A pilot writes up two instruments after a flight: the attitude indicator and the heading indicator were slow and wandering by the end of it. The turn coordinator worked the whole time. The shop quote lists two overhauled gyro instruments.
The two instruments that wandered may share one power source, and a technician who checks that source first fixes one fault instead of replacing two good instruments.
Gyroscope (gyro)
A wheel, called a rotor, that spins at high speed inside a set of rings. While it spins it holds its position in space. Gyro is the short name, and the instruments built around one are the gyro instruments.
Write-up
A pilot’s written description of a problem with the airplane, given to the shop.
Overhaul
Taking an instrument apart, inspecting and repairing it, and testing it to its standards before it goes back in service.
Learning Objectives
When you finish this lesson you can do the four things below. The check at the end asks about each one.
1ExplainExplain rigidity in space and precession in plain words.
2DescribeDescribe how the attitude indicator, the heading indicator and the turn coordinator use those properties, and what each one shows.
3IdentifyIdentify the parts of the vacuum system and the electric power source for gyros, and state the normal suction range.
4PredictPredict which gyro instruments a vacuum failure or an electrical failure will take out.
The Gyro Instruments on the Panel
The three gyro instruments in the basic T: the attitude indicator top center, the heading indicator bottom center, the turn coordinator bottom left.
Lecture Video
Watch the lecture. Continue appears when it ends. You can skip it for now and reopen it from this page later.
Two Instrument Families
In the Pitot-Static lesson you learned that three of the six basic flight instruments read air pressure from the pitot tube and the static ports. Choose those three.
The six basic flight instruments.
Recall
Which three instruments in the basic six run on pitot and static pressure?
The magnetic compass is a seventh instrument: a magnet floating in fluid that needs no power from the airplane and belongs to neither family. The heading indicator is set to agree with it.
The Gyroscope
Any spinning object has gyroscopic properties. A gyroscope is a wheel, or rotor, built and mounted to use them. An instrument gyro has two design features: great weight for its size, and high speed on low-friction bearings.
Panel A
Rotor
The rotor is the wheel, with its mass concentrated around its rim. Bearings, the parts that let a shaft turn with very little friction, let it spin at high speed. The line through its axle is the spin axis.
FIG 1 Gyroscopes, from a rotor alone (A) to a free gyro (D). Source: FAA-H-8083-31B, Figure 10-93.
Four Design Features
These four features make a spinning rotor hard to move off its axis. The next page names that property.
Rigidity in Space
Rigidity in space is the property of a spinning rotor to stay in the plane it is spinning in, with its spin axis pointed in one direction, however its base is moved. The faster it spins, the harder it resists any change. Mounted in gimbals, the rotor holds still while the case and the airplane turn around it.
Drag to turn the view. Roll and pitch the case with the sliders. Then slow the rotor or stop it and move the case again.
Spin axisLevel referenceFixed to the case
The 3D model cannot run on this device. The picture shows the same gyro, and the numbers, sliders and readout still work.
FIG 2 A free gyro in an instrument case, with the rotor spinning flat as it does in an attitude indicator.
Case: level. Spin axis: 0° from vertical. The rotor is at full speed.
Select a number on the model to name the part.
Check your understanding
Why does a gyro rotor spin at high speed?
Precession
Precession is the tilting or turning of a gyro in response to a force. The reaction does not appear at the point where the force is applied. It appears at a point 90 degrees later in the direction the rotor spins. The rate of precession goes up with the force and goes down as the rotor speed goes up.
Tap a numbered point on the rim to push it away from you. Watch where the motion appears. Then change the rotor speed or the push and try again.
Spin directionYour pushWhere it moves
The 3D model cannot run on this device. The picture shows the same gyro, and the numbers and the result still work.
FIG 3 A free gyro on a stand, its rotor standing on edge with the axle level.
Tap a numbered point on the rim to push it.
Precession rate1x
FIG 4 A force applied to a spinning rotor acts as though it came from 90 degrees further around in the direction of spin. The plane of the force, the plane of rotation and the plane of precession are all perpendicular to each other. Source: FAA-H-8083-31B, Figure 10-95.
Used and Unwanted
The turn indicators use precession on purpose. The heading indicator suffers from it: friction in its bearings is a small force that never stops.
