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This answer shows a picture of and describes a "modern" glider instrument panel, with two variometers, airspeed indicator, altimeter, compass, temperature gauge and radio controls.

Glider instrument panel

Notably (at least to me) missing from this is an attitude indicator (or "artificial horizon"). I had the impression that an AI was one of the more important indicators in an aircraft.

Is it frequently left off on gliders? How about other aircraft? Is this related to always avoiding flight outside of VFR (visual flight rules)? Is an AI not very useful or not useful at all in VFR conditions?

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    $\begingroup$ You don't need an artificial horizon when you can look out the window and see the real thing $\endgroup$ – J. Hougaard May 3 at 9:05
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    $\begingroup$ Certainly 3 decades ago, many small aircraft had no artificial horizon for cost & complexity reasons. $\endgroup$ – copper.hat May 3 at 15:17
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    $\begingroup$ What is missing from the picture is the yaw string. That's the most important instrument in a glider. $\endgroup$ – Martin Argerami May 3 at 16:09
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    $\begingroup$ @copper.hat: And even when small planes have/had attitude indicators, about the only time a VFR pilot looks at them is in training. $\endgroup$ – jamesqf May 3 at 17:31
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In most countries, for contest flying in gliders, the rules prohibit any kind of attitude indicator or turn rate indicator, to ensure that pilots do not illegally enter clouds and gain an unfair advantage.

However, some appropriately-rated pilots do legally fly appropriately-equipped gliders in clouds. In many cases this is done in the context of "wave soaring" at very high altitudes-- above 18,000' -- where the pilot must be in contact with Air Traffic Control and operating under Instrument Flight Rules anyway, unless he is operating within a special "wave window". In the United States under current regulations, an attitude indicator would be required for flight under Instrument Flight Rules.

There are some countries that still allow cloud flying in gliders without special coordination with air traffic control (i.e. without operating under Instrument Flight Rules) in significant portions of the airspace. England is one such country. Here's a publication from a gliding club in England re controlling a glider using only a turn rate indicator, with no artificial horizon -- http://www.dartmoorgliding.co.uk/Instrument_Flying_course_notes.pdf . It takes some specialized training to be able to carry this off safely.

Back in the 1940's through 1960's, this type of cloud flying used to be much more common throughout the gliding world than it is today. The primary reasons for including a turn rate indicator on the panel, but no attitude indicator, were cost, weight, panel space, reliability, power requirements, and resistance to "tumbling" if excessive pitch or bank attitudes were attained. In some cases the turn rate indicator was powered by a venturi that was exposed to the airflow when a special panel in the side of the fuselage was opened-- though this was undoubtedly a poor solution in cases where icing was possible.

The "Bohli compass" is an exotic compass for sailplanes that was specially designed to function somewhat like an attitude indicator-- http://www.hkavionics.com/Bohli_man/ba_kompi_e4.pdf . Some other specialized compasses have been developed for gliding that are less exotic than the Bohli, but do minimize compass errors and help allow the compass to be used as a directional gyro to some degree-- see photo https://www.reddit.com/r/Gliding/comments/6kb7li/what_is_the_name_of_this_type_of_compass/ -- also google "Cook compass".

Never try anything like this without wearing a parachute. Gliders have broken up in clouds.

In terms of state-of-the-art instrumentation, things have come a long way from the old days. Now there are sophisticated computerized variometers (sensitive rate-of-climb indicators) that incorporate linear and rotational accelerometers as well as pressure sensing, and in some of these instruments the accelerometers can be used to drive a AHRS-based attitude-indicator-type display on the screen of the instrument. See for example https://www.air-avionics.com/?page_id=552 . (How is the issue of unfair use in a contest addressed?) As all pilots know, this is not a simple task-- you can't just dangle a weight on a string (or the electronic equivalent thereof) and find out which way is "up" -- a lot of computer processing is involved -- see for example https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3279199/ .

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  • $\begingroup$ "Never try anything like this without wearing a parachute. Gliders have broken up in clouds." In many instances gliders are not flying high enough to allow time for the parachute to fully deploy before the pilot hits the ground... $\endgroup$ – Juan Jimenez May 10 at 8:31
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    $\begingroup$ If the cloud bases are so low that you won't have time to open a parachute, it's unlikely that a glider would be entering the cloud in the first place. Cloud base of the cumulus clouds formed by thermal updrafts are typically at least several thousand feet above ground level. Of course there are exceptions. $\endgroup$ – quiet flyer May 10 at 13:34
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Yes, it is normal. With very rare exceptions, gliders are not allowed to operate in IMC (instrument Meteorological Conditions) and must remain clear of clouds. There are also powered aircraft that do not have attitude indicators, mostly for the same reason (VFR, Visual Flight Rules only).

