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Following extensive EUROCONTROL input and pressure, a revised TCAS II Minimum Operational Performance Standards (MOPS) document has been jointly developed RTCA and EUROCAE.
During a joint session in March 2008, the RTCA SC (Special Committee) 147 and EUROCAE WG (Working Group) 75 agreed on the final version of the TCAS II MOPS, to be known as TCAS II version 7.1. The MOPS were approved the RTCA Program Management Committee in June 2008 (document DO-185B) and by the EUROCAE Council in September 2008 (document ED-143).
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The MOPS were revised following the identification by EUROCONTROL of two safety issues in the existing TCAS logic (one relating to the performance of the RA-reversal logic, and the other involving incorrect responses to Adjust Vertical Speed RAs). See below for the description of the changes.
The FAA has already published the TCAS II version 7.1 Technical Standard Order (TSO).
On 3rd June 2009 the following information was presented to EUROCONTROL by the European Aviation Safety Agency (EASA):
EASA plans to mandate carriage of Version 7.1:
- The Agency plans to initiate and progress a rulemaking task to develop the necessary text for an IR requiring the carriage of TCAS II with software version 7.1 as the minimum standard for aircraft operating within European Airspace (without prejudice to existing limitations and exceptions).
On 25 March 2010 EASA published Notice of Proposed Amendment (NPA) No. 2010-03 pertaining to the introduction of ACAS II software version 7.1. This NPA was open for comments until 7 May 2010.
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Although TCAS II is an aircraft system, it has been implemented to improve ATM safety. Studies conducted for EUROCONTROL, using recently recorded operational data, indicate that currently the probability of a mid-air collision in European airspace is 2.7 x 10-8 which equates to one in every 3 years. When TCAS II version 7.1 is implemented that probability will reduce by a factor of 4.
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See: Decision criteria for regulatory measures on TCAS II version 7.1 study |
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The results of the above mentioned study indicate that implementation of TCAS II version 7.1 in the aircraft fleets is urgently required. These views are shared by the EUROCAE WG75 and were subject of their letter to EASA in April 2008. EUROCONTROL briefed EASA in detail on the issue and the safety implications in October 2008. Following this meeting EASA undertook to publish an action plan for the implementation of TCAS II version 7.1.
EUROCONTROL believes that prompt action by Regulators is required to mandate forward and retrofit of TCAS II version 7.1 in European airspace as rapidly as possible. However, the implementation schedule (both forward- and retrofit) has not yet been established.
Separately, a proposal has been made to amend the ICAO standard to require TCAS II version 7.1 for compliance with ACAS II SARPs. EUROCONTROL supports this initiative.
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Version 7.1 – summary of changes |
The new version 7.1 will incorporate a number of changes - the two most important changes are described below. TCAS II version 7.1 is backwards compatible with TCAS II version 7.0.
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CP112E (Change Proposal 112E) |
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The issues with the reversal logic were resolved through a significant code change for TCAS II logic version 7.0. This change is known as Change Proposal 112E (CP112E).
CP112E brings improvements to the reversal logic of TCAS II logic version 7.0 by detecting geometries close to that of the 2002 Überlingen mid-air collision, and by easing the triggering thresholds of reversal RAs in encounters in which the aircraft remain vertically within 100 ft of each other. The basic principle is to detect that two aircraft are climbing, or descending simultaneously. Two mechanisms are used to ensure that reversal RAs are triggered when necessary.
CP112E first adds a feature which monitors RA compliance. When it is detected after a certain period of time that an aircraft is not responding correctly to an RA, it circumvents the "100 ft box" rule, allowing reversal RAs for aircraft closer than 100 ft vertically.
CP112E also adds a prediction of the vertical separation at the closest point of approach, based on current vertical speeds, to detect the need for a reversal RA. Indeed, when this prediction shows that the aircraft are probably going to be closer than a predefined threshold, reversal RAs are considered as a valid option for aircraft closer than 100 ft vertically.
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Reversal RAs are not triggered too early in an encounter, to leave time for the initial RAs to be efficient before reversing. In addition, reversal RAs are not triggered too close to the predicted closest point of approach, to avoid useless reversal RAs.
