getting to grips with surveillance(47)

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The OTHER volume is the volume outside the PROXIMATE volume, from –9900 ft to +9900 ft, and up to the maximum horizontal range. The PROXIMATE volume covers the vertical range from –1 200 ft to +1 200 ft, and the horizontal range up to 6 NM.
The dimensions of TA and RA volumes depend on TAU value. The penetration of the TA or RA volumes triggers Traffic Advisories or Resolution Advisories respectively. TAU varies according to the own aircraft altitude and the TCAS sensitivity level. The higher the own aircraft altitude and the sensitivity level, the higher the time to intercept TAU.
The sensitivity level permits a balance between necessary protection and unnecessary advisories. The sensitivity level goes from 1 to 7:
- 
Level 1 when TCAS is set in STAND BY, failed or the aircraft is on ground. In level 1, TCAS does not transmit any interrogations

- 
Level 2 when TCAS is set in TA ONLY mode (RA are inhibited)

- Levels 3 to 5 automatically selected according to the own aircraft altitude when TCAS is in TA/RA mode.
Example:  For own aircraft altitude from 5 000 to 10 000 ft, the sensitivity level is SL5 and TAU for TA is 40 seconds and TAU for RA is 25 seconds.
For more details, please refer to Introduction to TCAS II Version 7 (see References).
3.1.6. TCAS II CHANGE 7.1
The TCAS II Change or Version 7.1 is the result of a workshop composed of DSNA (French Air Navigation Service Directorate) and Egis Avia experts, and sponsored by Eurocontrol. The TCAS II Change 7.1 introduces two major changes:
- 
The Change Proposal 112E (CP112E) about the reversal logic

- 
The Change Proposal 115 (CO115) related to the RA “ADJUST VERTICAL SPEED, ADJUST”.

3.1.6.1. CP112E – SOLUTION TO THE REVERSAL LOGIC ISSUE
In the TCAS II Change 7.0, an issue has been identified that leads to non-issuance of reversal RA in specific situations. Indeed, in the TCAS II Change 7.0, when the vertical separation is less than 100 ft, the TCAS does not trigger reversal RA. This issue is also known as the “100 ft box” issue.

This issue has been observed in several in-service events. The geometry is always the same: both aircraft are either descending or climbing. The issue has been identified in:
- 
The accident that occurred in January 2001 in Japan where several passengers were injured. Both aircraft got near to each other approximately by less than 180 m (600 ft) laterally and less than 60 m (200 ft) vertically.

- 
The collision of Uberlingen over the Lake Constance in July 2002. All passengers and crewmembers were killed.

In both events, the ATC controller had instructed one of the aircraft to maneuver in the opposite direction as ordered by TCAS.
The CP112E introduces a monitoring of the compliance to RA. When the own aircraft does not follow the RA and goes in the opposite direction for a certain time, the “100 ft box” rule is inhibited: reversal of RA is then possible when aircraft get vertically closer than 100 ft.
　
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