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    $\begingroup$ It may be worth mentioning that some gliders are actually equipped and approved for flying into clouds, although most are not $\endgroup$ – J. Hougaard May 3 at 11:08
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    $\begingroup$ Is this the moment for the old joke that VFR and IFR stand for, "I Follow Roads" and "Visually Follow Roads"? $\endgroup$ – Oscar Bravo May 4 at 10:43
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As pointed out by Juan's answer, they are not required for the kind of operations allowed for gliders, so they are left out for cost reasons.

Furthermore, artificial horizons are relatively complex and expensive instruments, at least in contrast with most other instruments you see in a glider cockpit.

Older models need to keep their gyroscopes running in the face of their internal friction losses, which was usually achieved using compressed air directed at the flywheels. This compressed air can come form a pump or a venturi tube; the former is seldom found on gliders and the later represents a compromise with respect to the all-important glide ratio.

More modern iterations do not have moving parts, relying on laser gyroscopes instead, but they need a power source to run.

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    $\begingroup$ Pointing out the cost (especially that there's performance, as well as financial cost) of having the instrument was very illuminating. Thanks! $\endgroup$ – Curt J. Sampson May 3 at 10:30
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    $\begingroup$ I'm glad you found it useful! If you could wait a bit with accepting this answer, you're likely to get some better info; the usually recommended time is 24 hours to allow all timezones to see the question, and there are people here much more knowledgeable about gliders than myself. $\endgroup$ – AEhere supports Monica May 3 at 11:03
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    $\begingroup$ @CurtJ.Sampson -- some people will see that there's an accepted answer and skip the question. That's why it's usually better to wait a bit. $\endgroup$ – Pete Becker May 3 at 12:43
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    $\begingroup$ The attitude indicator only really tells you one thing -> which way is down. If you're flying VFR and you cannot determine where "down" is without an instrument, you're breaking the rules in spectacular fashion. A slip indicator only tells you which way 'feels' like down. A standalone slip indicator is often fitted in light aircraft (not in combination with an attitude indicator) since this is a completely passive device. With that said, one's bottom, sat in the pilot's seat, is a pretty good substitute for a slip indicator. If the plane is slipping, so too will your bottom...in the seat. $\endgroup$ – J... May 3 at 12:51
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    $\begingroup$ @J... "If you're flying VFR and you cannot determine where "down" is without an instrument, you're breaking the rules in spectacular fashion." In fairness, inadvertant VFR entry into IMC does happen on occasion, and I feel pretty safe in saying that very few VFR-only pilots in VFR-only aircraft plan to enter IMC. In such a situation, an attitude indicator, even if not normally used as a primary instrument, can definitely help in getting out of IMC in a somewhat safe manner if the pilot knows how to use it. No guarantees (for a high-profile example, see N9253N), but it does help the odds. $\endgroup$ – a CVn May 4 at 9:48
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I have never seen an AI in a glider, probably from several reasons. It is common however to see a turn and slip indicator.

The AI would basically never be used in normal flight, you need to keep your eyes outside the plane to see and avoid. The two most import instruments generally are the variometer which "beeps" (low tones when descending, high tones when ascending) and the yaw string showing if the flying is clean. Speed is heard from the wind noise. The horizon, well it is outside there to look at. At times you would look at the speed.

In IMC flying, inside clouds, you would switch to instrument flying and here the AI would be very useful. But the problem is that older type of AI instruments would not work very well in a glider as they could lock up in too high pitch or roll. This does not happen with a slip and turn instrument. It is quite possible to fly in clouds using speed / variometer / turn (although I never was trainee to do it). Instrument flying in gliders is generally quite rare as, at least in Sweden, it is not allowed in competitions (imagine what happens when there are lots of gliders in the same cloud).

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    $\begingroup$ "(imagine what happens when there are lots of gliders in the same cloud)." -- well said. $\endgroup$ – quiet flyer May 3 at 22:09

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