Gyroscopic instruments

Two gyro properties provide different information

A mechanical gyro contains a rapidly spinning rotor. Rigidity in space is its tendency to keep its spin-axis orientation when the instrument case and aircraft move around it. Gimbals let an attitude or heading instrument compare aircraft motion with that reference. The instrument must still be powered, properly aligned, and within its limitations.

Precession is the response of a spinning gyro to an applied torque: its axis moves in a direction determined by the rotor's rotation and the applied force. Rate instruments deliberately use that response, restrained and calibrated by springs, to indicate turning. In an attitude or heading gyro, unwanted torque from friction can cause drift; an erection or correction system is needed to maintain the intended reference.

Do not confuse rigidity with an error-free instrument. A slowly spinning rotor loses the behavior on which the instrument depends. A failing gyro can move smoothly and look credible before it becomes clearly unusable. Solid-state AHRS systems supply similar flight information using different sensors and processing; their dependencies are taught in the glass-cockpit lesson.

IFH 5-16–5-21: gyroscopic systems, rigidity, precession and rate instruments

Attitude, heading, turn rate and power answer different questions

The attitude indicator displays aircraft pitch and bank relative to its reference horizon. Read both the pitch markings and the bank index; learn the presentation used by the installed instrument. The small airplane and horizon are reference symbols, not an outside view or proof of climb performance.

An unslaved heading indicator provides a stable directional reference after the pilot sets it. It does not seek magnetic north. A slaved heading system receives a magnetic reference and corrects the indication automatically under specified conditions; it still has failure modes and limitations.

A turn instrument indicates direction and rate of turn, and a turn coordinator also responds to roll rate. It does not give the same pitch and bank-angle information as an attitude indicator. Power comes from the appropriate engine instruments, such as tachometer or manifold-pressure indications; it is not measured by the gyro. Compare attitude, power, and resulting altitude, airspeed, heading and turn trends.

For example, a bank indication should be consistent with developing turn rate and changing heading during an ordinary coordinated turn. If the attitude instrument slowly banks while independent turn and heading evidence stay stable, investigate the disagreement while maintaining control. Do not create a turn simply to center a suspect horizon.

IFH 5-19–5-21: attitude, heading and turn instruments · IFH 6-2: interpretation of pitch, bank and power instruments

Read the miniature airplane as a rate indication

A turn-and-slip indicator senses yaw rate. A turn coordinator's canted gyro responds to both roll and yaw: as the airplane rolls into a turn, its first movement includes roll rate; once the bank is stabilized, it indicates turn rate. The banked miniature airplane is not a calibrated bank-angle display. Many instruments explicitly say “no pitch information.”

On a conventional instrument marked 2 MIN, the specified turn-rate index corresponds to a standard-rate turn of 3° per second, which takes two minutes for 360°. Check the actual markings and manual; other rate scales exist. The bank angle needed for that rate changes with airspeed, so an index deflection does not establish a fixed bank angle.

The ball in the inclinometer is a separate, gravity/inertia device. It responds to lateral acceleration and helps assess coordination. A centered ball does not prove wings-level flight: it can be centered in a coordinated turn. An off-center ball is not a heading indicator. In ground preparation, identify which part is gyro-driven, which part is the inclinometer, and what each can and cannot tell you.

Common error: substituting the turn coordinator's miniature airplane for the attitude indicator and treating it as pitch or exact bank information. Use each indication for the information it actually measures.

IFH 5-20–5-22: turn-and-slip indicator, turn coordinator and inclinometer

Map the power sources before trusting redundancy

Mechanical gyros may be driven by air flow from a vacuum or pressure system, by electric motors, or by another approved arrangement. In a familiar conventional panel, attitude and heading gyros often share one engine-driven vacuum pump while the turn coordinator is electrical. That is an example, not a rule for all airplanes.

Consult the actual AFM/POH, equipment list and approved supplements. Identify the instruments on each pneumatic source and electrical bus, the meaning and acceptable range of each system gauge or warning, and the standby supply. An electric standby attitude indicator may depend on a separate battery, the aircraft bus, or both. A second instrument is only useful redundancy to the extent its sensor, power and supporting systems remain available.

Low suction or pressure can cause shared gyros to slow and wander progressively. A normal suction gauge does not prove that an individual gyro is sound. Similarly, an electrical power flag identifies a power condition; the absence of a flag is not proof of correct attitude. Include source-status indications in the cross-check without letting troubleshooting displace aircraft control.

IFH 5-16–5-19: pneumatic and electrical power sources · IFH 11-7–11-8: progressive pneumatic failure

Recognize alignment, erection and operating limits

At startup, a mechanical attitude gyro needs time and adequate power to erect and settle. Check its behavior under the aircraft's procedures before using it as a reliable reference. Adjusting the miniature airplane changes the reference presentation; it does not repair an incorrect gyro attitude. A caging control, if installed, must be used only as directed.

Mechanical erection systems can produce small transient attitude errors during rapid acceleration, deceleration or after turns. Older gyros can tumble if their pitch or roll limits are exceeded; other designs have different limits. Do not apply a universal angle or recovery time. Know the instrument's published limitations and compare its indication with other reliable information.

Heading gyros drift from real precession and apparent motion as the Earth rotates. Drift rate depends on the design and conditions; a single numerical Earth-rotation value is not a universal heading-error rate. Unusual or rapid drift, sluggish response, or continuing disagreement needs investigation rather than repeated adjustment that hides the defect.

Normal instrument characteristics have an expected size and recovery behavior. A steadily worsening attitude disagreement accompanied by loss of suction is not simply an erection error to be ignored. Use the source-status evidence and the remaining reliable instruments.

IFH 5-19–5-20: attitude erection, errors, tumble limits and heading drift · PHAK Chapter 8: gyroscopic principles, attitude indicator and heading indicator

Set a heading reference only from a valid comparison

Compare an unslaved heading indicator with the magnetic compass after startup and periodically in flight under the applicable procedures. Make the airborne comparison in stabilized, straight, level, unaccelerated flight. Allow the compass to settle and account for the aircraft's deviation information; compass turning and acceleration errors can otherwise make a serviceable gyro look wrong.

The IFH describes periodic heading comparisons, commonly about every 15 minutes for conventional unslaved instruments. Treat that as a reminder to establish a recurring check, not permission to wait when a discrepancy appears or a replacement for the installed procedures. Slaved systems may need different checks and correction methods.

A GPS track is the path over the ground, while heading is the direction the nose points. Crosswind can make them differ with both instruments working correctly. Do not reset a heading indicator to ground track just to make the two numbers match. The compass lesson develops variation, deviation and dynamic compass errors.

IFH 5-20: heading indicator checks · IFH 5-13–5-14: magnetic compass errors

Use ground checks to learn the evidence

Before instrument training, label the actual panel's attitude, heading and turn instruments, their power sources and warnings. With an instructor, use the published startup, taxi and run-up checks to observe expected responses, permissible error and settling. Do not perform aggressive ground maneuvers or disable equipment as an improvised test.

Prepare a short explanation of what would remain available after loss of each source. Include engine power instruments and pressure instruments, not only the gyros. Discuss whether the autopilot or flight director uses the suspect sensor; an apparent spare gyro may still share the input that drives automation.

Three habits prevent common mistakes: compare several kinds of evidence, distinguish normal instrument error from a worsening inconsistency, and verify the dependency before calling an instrument independent. Reading this lesson does not establish the control skill needed to fly with a failed attitude or heading indicator; that requires instructor-led training.

IFH 6-10: instrument cross-check and interpretation · IFH 11-4, 11-7–11-8: checks and system failures

FLIGHT SCENARIO

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