Here's What Air Traffic Collision Avoidance Systems Can And Can't Do

Yahoo

30-01-2025

Yesterday's tragic mid-air collision between an American Eagle Bombardier CRJ-700 regional jetliner and a U.S. Army H-60 Black Hawk helicopter close to Reagan National Airport, near Washington, D.C., has led to many questions about the role played by the Traffic Collision Avoidance System, or TCAS. While you can read our report of the incident here, it should be restated that, at this early stage we have no clue as to exactly what might have gone wrong. In the meantime, however, it's worth looking at TCAS in more detail, and, especially, what it can and can't do to prevent mid-air collisions.
Fundamentally, TCAS exists to help maintain separation between aircraft while they are flying. According to the International Civil Aviation Organization (ICAO), which regulates international air transport, aircraft need a minimum of 1,000 feet of vertical separation at the point at which their paths cross, provided they are flying at 29,000 feet or below, under Instrument Flight Rules (IFR). Flying above 29,000 feet, the separation requirement increases to 2,000 feet or greater, although there are some exceptions, in specific busy air corridors.For the most part, the responsibility for ensuring this vertical separation — and therefore removing the chance of a collision — falls upon the relevant air traffic control authorities and, in some cases, the aircrew. TCAS exists, above all, as a backup to this, providing the aircraft crew with a warning that is independent of aircraft navigation equipment and ground-based systems.
TCAS emerged from the lessons learned from a much earlier fatal collision, the 1956 accident involving a United Airlines Douglas DC-7 and a Trans World Airlines Lockheed L-1049 Super Constellation over Grand Canyon National Park, Arizona. This led to an overhaul of how air traffic control was operated in the United States. In particular, there was recognition of the need for a backup collision avoidance system that could operate even if air traffic control failed.
Early efforts during the late 1950s and early 1960s focused on passive and non-cooperating systems, but over the years, the TCAS concept has been considerably refined, and this collision-avoidance aid is now a fundamental part of aircraft operations not only in the United States but also around the world.
A major development in the 1970s concerned the emergence of the Beacon Collision Avoidance System (BCAS), using Air Traffic Control Radar Beacon System (ATCRBS) transponders that began to be installed in large numbers of airliners, military aircraft, and also general aviation aircraft. In 1978, the collision between a light aircraft and an airliner over San Diego spurred efforts to refine this technology, leading to TCAS II, the development of which began in 1981. This built on BCAS, providing the same kind of transponder-based interrogation and tracking, but adding some additional capabilities.Modern TCAS works using transponder signals from nearby aircraft, meaning it's independent of air traffic control, but also that it requires aircraft with transponders fitted and switched on in order to function. Using the transponder signals, the system builds a three-dimensional map of the surrounding airspace, with aircraft movements plotted within this. Based on respective aircraft and flight paths, speeds, and altitudes, TCAS then automatically provides the crews with an alert of a possible mid-air collision danger. In some modern airliners, the process of avoiding action will be taken without any pilot action, using a technology known as auto-TCAS.
The crews of the aircraft in question will receive an audible and visible warning, directing them to take the appropriate action: either climbing or descending to avoid a potential collision.
For various reasons, however, and especially in the case of the collision at Reagan National Airport, TCAS is not a major guarantee of flight safety across all parts of an aircraft's flight profile.
To begin with, the H-60 Black Hawk wouldn't necessarily have had TCAS fitted to start with, although it is an option for the Black Hawk. ICAO demands that TCAS be fitted to an aircraft that has a capacity of more than 19 passengers or a maximum takeoff weight of more than 5,700 kilograms, but these regulations only cover civil aviation and apply specifically to fixed-wing, turbine-powered aircraft.
It's worth noting that, despite this, TCAS is also found on military aircraft, especially larger ones, such as tankers and transports. An added impetus for this came after the mid-air collision between a German Luftwaffe Tupolev Tu-154 and a U.S. Air Force C-141 StarLifter, off the coast of Namibia in September 1997.
Furthermore, TCAS is intended to ensure flight safety at higher altitudes, thereby reinforcing the requirement to maintain at least 1,000 feet of vertical separation — or more, depending on flight level. For aircraft operating at lower altitudes, TCAS is inhibited. Specifically, 'Increase Descent' warnings are inhibited below 1,550 feet AGL, 'Descend' warnings are inhibited below 1,100 feet AGL, and all types of warnings are inhibited below 1,000 feet AGL. Terrain and other obstacles can also interfere with transponder signals during very low-level flight.
There is a key safety reason behind these limitations, namely that providing warnings at lower altitudes could result in aircraft crews making rapid maneuvers, which could be very dangerous during low and slow flight. For example, a flight crew receiving a 'Descend' warning at low level could quickly take action that would end up with the aircraft flying into terrain.
In these situations, therefore, TCAS isn't used to ensure collision avoidance.
Initial reports suggest that Flight 5342 and the Black Hawk helicopter collided at an altitude of somewhere between 200 feet and around 400 feet, although this is not confirmed.
Moreover, even at flight levels in which TCAS is engaged and is working as intended, the system is not foolproof.
In one tragic example, in 2002, a Tupolev Tu-154 and a Boeing 757 collided over southwest Germany, killing 71. An investigation found that, although they received TCAS warnings, the crew of the Tu-154 relied instead on contradictory instructions from air traffic control, leading to a collision.
There have also been incidents of mid-air collision in recent years in which TCAS has not provided warnings since one or more of the aircraft involved were flying without their transponders activated.
Since the accident yesterday, there's been much speculation about what might have exactly happened and questions have been asked about whether TCAS could have prevented it. Again, we really have no idea at this point what happened or what role TCAS did or didn't play. Regardless, even when aircraft are fitted with fully functional Traffic Collision Avoidance Systems, these will not necessarily prevent or even warn of a mid-air collision, especially at low altitude.
In the meantime, we will have to await more official information before we can better understand what might have gone wrong.
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