The Attitude Indicator
The attitude indicator shows the airplane’s attitude: its pitch, nose up or down, and its bank, wings tilted left or right, against an artificial horizon. Its gyro spins in the horizontal plane and works by rigidity in space. The horizon bar is attached to the gyro’s gimbals and stays level. The miniature airplane is fixed to the case and moves with the airplane.
Case: level. Reference: 0°. The display shows the difference between them, not a picture of the outside horizon.
Predict
An airplane rolls into a bank. What is the attitude indicator actually showing?
Tumble
Many attitude indicators have limits, usually 100 to 110 degrees of bank and 60 to 70 degrees of pitch. Past a limit the gyro tumbles and gives wrong indications until it is realigned, often with a caging knob that locks the gimbals. Many modern attitude indicators do not tumble.
The Heading Indicator
The heading indicator shows the airplane’s heading, the direction its nose points. Its rotor spins in a vertical plane, a compass card is geared to it, and rigidity holds the card steady while the airplane turns around it. It does not seek north. It is set to agree with the magnetic compass.
Two things make it drift, or creep, off its setting. Friction in its bearings precesses the rotor. And the earth turns 15 degrees every hour under a gyro that holds its place in space, so that alone can put the card off by up to 15 degrees an hour. Compare it with the magnetic compass at least every 15 minutes and reset it.
Heading indicator 090Magnetic compass 090
Time flown0:00
Push the knob in and turn it
Both read 090. Fly 15 minutes and watch the heading indicator.
CautionSome drift is normal. How much is acceptable is stated in the maintenance manual for that instrument and airframe. Do not carry a number from one airplane to another.
FIG 5 A vacuum-powered directional gyro, another name for the heading indicator. The rotor in its gimbals turns the compass card through gears. Source: FAA-H-8083-31B, Figure 10-103.
The First Gyroscope and the First Blind Flight
FIG 6 A spinning gyro rotor stays pointed the same way in space wherever its base is carried around the earth. Source: FAA-H-8083-31B, Figure 10-94.
1852
Léon Foucault, French physicist
Foucault spun a heavy wheel in a set of rings to show the earth turning. He named the device a gyroscope, from Greek words for rotation and for viewing.
The wheel held its place in space while the earth, and the room, turned around it, which is what the rotor in an attitude indicator does while the airplane turns around it. Friction slowed his rotor within about 8 to 10 minutes, and the effect faded with it.
Photograph of Léon Foucault by Bertall & Cie., before 1868, public domain, via Wikimedia Commons.
FIG 7 A vacuum-driven attitude indicator in level flight (left) and in a climbing right turn (right). Source: FAA-H-8083-31B, Figure 10-100.
1929
Lieutenant James H. Doolittle, U.S. Army Air Corps
On 24 September 1929, at Mitchel Field, New York, Doolittle took off, flew a set course and landed in a Consolidated NY-2 biplane with his cockpit covered, flying by instruments alone.
A Sperry Gyroscope Company artificial horizon and directional gyro gave him attitude and heading, and a Kollsman sensitive altimeter gave him altitude. They are the ancestors of the attitude indicator and the heading indicator in this lesson.
Photograph of Jimmy Doolittle, U.S. Air Force, public domain, via Wikimedia Commons.
Today
Many airplanes now replace the attitude and heading gyros with an attitude and heading reference system (AHRS), a solid-state unit with no spinning rotor. The EFIS lesson teaches it.
Sources: IEEE Milestone, First Blind Takeoff, Flight and Landing, 1929; Smithsonian National Air and Space Museum, Sperry artificial horizon (A19711850000); “Foucault’s gyroscope,” Wikipedia; FAA-H-8083-25B, page 8-20 (AHRS).
Turn Indicators
The turn indicators use precession on purpose. Their gyro is held by a single gimbal and a spring. When the airplane yaws, which is turning about its vertical axis, the rotor precesses against the spring, and how far it moves against the spring shows the rate of turn, in degrees per second. It does not show the angle of bank.
There are 2 types: the turn-and-slip indicator and the turn coordinator. Both show the direction and rate of turn and the quality of the turn, and both back up the attitude indicator for bank information if it fails.
FIG 8 A turn coordinator. Source: FAA-H-8083-31B, Figure 10-104.
Select a number on the face to read about that part.
The turn coordinator’s gimbal is canted, tilted about 30 degrees, so its gyro senses roll as well as yaw. It shows the rate of roll first, then the rate of turn once the bank is steady.
