Traffic Collision Avoidance Systems (TCAS)
TCAS (Traffic Alert Collision Avoidance System) is a surveillance and collision avoidance system that alerts aircrews when other aircraft enter a specified airspace envelope surrounding an aircraft. It's a backup radar station (transmits to, and receives transmissions from, other TCAS equipped aircraft). TCAS operate independently of the ground-based air traffic control (ATC) system and provide collision avoidance protection for a wide range of aircraft. TCAS systems all include some form of collision threat alerting as well as a traffic display.
TCAS Description & Operations
On the ND, or Navigation Display, the analysed traffic information is displayed as symbols. It's important to note that altitude and vertical motion data are only available if the signal is received from a mode C or mode S transponder. Otherwise, there will be no altitude information and no vertical motion arrow on the related symbol on the ND.
TCAS can calculate the other aircraft's closure rate relative to the own aircraft since it checks the other aircraft's relative distance in small time intervals. The closure rate is the most crucial variable, because it is a fail-safe key to predicting a meaningful collision. Complicated trigonometric computations of flight trajectories and ground speeds are unnecessary and can lead to erroneous extrapolations. It's worth noting that no heading or bearing information is needed to calculate a TCAS alarm.
When TCAS detects that an aircraft's distance and closure rate have reached critical levels, it issues audible and visual alerts to the pilots. It also computes aural and visual pitch directives if necessary to resolve a dispute. If the other aircraft also has TCAS II, these pitch commands are synced with the other aircraft's pitch commands to ensure that both aircraft do not ‘escape' in the same direction. It is possible to coordinate three aircraft.
It's important to note that TCAS only gives vertical guiding and not lateral guidance. TCAS also disregards performance limitations. In other words we can say, TCAS may still issue a climb command when flying at maximum altitude!
TCAS Components
TCAS requires several other systems in addition to a Transponder to function, including:
- Inertial Reference System (IRS) data as attitude & vertical motion data
- Position sensors for gears (as the extended gear disturbs the lower directional antenna, bearing detection of traffic flying below the own aircraft must be inhibited)
- Radio Altimeters (TCAS must know the radio altitude as the alarm logic varies with the height above ground)
- GPWS (disregards TCAS advisories in the event of a wind shear or ground proximity warning)
- Separate Display unit in case of Conventional aircraft. It may be integral part of digital VSI. (NOTE: In modern aircraft its displays on ND)
TCAS raises an alarm when an intruder aircraft reaches the protected region around an aircraft. The time to the Closest Point of Approach (CPA) is used to define the area's threshold (the time-to-go is distance divided by closure rate, both combined vertically and horizontally). When an intruder arrives, the protective area can be separated into two regions: one in which a Traffic Advisory (TA) message will be issued, and one in which a Resolution Advisory (RA) message will be generated.
TCAS Display on ND
Traffic Advisory (TA)
The pilot receives the Traffic Advisory (TA) warnings in the form of the word 'TRAFFIC' shown in yellow on the ND and the aural voice annunciation, 'Traffic, Traffic.' This isn't the highest level of alarm. Its sole objective is to draw attention to an impending conflict.
As soon as an intruder aircraft enters the TA region, TCAS initiates a TA. TCAS assumes the intruder aircraft's relative height is within 1200 feet if no altitude data from the intruder aircraft is provided. If bearing information is given, a yellow solid circle on the ND can be used to identify the intruder. Otherwise, the circle is removed, and lateral distance and relative height are displayed in yellow numerals under the phrase ‘TRAFFIC,' along with a vertical motion arrow (if motion is detected).
Resolution Advisory (RA)
At the highest TCAS alert level, Resolution Advisory (RA) signals are sent, which provide the pilot with aural and visual pitch commands. Because the escape manoeuvre must be flown manually, the pilot must disconnect the autopilot. Any flight director's orders (as well as ATC advisories may have to be ignored). An RA's pitch command is always given the top emphasis. An RA will not occur if no altitude data from the target is provided. If bearing information is given, a red solid square on the ND can be used to identify the intruder. If bearing information is not available, the square is removed, and lateral distance and relative altitude are displayed in red figures under the word 'TRAFFIC', along with a vertical motion arrow (if motion is detected).
TCAS is well acclaimed, but until TCAS IV/ ACAS III is developed and proven, it will not have an angular determination warning. Several solutions are available, including the use of GPS systems and Automated En-route Air Traffic (AERA) systems with accuracy rates of up to 99.99 percent. All aircraft locations, altitudes, and speeds will be evaluated by AERA. The goal is to increase aircraft autonomy and reduce ATC intervention dramatically.
Next Generation System
ADS-B Collision avoidance is a significant part of the FAA’s NextGen plan for transforming Airspace System. ADS-B is an integral part of NextGen program. We will discuss separately in next article about ADS-B.
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