A system carried on many aircraft to warn the pilot that the aircraft may be in danger of inadvertent contact with the ground. It is intended to reduce the occurrence of controlled-flight-into-terrain (CFIT) accidents, in which aircraft with no apparent mechanical difficulty or defect strike the ground while under the direct or indirect control of the pilot. These accidents usually occur in conditions of poor visibility (due to atmospheric obscuration such as fog or rain, or darkness of night) where external visual cues are not available to provide added assurance of terrain clearance. Since 1975, federal aviation regulations have required installation of the system on large turbine-powered aircraft in commercial service.