The Attitude Indicator, called the artificial horizon or gyro horizon, is a gyroscopic flight instrument mounted in the cockpit. It displays aircraft orientation relative to Earth's horizon, giving pilots an immediate, direct indication of the smallest attitude change. A miniature aircraft symbol and horizon bar replicate the airplane's actual attitude relation to the real horizon, making the AI instantly interpretable and keeping it near the center of the instrument panel.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is an attitude indicator on a plane?
The attitude indicator (AI), called the gyro horizon or artificial horizon, is a device that informs the pilot of the aircraft orientation relative to Earth's horizon, and its pitch indicator portion measures how much the aircraft's nose is angling up or down relative to the horizon.
An attitude indicator on a plane is a flight instrument that shows the aircraft's pitch and bank relative to the horizon. It is one of the six basic flight instruments and is driven by a gyroscope mounted in a horizontal plane, spun at high speed by an electric motor. The gyro's rigidity in space gives an immediate and direct indication of aircraft orientation. A horizon bar fixed to the gyroscope mimics the relationship of the aircraft to the actual horizon. A card mounted to the gyro arms has lines depicting degrees of pitch and bank so that pitch and roll are displayed in degrees. Long lines represent 10 degrees and short lines represent 5 degrees. A bank index at the top shows angles of 10, 20, 30 and 60 degrees. A self-erecting mechanism, actuated by gravity through pendulous vanes, keeps the gyro vertically upright. If the gyro is not erect, slight pitch-up indications appear during accelerations. In modern cockpits the attitude indicator is part of the EFIS display, while older panels show it as a separate dial. The helicopter attitude indicator is mounted in the airframe beneath the main rotor system and works on the same gyroscopic principle.
What is an attitude director indicator?
An attitude director indicator (ADI) is the primary control instrument for recovery from an upset. It is an attitude indicator combined with a Flight Director System. The same instrument is alternatively called an artificial horizon, a gyro horizon, or simply the attitude direction indicator. The ADI combines flight director command bars with a standard gyroscopic attitude indicator and displays pitch and roll data received from flight guidance computers. In glass-cockpit aircraft the Electronic Attitude Direction Indicator performs the same functions within the Primary Flight Display. It adds slip/skid indicators, flight-path-vector symbology, and flight-director guidance so the pilot receives a single, consolidated picture of required aircraft attitude. Because the ADI is no longer tied to spinning rotors, attitude and heading reference system sensors on three axes - roll, pitch, and yaw - feed the display through solid-state ring-laser gyros and accelerometers. The same AHRS information drives other electronic flight instrument systems and continues to drive a standby mechanical ADI if the primary display fails.
What does the attitude indicator measure?
The attitude indicator measures aircraft pitch and bank, displaying pitch from the artificial horizon line in degrees and roll from bank index marks in degrees. It informs the pilot of aircraft orientation relative to Earth's horizon and gives an immediate indication of the smallest orientation change. Because the gyro horizon - called the artificial horizon - shows the aircraft's position relative to the horizon, the pilot can tell if the airplane is flying level and can maintain straight-and-level flight without external visual cues.
Why is the attitude indicator important? The attitude indicator is important because flight without reference to a visible horizon can be safely accomplished by gyroscopic instrument systems. The AI provides real-time information about aircraft attitude, helping pilots execute coordinated turns and maintain control in clouds or darkness.
Preflight actions minimize the chance of inaccurate information. The gyro must erect itself using gravity on the ground, and the pilot verifies that the instrument displays the horizon bar correctly and that the bank index marks the expected zero-degree indication before taxi.
How does the attitude indicator work?
The attitude indicator works on the principle of rigidity in space. A small internal gyroscope, spun at high speed by an electric motor or a stream of air, is mounted in a horizontal plane and remains vertically upright while the aircraft rolls and pitches around it. Because the gyroscope maintains its orientation, the instrument case-fixed to the fuselage-rotates relative to the gyro, so the gyroscopic system gives an immediate and direct indication of aircraft orientation. Attached to the gyro arms are a horizon bar and a two-sided card whose lines depict degrees of pitch and degrees of bank. The miniature aircraft painted on the instrument glass then moves vertically up or down relative to this artificial horizon when the aircraft pitches, while the bank index at the top shows the angle of bank in 10, 20, 30 and 60 degree increments up to the mechanical bank limit of about 100 -110 degrees and pitch limit of roughly 60 -70 degrees.