FIG 9 A turn-and-slip indicator. Source: FAA-H-8083-31B, Figure 10-105.
Select a number on the face to read about that part.
FIG 10 The turn-and-slip gyro’s reaction in a right turn. Source: FAA-H-8083-31B, Figure 10-106.
In a right turn the yaw force acts on the gyro 90 degrees around in its spin direction, so the top of the rotor tilts left against the spring. Linkage moves the needle right, the same way as the turn.
The Inclinometer
The inclinometer is the ball in the curved, liquid-filled glass tube at the bottom of a turn indicator. Gravity and the force of the turn act on it. In a coordinated turn, where the bank and the rate of turn match, the ball stays centered between the two reference lines.
A right turn at standard rate
Coordinated
The bank and the rate of turn match, and the ball stays centered between the reference lines.
RuleStep on the ball. Press the rudder on the side the ball has moved to.
CautionA work order that says the turn indicator is bad can mean the turn coordinator’s gyro, the heading indicator the pilot used to check the turn, or the ball, which has no power and no electrical fault to find. Before any instrument job, find out exactly which instrument and which indication.
Standard-Rate Turns
A standard-rate turn is a turn of 3 degrees per second. Because a turn indicator shows a rate, you can work out how long a turn takes: divide the degrees of turn by the rate.
Time in seconds = degrees of turn ÷ degrees per second
Worked example
A full 360 degree turn at standard rate.
360 ÷ 3 = 120 seconds
120 ÷ 60 = 2 minutes
That is why the face says 2 MIN.
Your turn
How many seconds does a 90 degree standard-rate turn take?
Heading 360Turn coordinator
Stopwatch0 s
Fly a 90 degree right turn at standard rate, in real time, from 360 to 090.
Your turn
A 4-minute turn indicator’s mark means 1.5 degrees per second. How many seconds does a full 360 degree turn take at that rate?
Gyro Power Sources
Every gyro needs its rotor spinning, and the power comes from one of two sources. A vacuum system pulls air through the instrument and against vanes, or buckets, cut in the rotor’s rim, the way water turns a waterwheel. An electric gyro has an electric motor whose armature, the turning part of the motor, is the rotor.
Inches of Mercury (inHg)
The unit of the suction gauge. Suction, or vacuum, is how far the pressure in the system is below the outside air pressure.
FAA-H-8083-25B, Pilot’s Handbook of Aeronautical Knowledge
Chapter 8, Sources of Power
Select a highlighted phrase to read what it means.
In some aircraft, 1all the gyros are vacuum, pressure, or electrically operated. In other aircraft, 2vacuum or pressure systems provide the power for the heading and attitude indicators, while the electrical system provides the power for the turn coordinator. Most aircraft have 3at least two sources of power to ensure at least one source of bank information is available if one power source fails. The vacuum or pressure system spins the gyro by 4drawing a stream of air against the rotor vanes to spin the rotor at high speed, much like the operation of a waterwheel or turbine. The amount of vacuum or pressure required for instrument operation varies, but is 5usually between 4.5 "Hg and 5.5 "Hg.
Select a highlighted phrase.
Source: FAA-H-8083-25B, page 8-16. Text as printed.
Vacuum or Electric
Vacuum
Air drawn through the instrument
Tap the bucket where the card belongs. On a computer you can also drag the card.
Electric
A motor on the bus
Which bucket does this card belong in?
CautionDo not run a vacuum-driven gyro on shop air or blow into its ports. It is built to run on clean, controlled suction. Dirt carried in collects in the rotor bearings, and air pushed backward through the system can damage the instruments.
The Vacuum System
A typical vacuum system has an engine-driven vacuum pump, a relief valve, an air filter, a suction gauge and the tubing that connects them. The pump pulls air in through the filter, through the attitude and heading indicators, where it spins their rotors, and out through the pump.
FIG 11 A typical pump-driven vacuum system. Blue lines carry filtered air to the instruments; red lines carry it on to the pump. Source: FAA-H-8083-31B, Figure 10-91.
Select a part to ring it on the figure and read what it does.
The Suction Gauge
The suction gauge, also called the vacuum gauge, shows how far the pressure in the instrument system is below the outside air pressure, in inches of mercury. The rotors’ speed depends on that suction, so the gauge tells you whether the vacuum gyros are spinning fast enough to be trusted.
FIG 12 A suction gauge, marked in inches of mercury. Source: FAA-H-8083-31B, Figure 10-19.
