9.3 Flight Instruments: Magnetic Compass, Altimeter, Airspeed Indicator & IMU
Key Takeaways
- Magnetic variation is the angle between true north and magnetic north at a location; the agonic line is where variation is zero, and isogonic lines join points of equal variation.
- Deviation is the compass error caused by the aircraft's own magnetic fields, which is why calibration must be done on clean open ground away from rebar, vehicles and power lines.
- An RPAS barometric altimeter is zeroed at the launch point, so it displays height above launch, not true AGL over sloping terrain.
- Flying from high pressure or warm air toward low pressure or cold air makes the aircraft lower than indicated: high to low, look out below.
- Most small multirotors carry no airspeed indicator and infer speed from GNSS groundspeed, so a stationary hover in a 20 km/h wind is 0 km/h groundspeed but 20 km/h airspeed.
9.3 Flight Instruments: Magnetic Compass, Altimeter, Airspeed Indicator & IMU
Exam Focus: TP 15263 ticks the magnetic compass, the altimeter, the airspeed indicator and the inertial measurement unit in the Basic column, and asks for principles of operation and errors and malfunctions for each. On a small RPA these appear as solid-state sensors rather than dials, but the physics and the failure modes are identical — and the failure modes are what crash drones.
1. The Magnetic Compass (Magnetometer)
Principle of Operation
A magnetic compass aligns itself with the horizontal component of the Earth's magnetic field. A mechanical compass uses a pivoted magnetized needle; an RPA uses a solid-state magnetometer that measures field strength on three axes and computes a heading. Either way, the instrument points at the magnetic pole, not the geographic pole.
Variation
Variation is the angular difference between true north and magnetic north at a given place.
- Lines joining points of equal variation are isogonic lines; the line along which variation is zero is the agonic line.
- Canada spans an extraordinary range of variation because the magnetic pole sits within the Canadian Arctic. Variation runs roughly west in the east of the country and east in the west, reaching well over 20 degrees in parts of Atlantic Canada and the far north.
- The conversion rule is the one every Canadian pilot memorizes: East is least, West is best — subtract easterly variation and add westerly variation when converting a true direction to a magnetic one.
- Variation is printed on the VNC as isogonic lines, and it changes slowly over the years, which is why charts carry a date.
Deviation
Deviation is compass error caused by magnetic fields within the aircraft itself — ferrous fasteners, magnets in a gimbal or motor, and above all the large currents flowing through the power distribution board when the motors spool up. Deviation changes with the aircraft's own configuration, which is why it must be resolved by calibration rather than by a chart.
Factors That Corrupt a Magnetometer
| Source | Why It Matters |
|---|---|
| Reinforced concrete (rebar in a slab, deck or balcony) | The single most common cause of a corrupted calibration |
| Vehicles, shipping containers, railway tracks, bridges | Large ferrous masses bend the local field |
| High-voltage lines and substations | Current-carrying conductors generate their own field |
| Ferrous tools, magnets, phone cases, tripods | Left near the aircraft during calibration, they are baked into the result |
| Motor current | High throttle produces a heading shift the controller must compensate for |
| Large change of geographic position | A magnetometer calibrated in one region may need recalibration after a long move |
Importance of Calibration
Calibrate on clean, open, level ground, well away from structures, vehicles and buried services, with no metal objects on your person. A bad calibration is the classic cause of "toilet bowling" — an expanding spiral in which the compass heading and the GNSS track vector disagree and the controller chases the error. The recovery is to switch immediately to ATTI mode and fly the aircraft home manually; engaging Return-to-Home with a corrupted compass simply automates the spiral.
2. The Altimeter
Principle of Operation
A barometric altimeter is an aneroid barometer calibrated in feet. A sealed capsule expands as the surrounding static pressure falls; that expansion is converted to an altitude reading using the standard atmosphere relationship of roughly 1 inch of mercury per 1,000 feet in the low levels.
How an RPA Differs
Almost every small RPA zeroes its barometer at the launch point. The telemetry therefore shows height above the take-off point, not height above the terrain below the aircraft. CAR 901.25(1)(a) is written in AGL — the distance between the aircraft and the surface directly beneath it. Fly from a hilltop out over a valley and the two numbers diverge immediately.
