On July 24, 2000 in Sumner, Georgia, an AS350B Astar collided with the ground in a heavily wooded area during a clear dark night. According to the NTSB, no pre-existing airframe or engine malfunctions were identified during the post-crash examination. They determined the probable cause as the pilot experiencing spatial disorientation, which resulted in a loss of control of the helicopter. A contributing factor was the dark night.
Threats, clearly visible during the day, are masked by the darkness. In fact, Controlled Flight into Terrain (CFIT) at night is a major problem for fixed and rotor wing operations. CFIT is defined as colliding with the earth or a man-made object under the command of a qualified flight crew with an airworthy aircraft.
Due to the unique low flying operation of helicopters CFIT at night during VMC has been troublesome, especially for the aeromedical industry. According to the Air Medical Physician Association, half of all EMS accidents happen at night.
For example, on November 14, 2001 the pilot of a BO 105 LS reported that while performing a normal take-off from a non-airport shortly after midnight on a dark moonless night, he lost visual references outside the helicopter, became disoriented, and collided with terrain. He further stated that no mechanical malfunction or failure had occurred during the accident flight. The NTSB determined the probable cause was the pilot’s failure to maintain adequate separation from terrain during the initial climb. Factors include spatial disorientation and a dark moonless night.
Obviously, the key to preventing these accidents is improving the pilot’s ability to see obstructions–the idea behind Night Vision Goggles (NVG). In the late 1990s, the idea of using NVGs in the civilian sector was starting to evolve. By early to mid 2000, EMS operators started using NVGs in their operations. Today, the vast majority of EMS operators and law enforcement units fly their helicopters under NVGs. With the proper training, use of NVGs increases the safety margin associated with flying helicopters at night.