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Home > Air Safety > How airplanes avoid colliding in the air?
How airplanes avoid colliding in the air?
Aviation - Air Safety
Tuesday, 11 December 2007 21:26

Traffic alert and Collision Avoiding System

The Traffic alert and Collision Avoiding System or TCAS is a system that basically identifies the location and tracks the progress of another aircraft equipped with beacon transponders. It is one of the primary system that assists a pilot to avoid colliding with other airplanes in the air. Most modern aircraft, including the Boeing 777, are installed with TCAS that alerts the pilots of any traffic that is on a collision course.

The TCAS is an airborne equipment that sends a signal to another aircraft that is also equipped with a similar equipment. From the replies, the system analyze the intruding aircraft's altitude, closure rate, projected flight path to predict the penetration of the airspace above the threaten aircraft.

The system then display the intruding aircraft visually in the cockpit instrument panel and aurally alert the pilot, such as, "Climb, Climb, Climb!' or 'Descent, Descent, Descent!' of the potential threat. When the threat situation has passed, the aural alert 'Clear of Conflict !' will notify the pilot that the dangers have been averted.

There are two versions of the TCAS system in use, TCAS I and II.

TCAS I is the simplest and less expensive of them but is also less capable than the others. It was designed primarily for general aviation use. It sends signals and interrogates transponders from another airplane. The TCAS I receiver and display indicates approximate bearing and relative altitude of all aircraft within a certain range. When a pilot receives a warning that an aircraft in the area pose a potential threat, it is up to him to visually identify the intruder and notify air traffic control for assistance in resolving the conflict.

TCAS II on the other hand is a more comprehensive system than TCAS I. This system was required to be installed on all commercial air carriers operating in the United States by December 31, 1993. It offers all of the same benefits but issues a more detailed warning to the pilot. The intruder target is plotted and the system is able to tell whether the aircraft if climbing, diving, or in straight and level flight. Once this is determined, the system will advise the pilot to execute an evasive maneuver that will avoid the intruder.

The next generation TCAS III will be almost the same as TCAS II but it will allow pilots who receive warnings, to execute lateral deviations to evade conflicting traffic. This new system will hopefully cut down false alarms since it can more accurately determine an intruder's location.

Over China recently, two Boeing 747's were flying toward each other along the same airway but separated vertically by 2,000 feet. As the jets drew closer, the TCAS from the lower jet warned the pilot, "Climb! Climb!" The pilot did comply but to within just a few hundred feet of the conflicting airliner's belly before he realized it was an erroneous warning. Fortunately, the 422 passengers and crew were able to escape by the Captain's quick evasive action and the worst mid-air collision in history was averted!

Some time last year, a mid air collision between a TU-154 and Boeing 757 would have been prevented by TCAS had the Swiss air traffic official not interfered. It was apparently stated that the command to maneuver by the ATC was given to the Boeing 757 and the TU-154 was recommended to remain at its current level. At that moment prior to the collision, the TCAS was giving the right evasive commands but it seems that the interference of the air traffic official, who twice commanded the airplane to dive within a very short period of time, resulted in the collision.

Then there was the mid-air disaster in November 1996 when a Saudi Arabia Airlines Boeing 747 and a Kazakhstan National Airways Ilyushin IL-76 cargo plane that collided and killed 349 people over India.. In that incident, air traffic controllers had warned the pilot of the incoming Kazakh plane about the outgoing Saudi Arabian Airlines Boeing 747 approaching through the clouds. The Saudi plane was supposed to be at 14,000 feet, the Kazakh plane at 15,000 feet.

It was speculated that the accident was caused by the confused instructions between air traffic controllers and the pilot of an incoming Kazakh plane. The Saudi Captain wanted to climb but the Kazakh plane sought permission to descend. Just before two planes collided, ATC told the approaching Saudi jet to climb to 14,000 feet and instructed the Kazakh plane to descend to 15,000 feet. It appears that the Saudi pilot might have misinterpreted the controller's instructions and climbed higher than 14,000.

One of the plane's engines was found on top of the Ilyushin's tail, indicating the possible point of impact. It was suggested that the Boeing 747 had been flying higher than the Kazakh plane at the time of collision. The cockpit instruments showed that impact occurred at 14,800 feet, which means the Ilyushin was some 200 feet below its prescribed height and the Boeing 800 feet above it. A TCAS II installed on both the airplanes would have averted this disaster!

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