With this instrument the pilot receives instantaneous indication about pitch and roll of the aircraft relative to the horizon. It is shown on the face of the instrument with an aircraft image. Roll indications are indicated at the top and pitch with the aircraft image relative to the background in blue and brown.
This instrument is located at the top center of the basic six and it is paramount for flying in instrument meteorological conditions, also known as IMC.
During preflight, taxi and cruise the pilot needs to look out for correct instrument indications and verify, if possible, with the real world by looking outside. Any failure during IMC means an emergency, should backup instruments not be installed or properly functioning.
Inside the instrument you will find an earth gyro spinning around and it is kept in the horizontal plane through rigidity and precession with a pendulous unit mounted on the bottom of the gyro.
The aircraft image on the instrument is fixed (but adjustable up and down for different level flight attitudes/speeds) and the blue/brown background is able to move up and down. Some East-European types have the brown (showing the Earth) part above the blue, it is peculiar to say the least.
As shown on the image to the right the gyro lies horizontally (vertical axis) in the inner gimbal ring. The outer gimbal is pivoted wingtip to wingtip and moves the background plate.
The center of gravity of the gyro and pendulous unit is below the suspension points thus making sure that the pendulous unit can erect and is able to keep the gyro in the horizontal plane.
In vacuum driven attitude indicators a system of pendulous vanes at the bottom is used to erect the gyro. When the gyro is horizontal all vanes cover their ports halfway so that air exits all ports evenly.
There are four ports and opposite vanes are mechanically attached to each other. When the gyro tilts for any reason to left or right the vanes of the ports fore and after will move and either close or open port fore or aft. One of the air exits is then closed more than the other three and this creates a precession force which will try to erect the gyro again until the four ports have a equal force.
Other systems use steel balls which spread out evenly when the gyro is spinning vertical. Should the gyro become tilted the balls will move to the low side and this unbalanced force results in a precession force erecting the gyro.
Most electric attitude indicators use mercury switches and torque motors to keep the gyro horizontal.
Limitations and errors
Basically the attitude indicator is subject too two errors: acceleration and turning error. Both result from the same cause: pendulosity and erection to a false vertical.
When an aircraft accelerates to a higher speed the gyro moves to the right due to precession. This will show up on the instrument as a climbing right turn. During deceleration the AI shows a descending turn to the left. This error will be more pronounced with higher performance aircraft as these usually have more power and thus a greater acceleration.
When initiating a turn (either direction) the gyro will swing or move to the rear, again due to precession of centrifugal force against the gyro. Both errors are small and will correct themselves. The error will show itself as a pitch error and a bank indication.
As the vanes are suspended above the ports, they are held vertical by gravity. When the aircraft accelerates or turns, inertia of the vanes makes sure that they lag just a little bit. Thus creating a force making the gyro tilt to the back when accelerating and tilting opposite to the turn the aircraft makes, as a result you will see the AI background move up and down.
When suction falls below 3½ inHg the instrument will become sluggish to move. Above 4½ inHg the attitude indicator will become more lively and it will wear out that much sooner.
Normal vacuum driven AIs have a pitch and bank limit of 55° in pitch and 90° in roll. More modern types are limited to 60°pitch and to about 110° of roll. When these limits are exceeded the gyro will topple and you will see the instrument oscillating. If you have a cage knob then do so, if you do not, the gyro is designed to erect itself but this will take more time.