Collision Avoidance Logic

Collision avoidance logic is the critical part of TCAS system, the aircraft surveillance data is passed to collision avoidance logic to determine which type of advisory should be initiated. This sub-chapter aims to provide the basic knowledge of CAS logic and describes the logic function of TCAS II, Version 7.1. TCAS II, Version 7.1 is the latest TCAS system implemented world widely and mandatory by the authority. It improves aural prompt and add improved reversal logic compared with version 7.0.

Basic Concepts Introduction • Traffic Advisory:

A TA is the particular indication (Aural prompt and visual caution on traffic display ) issued by TCAS system to inform pilots or flight crews that the nearby traffic has entered or projected to the protected volume of own aircraft, threat maybe happen if the situation continues to deteriorate. It reminds the pilots and crews to raise the attention. • Resolution Advisory:

A RA issued by TCAS system that providing advices to pilots and flight crews and requiring them to react immediately (climb or descend, etc.) in order to maintain the safe separation from the intruders. • Closest Point Approach (CPA):

CPA locates at the centre of aircraft protective volume that is referred to decide the threat level.

• Sensitivity Level (SL):

Define CAS protection level based on its own aircraft altitude. It has 7levels utterly, from level 1 to level 7, the high SL suggests high degree protection is served. The definition of SL and the alarm thresholds accompany with it are shown in Figure 2.14. SL is used to determine the category of ‘protected volume’. Figure 2.15 and Figure 2.16 present TCAS horizontal and vertical dimension protection volume. It was defined safety area around own aircraft according to time.

• Warning Time - T au:

Tau is the estimated time (second) to CPA calculated by aircraft speed and attitude. It is the principle term to issue TCAS RA and TA and works together with SL.

Figure 2.14:Sensitivity Level Definition and Alarm Thresholds[33]

Collision Avoidance System Function for TCAS II

CAS logic function can be presented as Figure 2.17, the operation of TCAS II is summarized as below:

• Surveillance

Figure 2.15: TCAS Protection Volume (horizontal)[35]

Figure 2.16:TCAS Protection Volume (vertical)[35]

• Threat detection

• Resolution Determination and Coordination

The following paragraphs will discuss these operations separately.

Surveillance TCAS can be looked as a mini Secondary Surveillance Radar (SSR) on aircraft, surveillance is its fundamental function that re- sponsible to provide information of the proximate aircraft with slant range, altitude and bearing got by transponder interrogating. Typically, the re- quired reliable surveillance range is 14nmi. Actually, the maximal range that TCAS could cover is up to about 30nmi and could track up to 30 nearby traffic simultaneously.

Nearby Traffic Tracking Once the other traffic is determined to be the target aircraft, TCAS system will monitor (tracking function) it until outside of the surveillance range. The tracking function gets partial inputs from surveillance function and cooperate with CAS logic to assess the inva- sion geometry, including time to CPA and the horizontal distance at CPA. While the altitude, relative altitude and vertical speed are calculated by CAS logic for both target aircraft and own aircraft. The nearby traffic tracking function keep all the target aircraft under watch to locate and track them.

Figure 2.17:CAS Logic Functions[35]

Threat Detection Threat detection is responsible to determine whether a TCAS TA or RA need be issued or not. Time to CPA received from nearby traffic tracking function is compared with tau to decide the alter. As Fig- ure 2.14 presented, tau values change along with SL - aircraft altitude, the higher aircraft altitude with the bigger tau value. Miss distance is another threshold of TCAS used to declare a threat. At some particular condition, such as one aircraft is level off, the other aircraft is descending with a slow vertical speed, while these two aircraft are in the collision scenario. In or- der to avoid the tau value never meet, the miss distance is applied. Thus how to decide what kind of threshold is used depends on which of them is meet firstly. TCAS continuously calculate the relative range and altitude between the target aircraft and own aircraft, the alert only will be issued when horizontal and vertical threat are both satisfied.

Resolution Determination and Coordination If TCAS TA is trig- ger, the traffic information on traffic display will change to yellow and aural prompt will provide to flight crew to get awareness. If a threat is affirmed, a TCAS RA will be selected. Correspondingly, the RA will be annunciated to the pilots and the required reaction will be presented on traffic display. TCAS only can perform its function with the aircraft install TCAS system. In order to select an appropriate RA, two steps are considered. Firstly,

deciding the RA sense, such as upward or downward. Then, choosing the strength for the advisory. As Figure 2.18 shown, suppose CPA is calculated, the vertical separation between two conflict aircraft is the marked as ‘A’ and ‘B’. The downward sense logic can provide greater vertical separation compare to the upward one, therefore the descending RA is selected to own aircraft.

Figure 2.18:RA Sense Determination[35]