Gyroscopic Instruments

The two gyroscopic properties

• Rigidity in space — a spinning gyro tends to maintain its orientation regardless of how the case around it moves. The AI and HI use this — the gyro stays put while the airframe rolls, pitches, or yaws around it.
• Precession — a force applied to a spinning gyro produces a reaction 90° later in the direction of rotation. The turn coordinator uses precession by design — it's mounted at a 30° angle so both yaw and roll cause it to precess the wing display.

Power sources — and why the split matters

Gyros are powered by either:

• Vacuum / pneumatic — engine-driven vacuum pump pulls air across the gyro vanes. Drives the AI and HI on most older helicopters. Vacuum failure = AI and HI fail together.
• Electric — drives the turn coordinator on most aircraft. Survives vacuum failures, and vice versa.

Why split? Redundancy. A vacuum failure (pump fails, hose blows) leaves you with the turn coordinator and pitot-static instruments — enough to fly partial panel. An electric failure leaves the AI and HI working. Either failure mode preserves a usable instrument set.

Modern glass cockpits often use electric solid-state attitude sensors (no mechanical gyros) with battery backup. The redundancy story changes — read the POH for your specific aircraft.

Attitude Indicator (AI)

• Displays pitch and bank using a horizon bar that remains fixed in space (rigidity) while the aircraft moves around it.
• The gyro spins on a vertical axis; gimbals allow the case to move while the gyro stays level.
• Errors: small acceleration/deceleration errors at very high G; precession over long sustained turns; topple if pitch or bank exceeds gyro limits (~60° pitch / 100° bank for older mechanical gyros — modern AIs are non-tumbling).
• The most critical IFR instrument — and the most distracting when failed. Cover or omit it from the scan if it fails.

Turn Coordinator (TC) / Turn-and-Slip

• The gyro is mounted at a 30° angle, so it senses both yaw and roll — the indication leads the AI's bank by a fraction of a second.
• The "L" / "R" wings show direction and rate of turn. The lower symbols indicate standard rate (3°/sec, 360° in 2 minutes) when the wing aligns with the index mark.
• The inclinometer ball below shows coordination (slip / skid). "Step on the ball" — apply rudder/anti-torque pedal toward whichever side the ball is leaning.
• Electric — survives vacuum failures. The instrument flag drops when power fails.

Heading Indicator (HI) / Directional Gyro (DG)

• A free-spinning gyro that maintains its orientation as the aircraft yaws around it.
• Drifts due to friction and earth's rotation — must be cross-checked against the magnetic compass every 15 minutes in level, unaccelerated flight.
• No turn errors, no acceleration errors — drift is its only weakness.

Horizontal Situation Indicator (HSI)

Combines a heading indicator with a CDI (course deviation indicator). One instrument shows aircraft heading, course set, course deviation, and to/from in a single view.

• Eliminates reverse sensing on back-course localizers (the HSI rotates the CDI to match the OBS-set course, so it always reads the right way).
• The most operationally useful upgrade from a basic six-pack — modern glass cockpits effectively are HSIs by default.

Partial panel — what you have left

If the AI fails (most likely from vacuum failure), the remaining instruments give you bank and pitch information:

• Bank — turn coordinator (direct), heading indicator (rate of yaw), magnetic compass (cross-check).
• Pitch — VSI (rate of climb/descent), altimeter (trend), airspeed indicator (pitch effect on speed).

Cover the failed AI to remove the visual distraction. Use the remaining instruments deliberately — primary/supporting scan technique. See IFR Scan for the full methodology.