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TCAS — Traffic (alert and) collision avoidance system — is an airborne system, created to reduce possibility of mid-air collisions and near-misses.

Aviation authorities became concerned about designing TCAS since late 1950-s. As aviation transport grew in size, volume and speed, mid-air collisions became a serious threat, which started to kill people and it needed to get stopped. First generation of TCAS was ready by the end of the 1990-s and it was installed on most commercial aircraft.

How does TCAS work? It gets signals from other aircraft, equipped with RBS or Mode-S  transponders, shows these aircraft to the pilots on a special indicator (usually on a Navigational Display or EHSI — Electronic Horizontal Situation Indicator), monitors the airspace around an aircraft, analyses movements of other traffic and if necessary, gives commands either to climb or to descend in order to avoid near-miss or collision with other aircraft. In other words, TCAS is based on secondary surveillance radar transponder signals, and operates independently of ground-based equipment to provide advice to the pilot on potential conflicting aircraft.
ICAO demands all aircraft with maximum take-off mass (MTOM) of over 5700 kg or authorized to carry more than 19 passengers to be fitted with TCAS.
Current version — TCAS II is the second and current generation of instrument warning TCAS, used in the majority of commercial aviation aircraft. It offers the pilot direct, vocalized instructions to avoid danger, known as a «Resolution Advisory» (RA). The suggestive action may be «corrective», suggesting the pilot change vertical speed by announcing, «Descend, descend», «Climb, climb» or «Adjust Vertical Speed Adjust» (meaning reduce vertical speed). By contrast a «preventive» RA may be issued which simply warns the pilots not to deviate from their present vertical speed, announcing, «Monitor vertical speed» or «Maintain vertical speed, Maintain». TCAS II systems coordinate their resolution advisories before issuing commands to the pilots, so that if one aircraft is instructed to descend, the other will typically be told to climb — maximising the separation between the two aircraft.
TCAS II issues the following types of aural annunciations:
•Traffic advisory (TA)
•Resolution advisory (RA)
•Clear of conflict
\"\"When a TA is issued, pilots are instructed to initiate a visual search for the traffic causing the TA. If the  traffic is visually acquired, pilots are instructed to maintain visual separation from the traffic. The pilot training programs also indicate that no horizontal maneuvers are to be made based solely on information shown on the traffic display. Slight adjustments in vertical speed while climbing or descending, or slight adjustments in airspeed while still complying with the ATC clearance are acceptable.
When an RA is issued, pilots are expected to respond immediately to the RA unless doing so would jeopardize the safe operation of the flight. This means that aircraft will at times have to manoeuver contrary to ATC instructions or disregard ATC instructions. In these cases, the controller is no longer responsible for separation of the aircraft involved in the RA until the conflict is terminated. Although the system occasionally suffers from false alarms, pilots are now under strict instructions to regard all TCAS messages as genuine alerts demanding an immediate, high-priority response.
\"\"The Überlingen mid-air collision is a notorious disaster, which occurred at 21:35 UTC on 1 July 2002 between Bashkirian Airlines Flight 2937 (a Tupolev Tu-154M passenger jet carrying 60 passengers – mostly children – and 9 crew from Moscow to Barcelona) and DHL Flight 611 (a Boeing 757-23APF cargo jet manned by two pilots from Bahrain to Brussels) over the town of Überlingen in southern Germany. 71 people on board the two aircraft were killed.
Both aircraft were maintaining flight level 360 (about 11000 m) on a collision course. Despite being over Germany, that part of airspace was controlled from Zürich, Switzerland, by the private Swiss airspace control company «Skyguide». The only air traffic controller handling the airspace, Peter Nielsen, was working two workstations at the same time. He did not realize the problem in time and thus failed to keep the aircraft at a safe distance from each other. Only less than a minute before the accident he realized the danger and contacted Flight 2937, instructing the pilot to descend by a thousand feet to avoid collision with crossing traffic. Seconds after the Russian crew initiated the descent, however, their TCAS instructed them to climb, while at about the same time the TCAS on Flight 611 instructed the pilots of that aircraft to descend. Had both aircraft followed those automated instructions, it is likely that the collision would not have occurred. Flight 611\’s pilots on the Boeing jet initially followed the TCAS instructions and initiated a descent, but could not immediately inform the controller due to the fact that he was dealing with Flight 2937. About eight seconds before the collision, Flight 611\’s descent rate was about 2,400 feet per minute (12 m/s), not as rapid as the 2,500 to 3,000 ft/min (13 to 15 m/s) range advised by TCAS. The Russian pilot on the Tupolev disregarded the TCAS instruction to climb and instead began to descend, as instructed by the controller, thus both planes were now descending. Unaware of the TCAS-issued alerts, Nielsen repeated his instruction to Flight 2937 to descend.
