Nap-of-the-earth is a type of very low-altitude flight course used by military aircraft to avoid enemy detection and attack in a high-threat environment. During NOE flight, geographical features are used as cover, exploiting valleys and folds in the terrain by flying in, rather than over, them. This keeps below enemy air defenceradar coverage, avoiding being silhouetted against the sky. Other, mostly older terms include "ground-hugging", "terrain masking", "flying under the radar" and "hedgehopping".
Purpose
NOE is used to minimize detection by hostile aircraft, AWACS surveillance and control systems, ground-based radar, or attack targets. A high-flying aircraft can be detected by defense systems at long range, giving an air defense system time to react, alerting SAM and AAA missile systems and fighter aircraft. Using NOE flight, the approach may be undetected; the aircraft "pops up" to attack the target and then turns to escape before the enemy can respond. Doppler radar has the potential to detect NOE flight, but the incoming aircraft has to be within radar range in the first place, and low flight minimizes this possibility by using hills and mountains to break the line of sight, defeating terrestrial air defense radar and in rough enough terrain also airborne early warning.
Sensors for NOE
Most NOE flying is done during the day using visual reference by pilots who are experienced in low flying. Data from a radar altimeter or terrain-following radar system is also used, the latter enabling low flying in adverse weather where it would not be possible by visual reference and manual pilot control. At night, a night-vision device may be used.
The lowest NOE flying is by helicopters because they have lower speeds and more maneuverability than fixed-wing aircraft, particularly fast-jets. Helicopters can fly at treetop levels or even below the height of surrounding trees where there are clear areas, flying under wires rather than over them. Attack helicopters can hide behind trees or buildings, "popping up" just enough to use their radar or other sensors and then minimally exposing themselves to launch weapons. The required altitude depends on the type of terrain, such as low vegetation or low buildings, tree canopy layer of less than, presence of tall buildings or concrete pylons with heights of up to international standard, or emergenttree layer canopy and lattice steel electricity pylons to being present. Over populated areas with tall radio transmission towers, an altitude of AGL applies. In May 2011, four U.S. military helicopters evaded the Pakistani air defense system during the Osama bin Laden mission. Officials in Pakistan attributed the failure to detect the aircraft to their use of nap-of-the-earth flying techniques.
Height Above Ground Level in NOE and low flying generally vary with the aircraft speed, aircraft maneuverability and the ruggedness of the terrain. Helicopters are capable of flying with no more than a few feet of clearance below the helicopter's skids or wheels. Fast jets are more constrained and at a typical low-flying speed of 450 knots, 200 feet is not unusual and 50 feet is possible in relatively flat terrain. Power wires are a danger to all aircraft flying at low level and "wire strikes" are common, such as the Cavalese cable car disaster. Special maps are produced that plot the routes of these wires but these are difficult to keep up-to-date, especially for foreign/enemy countries. Pilots are trained to scan for the pylons or power-poles that support these wires, because they can be seen at a distance where the wires themselves cannot.
