Collision Avoidance Systems, Airplane

Collision avoidance involves prevention of unwanted contact with other planes, obstacles, or the ground. Over half of midair collisions occur within 5 miles of an airport; 96 percent occur below 3,000 feet, and 40 percent occur below 500 feet. Most happen during the day; less than 2 percent occur after sundown. Most are in the traffic pattern, and high-speed head-on collisions account for only 14 percent of midair collisions. Side impact accounts for 39 percent, with 47 percent due to one plane overtaking another. Some routes and major commercial hubs are congested airspace, and midair collision risk increases with volume of aircraft in finite space. Commercial airlines continue to add new aircraft to an already congested system.

The first layer of accident avoidance is proper operational procedures. Traffic is assigned lanes—a specific airway at a specific altitude for each craft—and air traffic controllers monitor the various craft and lanes to predict and prevent conflicts. Standard distances are 3 to 5 miles laterally and 1,000 feet vertically. Air traffic controllers advise pilots of measures to take as surrounding traffic dictates. When air traffic control (ATC) fails, an airborne collision avoidance system takes over, giving the pilot what it takes to avoid the collision.

Types of collision avoidance systems include airborne radar (available since World War II), but nonmilitary aircraft that routinely have weather radar rarely carry sensitive anticollision radar. An airborne collision avoidance system (ACAS) may be defined as a system on the aircraft working independently of air traffic control and ground-based equipment to warn pilots that other aircraft are in position for a potential collision. The system may, if the danger is imminent, suggest a maneuver to avoid the collision.

ACAS standards are published in the Convention on International Civil Aviation, annex 10, volume IV. As of 2009 only two versions of the Traffic Alert and Collision Avoidance System (TCAS II) met the International Civil Aviation Organization (ICAO) standard.

ACAS differs from the airborne separation assurance system (ASAS) in that ACAS means a short-range system for prevention of metal-to-metal collisions. ASAS pertains to longer range systems to preserve standard en route separation of 5 nautical miles (9.25 kilometers) horizontal and 1,000 feet (305 meters) vertical.

Airborne collision avoidance systems have the goal of providing the pilot adequate information to take appropriate action in a dangerous situation. Current collision avoidance systems offer proximity warning (PW), traffic advisory (TA), and resolution instruction (RI). PW says an aircraft is near, TA says a particular plane is a threat, and RI tells what to do to avoid the threat.

PW devices were used in the early 1950s, but they quickly became inadequate to handle the increased volume and associated increased collision risk. Collision avoidance systems of various sorts were tried [Page 384]from the 1950s to the mid-1980s, at which time the TCAS became the standard.

In 1956 a Lockheed Constellation and a DC-7 collided in visual flight conditions over the Grand Canyon with a loss of 128 lives. The collision led to demands for modernizing the air traffic control system, producing the current system for preventing midair collisions. After the Grand Canyon collision, in the 1950s and 1960s the FAA allowed various approaches to collision avoidance before settling on the Beacon Collision Avoidance System (BCAS), an airborne transponder system, in 1974. In 1978 over San Diego a Boeing 727 and a Cessna 172, both under radar control, collided and 144 people died. Restrictions on flight in heavy traffic areas were tightened. The San Diego collision of 1978 led to an expansion of the BCAS, renamed TCAS in 1981.

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