Gyroscopic Instruments

How gyroscopes work

Definition

A fast-spinning rotor has two useful properties. Rigidity in space: it holds its orientation, giving a steady reference no matter how the airplane moves around it. Precession: when a force is applied to a spinning gyro, the effect shows up 90 degrees around in the direction of spin — and comes out in the same direction as the force you applied.

Explanation

Rigidity is what the attitude and heading indicators use — the gyro stays put while the airplane turns and pitches around it. Precession is what the turn coordinator uses — a turn feeds a force into the gyro, and the gyro's 90-degrees-later reaction is what deflects the needle to show your rate. Understanding these two properties is the key to every gyro instrument and every gyro failure.

Figure 8-18. Regardless of the position of its base, a gyro tends to remain rigid in space, with its axis of rotation pointed in a constant direction. — Figure 8-18 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 217)

Why it matters

Every gyro instrument and every gyro failure traces back to these two properties. Get rigidity and precession, and the rest of this lesson follows.

A DPE might ask

What are the two properties of a gyroscope?
Which instruments use rigidity, and which uses precession?

PHAK ch. 8 — Gyroscopic principles

The attitude indicator

Definition

The attitude indicator — the artificial horizon — uses rigidity. Its gyro holds level while the airplane moves around it, so the instrument shows your pitch and bank directly against a horizon line, with a fixed miniature airplane you fly to the horizon. It's the most direct picture of what the airplane is doing, and it's typically vacuum-driven.

Explanation

Read it like a window: blue is sky, brown is ground, and the miniature airplane's relationship to the horizon is your attitude. Bank is shown by the pointer against the scale at the top; pitch by the airplane above or below the horizon. Modern attitude indicators have small, well-corrected acceleration and turn errors you rarely notice.

Figure 8-23. Attitude indicator. — Figure 8-23 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 220)

Why it matters

The attitude indicator is your primary reference in reduced visibility. Reading it instantly — and knowing it's usually on vacuum — is fundamental instrument airmanship.

A DPE might ask

What does the attitude indicator show, and what powers it?
What gyroscopic property does it use?

PHAK ch. 8 — Attitude indicator

The heading indicator

Definition

The heading indicator (directional gyro) uses rigidity to give you a stable heading reference — steadier than the magnetic compass, which swings and lurches in turns and turbulence. You read your heading at the top lubber line. But the gyro slowly precesses (drifts) from bearing friction and the Earth's rotation, so it must be reset to the magnetic compass.

Explanation

Reset the heading indicator to the magnetic compass about every 15 minutes, and only in straight-and-level, unaccelerated flight — that's when the compass reads true. Between resets you fly the steady gyro; you just don't trust it to stay perfect on its own. It's typically vacuum-driven, so a vacuum failure takes it down along with the attitude indicator.

Figure 8-25. A heading indicator displays headings based on a 360° azimuth, with the final zero omitted. For example, “6” represents 060°, while “21” indicates 210°. The adjustment knob is used to align the heading indicator with the magnetic compass. — Figure 8-25 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 222)

Why it matters

An un-reset heading indicator can be off by ten or twenty degrees without you noticing — a real navigation error. The reset habit is basic discipline a DPE will check.

Common error

A common mistake is trusting the heading indicator indefinitely, or resetting it in a turn. It drifts over time, and the compass only reads correctly when you're straight, level, and unaccelerated — so reset the gyro to the compass under those conditions, roughly every 15 minutes.

You should reset the heading indicator to the magnetic compass:

A DPE might ask

Why and when do you reset the heading indicator?
Why is the heading indicator steadier than the compass?

PHAK ch. 8 — Heading indicator

The turn coordinator

Definition

The turn coordinator uses precession to show your rate of turn. Bank into a turn and the gyro's reaction deflects the little airplane; line its wing up with the index mark and you're in a standard-rate turn — 3 degrees per second, a full 360 in two minutes. It shows turn rate, not pitch. Below it sits the inclinometer — the ball — which shows whether the turn is coordinated.