To keep the gyro spin axis truly vertical in flight, a self-erecting mechanism applies a precessing force whenever the gyro tilts from its vertical position. On the ground the mechanism uses gravity while in the air the attitude indicator uses pendulous vanes that control tiny exhaust ports for the suction air or use torque from the electric spinner. These vanes purposely introduce a controlled amount of gyroscopic precession to correct for banking errors and to raise the gyro back to the vertical, assuring that the instrument reading remains directly derived from a gyroscopically-leveled, essentially precession-free sensor. Rigidity still predominates, so pilots can see the airplane axis relative to Earth's horizon and can reference aircraft pitch and bank about the artificial horizon under both VFR and IFR conditions. In glass-cockpit aircraft the same attitude information is calculated inside an Inertial Reference Unit that contains laser-ring gyros, accelerometers and a magnetometer, but the fundamental principle - rigidity in space - remains unchanged.
Gyroscopic fundamentals reign inside the instrument, offering a steady, present portrayal of pitch and rotation. A small cockpit sign, sided with a simulated horizon line, coordinates bank inclinations and pitch postures through delicate motions of the orientation device. The association between the aircraft symbol and the horizon bar forms a natural real-time response circuit and any deviation generates a command signal that rectifies the aircraft's orientation.
How is the attitude indicator powered?
The attitude indicator is powered by one of two principal sources: aircraft vacuum or electrical energy. In the traditional vacuum attitude indicator, the gyroscope is spun by suction delivered from an onboard vacuum pump. Air pulling through the rotor causes the gyroscope to spin at high speed. In the electric attitude indicator, an electric motor replaces the vacuum turbine. The gyro rotor is driven by a high-speed motor energised from the aircraft DC bus. During normal flight, the electric attitude indicator runs off the DC bus, while the standby attitude instrument receives power from the same bus yet is isolated by an emergency switch that connects a dedicated battery pack when the main electrical system fails. Thus, the electric AI can operate on direct current from the electrical system and, equipped with a backup battery, can serve as a backup instrument with battery power if the primary supply is lost.
The attitude indicator is powered by an inner gyro supplied by energy from the aircraft's electrical system. Should the primary electrical system fail, the instrument automatically reverts to a simpler battery-powered substitute orientation device, a little self-contained replacement unit that continues to offer vital pitch and bank data. In older aircraft, a vacuum body supplied the main orientation dial, with gyrostabilizer-generated inflexibility keeping the gyroscope spun.
What are the operating characteristics of a vacuum driven attitude indicator?
A vacuum-driven attitude indicator draws a stream of air through a scooped rotor, using suction supplied by a vane-type engine-driven pump mounted on the accessory case. The airflow spins the gyroscope at high speed. The gyro remains rigid in space while the aircraft pitches and rolls about it, so the horizon disc attached to the gimbals stays fixed relative to the earth. Pitch limits are approximately 60 to 70 degrees and roll limits are 100 to 110 degrees; newer instruments do not have these restrictive tumble limits.
Inside the case, pendulous vanes hang vertically on the ground. When the aircraft turns, the vanes move from the vertical, causing precession toward the inside of the turn. This motion applies a correcting force that keeps the gyro upright and eliminates bank error. A relief valve prevents vacuum pressure from exceeding prescribed limits, and the vacuum gauge attached to the lines gives an indication that adequate suction - at least 4.6 inches (116.8 mm) - is being generated.
What are the components of an attitude indicator?
The components of an attitude indicator include a double gimbal, a miniature airplane, roll scale, and a caging knob. The attitude indicator is built around a gyroscope spun by an electric motor; the gyro spins and provides rigidity. The gyro is mounted in a double gimbal so the aircraft can pitch and roll about the gyro while the gyro itself remains spatially fixed. A miniature airplane fixed to the case moves with the aircraft and shows pitch and roll against the stationary disc. Roll attitude is read on a roll scale that is part of the disc assembly. Pitch attitude is taken from the artificial horizon line, with markings in degrees. A caging knob at the bottom center of the instrument case can lock the gimbals and hold the rotor spin axis vertical during manoeuvres or erection. Newer instruments, however, do not have a caging mechanism because they are designed without tumble limits.
What are the colors of an attitude indicator?
A traditional attitude indicator uses a blue-and-brown horizon card attached to the gyroscope. The top half of the instrument is blue to represent the sky, and the bottom half is brown to represent the ground. A horizon line separates the colored sky from the ground, making the blue-above, brown-below pattern instantly readable. Modern flight displays are capable of showing many other colors, yet the standard still favors the classic scheme because the blue-brown combination is easier to distinguish than blue-green, blue-red, or any other color combination.
What are the types of attitude indicators?
The types of attitude indicators are outlined below.
- Traditional Attitude Indicators
- Electric Attitude Indicators
- Digital Attitude Indicators
- All-in-One Attitude Indicators
What are the errors of the attitude indicator?
The errors of the attitude indicator are outlined below.
