Editing Attitude indicator (section)

==Operation==
[[File:Vacuum Pump system.png|thumb|upright=1.5|Vacuum system using a vacuum pump]]
[[File:Venturi vacuum.png|thumb|upright=.8|Vacuum system using a venturi]]
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 (mechanical)|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.<ref name=FAA1/><ref name=FAA2/><ref name=":3">{{Cite web|url=https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/|title=AMT Handbook - Chapter 10. Aircraft Instrument Systems|last=Federal Aviation Administration (FAA)}}</ref>

Attitude indicators have mechanisms that keep the instrument level with respect to the direction of gravity.<ref>{{cite web|url=http://www.faatest.com/books/IFRH/4-4.htm|title=4-4|first=alan|last=murphy|website=www.faatest.com|access-date=22 March 2018}}</ref> 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 [[precession|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.<ref>{{cite web|url=http://www.faatest.com/books/IFRH/4-5.htm|title=4-5|first=alan|last=murphy|website=www.faatest.com|access-date=22 March 2018}}</ref>

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.<ref name=FAA1/><ref name=FAA2/>

Attitude indicators are free from most errors, but depending upon the speed with which the erection system functions, there may be a slight nose-up indication during a rapid acceleration and a nose-down indication during a rapid deceleration. There is also a possibility of a small bank angle and pitch error after a 180° turn. These inherent errors are small and correct themselves within a minute or so after returning to straight-and-level flight.<ref name=FAA2 />