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Home | Update | LATEST ISSUE | Gallery | FR Profile | Datafiles | FR 1/98 TCAS T O REDUCE COLLISSION RISKS by Claude Pascal The traffic on the airways is continuously increasing. Will flight safety be already at stake in the near future? TCAS, a collision avoidance system, is being praised everywhere as the solution. Here is a case study that was done by Claude Pascal, an experienced airline pilot. Vacation time. Low-cost flights. The sky is full of aircraft and the airways are crowded. The slot coordination center in Brussels is very busy. We are sitting in our Boeing and are still waiting for the start-up clearance although all of the passengers are on board. 40 minutes delay for our flight to the Canary Islands. Finally we get the clearance to start the engines and, ten minutes later, to take-off. We don't get immediately cleared to our desired altitude of 37000 ft for which we have filed the flight plan. Instead, we are told by the controller to increase our speed. The air traffic controller knows our exact altitude (to an accuracy of 30 meters), as well as, our flight path and speed. In the departure and approach sectors, the aircraft are spaced with no less than four to six nautical miles, depending on the size of the aircraft. Later, in cruise flight, the spacing is furthermore increased. Only a few minutes after take-off, we are handed over to another controller on a new frequency. The next sector, the next controller. Since the flightplan was relayed to all the sectors on our route before take-off, the controllers know our intended route and speed. Also, the controllers of neighboring sectors are connected via a standing phone line. A perfect organized system - once you get airborne. France. A female voice on the radio. She is talking to several other aircraft in her own language. The same thing happens in Greece or in Spain. This has nothing to do with what one learned in school. One only understands little - too little. Often, we can't get the situational awareness where the other aircraft are and which course and altitude they are flying. Is there a possibility of an undesired approach which is out of our control? We are reporting - like always - in English, the international aviation language, and receive, also in English, the clearance to cross southern France up to the Pyrenees. After being instructed to squawk 1035, the combination is set in the transponder. Whenever we are scanned by a radar beam from a ground station, which is reflected by the aircraft and received by the ground station as a weak signal, the transponder is sending out a strong radar signal to the ground station to amplify the reflected signal. The signal is coded with a four-digit number and the aircraft's altitude. This gives the controller a much better signal and the radar display shows not only our exact position but, also our altitude and speed. For some years now we have been carrying equipment on board that also works with the transponder principle and which is probably one of the biggest innovations in respect to flight safety: TCAS - traffic alert and collision avoidance system. It is an on-board system to avoid undesired approaches and collisions in midair. The system can receive the transponder signals of up to 45 other aircraft in a distance of up to approximately 40 nm. It then assesses the flight passes in relation to the aircraft's own flight pass. The cockpit display shows the other aircraft as symbols, allowing us to monitor their flight passes and altitudes. Would we come too close to another aircraft that is equipped with TCAS or at least with a altitude encoded transponder, we would first be getting an aural warning. Further approaching the other aircraft, with the danger of a collision, the system would give us avoidance guidance which, if both aircraft are TCAS equipped, would be coordinated between each other. The resolution advisories, as the avoidance signals are called, are also displayed on the vertical speed indicator. TCAS allows us to avoid up to three aircraft simultaneously. The maneuvers are always flown in the vertical and are coordinated between the aircraft - as long as all involved aircraft are equipped with TCAS. This is the main problem: aircraft that do not have a transponder are invisible to TCAS. If TCAS doesn't receive an altitude code, it is not able to give resolution advisories. Evasion maneuvers must then be initiated by the pilot and are not automated. We have passed the Pyrenees and have reached our final cruise altitude of 37000 feet. Crossing Spain, we have to change frequencies five times. We have to deviate from the planned route slightly because of "military activity". We can't hear any of that activity on the radios since the military aircraft are transmitting on a frequency band that we can't use. There is no coordination between us and these aircraft. Passing Portugal we are reaching the airspace of Marocco. The controller in Lisbon says good-bye: "radar service terminated, change to Casablanca, good-bye." This transmission reveals another problem that is prevalent in large parts of the world: There is no radar monitoring! The controller from Casablanca speaks bad English and the transmission quality is well below standards. He is talking in French with some unknown north African aircraft and has difficulties to understand us when we call him. It is good that there is only a small number of reporting points on the few airways in this controllers sector and luckily they sound similar in all languages. We have just passed a sector boundary and have reported our altitude and position when suddenly something happens that, statistically, occurs only seldom: An Airbus A340 is coming opposite at the same airway and, although the airliner should pass us either 2000 ft underneath or above our altitude, it has a clearance to climb to 39000 ft. The controller in Lisbon expects the Airbus to be at 39000 ft, not knowing that it had just shortly prior received the clearance to climb from Casablanca Control. And that is why we are not aware of the Airbus either when our TCAS suddenly sets off a warning: "Traffic, Traffic, Descend, Descend". We are reacting immediately, switch