12.2 Gyroscopic Instruments, Vacuum Failure, and Magnetic Compass Errors

ACS PA.I.G.K1h pairs the pitot-static set with vacuum/pressure systems and their flight instruments. PA.VIII then asks you to fly those instruments when the windshield is not the truth. Two physical laws, three gyro gauges, one wet compass, and a suction gauge are the whole unit.

Rigidity in space and precession

A spinning rotor wants to stay pointed at the same place in space. That is rigidity in space. It is why an attitude indicator can hold a tiny airplane against a horizon bar while the airframe banks and pitches around it, and why a heading indicator can hold a card after you set it to the compass.

Apply a force to a spinning gyro and the gyro does not yield in the direction you pushed. The result appears 90° later in the direction of rotation. That is precession. The turn coordinator and the older turn-and-slip indicator are precession instruments: yaw (and, on a turn coordinator, roll) tilts the gyro, and the needle or miniature airplane is the 90° result.

Rigidity holds a reference. Precession both senses rate and, over time, drifts a rigidity instrument. Heading indicators precess and must be reset. Attitude indicators have erection mechanisms that slowly fight precession and acceleration errors.

Who is usually vacuum, who is usually electric

In the classic single-engine trainer PHAK describes:

That split is typical, not a certification law. Some airplanes put the attitude gyro on electricity, some use a pressure pump instead of suction, and glass panels replace both with an AHRS. For PAR steam-gauge stems, treat AI + HI = vacuum and turn coordinator = electric unless the question gives you a different POH.

The suction gauge (often about 4.5 to 5.5 inches of mercury when healthy — use the green arc on that airplane) is how you know the pump is feeding the vacuum gyros. Low suction is the first failure flag, not a slowly lying horizon.

Turn coordinator versus turn-and-slip, and 3° per second

A turn coordinator mounts the gyro canted. It therefore senses roll rate as you enter the turn and yaw rate once the bank is established. The display is a miniature airplane. Tick marks are usually standard rate (and sometimes half-standard). The ball is not a gyro; it is an inclinometer. Step on the ball. Coordinated flight is the ball centered, not the miniature airplane level.

A turn-and-slip (turn-and-bank) indicator mounts the gyro vertically. It shows rate of turn only. It does not show roll rate, so it is slower to come alive when you first bank. The needle is not a tiny airplane. Same ball.

Standard rate is 3° per second — a two-minute 360°, a one-minute 180°. That is the PA.VIII number. A rough bank-angle rule of thumb is about 15 percent of TAS, or TAS ÷ 10 + 5, in degrees; the knowledge test wants the 3°/sec definition more than a private formula. Do not call a 30° bank “standard rate” at every speed.

Vacuum failure versus electrical failure

Vacuum-pump failure (belt, pump, line, or filter):

• Suction gauge falls out of the green.
• The typical attitude indicator becomes unreliable and may slowly topple or sit at a false bank/pitch.
• The typical heading indicator freezes or drifts without a usable reference.
• The electric turn coordinator still works.
• Pitot-static instruments still work.
• The magnetic compass still works.

That is partial-panel flying: TC + ball + ASI + altimeter + VSI + compass. Do not chase a dying vacuum AI.

Electrical failure (alternator, then battery):

• The typical turn coordinator fails (it is electric).
• Radios, transponder, and flap motor follow as the battery dies.
• The vacuum attitude indicator and heading indicator keep working as long as the engine-driven pump still has a belt and oil.

The PAR trap is “lost the electrics, so the attitude indicator is dead.” In the airplane the test is picturing, the vacuum AI is the instrument that survives an electrical failure. The opposite failure (vacuum gone, electrics up) is when you lose the horizon bar and keep the miniature airplane.

Reset the heading indicator to the magnetic compass only in straight, unaccelerated flight. Precession of a few degrees in 15 minutes is normal; flying a 10°-wrong HI for an hour is how a cross-country becomes a search.

Magnetic compass: variation, deviation, dip

The wet compass is the only heading instrument that does not need a pump or a bus — and the one that lies the moment you accelerate or turn.

