Sailplane Altimeter Tools to Show Flight Levels

In aviation, altimeters provide the height in the ISA that corresponds to the air pressure at the level at which the aircraft is flying. Most altimeters give the height in feet; sailplane altimeters in non-Anglo-Saxon countries usually give the height in meters. Air Traffic Control uses the ISA height expressed in units of 100 ft; these FLS are the common measure used for height separation of aircraft en route. Pilots in aircraft taking off and descending for landing—and sailplane pilots in particular—need another height indication, namely their actual height above MSL, which is usually called the altitude. This altitude can be provided by the altimeter when its interior is rotated within its casing so that the altimeter in theory shows 0 m at MSL. The theoretical air pressure at 0 m MSL is known as the QNH (pressure) and that pressure is shown in the little window in the scale of the altimeter when the interior of the altimeter is rotated. In that situation the FL indication on the altimeter is no longer available. By using either of the new tools, the FLS-disk or the FLS-ring, the FL indication can be restored.

Reference: Aerokurier, June, 2008 p. 92.