The attitude indicator (often referred to as the AI and also called
the artificial horizon or vertical gyro) is one of three gyroscopic
instruments in an aircraft, the other two being the heading indicator
(directional gyro or DG), and the turn coordinator (or horizontal
gyro). As the only instrument that provides immediate, precise
indication of the airplane?s pitch attitude and bank angle, the
attitude indicator is of critical importance, particularly in flight
in instrument meterological conditions (IMC) under instrument flight
rules (IFR). Understanding its operation and use is of vital to safe
These instruments can be powered in one of several ways ?
mechanically, electrically, via lasers or the newest, solid state. On
smaller aircraft they?re usually powered by the airplane?s vacuum
system. The vacuum system draws air in through a filter assembly,
which then moves through turbines in the instruments, where it causes
the gyros to spin at high speed. The air then continues on to the
engine-driven vacuum pump, where it is expelled. There?s a relief
valve that prevents the pressure from getting too high.
Another, more expensive option, used in many larger aircraft are
electrically powered instruments. Despite the expense, some smaller
aircraft are now employing these electrically powered gyroscopic
systems as well, either alone or as backup systems, in the wake of
several well-publicized vacuum system failures in recent years that
resulted in crashes. A third means is new: laser gyros.
Since your clarification indicates you?re interested mainly in the
gyroscopic aspect of how this instrument works, I?ll touch only
briefly on the differences between the traditional vacuum-powered
gyros and electrical ones.
A gyroscope is operated on the principle of ?rigidity in space,? which
describes the principle that a rapidly spinning wheel with a heavily
weighted rim that tends to remain fixed in the plane in which it is
spinning. By mounting this wheel, or gyroscope, on a set of gimbals or
surrounding frames, the gyro is able to rotate freely in any plane.
So then if the gimbal?s base tilts, twists, or otherwise moves, the
gyro nevertheless remains in the plane in which it was originally
spinning, This principle is what makes it possible for a gyroscope to
be used to measure changes in the attitude or direction of the
So then, the basic feature of the AI is a gyroscope that spins in the
horizontal plane, parallel to the earth. Because it is mounted on two
gimbals, it can remain in that horizontal plane regardless of the
position of the airplane. What that means is that essentially the
airplane revolves around the AI, which in turn remains fixed and
spinning in its horizontal plane and parallel to the horizon. The
gyroscope stays where it is regardless of whether the wings are level,
or banked, or whether the aircraft is ascending or descending.
When the aircraft is started, the gyro begins to spin; then when the
airplane is taxiing, gravity gives the force that levels the gyro
through the action of ?pendulous vanes? on a vacuum-driven system.
How this works is through the momvement of air. The air goes out of
the instrument through four ports near the base of the unit. These
ports are at right angles to each other, and can be individually and
alternately blocked and opened by these vanes, which swin in front of
them. If the gyro tilts in such a way that it is not parallel to the
ground, then the force of gravity causes these vanes to either open or
close. The differential in thrust from the air moving through the
ports is the action that tilts the gyro back to level.? On electrical
systems, electrical power rather than the movement of air ?spins up?
There is one other principle that needs to be mentioned in order to
fully understand this operation, and that is precession. Precession
states that when a force from the outside tries to tilt a spinning
gyro, that force is actually felt by the gyro as if it had been
applied at a point 90 degrees away, in the direction of the rotation.
In other words, ?the cause precedes the effect by 90 degrees.?
Unwanted precession is caused by factors such as friction in the
moving parts of the instrument, or sometimes
acceleration/deceleration, and can eventually result in a slow
drifting of the instrument indications and small errors. Modern
instruments, however, are quite accurate, with errors being usually
very minor, i.e. less than 5 degrees of bank error and/or 1 bar width
of pitch error in a 180 degree turn.
Now then, you can see that the gyro remains in its position, taking
into account precession, because of its being able to move freely.
However, the gimbals to which it is attached, are in turn attached to
the airframe, and that is what makes it possible for the instrument to
give guidance as to bank and pitch. The bank indicator part of the
instrument is attached directly to the longitudinal gimbal mount. The
pitch indicator portion is linked in such a way as to reverse the gyro
movement, so that the pitch indicator bar moves down to indicate a
nose-up attitude, and up to indicate nose-down.
All this is kind of confusing to understand without visual aid, so
I?ve compiled a list of links for you, to sites that not only expand
upon this explanation, but have clear illustrations as well, enabling
you to see what the descriptions refer to.
Here is a very good page with detailed, illustrated explanations of
aircraft gyroscopic systems and instruments:
Here is another excellent article, with clear illustrations on the
operations of the variously powered gyros, including the new laser
The FAA?s ?Pilot?s Handbook of Aeronautical Knowledge.? See
especially 6-9, ?Gyroscopic Flight Instruments:?
Note this reference is in pdf format, for which you?ll need Adobe
Acrobat Reader to view. If you don?t already have it, you can
download a free copy here:
Here?s another pdf document that also has a good, illustrated
explanation of the working of gyroscopic flight instruements,
including the AI (see p. 6):
Another good explanation of laser gyros:
Brief illustrated explanation of the gyroscopic instruments, including
the AI. Quite simple, but good for a brief overview of the principles
Interesting article debating the efficacy of vacuum-powered mechanical
gyros vs. electric: http://www.sbw.org/suctionbad/
This is a very complex topic, as I?m sure you already know. So if
there is anything that remains unclear, please do use the ?Request
Clarification? feature to ask before rating and closing your question,
so I can be sure you?re satisfied with the information given, and that
you have a good understanding of how this instrument works. I?ll be
happy to work with you as necessary to ensure that!
Oh - and one other thing: despite care taken to ensure that all links
are working, sometimes they still don't post correctly, so if you have
any trouble with a link, try copying/pasting into your browser. If
you still have a problem, let me know in a clarification request, and
I'll repost it.
Search strategy: As an instrument-rated Commercial pilot, I?ve relied
heavily on my own knowledge of this topic, as well as on my personal
collection of bookmarks, and reference material from my own library,
in particular information found in Chapter 2, Section A ?Flight
Instrument Systems,? pp. 2-3 through 2-7 of ?Instrument/Commercial
Manual,? Jeppesen Sanderson, Inc. 1999.
However, I also used the search terms below to gather additional
links, and should you desire more reference material, I?m sure you
will be able find plenty by using the same queries, as I certainly did
not cite every possible source:
[flight instrument systems operation ?attitude indicator? OR ?artificial horizon?]
[operation ?attitude indicator? OR ?artifical horizon?]
[gyroscopic flight instruments]
[electrical gyroscopic flight instruments]
[?how does it work? ?attitude indicator? OR ?artifical horizon?]
[aircraft vacuum system power source]