Directional Gyro, Remote Compass and AHRS - ATPL 022 — Instrumentation

The module's closing lesson performs the marriage the magnetism module promised: the directional gyro, steady but forgetful, is wedded to the flux valve, accurate but jittery, and the remote-reading compass is the result. The lesson ends with the box that made the whole spinning family historic: the solid-state AHRS.

The directional gyro

The directional gyroscope exists to hold a heading reference that no manoeuvre can disturb: a two-degrees-of-freedom gyro with its spin axis horizontal, driving a compass card or needle. Unlike the magnetic compass it neither dips, swings, nor lies in turns; and unlike the compass it has no idea where north is: it only holds whatever direction it was set to. Two display generations carry it:

Direction indicator (DI) — the plain generation: a card read against a lubber line.
Horizontal situation indicator (HSI) — lays the heading card under a navigation display: course pointer, deviation bar and glideslope index in one face.

Why the directional gyro drifts

Set the gyro on a heading and walk away, and its reading decays for the three reasons the gyro principles lesson built:

Rotation of the Earth — turns the meridians under the space-stable axis: apparent drift, at its fastest near the poles.
Aircraft manoeuvring — adds real drift: gimbal friction and the small forces of turns and accelerations precess the axis away from its setting.
Movement over the Earth's surface — adds transport drift: flying long distances, especially east-west, carries the gyro to places where local north points differently.

The sum is why an unaided directional gyro is a short-memory instrument: excellent over minutes, wrong over an hour.

Realigning it

The unaided gyro therefore leans on the pilot, and the procedure is deliberately plain: in steady, straight and level flight (so the magnetic compass is telling the truth), read the compass heading, press or pull the setting knob to cage the card (locking the gimbals so the card can be turned by hand), rotate it to the compass's value, and release. Repeated every ten to fifteen minutes, the drift never accumulates beyond a few degrees: the manual version of what the remote-reading compass does continuously and automatically.

Cruising straight and level, a pilot flying a simple directional gyro checks it against the magnetic compass: the DG says 095, the compass, steady in the calm air, says 092.

Three degrees in the quarter-hour since the last check is ordinary drift: apparent wander from the turning Earth, whatever real wander the bearings contribute, and any transport effect from the leg flown. The DG has no idea where north is; it only holds what it was last told.
The drill from the lesson: read the compass in steady, straight and level flight, where its turning and acceleration errors sleep, then cage, rotate the card to 092, and release.
The habit is the instrument's operating cost: realign every 10 to 15 minutes, always from steady flight. A remote-reading compass automates exactly this loop, which is why the lesson's slaving system exists at all.

The remote-reading compass

The remote-reading compass system automates that realignment into a closed loop: the flux valve supplies magnetic north continuously, and the gyro supplies steadiness, each covering the other's weakness. The principle: the flux detection unit (the flux valve in its system role) senses the field's direction; a transducer compares the gyro's current heading with the valve's; any disagreement becomes an error signal that the precession amplifier strengthens and feeds to a torque motor, which precesses the gyro toward the valve's heading, a correction called slaving (the slave position of the mode switch), done slowly on purpose so the valve's manoeuvre jitter is averaged away while its long-term truth wins. The annunciator shows whether gyro and valve currently agree, and the display unit carries the compass card, the synchronising and set-heading knob for manual alignment after power-up, and the mode switch (DG/compass, or slave/free) that disconnects the valve, leaving a plain directional gyro for polar regions where the magnetic reference fails.

Remote compass versus the bowl compass

The syllabus asks for the trade, point by point:

Design — the remote system needs electrical power and weighs and occupies more, against the bowl compass's self-contained simplicity: the one comparison the bowl wins. Everywhere else the remote system takes the column.
Deviation — smaller (wingtip or fin sensor) and compensated electrically.
Turning and acceleration errors — largely absent, the gyro riding through what fools the pendulous card.
Attitude errors — similarly small, the pendulous element's transients smoothed by the gyro.
Accuracy and stability — of the displayed heading, far higher.
Available to several systems at once — the compass card, the radio magnetic indicator (RMI, a bearing pointer laid over the heading card), and the automatic flight control system all drink the same electrical heading, which no liquid-filled bowl could ever feed them.

The AHRS

The attitude and heading reference system (AHRS) retires the spinning rotor altogether: a single solid-state unit with no moving parts, replacing the classic gyros. Inside sit three families of sensors:

solid-state accelerometers.
solid-state rate-sensor gyroscopes — laser or vibrating-structure devices that measure rotation rate without a wheel.
solid-state magnetometers — measuring the Earth's magnetic field: the flux valve's job done by electronics.

The unit senses rotation and acceleration about and along all three axes and the direction of the magnetic field, and computes from them the whole classic panel at once: attitude, heading, turn rate. The indications go where modern indications go: to the electronic flight instrument system (EFIS) screens, where one AHRS feeds the attitude sphere, the heading display and everything between — the three gyro instruments of this module, reborn as software.

What the exam asks

The drift causes and the remote-compass component chain are the bank's favourites here. The overview, following the learning objectives:

Explain the directional gyro's purpose and identify the DI and HSI

Explain the drift from Earth rotation, manoeuvring, and movement over the surface

Describe the pilot's alignment procedure

Describe the remote-reading compass principle and explain each block-diagram component

State its advantages and disadvantages versus the direct-reading compass

Explain the AHRS as a solid-state replacement and what it senses and displays