The Compass and the Flux Valve - ATPL 022 — Instrumentation

Two instruments read the Earth's field aboard:

direct-reading magnetic compass — the simple standby that needs no power and lies during every manoeuvre.
flux valve — the electrical sensor that feeds the aircraft's heading systems while quietly avoiding most of the compass's vices.

This lesson covers both, including the technique that works around the compass's turning errors and the pre-flight check that decides whether to trust it at all.

The direct-reading compass

The purpose of the direct-reading compass is deliberately humble: an independent, self-contained magnetic heading reference that works with every electrical system dead, which is why certification keeps one on the flight deck long after everything else has gone digital. Its construction serves that independence: a magnet assembly carrying the compass card hangs pendulously below a pivot in a bowl of liquid, the weight hanging low so the assembly stays roughly level against the dip's pull, and the liquid damping the oscillation so the card settles quickly. The pilot reads the card against the lubber line, a reference mark on the bowl aligned with the aircraft's nose: the card stays put, holding north, while the aircraft, and the lubber line with it, turns around the card.

When the compass lies

That pendulous hang is also the instrument's weakness, because a weight hanging below its pivot responds to every acceleration, not just gravity, and the dip-tilted magnets turn each disturbance into a false heading. The compass indicates correctly only in straight, level, unaccelerated flight; three manoeuvres corrupt the indication:

Accelerating or decelerating — the hanging assembly swings fore or aft, and the tilted magnet system registers a phantom turn.
Turning — the assembly banks with the manoeuvre's apparent gravity (the combined pull of gravity and the turn) while the vertical component of the Earth's field keeps pulling on it: passing through some headings the card lags the real turn, through others it races ahead, so rolling out exactly on the card's say-so puts the aircraft on a wrong heading.
Pitching up or down — tilts the pendulum along the aircraft's axis, stirring the same false indications whenever the attitude is changing.

The pattern behind all three: the errors are worst where the dip is strong (high magnetic latitudes) and vanish only where the field is horizontal.

The timed turn

The escape from the turning errors is not to read the compass while turning at all. In a timed turn the pilot notes the heading change required, turns at a known rate, and holds the turn for the computed time by the clock, rolling out on time rather than on the misbehaving card: once wings-level and settled, the compass tells the truth again and confirms the result. The standard rate is the rate-1 turn of the steady-turn lesson: three degrees per second, a full circle in two minutes.

The serviceability check

Before flight the compass earns its trust in two steps:

Physical check — the instrument is intact and readable, the liquid free of bubbles and discolouration, the card sitting level and rotating freely without sticking when the aircraft turns during taxi.
Comparison check — the indication is compared, the course's cross-checking habit again, against a known direction (another compass or heading system aboard, or the runway's published direction when lined up), and the residual disagreement must sit within the deviation card's few degrees.

A compass that fails either check is not used as a heading reference, and the defect is recorded before flight.

The flux valve

The flux valve exists to give heading to everything the pilot does not read directly: its purpose is to sense the direction of the Earth's magnetic field electrically, as a signal rather than a needle, so that remote systems can use it. The principle turns a transformer trick into a compass: a soft-iron core of three spokes is driven in and out of magnetic saturation (the state in which the core can carry no more magnetic flux) by an excitation coil fed with alternating current. The gating is the trick: each time a spoke saturates it stops admitting the Earth's flux, and as it comes out of saturation that flux floods back in, so the Earth's steady field is chopped into a changing one, and a changing flux is what induces a voltage. The repeatedly opened and closed flux gate is why the device is called a valve. The Earth's field, threading through the spokes, biases how each spoke saturates, and the pick-off coil on each spoke reads a voltage that depends on how that spoke lies relative to the field. Three spokes at 120 degrees give three such voltages, and together they fix the field's direction unambiguously. The element hangs pendulously in its case to stay near-level, but it is fixed in azimuth: nothing swings toward north; the electronics compute north from the three voltages.

Where it lives, and who uses it

Location is half the design: flux valves mount in the wingtips or the fin, the aircraft's magnetically quietest corners, as far as possible from the engines, wiring looms and steel mass that write deviation into any sensor near them. The classic application is the remote-reading compass system, where the valve's signal steadies a directional gyro into a heading indicator: the marriage the next module's gyro lessons perform in full. Two consequences of the electrical architecture close the syllabus's list. First, the deviation that remains at the valve's location is compensated electrically in the system, corrected once and applied continuously, which eliminates the need for a deviation correction card for the systems it feeds. Second, though the pendulous element feels manoeuvres too, the flux valve does not suffer the same magnitude of errors as the direct-reading compass when turning, accelerating or pitching: there is no freely swinging magnet system to mislead, the transient errors are smaller, and the downstream gyro smooths what remains. That is why the systems the crew actually steers by are built on the valve, and the bowl compass is the standby.

What the exam asks

The manoeuvre-error catalogue and the rate-1 timing calculation are the bank's favourites here. The overview, following the learning objectives:

Explain the purpose of the direct-reading compass

Describe why it indicates correctly only in straight, level, unaccelerated flight, and the manoeuvre errors

Explain timed turns and calculate a rate-1 turn's duration

Describe the pre-flight serviceability check

Explain the flux valve's purpose and operating principle

Indicate its locations, its remote-compass application, the electrical deviation compensation, and its smaller manoeuvre errors