\"\"The aircraft collided at almost a right angle at an altitude of 34,890 feet (10,630 m), with the Boeing’s vertical stabilizer slicing completely through Flight 2937’s fuselage just ahead of the Tupolev’s wings. The Tupolev exploded and broke into several pieces, scattering wreckage over a wide area. The crippled Boeing, now with 80% of its vertical stabilizer lost, struggled for a further seven km before crashing into a wooded area.
Only one air traffic controller, Peter Nielsen of ACC Zurich, was controlling the airspace through which the aircraft were flying. The other controller on duty was resting in another room for the night. This was against the regulations, but had been a common practice for years and was known and tolerated by management. Due to maintenance work, Nielsen had a stand-by controller and system manager on call.  Nielsen was unaware of this. When Nielsen realized that the situation had subtly increased beyond his span of control, it was too late to summon assistance.
\"\"In the minutes before the accident, Nielsen was occupied with an Airbus on a delayed Aero Lloyd, Flight 1135, approaching Friedrichshafen Airport. Handling two workstations at once, Nielsen struggled with the malfunctioning phone system that he was trying to use to call the Friedrichshafen airport to announce the approaching Aero Lloyd. The main phone lines at «Skyguide» were down due to maintenance work, and the backup line was defective. This caused Nielsen to spend more time than he expected coordinating the Airbus’ arrival into Friedrichshafen, and to miss several calls from aircraft. The faulty phone lines also prevented adjacent air traffic controllers at Karlsruhe from phoning in a warning. Due to these distractions he did not spot the danger until about a minute before impact. Had he been aware of the dangerous situation earlier, he could have kept the aircraft at a safe distance from each other. They would have been separated and their collision avoidance systems would not have issued instructions. Additionally, after Nielsen instructed the Russian crew to descend, he returned to the situation with the Airbus bound for Friedrichshafen, and did not hear the DHL aircraft report of its TCAS descent.
Another factor was that the ground-based collision warning system, which would have alerted the controller to imminent collisions early, had been switched off for maintenance; Nielsen was also  unaware of this.
One of the factors was confusion and hesitation of Russian pilots, who received 2 conflicting commands: to descent (from the controller) and to climb (from TCAS). Due to the fact that TCAS implementation in Russia was still in progress at that time, this system was considered only as «additional», while controller’s instructions had priority. When pilots of Tu-154 received contradictory commands, they didn’t know which order to follow, and after some moments of arguing among the crew, they decided to follow controller’s order. At first «Skyguide» tried to use this hold-up with Russian pilots’ response to blame the crew for poor English and not understanding instructions.
The accident raised questions on how pilots must react when they receive conflicting orders from the TCAS and from air traffic control. The TCAS is programmed to assume that both crews will promptly follow the system’s instructions. The operations manual now clearly states that TCAS should always take precedence over any ATC commands: «If an instruction to manoeuvre is received simultaneously from an RA (resolution advisory, the command issued by the TCAS) and from ATC, the advice given by RA should be followed.»
When an RA occurs, the pilot flying should respond immediately by direct attention to RA displays and maneuver as indicated, unless doing so would jeopardize the safe operation of the flight, or unless the flight crew can assure separation with the help of definitive visual acquisition of the aircraft causing the RA. In responding to a TCAS RA that directs a deviation from assigned altitude, communicate with ATC as soon as practicable after responding to the RA. When the RA is removed, the flight crew should advise ATC that they are returning to their previously assigned clearance or should acknowledge any amended clearance issued.
Prior incidents
\"\"About a year before the Bashkirian Airlines-DHL collision there had already been another incident involving confusion between conflicting TCAS and ATC commands. During the 2001 Japan Airlines  mid-air incident, two Japanese airliners nearly collided with each other in Japanese skies. Both aircraft had received conflicting orders from the TCAS and ATC; one pilot followed the instructions of the TCAS while the other did not. A collision was only averted because one of the pilots made evasive maneuvers based on a visual judgement. The aircraft missed each other by less than 100 metres (330 ft), and the abrupt maneuver necessary to avert disaster left about 100 occupants injured on one aircraft, some seriously. As a consequence Japan called for measures to prevent similar incidents. However, ICAO did not take action until after the crash over Germany. In addition four near misses in Europe occurred before the German disaster, because one set of pilots obeyed the air traffic controllers while the other obeyed TCAS. ICAO decided to fulfill Japan\’s request 18 months after the Japan Airlines incident.
Nearly two years later, on 24 February 2004, Peter Nielsen, the air traffic controller on duty at the time of the accident, was stabbed to death by an architect, Vitaly Kaloyev, who had lost his wife and two children in the accident.
On 19 May 2004, the German Federal Bureau of Aircraft Accidents Investigation (BFU) published its determination that the accident had been caused by shortcomings in the Swiss air traffic control system supervising the flights at the time of the accident and by ambiguities in the use of TCAS.

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