from The Warzone
... 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...
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...
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...
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