• Variation is the angle between true north and magnetic north. It is a location error, published on isogonic lines. The agonic line is where variation is zero. East variation is subtracted from true to get magnetic (East is least); west is added (West is best). Variation does not care which airplane you sat in.
• Deviation is the compass error caused by this airplane’s magnets — radios, a yoke, a speaker. It is reduced by a compass swing and a compass correction card. Deviation changes with heading.
• Magnetic dip is the Earth’s field lines diving toward the poles. The compass magnets try to follow that dive. Dip is the parent of acceleration error and northerly turning error. It is worst near the poles and least near the equator.

ANDS — Accelerate North, Decelerate South

In the Northern Hemisphere, on a easterly or westerly heading, acceleration and deceleration tilt the compass card through dip:

• Accelerate and the compass indicates a turn toward north.
• Decelerate and the compass indicates a turn toward south.

ANDS. The trap item is “accelerate south.” That is the wrong hemisphere-mnemonic. You are not actually turning. The airplane is still on 090 or 270; the card is lying. On north or south headings, acceleration error is essentially nil. In the Southern Hemisphere the sense reverses; PAR stems are Northern Hemisphere unless they say otherwise.

UNOS — Undershoot North, Overshoot South

Northerly turning error is also a dip product, and it is largest when you turn through north or south.

PHAK: if you are on a northerly heading and start a turn toward east or west, the compass lags — it may even indicate a turn in the opposite direction at first. If you are on a southerly heading and start a turn toward east or west, the compass leads (runs ahead of the real heading). On east or west headings the lead/lag from dip is at a minimum as you start the turn.

The rollout mnemonic is UNOS:

• Turning to a north heading: undershoot — start the rollout before the compass reaches the desired northerly heading, because the card is lagging.
• Turning to a south heading: overshoot — wait until the compass has passed the desired southerly heading, because the card is leading.

A practical Northern Hemisphere rule of thumb is to lead or lag by roughly the latitude; do not treat that as a published AFM number. What PAR wants is the direction: lag on north, lead on south, undershoot north, overshoot south.

Oscillation is the card swinging in turbulence. Compass lag in casual speech is the pile-up of dip, fluid damping, and your own impatience. None of these errors go away because you bought a fancy heading indicator; they go away when you read the compass in straight, unaccelerated, reasonably level flight and then transfer that heading to the HI.

Scenario: Luis loses suction at night

Luis is in a vacuum-AI / electric-TC 172. The suction gauge slides out of the green. The horizon bar begins a slow, uncommanded right bank. He does not pull the airplane into that bank. He covers the AI, flies turn coordinator, ball, and pitot-static, and uses the magnetic compass only in straight, unaccelerated segments to keep the heading indicator honest — except the heading indicator is vacuum too, so tonight the compass is the heading instrument, in wings-level cruise only. Ten minutes later the alternator light is a different airplane: if he had lost the bus instead of the pump, the miniature airplane would have died and the vacuum attitude indicator would still have been the truth.

Unusual attitudes — recognition and recovery on instruments

ACS Task VIII.E covers unusual attitudes: their prevention, and the procedure for recovery in flight. For a VFR private pilot this is survival material, because the usual entry is inadvertent flight into instrument conditions followed by the spatial disorientation described in Section 17.2.

Prevention comes first in the ACS wording: causal factors are flight-related (distraction, task saturation, an unnoticed trim change), physiological (the somatogravic and somatogyral illusions), and environmental (loss of the horizon in haze, at night, or in cloud), plus system and equipment failures — a toppled attitude indicator after a vacuum failure is a classic unusual-attitude generator.

Recognition is by instrument cross-check, not by feel. Read the airspeed and the altimeter/VSI trend together:

The order matters and is a favorite test point. In the nose-low case you level the wings before pulling, because pulling in a bank tightens the spiral and multiplies load factor (Section 10.1). In the nose-high case you unload before rolling, because rolling near the critical angle of attack invites a spin (Section 10.3).

Recover by reference to the primary instruments, and be skeptical of the attitude indicator itself until you have confirmed it against the airspeed, altimeter, VSI, and turn coordinator — if the gyro failed, it is the instrument that put you there.