4. Relocate to a magnetically clean open patch—away from the van, fence, and steel.
5. Recalibrate per manufacturer instructions.
6. Hover-test far from people; only then reconsider the job with conservative buffers.
7. If errors persist, do not fly Near People. Seek maintenance, check for magnetic accessories on the airframe, or cancel.

Exam trap: "Compass error mid-job—continue at 5 m because low-speed mode is on." False. Low-speed mode caps speed; it does not fix heading. A circling aircraft at 3 m/s can still enter a person bubble and still breaches control expectations.

Why Magnetic Interference Causes Flyaways

A flyaway (section 9.3) is an uncommanded departure where the aircraft stops following intended control and moves off. Compass faults feed flyaways through a simple chain:

1. Magnetometer reports a false heading.
2. The controller computes a false position-hold or yaw error (it thinks it is pointed differently than it is).
3. Motor commands try to "correct" an error that does not exist in the real world.
4. The aircraft accelerates or circles away from the intended hover, sometimes faster than a surprised pilot can interpret on sticks and screen.
5. Near people, the uncommanded path may intersect the separation bubble or an overflight line.

GNSS may still show a moving map track (the aircraft really is moving), which confuses pilots who stare at the phone instead of the sky. VLOS and readiness to switch to manual control remain essential, just as with GNSS loss—but here the root is magnetic, not satellite count.

Calibration on metal bakes the error into the model, so the flyaway can start on the very next take-off even in an open field. That is why "never calibrate on metal / near speakers" is not pedantry—it is flyaway prevention.

Compass vs GNSS — Keep the Sensors Distinct

A pilot who "has 18 satellites" can still have a deadly bad compass beside a steel bridge. Do not use satellite count to dismiss magnetic warnings.

Pre-Flight Compass Hygiene for A2 Sites

1. Choose a take-off pad with open sky and low metal—not the van roof, not the steel staircase landing.
2. Remove magnetic phone holders, magnetic lens caps, and spare magnets from the aircraft bag and pockets near the airframe during calibration.
3. Complete calibration before stacking the team and public close to the pad.
4. Watch for interference warnings during the first hover; abort early.
5. After changing cameras or major metal accessories, check whether the manual requires re-calibration.
6. Log repeated compass issues at a site—some industrial yards are simply unsuitable for precise Near People work.

Realistic Scenarios

Scenario A — calibration on the steel deck. Pilot calibrates on a metal loading bay, launches for a 30 m façade survey, and the aircraft toilet-bowls toward a footpath. Root cause: corrupted calibration. Correct prevention: calibrate on clean ground away from the bay.

Scenario B — van-side launch. Aircraft sits 1 m from the work van. No full re-calibration, but local field is distorted. On lift-off the nose yaws hard. Correct action: move the pad, re-check compass, hover-test clear of people.

Scenario C — stage PA speakers at an event. Even if Open rules and authorisations otherwise allow a flight (often they will not near assemblies), speaker magnets destroy heading. Exam answer: magnetic interference risk is severe; do not treat the site as a normal A2 pad.

Scenario D — "I’ll ignore the yellow compass warning because the client is waiting." Wrong. Client schedule is not a performance evaluation pass. Land or do not take off; protect uninvolved persons first.

Memory Hooks for Section 9.2

1. Compass = heading; bad heading → toilet-bowl, yaw, flyaway.
2. Never calibrate on metal or near speakers/magnets.
3. Vehicles, steel, power lines, rebar, camera magnets = interference.
4. Symptoms mid-job → land, clean site, recalibrate or cancel—not tighter 5 m work.
5. Good GNSS does not prove a good compass.

If a stem mentions steel, power lines, circling hover, or calibration location, start your answer with magnetic interference / compass, then apply the A2 distance and abort logic.