On March 24, an Air India Airbus A319, flying to Kathmandu from New Delhi, and a Nepal Airlines Airbus A320 came close to a mid-air collision. The Air India aircraft, which was flying at 19,000 ft, descended to 15,000 ft following Kathmandu ATC “clearance”. The Nepal Airlines aircraft, flying from Kuala Lumpur, was also at 15,000 ft, according to the reports. While the Air India plane climbed sharply, the Nepal Airlines aircraft descended to 7,000 ft to avoid a collision, the reports added. The Civil Aviation Authority of Nepal (CAAN) has suspended three air traffic controllers for “carelessness”, according to CAAN spokesperson Jagannath Niroula, PTI reported. What probably went wrong? Whether an aircraft is flying in terminal airspace (airspace near an airport which is mostly used by arriving and departing traffic) or is in enroute airspace (away from an airport; for example, in cruise), planes are separated both vertically and horizontally for safety. In the incident involving the Air India and Nepal Airlines aircraft, either the vertical or horizontal separation minima or both were breached. What are the rules of separation between aircraft? * Vertical separation Aircraft flying below 29,000 ft are separated vertically by 1,000 ft. Aircraft flying above 29,000 ft are separated by 2,000 ft while supersonic aircraft flying above 45,000 ft are separated by 4,000 ft (Currently, there are no supersonic commercial jet in service). However, between 29,000 ft and 41,000 ft, modern jets are allowed to fly with vertical separation of 1,000 ft under Reduced Vertical Separation Minima (RVSM) rules. RVSM allows more aircraft to fly at fuel-efficient high altitudes, thus increasing airspace capacity and reducing fuel burn. An airline is allowed to fly under RVSM rules only after meeting regulatory requirements as it requires further training of the crew and onboard equipment. * Horizontal separation Horizontal separation between aircraft is of two types: Longitudinal and lateral. Air traffic controllers calculate longitudinal separation between aircraft using both distance (in radar-covered airspace) or time (in non-radar airspace). In airspace with radar coverage, longitudinal (one aircraft flying in front and the other behind it) separation of 5 nautical miles to 3 nautical miles is maintained between aircraft flying at the same altitude and in the same direction. If based on time, a separation of 15 minutes to 10 minutes is maintained between two aircraft depending on how frequently a pilot can relay his ‘position’ (where the aircraft is) and speed to the controller. Lateral separation between aircraft refers to the distance between two aircraft flying side by side. The degrees by which the tracks of two aircraft diverge from a navigational facility and the distance of the aircraft from the navigational facility help air traffic controllers know if the lateral separation is safe. Can two aircraft cross each other’s paths? Yes, as long as the separation minima is not breached. A 15-minute — or 10-minute if navigational aids allow frequent determination of aircraft ‘position’ and speed — longitudinal separation is maintained for two aircraft whose paths cross. Similarly, if an aircraft climbs or descends through the path of another aircraft, a 15-minute, 10-minute or 5-minute longitudinal separation is maintained. Radar-controlled airspace & role of Air Traffic Control (ATC) Air traffic controllers are primarily responsible for ensuring safe separation between aircraft. In radar-covered airspaces, the position of all aircraft can be seen on the controller’s screen. An aircraft’s transponder also relays its altitude, speed and heading to the ATC, all of which help the controller ensure vertical and horizontal separation between aircraft. If a potential conflict arises, the controller ‘vectors’ (issue a different heading) an aircraft away from the other. Moreover, as there are also smaller, slower aircraft out there, which can find themselves in potential conflict with heavier, faster jets, all aircraft adhere to a speed restriction of 250 knots below 10,000 ft. In any incident involving violation of separation standards, investigators not only look at the role of ATC but also examine crew action, equipment, existing procedures (if they can be improved further), human factors, among others. Non-radar controlled airspace While radar-covered airspaces allow ATC to “see” the position of aircraft, the distance between them, heading, altitude etc, making the task of ensuring separation easier, large parts of the world do not have radar coverage. The oceans are not covered by radar. Large parts of South and Central America, Africa and Asia too do not have radar coverage. Where there is no radar coverage, procedural separation is followed. To ensure longitudinal and lateral separation between aircraft, en route controllers depend on ‘position’ reports from pilots, where the pilot relays the aircraft’s position, altitude, ETA at next reporting point, among others. From the ‘position’ reports, controllers are able to calculate the time separating two aircraft (longitudinal separation) and its geographical position (lateral separation). As controllers cannot “see” the aircraft on radars, in such phases of flight, the longitudinal and lateral separation are far greater. Longitudinal separation between two aircraft flying the same track at the same altitude over oceans can be up to 30 minutes and lateral separation 60 miles. But vertical separation remains same at 1,000 ft. Remember Air France 447, which plunged into the Atlantic Ocean in 2009, killing 228 people? It was flying over the oceans, out of radar coverage, when it ran into trouble. Oceanic control on the ground was relying on ‘position’ reports from the crew to track the flight. Uncontrolled airspaces In uncontrolled airspaces, ATC is not responsible for separation between aircraft. It’s up to pilots to maintain separation by following the ‘see-and-be-seen’ rule. TCAS It is unclear from media reports if the crew of Air India and Nepal Airlines took action after being alerted by ATC or onboard TCAS (Traffic Alert & Collision Avoidance System) resolved the potential conflict. TCAS, working independently of ATC, is an onboard system that alerts pilots if it senses a potential mid-air collision. While one aircraft’s TCAS will ask the crew to “climb, climb”, the other aircraft’s TCAS will order its crew to “descend, descend”. In such a situation, the TCAS’ commands are followed and ATC’s instructions ignored to avoid confusion. What will happen if two aircraft violate separation standards? One, which is obvious, there is risk of a mid-air collision. Second, is the risk posed by ‘wake turbulence’. An aircraft’s wingtip produces vortices called ‘wake’. Bigger the aircraft, more powerful the ‘wake’. Called ‘wake turbulence’, it has led to loss of control of aircraft flying into it and, in worst cases, even resulted in crashes. Even during takeoff, for this reason, the ATC gives a gap of a few minutes — usually 2 minutes — between two departing aircraft, so that one aircraft doesn’t encounter the ‘wake’ of the one that just took off. The crash of an IAF C-130J Super Hercules on March 28, 2014 is believed to have been caused by the huge transport inadvertently flying into the ‘wake’ of the lead plane during a tactical training mission, leading to a loss of control at low altitude and the accident that killed all five crew members, The Indian Express reported on April 23, 2014. Preliminary findings of the detailed inquiry under way point to a “wake turbulence” incident in which the C-130J, which was part of a two-aircraft formation practising insertion of paratroopers, stalled at a low level after hitting the ‘wake’ of the lead aircraft, the report said.