4.5 to 5.5 inHg
The usual normal range
Marked by a green arc on the gauge, in a light airplane such as the Cessna 172.
The range differs by airplane and instrument. The Airframe handbook gives 3.5 to 4.5 inches for the systems it describes, and older systems ran at about 4. Read the green arc and the maintenance manual for the airplane in front of you.
AC 43.13-1B, Acceptable Methods, Techniques, and Practices
Paragraph 12-38, System Filters
Select a highlighted phrase to read what it means.
The life of an air-driven gyro instrument is determined to a great extent by 1the cleanliness of the air that flows over the rotor. In vacuum systems, this air is taken from the cabin where there is usually a good deal of dust and very often tobacco smoke. Unless all of the solid contaminants are removed from the air before it enters the instrument, they will 2accumulate, usually in the rotor bearings, and slow the rotor. This causes an inaccurate indication of the instrument and will definitely 3shorten its service life. 4Dry air pumps are also subject to damage from ingested contaminants, and all of the filters in the system must be 5replaced on the schedule recommended by the aircraft manufacturer, and more often if the aircraft is operated under particularly dusty conditions, especially if the occupants of the aircraft regularly smoke while flying.
Select a highlighted phrase.
Source: AC 43.13-1B CHG 1, paragraph 12-38, page 12-17. Text as printed. Range: FAA-H-8083-25B, page 8-16; FAA-H-8083-31B, page 10-45; Cessna Model 172 Maintenance Manual, 37-10-00.
Read the gauge
At cruise power the suction gauge reads 3.0 inches of mercury, and its green arc runs from 4.5 to 5.5. What does the reading tell you?
Power Failures
Gyros fail by power source. A symptom that hits every instrument on one source points at that source, not at each instrument on it. Predict first, then fail the source and watch the panel.
Suction LOW VAC
Step 1. Tap each instrument you expect to go bad when the vacuum pump fails. Then fail the vacuum pump.
Vacuum pump
Electrical bus
Yaw
Everything is working. The suction is inside the green arc.
Applying the Procedure
Back to the write-up: the attitude indicator and the heading indicator were slow and wandering, and the turn coordinator worked. Trace it from the power source.
AttitudeHeadingTurn DC ELECSuction not read
Select a step to see it on the panel.
ResultBoth instruments stay in the panel. One vacuum fault is repaired instead of two good gyros being replaced.
Check
Five questions, three answers each. Each answer tells you why it is right or wrong.
4 of 5
Needed to pass
1ExplainExplain rigidity in space and precession in plain words.
2DescribeDescribe how the attitude indicator, the heading indicator and the turn coordinator use those properties, and what each one shows.
3IdentifyIdentify the parts of the vacuum system and the electric power source for gyros, and state the normal suction range.
4PredictPredict which gyro instruments a vacuum failure or an electrical failure will take out.
Question 1 of 5
0 of 5
ResultsResults
From This Lesson
01A force on the rim of a spinning rotor shows up 90 degrees later, in the spin direction.
02The attitude indicator shows the case moving around a rotor that holds still.
03A turn indicator’s spring measures a rate of turn, never an angle of bank.
04The suction gauge shows the vacuum system, usually 4.5 to 5.5 inches of mercury.
05Instruments that fail together point at the power source they share.
Select any row to re-read its feedback.
Field Card
1You can now explain rigidity in space and precession.
2You can now say what the attitude indicator, the heading indicator and the turn coordinator each show, and why.
3You can now find the vacuum pump, filter, relief valve and suction gauge, and know the usual 4.5 to 5.5 inHg range.
4You can now trace a gyro symptom to its power source before you touch an instrument.
Properties
Rigidityrotor holds, case moves
Precession90 degrees later in the spin direction
Speedmore rigidity, slower precession
Frictionmakes the heading indicator drift
Instruments
Attituderigidity, pitch and bank
Headingrigidity, reset to the compass every 15 minutes
Turnprecession, rate of turn, not bank
Ballcoordination, no power
Power
Vacuumpump, filter, relief valve, suction gauge
Suctiongreen arc, about 4.5 to 5.5 inHg
Electricmotor on the DC bus, OFF flag
Splitone failure leaves bank information
Rule
A symptom that hits every instrument on one power source points at that source. Read the suction gauge before you pull a gyro.
FIG 13 Rigidity in space and precession, the summary drawing from the original lesson.
