Aircraft Collision Avoidance Systems

Modern aircraft have a number of systems to their disposal to enhance flight safety. The common term is aircraft collision avoidance systems.

These range from radar, TCAS, portable CAS, terrain awareness system (TAWS), ground proximity warning system (GPWS), synthetic vision in EFIS and the obstacle collision avoidance system.

TCAS or Traffic Collision Avoidance System interrogate other transponders and process these replies from other aircraft enabling them to show these on the PFD, MFD displays or a special IVSI indicator.

Some even issue aural warnings to the pilot helping him to avoid a potential midair collision.

Mode S transponders even exchange data with them so that aircraft equiped with these systems know where you are and are able to remain at a safe distance from you and other traffic.

For the experimental homebuilt aircraft pilot these are usually quite expensive systems, except for the portable CAS and the good old see and be seen system (which has its flaws).

Traffic Detection with Transponders, TCAS

These systems can only operate 100% reliable if all aircraft are equipped with at least a Mode C SSR tranponder. The TCAS system will interrogate aircraft transponders via 1030 MHz and receive replies on 1090 MHz. With this information it is aware of other, possibly conflicting, traffic in the vincinity.

And by constantly interrogating, the TCAS system can see in which direction traffic is moving (speed and time) and if it might be coming too close. If so, then the pilot receives a visual or aural warning of that traffic with an advisory.

As more and more aircraft are being equipped with transponders, mandatory or not, this system works quite well.

Portable systems, PCAS

Portable Collision Avoidance System

A number of manufacturers sell portable passive traffic detectors based on receiving transponder transmissions by other aircraft. It is basically a listening device with indicators on the frontpanel displaying signal strength, translated into range. These devices detect traffic within a 5 to 7 nm radius and relate signal strength to distance.

When traffic gets really close (within 3 nm or so) the indications usually change to be more pronounced. Mode S transmissions can sometimes be displayed in a different color indicator.

More sophisticated models use the mode C altitude transmission of other aircraft to compare that with their own mode C to indicate the relative altitude of the target.

Integration with the aircraft intercom system is also possible. Note that some models are battery powered where others need a connection to the aircraft avionics bus (fused).

Terrain Avoidance

These are used to prevent controlled flight into terrain, CFIT, accidents. Based on radar altimeters in ground proximity warning system (GPWS) and GPS based terrain awareness systems (TAWS). Modern EFIS systems include a form of TAWS by using GPS data to create synthetic vision, a 2 or 3D view of the landscape around the current position of the aircraft.

TAWS

Terrain Awareness and Warning System is the common name of a system to avoid CFIT accidents. It is divided into classes A, B and C where A is the most sophisticated in terms of detection of, for example: imminent impact, reduced terrain clearance, very high rates of descent or negative climb rates after take-off and more.

Class A systems are able to indicate excessive closure rates, aircraft not in landing configuration during descent and ILS deviations, all of this is displayed to the pilots showing all relative data.

GPWS

Based on a radar altimeter system, where the altitude of the aircraft above terrain is presented to the pilot as an indication of absolute altitude of the aircraft above the terrain below (AGL - Above Ground Level). The system transmits a radar pulse vertically downward and listens for the response, time is translated into altitude and indicated. The range is usually from 40 ft to 2500 ft.

Synthetic Vision Systems, SVS

Developed in the late 70s and 80s it is primarily used to enhance pilot situational awareness around the aircraft in relation to terrain and obstacles. The system can be connected to databases with geographical, terrain, aeronautical and other information sets to display that on the PFD or Multifunction display.

For position information a SVS uses inertia or GPS systems, where GPS is used in low cost and entry level systems. As a navigation enhancement socalled highways in the sky (HITS) are pictured on the display to help the pilot navigate easily through rectangles and he/she can remain clear of any obstacles by flying inside these rectangles.

Data from forward looking radar can be added to the SVS to produce a more detailed picture regardless day or night. This way objects not in one of the databases but detected by radar are shown real time on the displays.