TCAS TRAFFIC DISPLAY IS NOT RADAR DISPLAY

TCAS in aviation was mandated to facilitate greater situational awareness to the flight crew and serve as a last safety net to avoid mid-air collisions. Over the years, design Improvements in accuracy and response capability of TCAS systems are able to generate Traffic alerts and Resolution Advisories with manoeuvre guidance in case of RAs.

 Of late there have been several reported incidents of crew resorting to pre-emptive manoeuvres solely based on TCAS displays showing traffic proximity. Manoeuvres initiated to achieve self-separation or sequencing, solely on the information displayed on TCAS systems can often result in degraded safety margins and should never be attempted by the flight crew.

 "An A340 reduced its speed on its own, miles too early on approach, to increase the distance from the preceding aircraft [based on the TCAS traffic display]. It messed up the sequence and an A320 was then only 4 NM behind it was obliged to perform an "S" for delay".

A B737 is cleared to climb to 3000 ft. A VFR on an opposite track is level at 3500 ft, but offset horizontally. The controller provides traffic information to the B737. The pilot reports two targets on the TCAS traffic display and shortly after reports a left turn to avoid this traffic. Fortunately, the controller instructs the B737 to stop climb at 2500 ft, because the inappropriate turn reduced the separation.

Examples are many and continue to occur. Regulations on the use of TCAS traffic display at ICAO PANS-OPS Doc 8168, chapter 3, section 3.2, states that “Pilots shall not manoeuvre their aircraft in response to traffic advisories (TAs) only”. This point is also emphasised in ICAO ACAS II Training Guides for pilots. Confusing and nonstandard responses from pilots are also commonly encountered. Responses such as “TCAS Contact” or “We have the Traffic on TCAS”, on receiving traffic information from ATC, provide no added value. 

Flight crew need to appreciate that the TCAS traffic display is not designed to support self-separation manoeuvres, but to aid visual acquisition of an intruder. It gives only a snapshot of the relative horizontal and vertical position of other aircraft in the vicinity. The lack of speed vector and the possibility of rapidly changing relative bearings, it is extremely difficult to anticipate the evolution of the situation based solely on the TCAS traffic display.

Air traffic controllers, on the other hand, base their actions on the comprehensive information shown on the radar display, which enables them to provide a safe and expeditious air traffic flow. The radar display also provides velocity vectors and the controller is able to predictive information of crossing traffic. The TCAS traffic display does not provide the information necessary for the provision of self-separation and sequencing.

When operating IFR in a controlled airspace, ATC is responsible to provide safe separation between traffic. TCAS II is designed to trigger an RA command, should there be a potential or imminent risk of collision between two transponder equipped aircraft. Correct response to annunciated RAs will safely resolve such situations.

If an aircraft is close to its operational ceiling or where the indicated rate climb is excessive, it may simply be unable to follow the RA commands. In these cases, the pilot should continue to manoeuvre at the maximum rate possible consistent with safety and performance. If unable to climb, it may even maintain level flight( engine out conditions).The TCAS equipment in the target aircraft will be able to detect the reduced response and adjust the manoeuvre commands to achieve the desired separation.

Airline operators need to include an unambiguous policy in their Operations Manual procedures, prohibiting self-separation manoeuvres by the crew based solely on TCAS displays.