Traffic Collision Avoidance System
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The Traffic alert and Collision Avoidance System (or TCAS) is a computerised avionics device which is designed to reduce the danger of mid-air collisions between aircraft. It monitors the airspace around an aircraft, independent of air traffic control, and warns pilots of the presence of other aircraft which may present a threat of mid air collision (MAC). It is an implementation of the Airborne Collision Avoidance System mandated by International Civil Aviation Organization to be fitted to all aircraft over 5700 kg or authorised to carry more than 19 passengers.
In modern glass cockpit aircraft, the TCAS display may be integrated in the Navigation Display; in older glass cockpit aircraft and those with mechanical instrumentation, a TCAS display replaces the mechanical Instantaneous Vertical Speed Indicator (which indicates the speed with which the aircraft is descending or climbing).
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[edit] Impetus for a collision prevention system
Although research into collision avoidance systems has been ongoing since at least the 1950s, authorities such as the Federal Aviation Administration were only spurred into action after several major mid-air collisions involving great loss of life. Some of these tragic mid-air accidents include:
- Grand Canyon midair collision in 1956;
- The Zagreb mid-air collision in 1976;
- PSA Flight 182, a Boeing 727 which collided with a Cessna 172 in 1978;
- Aeroméxico Flight 498, a 1986 collision similar to PSA Flight 182, which finally spurred the US Congress and other regulatory bodies into action and led to mandatory collision avoidance equipment.
[edit] TCAS Basics
TCAS involves communication between all aircraft equipped with an appropriate transponder. Each TCAS-equipped aircraft "interrogates" all other aircraft in a determined range about their position (via the 1030 MHz radio frequency), and all other craft reply to other interrogations (via 1090 MHz). This interrogation-and-response cycle may occur several times per second.
Through this constant back-and-forth communication, the TCAS system builds a three dimensional map of aircraft in the airspace, incorporating their bearing, altitude and velocity. Then, by extrapolating current position data to anticipated future positions, it determines if a potential collision threat exists.
It should be noted that TCAS and its variants are only able to interact with aircraft that have a correctly operating transponder .
[edit] Relationship to Automatic Dependent Surveillance (ADS)
Automatic Dependent Surveillance-Broadcast (ADS-B) signals are transmitted from aircraft equipped with suitable transponders, containing information such as identity, current position, altitude, and airspeed. The signals are broadcast on the 1090 MHz radio frequency, the same frequency as TCAS responses.
TCAS equipment which is capable of processing ADS-B messages may use this information, in conjunction with normal TCAS messages, to enhance both prediction capabilities and the situational display. This method is known as "Hybrid Surveillance".
Prediction capabilities are improved by the additional information present in ADS-B messages (e.g. airspeed), as well as the fact that ADS-B messages can be passively received over a distance of approximately 100 NM, compared to the 40 NM range of active TCAS surveillance. The identity information present in ADS-B messages can be used to label other aircraft on the cockpit display (where present), improving situational awareness.
The underlying collision avoidance functions in TCAS are not altered while using Hybrid Surveillance.
[edit] Versions of TCAS
[edit] TCAS I
TCAS I is the first generation of collision avoidance technology. It is cheaper but less capable than the modern TCAS II system, and is mainly intended for general aviation use. TCAS I systems are able to monitor the traffic situation around a plane (to a range of about 40 miles) and offer information on the approximate bearing and altitude of other aircraft. It can also generate collision warnings in the form of a "Traffic Advisory" (TA). The TA warns the pilot that another aircraft is in near vicinity, announcing "traffic, traffic", but does not offer any suggested remedy; it is up to the pilot to decide what to do, usually with the assistance of Air Traffic Control. When a threat has passed, the system announces "clear of conflict".
[edit] TCAS II
TCAS II is the second and current generation of TCAS, used in the majority of commercial aviation aircraft (see table below). It offers all the benefits of TCAS I, but will also offer the pilot direct, vocalised instructions to avoid danger, known as a "Resolution Advisory" (RA). The suggestive action may be "corrective", suggesting the pilot change altitude by announcing, "descend, descend" or "climb, climb". By contrast a "preventive" RA may be issued which simply warns the pilots not to deviate from their present altitude, announcing, "monitor vertical speed". 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 craft.
As of 2006, the only implementation that meets the ACAS II standards set by ICAO is Version 7.0 of TCAS II, produced by two avionics manufacturers: Rockwell Collins and Honeywell.
[edit] TCAS III
TCAS III is the "next generation" of collision avoidance technology which is currently undergoing active development by aviation companies such as Honeywell. TCAS III incorporates technical upgrades to the TCAS II system, and will have the capability to offer traffic advisories and resolve traffic conflicts using horizontal as well as vertical manouevring directives to pilots. For instance, in a head-on situation, one aircraft might be directed, "turn right, climb" while the other would be directed "turn right, descend." This would act to further increase the total separation between aircraft, in both horizontal and vertical aspects.
[edit] Current implementation
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. (Only Ground Proximity Warning System warnings have higher priority than the TCAS.) The FAA and most other country's authorities' rules state that in the case of a conflict between TCAS RA and air traffic control (ATC) instructions, the TCAS RA always takes precedence. (If one aircraft follows a TCAS RA and the other follows conflicting ATC instructions, a collision can occur, such as the July 1, 2002 Überlingen disaster. In this mid-air collision, both airplanes were fitted with TCAS II systems which functioned properly, but one obeyed the TCAS advisory while the other ignored the TCAS and obeyed the controller; both aircraft descended into a fatal collision.)
[edit] Regulatory situation around the world
| Jurisdiction (Agency) | Classification of aircraft | TCAS mode | Date of mandate |
|---|---|---|---|
| USA (FAA) | All commercial turbine powered transport aircraft with more than 30 passenger seats (or MTOW above 33000 pounds ~ 15000kg) | TCAS II | 1 January, 1993 |
| Europe (EASA) | All commercial turbine powered transport aircraft with more than 30 passenger seats (or MTOW above 15000kg) | TCAS II | 1 January 2000 |
| Europe (EASA) | All commercial turbine powered transport aircraft with more than 19 passenger seats (or MTOW above 5700kg) | ACAS II (Effectively TCAS II Version 7.0) | 1 January 2005 |
| Australia (CASA) | All commercial turbine powered transport aircraft with more than 30 passenger seats (or MTOW above 15000kg) | TCAS II | 1 January, 2000 |
| Hong Kong, China (CAD) | All aircraft in Hong Kong with more than 9 passenger seats (or MTOW greater than 5700kg) | TCAS II Version 7.0 | 1 January, 2000 |
[edit] References
- ↑ Introduction to TCAS II Version 7 (PDF)
- ↑ European ACAS II Mandate
- ↑ Explanatory Statement regarding TCAS for CASA(PDF)
- ↑ Airworthiness Notice No. 24 (PDF)
