Traffic collision avoidance system
It is a type of airborne collision avoidance system mandated by the International Civil Aviation Organization to be fitted to all aircraft with a maximum take-off mass (MTOM) of over 5,700 kg (12,600 lb) or authorized to carry more than 19 passengers.
ACAS/TCAS is based on secondary surveillance radar (SSR) transponder signals, but operates independently of ground-based equipment to provide advice to the pilot on potentially conflicting aircraft.
[2] Although ATCRBS airborne transponders were available, it was not until the mid-1970s that research focused on using their signals as the cooperative element for a collision avoidance system.
This technical approach enabled an independent collision avoidance capability on the flight deck, separate from the ground system.
In 1981, the FAA decided to implement the Traffic Alert and Collision Avoidance System (TCAS), which was developed based on industry and agency efforts in the field of beacon-based collision avoidance systems and air-to-air discrete address communication techniques that used Mode S airborne transponder message formats.
[3] A short time later, prototypes of TCAS II were installed on two Piedmont Airlines Boeing 727 aircraft, and were flown on regularly scheduled flights.
Although the displays were located outside the view of the flight crew and seen only by trained observers, these tests did provide valuable information on the frequency and circumstances of alerts and their potential for interaction with the ATC system.
[3] Later versions of TCAS II manufactured by Bendix/King Air Transport Avionics Division were installed and approved on United Airlines airplanes in early 1988.
Then, by extrapolating current range and altitude difference to anticipated future values, it determines if a potential collision threat exists.
Training programs also indicate that no horizontal maneuvers are to be made based solely on information shown on the traffic display.
Once the RA is reported by the pilot, ATC is required not to attempt to modify the flight path of the aircraft involved in the encounter.
Some countries have implemented "RA downlink" which provides air traffic controllers with information about RAs posted in the cockpit.
[6][2] One potential problem with TCAS II is the possibility that a recommended avoidance maneuver might direct the flight crew to descend toward terrain below a safe altitude.
Manufacturers and authorities recognized the need for an alternative to TCAS; this led to the development of the Traffic Advisory System.
The system structure, components, operation, traffic display and TA logic are identical, but the minimum operational performance standards (MOPS) of TAS allow some simplification compared to TCAS I:[14][15] The following documents contain all of the differences between TCAS I and TAS: In spite of all this, most of the manufacturers do not take the above-mentioned opportunities to make simplified devices.
Automatic dependent surveillance – broadcast (ADS–B) messages are transmitted from aircraft equipped with suitable transponders, containing information such as identity, location, and velocity.
This reduction in interrogations reduces the use of the 1030/1090 MHz radio channel, and will over time extend the operationally useful life of TCAS technology.
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.
Following extensive Eurocontrol input and pressure, a revised TCAS II Minimum Operational Performance Standards (MOPS) document has been jointly developed by RTCA (Special Committee SC-147[22]) and EUROCAE.
As a result, by 2008 the standards for Version 7.1 of TCAS II have been issued[23] and published as RTCA DO-185B[7] (June 2008) and EUROCAE ED-143 (September 2008).
;[25] and the improved handling of corrective/preventive annunciation and removal of green arc display when a positive RA weakens solely due to an extreme low or high altitude condition (1000 feet AGL or below, or near the aircraft top ceiling) to prevent incorrect and possibly dangerous guidance to the pilot (Change proposal CP116).
TCAS III was the "next generation" of collision avoidance technology which underwent development by aviation companies such as Honeywell.
In addition, some reliable source of position (such as Inertial Navigation System or GPS) is needed on the target aircraft in order for it to be encoded.
[30][31] 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.
This is mainly because of the TCAS-RA inherently possessing a more current and comprehensive picture of the situation than air traffic controllers, whose radar/transponder updates usually happen at a much slower rate than the TCAS interrogations.
On 25 September 2009 FAA issued Advisory Circular AC 20-151A[37] providing guidance for obtaining airworthiness approval for TCAS II systems, including the new version 7.1.
On 5 October 2009, the Association of European Airlines (AEA) published a Position Paper[38] showing the need to mandate TCAS II Version 7.1 on all aircraft as a matter of priority.
On 25 March 2010, the European Aviation Safety Agency (EASA) published Notice of Proposed Amendment (NPA) No.
[44] Rockwell Collins currently announces that their TCAS-94, TCAS-4000 and TSS-4100 TCAS II compliant systems are software upgradeable to Change 7.1 when available.
[51] Airbus and Honeywell have tested a proposed automated system, where if the pilot ignored a Resolution Advisory, the aircraft's autopilot would automatically take evasive action.
