Survival knives are knives intended for survival purposes in a wilderness environment, often in an emergency when the user has lost most of his/her main equipment. Most military aviation units issue some kind of survival knife to their pilots in case their aircraft are shot down behind enemy lines and the crew needs tools to facilitate their survival, escape, and rescue. Survival knives can be used for trapping, skinning, wood cutting, wood carving, and other uses. Hunters, hikers, and outdoor sport enthusiasts use survival knives. Some survival knives are heavy-bladed and thick. Other survival knives are lightweight or fold in order to save weight and bulk as part of a larger survival kit. Their functions often include serving as a hunting knife. Features, such as hollow handles, that could be used as storage space for matches or similar small items, began gaining popularity in the 1980s.