Explanation

Two instruments in one. The airplane shows rate (how fast you're turning). The ball shows coordination (whether your rudder and bank are balanced): centered is coordinated; off to one side means you're slipping or skidding — 'step on the ball' (add rudder toward the side the ball is on) to center it. The turn coordinator is typically electric, so it keeps working if the vacuum system fails.

Figure 8-21. Turn indicators rely on controlled precession for their operation. — Figure 8-21 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 219)

Why it matters

In a vacuum failure the turn coordinator is often the gyro you have left — the backbone of partial-panel flying. And the ball is your everyday coordination check on every turn.

A DPE might ask

What does the turn coordinator show, and what does the ball show?
What is a standard-rate turn?

PHAK ch. 8 — Turn coordinator & inclinometer

Power sources & failures

Definition

Gyros only work while they're spinning, so they need power — either engine vacuum (a pump pulls air across the gyro) or electricity. The usual split puts the attitude and heading indicators on vacuum and the turn coordinator on electric, so a single failure can't take all three. The dangerous case is the insidious vacuum failure: the pump quits and the attitude and heading indicators spin down slowly, giving subtly wrong readings before they obviously fail.

Explanation

Catch it early: check the suction gauge (a low or zero reading means the pump), and cross-check your gyros against the others — if the attitude indicator disagrees with the turn coordinator, altimeter, and airspeed, suspect vacuum. Then fly partial panel: use the electric turn coordinator plus airspeed, altimeter, and the magnetic compass to keep control while you get to better conditions. The full partial-panel technique is the next lessons' focus.

Figure 8-20. Typical vacuum system. — Figure 8-20 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 219)

Why it matters

A slow vacuum failure has killed pilots who kept trusting a dying attitude indicator. Knowing the power split, watching the suction gauge, and cross-checking is exactly the airmanship that prevents it.

Common error

The deadly mistake is trusting the attitude indicator as it slowly fails. A dying vacuum gyro doesn't snap to a flag — it drifts, so a wings-level airplane can show a subtle bank and lead you into a spiral. Cross-check: if the attitude indicator disagrees with the turn coordinator, ball, altimeter, and airspeed, believe the others and fly partial panel.

Your vacuum pump fails in flight. Which instruments are affected, and which do you fly?

A DPE might ask

How are your gyros powered, and why split them?
How would you recognize and handle a vacuum failure?

PHAK ch. 8 — Gyro power sources & failures

Wrap

Definition

Three gyro instruments, two properties. The attitude and heading indicators use rigidity for stable references; the turn coordinator uses precession to sense your turn rate, with the ball for coordination. Reset the heading indicator to the compass, cross-check the attitude indicator, and know which instruments a vacuum failure leaves you.

Explanation

It all comes back to how each gyro is powered and what property it uses: that tells you both its normal job and its failure mode. Reset the heading indicator, keep the ball centered, watch the suction gauge, and be ready to fly partial panel on the electric turn coordinator when the vacuum quits.

Instrument	Principle · power · if vacuum fails
Attitude indicator	Rigidity · shows pitch & bank · usually vacuum — fails with the pump.
Heading indicator	Rigidity · stable heading, but drifts — reset to the compass ~every 15 min · usually vacuum — fails with the pump.
Turn coordinator	Precession · turn rate + the ball for coordination · usually electric — survives a vacuum failure.
Vacuum failure Watch	Insidious — attitude & heading spin down slowly. Check the suction gauge, cross-check, fly partial panel.

Why it matters

These are the instruments you fly by when you can't see outside. Knowing how they work, how they're powered, and how they fail is the core of instrument airmanship the rest of Module 4 builds on.

A DPE might ask

Summarize the principle, power, and failure of each gyro instrument.
What survives a vacuum failure, and how do you fly it?

PHAK ch. 8 — Gyroscopic instruments summary