Errors and Malfunctions
| Error | Mechanism | Effect |
|---|---|---|
| Non-standard pressure | Sea-level pressure changes during a long flight or as a front passes | Indicated altitude drifts even in a perfect hover |
| Non-standard temperature | Cold air is denser, so pressure levels sit lower than standard | In cold air the aircraft is lower than indicated |
| Blocked or turbulent static port | Ice, water, insects, or propeller downwash over a vent | Erratic or frozen altitude; RTH may level at the wrong height |
| Gust-induced pressure change | A gust front or convective cell passing overhead | Sudden apparent altitude jump with no real climb |
| Launch-point datum | Barometer zeroed at take-off | Reads height above launch, not AGL over sloping ground |
Memory Aid: "High to low (or hot to cold), look out below." Flying toward lower pressure or colder air without resetting the datum leaves you lower than the instrument claims — the direction of error that hits terrain and obstacles.
3. The Airspeed Indicator
Principle of Operation
An airspeed indicator measures the difference between ram (pitot) pressure at a forward-facing tube and static pressure at a flush port. That differential is dynamic pressure, which the instrument displays as indicated airspeed (IAS). True airspeed (TAS) is IAS corrected for density: as density altitude rises, TAS exceeds IAS.
Errors and Malfunctions
- Blocked pitot, open static: the trapped ram pressure no longer changes with speed, and the ASI behaves like an altimeter — reading higher in a climb and lower in a descent regardless of actual speed.
- Blocked static, open pitot: the ASI under-reads in a climb and over-reads in a descent.
- Water or ice in the lines: intermittent, jumpy or zero indications; a pitot heater exists for exactly this reason on larger aircraft.
- Insects and covers: a mud-dauber nest or a forgotten pitot cover is the classic pre-flight catch.
The Small-RPA Reality
Most small multirotors have no airspeed sensor at all. They report groundspeed from GNSS and estimate wind by comparing commanded attitude with achieved track. That gap matters:
The Hover Trap: A multirotor holding a stationary hover in a 20 km/h wind shows 0 km/h groundspeed but is flying at 20 km/h airspeed. Aerodynamic forces, motor current and battery drain all depend on airspeed, not groundspeed. A downwind leg looks fast and cheap; the upwind return is where the battery disappears — and if the wind aloft exceeds the aircraft's maximum speed, the aircraft cannot come home at all.
4. The Inertial Measurement Unit (IMU)
Components
A MEMS IMU combines:
- Three-axis gyroscopes measuring angular rate about pitch, roll and yaw;
- Three-axis accelerometers measuring linear acceleration, including the constant 1 g of gravity that establishes which way is down.
Add the three-axis magnetometer and the package is often marketed as a nine-axis unit. The flight controller fuses IMU data with the barometer and GNSS to produce a single attitude and position estimate.
Errors and Malfunctions
| Error | Cause | Symptom |
|---|---|---|
| Bias drift | MEMS bias changes with temperature | Slow attitude drift; worse if launched immediately in extreme cold |
| Vibration saturation (clipping) | An unbalanced or chipped propeller, a loose motor mount | Wobble, uncommanded altitude loss, erratic behaviour |
| Bad initialization | Powered up on a slope, on a moving boat or vehicle, or while being carried | The controller adopts a false "level"; the aircraft leans on take-off |
| Thermal shock | Warm aircraft taken straight into a cold field | Attitude estimate drifts until the unit stabilizes |
Discipline: power up on level, stationary, solid ground, allow the warm-up and sensor initialization to complete before arming, and treat any propeller nick or motor-mount looseness as an IMU problem as well as a propulsion problem.
Integrated Failure Matrix
| Instrument | Primary Failure Signature | Immediate Pilot Action |
|---|---|---|
| Magnetometer | Expanding spiral ("toilet bowling"); heading and track disagree | Switch to ATTI mode and fly home manually; do not engage RTH |
| Altimeter | Altitude jumps or freezes with no real climb | Fly visually; reduce altitude; land and re-datum |
| Airspeed / groundspeed estimate | Aircraft hovers but drains battery fast; slow upwind progress | Turn back early; budget the upwind leg first |
| IMU | Wobble, oscillation, uncommanded pitch or roll | Land immediately; inspect propellers and motor mounts; recalibrate on level ground |
What is magnetic variation, and how does it differ from deviation?
A pilot launches from the edge of a plateau, zeroes the barometric altimeter at the launch point, and flies out over a valley 300 feet lower while the telemetry shows 250 feet. What is the true altitude above the ground beneath the aircraft, and why does this matter?
An airspeed indicator's pitot tube becomes completely blocked by ice while the static port remains clear. How will the instrument behave?
A multirotor is holding a stationary hover over a fixed ground point while a steady 20 km/h wind blows. What are its groundspeed and airspeed, and what is the operational consequence?