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The attitude indicator (AI), formerly known as the gyro horizon or artificial horizon, is a flight instrument that informs the pilot of the aircraft orientation relative to Earth's horizon, and gives an immediate indication of the smallest orientation change. The miniature aircraft and horizon bar mimic the relationship of the aircraft relative to the actual horizon. It is a primary instrument for flight in instrument meteorological conditions. Attitude is always presented to users in the unit degrees (°). However, inner workings such as sensors, data and calculations may use a mix of degrees and radians, as scientists and engineers may prefer to work with radians.


History

Before the advent of aviation, artificial horizons were used in
celestial navigation Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space (or on the surface of ...
. Proposals of such devices based on gyroscopes, or spinning tops, date back to the 1740ies. Later implementations, also known as ''bubble horizons'', were based on bubble levels and attached to a
sextant A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of cel ...
. In the 2010s, remnants of an artifical horizon using liquid mercury were recovered from the wreck of HMS Erebus.


Use

The essential components of the AI include a symbolic miniature aircraft mounted so that it appears to be flying relative to the horizon. An adjustment knob, to account for the pilot's line of vision, moves the aircraft up and down to align it against the horizon bar. The top half of the instrument is blue to represent the sky, while the bottom half is brown to represent the ground. The bank index at the top shows the aircraft angle of bank. Reference lines in the middle indicate the degree of pitch, up or down, relative to the horizon. Most Russian-built aircraft have a somewhat different design. The background display is colored as in a Western instrument, but moves up and down only to indicate pitch. A symbol representing the aircraft (which is fixed in a Western instrument) rolls left or right to indicate bank angle. A proposed hybrid version of the Western and Russian systems would be more intuitive, but has never caught on.


Operation

The heart of the AI is a gyroscope (gyro) that spins at high speed, from either an electric motor, or through the action of a stream of air pushing on rotor vanes placed along its periphery. The stream of air is provided by a vacuum system, driven by a vacuum pump, or a venturi. Air passing through the narrowest portion of a venturi has lower air pressure through Bernoulli's Principle. The gyro is mounted in a double gimbal, which allows the aircraft to pitch and roll as the gyro stays vertically upright. A self-erecting mechanism, actuated by gravity, counteracts any precession due to bearing friction. It may take a few minutes for the erecting mechanism to bring the gyros to a vertical upright position after the aircraft engine is first powered up. Attitude indicators have mechanisms that keep the instrument level with respect to the direction of gravity. The instrument may develop small errors, in pitch or bank during extended periods of acceleration, deceleration, turns, or due to the earth curving underneath the plane on long trips. To start with, they often have slightly more weight in the bottom, so that when the aircraft is resting on the ground they will hang level and therefore they will be level when started. But once they are started, that pendulous weight in the bottom will not pull them level if they are out of level, but instead its pull will cause the gyro to precess. In order to let the gyro very slowly orient itself to the direction of gravity while in operation, the typical vacuum powered gyro has small pendulums on the rotor casing that partially cover air holes. When the gyro is out of level with respect to the direction of gravity, the pendulums will swing in the direction of gravity and either uncover or cover the holes, such that air is allowed or prevented from jetting out of the holes, and thereby applying a small force to orient the gyro towards the direction of gravity. Electric powered gyros may have different mechanisms to achieve a similar effect. Older AIs were limited in the amount of pitch or roll that they would tolerate. Exceeding these limits would cause the gyro to tumble as the gyro housing contacted the gimbals, causing a precession force. Preventing this required a caging mechanism to lock the gyro if the pitch exceed 60° and the roll exceeded 100°. Modern AIs do not have this limitation and therefore do not require a caging mechanism.


Flight Director Attitude Indicator

Attitude indicators are also used on crewed spacecraft and are called Flight Director Attitude Indicators (FDAI), where they indicate the craft's yaw angle (nose left or right), pitch (nose up or down), roll, and orbit relative to a fixed-space
inertial reference frame In classical physics and special relativity, an inertial frame of reference (also called inertial reference frame, inertial frame, inertial space, or Galilean reference frame) is a frame of reference that is not undergoing any acceleration. ...
from an Inertial Measurement Unit (IMU). The FDAI can be configured to use known positions relative to Earth or the stars, so that the engineers, scientists and astronauts can communicate the relative position, attitude, and orbit of the craft.


Attitude and Heading Reference Systems

Attitude and Heading Reference Systems (AHRS) are able to provide three-axis information based on ring laser gyroscopes, that can be shared with multiple devices in the aircraft, such as "
glass cockpit A glass cockpit is an aircraft cockpit that features electronic (digital) flight instrument displays, typically large LCD screens, rather than the traditional style of analog dials and gauges. While a traditional cockpit relies on numerous me ...
" primary flight displays ( PFDs). Rather than using a spinning gyroscope, modern AHRS use solid-state electronics, low-cost
inertial sensor An inertial measurement unit (IMU) is an electronic device that measures and reports a body's specific force, angular rate, and sometimes the orientation of the body, using a combination of accelerometers, gyroscopes, and sometimes magnetometer ...
s,
rate gyro A rate gyro is a type of gyroscope, which rather than indicating direction, indicates the rate of change of angle with time. If a gyro has only one gimbal ring, with consequently only one plane of freedom, it can be adapted for use as a rate gyro t ...
s, and magnetometers. With most AHRS systems, if an aircraft's AIs have failed there will be a standby AI located in the center of the instrument panel, where other standby basic instruments such as the airspeed indicator and altimeter are also available. These mostly mechanical standby instruments may remain available even if the electronic flight instruments fail, although the standby attitude indicator may be electrically driven and will, after a short time, fail if its electrical power fails.


Attitude Direction Indicator

The Attitude Direction Indicator (ADI), or Flight Director Indicator (FDI), is an AI integrated with a Flight Director System (FDS). The ADI incorporates a computer that receives information from the navigation system, such as the AHRS, and processes this information to provide the pilot with a 3-D flight trajectory cue to maintain a desired path. The cue takes the form of V steering bars. The aircraft is represented by a delta symbol and the pilot flies the aircraft so that the delta symbol is placed within the V steering bars.


See also

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Acronyms and abbreviations in avionics Below are abbreviations used in aviation, avionics, aerospace and aeronautics. A B C D E F G H I J K L M N N numbers (turbines) O P Q R S T U V V speeds W X Y Z See also * List of aviat ...
*
Air India Flight 855 Air India Flight 855 was a scheduled passenger flight from Santa Cruz Airport, Bombay to Dubai International Airport, Dubai. On New Years Day in 1978, the Boeing 747 operating the flight crashed about off the coast of Bandra, Bombay (now M ...
* Korean Air Cargo Flight 8509 *
Peripheral vision horizon display The peripheral vision horizon display, also called PVHD or the Malcolm Horizon (after inventor Dr. Richard Malcolm), is an aircraft cockpit instrument which assists pilots in maintaining proper attitude. The PVHD was developed in the mid-1970s and ...
(PVHD) *
Turn and slip indicator In aviation, the turn and slip indicator (T/S, a.k.a. turn and bank indicator) and the turn coordinator (TC) variant are essentially two aircraft flight instruments in one device. One indicates the rate of turn, or the rate of change in the aircra ...


References

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