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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 actually 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".

Also, 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 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 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, and power requirements. 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.

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