off the autopilot and push over the aircraft. Only seconds later and with only a few meters of vertical separation, the Airbus passes overhead. The TCAS has saved the lives of 400 people. An isolated case? Not really. But not an accident either since both aircraft were fitted with TCAS and reacted fast enough - nothing for the front page but another case for the files. Since then, two years have gone by. Still, the situation of the air traffic in the airspace over Marocco hasn't changed. I myself have had three near-misses in five years and have heard of another four in my professional environment. Our aircraft were fitted with TCAS five years ago. But there are still many aircraft out there, especially outside of the USA and Europe that are not fitted with the collision avoidance system. Near-misses can happen anywhere. Studies revealed that it will take at least 12,5 seconds to accomplish the proper evasive maneuver after visually picking up another aircraft from the cockpit and realizing the potential for a collision. Airliners on opposing headings have a closure rate of 1600 k/h, approximately 450 meters per second. The evasive maneuvers must begin at a distance of at least two miles (3,7 kilometers). Otherwise, there is no escape. Flight safety can only be ensured if all airliners are equipped with TCAS, if more HF frequencies are available of the oceans (and in better quality), or, better, if satellite communications are established. Furthermore, small aircraft should have a transponder with altitude encoding and their flight should be known to the controller. Some airlines already fly offset of the published airways to reduce the potential for a near miss. But, is that really the right way? Two or more airliners, small aircraft, helicopters, balloons, or military jets come closer to each other than it is legally allowed, such creating the danger for a collision. The minimum distance of the aircraft depends on the airspace they are flying in. In the approach or departure sector, the spacing can be reduced to a minimum of approximately four nautical miles (7,4 kilometers); in the USA this minimum is sometimes further reduced to two miles (3,7 km) in the final approach. In Iran or other countries that don't have a civil operated radar (like in almost all of the African countries), the aircraft are even supposed to be spaced at a minimum of 10 minutes (approximately 80 nm). The vertical spacing almost anywhere in the world is 1000 ft for aircraft on opposite headings. Beginning at 29000 ft altitude, the vertical spacing between the flight levels is doubled. Since March of 1997, this does not apply to the North Atlantic any more where the aircraft are spaced with only 1000 ft vertical distance and with a lateral distance of 10 minutes (up to 37000 ft). The spacing Enroute is most often expressed in flight minutes since, without radar, this is the only practical way to control the distance. Aircraft flying according to visual flight rules (VFR) in Germany, are fitted in between the IFR flight levels. The danger arises when aircraft change flight altitudes without coordination. VFR aircraft can climb or descend through IFR flight levels without talking to a controller beforehand. Since light aircraft are often equipped with only a simple transponder (no altitude encoding), the TCAS of an airliner can't pick the radar signal up, such not being able to give the airliner crew a resolution advisory. Only the incidents that almost caused a serious accident or that caused an accident with many fatalities are being investigated. Many near-misses either go undetected by the crews or, are not being reported. Still, the possibility for a mid-air collision is small. For one, because aircraft are operating on published airways and are one way roads depending on the flight level. Furthermore, the airways are radar monitored by controllers. In order to avoid traffic pile ups in certain highly frequented regions, aircraft are given so called slot times or CTOTs (calculated take-off times) for their take-off. Also, at dedicated reporting points, pilots must report the time of their overflight, altitude, and the expected overflight time of the next waypoint, giving everybody else on the same frequencies a heads-up. When the airspace is radar monitored, the pilots are often released from the mandatory calls at the reporting points. Otherwise the frequencies would be blocked. If workload permits, good controllers, and there are many of them in the USA and in Europe, will always give traffic information anyway. By introducing modern systems, such as autopilot, INS, and satellite navigation, the navigational capabilities of the aircraft in maintaining the desired track, altitude and speed have made tremendous progress over the recent years. This makes aircraft fly much more precise both, laterally and horizontally. This capacity is highly desired for precision landings. Enroute, however, the ultimate precision is unintentionally generating a new risk. If aircraft fly on opposing tracks and at the same altitude, they will almost surely collide. In earlier times of air traffic, these aircraft would probably have passed each other with a mile of distance. The lateral distance of aircraft depends much on the country they are flying in and on the phase of flight. In German airspace, the spacing in the approach phase can be reduced to 4 nm since the aircraft speeds are much lower than in cruise flight. Slower aircraft are also more maneuverable since the turn radius is smaller. In the airport vicinity, it is possible to fly a turn with a diameter of 4 nm. In cruise flight at a speed of 860 km/h, using standard bank angles of 15 degrees, a 360 degree turn would have a diameter of 43,4 kilometers! In order to avoid an aircraft coming head-on by only 100 meters, it takes at least five seconds and would require an immediate banking of 25 degrees. During this short time, the two aircraft would have already come closer by 2,5 kilometers. From page 60 of FLUG REVUE 1/98 Home | Update | LATEST ISSUE | Gallery | FR Profile | Datafiles | FR 1/98 Copyright 1997/98 by Motor-Presse Stuttgart. All rights reserved. Last updated December 5, 1997 FLUG REVUE, Ubierstr. 83, 53173 Bonn, Germany