The fuselage of the aircraft provided insulation from the wind and some protection against direct exposure to cold. It did not provide heat. The survivors sleeping inside the fuselage were sleeping in temperatures that could cause hypothermia — a condition in which the core body temperature drops below the range required for normal metabolic function. Hypothermia causes confusion, loss of coordination, drowsiness, and, if it progresses, organ failure and death. It is a process, not an event; it unfolds over time and can be slowed by insulation and body heat but not stopped without warmth.
The survivors managed hypothermia by sleeping in physical contact with each other, using luggage and salvaged clothing as insulation, and later using the sleeping bag they constructed from aircraft materials. They were not warm. They were simply warm enough, most of the time, to maintain the core temperature required for survival. The margin between warm enough and fatally cold was not large.
Frostbite was a constant threat and a frequent occurrence. The extremities — fingers, toes, nose, ears — lose heat more rapidly than the core body. When tissue freezes, the ice crystals that form inside cells rupture the cell walls. Thawing frostbitten tissue is painful; the damaged cells die, and in severe cases the affected extremity must be amputated. The survivors had multiple cases of frostbite throughout the 72 days. Roberto Canessa, using his medical knowledge, managed frostbite cases as best he could — which meant preventing further freezing, allowing slow rewarming, and monitoring for the progression of tissue damage. Without surgical tools and proper medical facilities, there was a limit to what could be done.