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The vertical separation at CPA
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The time to reach CPA or time to intercept (TAU or τ3).
3.1.4.1. VERTICAL SEPARATION
Considering the vertical separation, the range and their variations for a surrounding aircraft, the own aircraft TCAS is able to predict whether the surrounding aircraft will trigger a TA or an RA at CPA.
At CPA, three zones are defined (refer to Figure 3-3). The vertical separation between the intruder and own aircraft at CPA defines the type of advisory to be triggered: Figure 3-3: Closest Point of Approach (CPA)
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Between T0 and T1: TA
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Between T1 and T2: Preventive RA, it instructs the flight crew to avoid certain deviations from current vertical speed. A red sector only is displayed on the Vertical Speed Indicator (VSI)
- Below T2: Corrective RA, it instructs to fly within a vertical speed range, displayed in green on VSI. A red sector on VSI indicates the forbidden vertical speed range.
Values of T0, T1 and T2 vary according to TAU values (refer to 3.1.4.2 – Time to Intercept (TAU)) and the sensitivity level (refer to 3.1.5 – TCAS Envelope).
In Figure 3-3, a corrective RA will be triggered because of Aircraft 1 and a TA will be triggered because of Aircraft 2.
3.1.4.2. TIME TO INTERCEPT (TAU)
TCAS determines the collision threat with TAU rather than the geometric position of CPA. For two aircraft approaching on the same axis, this time is the ratio of the distance between the aircraft by the sum of their speeds.
Distance Range
TAU = or more generally TAU = .
+ Range rate
V V
Own Aircraft Intruder
3 TAU refers to the Greek letter τ.
The collision threat increases when TAU decreases. The TCAS triggers advisories when TAU crosses predetermined time threshold.
This method based on TAU prevents advisories from being triggered if the TAU trend is inverted even though the range between two aircraft decreases (e.g. aircraft of parallel airways in opposite directions illustrated in Figure 3-4).
envelope is divided into four volumes: 2 700 ft and 9 900 ft above, 2 700 ft and 9 900 ft below. The horizontal range may vary from 14 to 100 NM according to the TCAS manufacturer. Refer to your FCOM for more details.
+9 900 ft ABOVE
+2 700 ft
ALL -2 700 ft BELOW
-9 900 ft The flight crew can restrict the TCAS display in a given volume. The flight crew selects the following settings: ALL, ABOVE, or BELOW as per Figure 3-5.
Protection envelopes are also defined to set the threat levels around the aircraft. There are 4 protection envelopes around the aircraft (from the farthest to the closest ones): OTHER, PROXIMATE, TA and RA. Refer to Figure 3-8. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：getting to grips with surveillance(46)