- Attitude indicator may develop small errors in pitch or bank during extended periods of acceleration, deceleration, turns, or due to the earth’s curvature
- Attitude indicator errors include being slow to erect or displaying incorrect indications inflight on long trips
- After a 180 degree turn, a small bank and pitch error may occur in an attitude indicator
Inherent errors are small and correct themselves within a minute or so after returning to straight-and-level flight. During manoeuvres, accelerations and decelerations cause precession errors: rapid acceleration causes a slight nose-up indication, while decelerations cause a slight nose-down indication. Preflight actions minimize the chance of inaccurate information, and standby units share the same error sources and tolerances.
What are the limitations of the attitude indicator?
Older attitude indicators were limited to about 60 degrees in pitch and 100 degrees in roll. If these limits were exceeded, the mechanism would lock up and the attitude reading would become inaccurate. The indicator stops at the limit until the aircraft returns within limits, or it tumbles and oscillates until the gyro re-erects. Newer instruments do not have these restrictive tumble limits.
Toppling is caused by the gyro precessing when G forces are not felt down the axis of rotation. Pendulous vanes act undesirably with a turn and produce precession that tilts the gyro. Restraint on the gyro from minimum friction or caging also produces precession, and the gyro must remain vertically upright while the aircraft rolls and pitches around it for the instrument to function properly.
Additional anomalies include slight nose-up indication during rapid acceleration and slight nose-down indication during deceleration. When the engine is first started, the gyro is not erect, and a 180 degrees turn causes a small bank angle and pitch error. The indicator becomes stuck, fails internally, or acts erratically after a vacuum failure. Preflight actions minimize the chance of inaccurate information, and caging is used especially during acrobatics to lock the gyro in its vertical position when limits are exceeded.
What causes an attitude indicator breakdown?
Attitude indicator breakdown could be caused by vacuum failure. Vacuum failure manifests the instrument acting erratically where low vacuum causes the attitude gyroscope to precess. The standby attitude indicator fails if its electrical power fails. Bearing lubrication failure follows when oil settles to the bottom of bearings when the attitude indicator is not exercised, and oil migration out of the bearing race leaves the bearing improperly lubricated. Bad bearings cause the gyro to not spin at the required rpm, producing bearing friction that causes precession and makes the indicator stick. Physical failure will likewise manifest in the indicator becoming stuck.
Exceeding pitch or roll limits causes gyro to tumble; exceeding 60-70 degrees of pitch or 100-110 degrees of bank causes gyro to topple when gyro housing contacts gimbals, applying precessing force. Older instruments had caging mechanisms that locked gyro vertically during maneuvers exceeding limits, preventing tumble. Newer instruments do not have restrictive tumble limits and therefore do not have a caging mechanism. Pendulous vanes, used to correct banking errors, act undesirably with turn because G's are not felt down the axis of rotation, causing precession and slight tilting. Earth’s rotation causes gyro alignment drift over time, and friction places restraint on gyro producing further precession. Accelerations cause slight pitch-up indication, while decelerations cause slight pitch-down indication.
How to read an attitude indicator?
To read an attitude indicator follow the instructions given below.
- Keep gyro vertically upright while aircraft rolls and pitches around it
- Measure aircraft pitch and roll using gyroscope
- Show aircraft angle of bank using bank index at the top
- Indicate degree of pitch using reference lines in the middle
- Align aircraft symbol along the horizon line in level unbanked flight
During level unbanked flight the miniature aircraft symbol lies exactly on the white horizon line. Any displacement above the line shows the nose pitching up, while a position below the line indicates the nose pitching down. Degrees of pitch are judged by the evenly spaced reference lines that parallel the horizon line: each mark above or below the horizon represents a fixed pitch interval, so the farther the symbol is from the line, the steeper the climb or descent. Roll information is taken from the bank index, a curved scale at the top of the instrument that carries marks at 10, 20, 30, and 60 degrees. When the aircraft is rolled, the horizon line tilts and the index pointer moves across these graduated tabs. The number aligned with the pointer gives the exact angle of the bank. Although the aircraft rolls and pitches around it, the gyroscope inside the indicator remains vertically upright, so the horizon line and bank index stay fixed to the earth reference and accurately portray aircraft attitude at every instant.
How do you adjust an attitude indicator?
Begin every adjustment on the ground while the aircraft is level. First, cage the gyro by pulling the PULL TO CAGE knob and keep the pull steady until the gyro presentation has stopped moving. After the gyro is caged, release the same knob; the spring-loaded mechanism returns the caging system to the normal uncaged position and the gyro is free to erect. Next, check the L bubble level on the bezel lip, tilt the instrument until the bubble is centered so the gyro casing is truly horizontal. With the miniature aircraft now visible, rotate the adjustment knob to raise or lower the aircraft symbol until its wings overlap the horizon bar. This aligns the presentation with the actual flight attitude and compensates for the pilot's viewing angle. Finally, verify that the gyro has erected itself upright. The self-erecting mechanism driven by pendulous vanes will have returned the spinning wheel to the vertical position, ready for flight.