CP112E has been extensively validated in Europe by the EUROCONTROL-sponsored team composed of DSNA and Egis Avia experts, and by several organizations in the USA (MIT Lincoln Lab, MITRE, FAA, and Johns Hopkins University). This validation shows that CP112E significantly improves the safety brought by TCAS. It triggers reversal RAs in time in the geometries in which the issue with reversal RAs was identified. In addition, side effects and performance deteriorations are minimal for CP112E and are considered acceptable compared to the collision risk with current versions of TCAS.
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CP115: solution for the “Adjust Vertical Speed, Adjust” (AVSA) issue |
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Operational monitoring has revealed that in some cases pilots do not respond correctly to the “Adjust Vertical Speed, Adjust” (AVSA) RAs. The AVSA RA requires the reduction of the vertical speed to 2000, 1000, 500, or 0 feet/min. In the cases involving an incorrect response, the pilots increased their vertical speed instead of reducing it, consequently, causing a deterioration of the situation.
It has been observed that enhancements in training alone can improve the behaviour of a flight crew when an “Adjust Vertical Speed, Adjust” (AVSA) RA is issued, but are not sufficient to avoid all opposite reactions. Therefore, to fully address the issue with AVSA RAs, a complete solution had to be envisaged, rather than only focussing on training.
Indeed, several organizations, including airlines and incident investigation authorities, have concluded that the “Adjust Vertical Speed, Adjust” aural message is too ambiguous and that the presentation (i.e. both the display and the aural message) of AVSA RAs to flight crews should be enhanced.
Replacing the several “Adjust Vertical Speed, Adjust” RAs with a single “Level-off” RA is the solution to the AVSA issue as the associated aural message is straightforward and the associated manoeuvre corresponds to the standard manoeuvre already performed in critical situations. This affects the RA display accordingly. The modification has the added benefit of simplifying the list of RAs posted by TCAS II, as RAs requesting a reduction of the vertical rate to 500, 1,000 or 2,000 feet/min would become unnecessary. Additionally, this replacement also simplifies the TCAS procedure and training.
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A change proposal to TCAS II logic version 7.0, referred to as Change Proposal 115 (CP115) was developed and evaluated by a EUROCONTROL-sponsored team composed of DSNA and Egis Avia experts.
Validation of CP115 showed that the proposed solutions bring substantial safety benefits when compared to the AVSA RAs of TCAS II logic Version 7.0, as it reduces the overall risk of collision. In addition, CP115 brings operational benefits as it reduces the RA alert rate and minimises the altitude deviations induced by TCAS. The potential issue of an induced conflict with a third party aircraft was investigated using European and US operational data. This investigation showed that introducing CP115 will not cause induced conflicts with third party aircraft more frequently than with the current version of TCAS.
The text above is based on the article “Traffic alert and Collision Avoidance System, over 15 years of work by DSNA to improve ATM safety” by Christian Aveneau from DTI Technical Review no. 6. Used with permission, with minor editorial changes. Full text available here:
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ACAS II Studies |
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Video illustration of CP115 effect |
The following video clips show an Adjust Vertical Speed RA event. The first clip is a recreation of the actual event in which one of the pilots responded to the Adjust Vertical Speed RA by increasing rather than decreasing the vertical speed. The same situation but with a correct response from the pilot is shown in the second clip. Finally, the third clip demonstrates the same event when the Adjust Vertical Speed RA is replaced with the Level-off RA (CP115).
The video clips show ATC radar picture and flight instruments from the aircraft. Radio exchanges between the pilots and controller are shown on the bottom of the screen.
The situation was as follows:
AVSA02 is an A320 is level at FL270, heading South. Another A320, AVSA01, is cleared to climb to FL260, heading North. Its rate of climb is about 3300 fpm. When passing through FL253, its TCAS triggers an initial "Adjust Vertical Speed" RA requiring a reduction in the rate of climb to 1000 fpm.
However, the flight crew misinterprets the RA and reacts opposite to it: the rate of climb is increased to more than 6000 fpm instead of being reduced. The closure rate increases between the two aircraft and the RA is strengthened to "Descend". The flight crew follows this second RA but the manoeuvre takes time to be effective. As a result of the wrong reaction to the "Adjust Vertical Speed" RA, AVSA01 busts its flight level by 1200 ft and AVSA02 receives a "Climb" RA, which the flight crew follows. The separation is reduced to 300 ft and 0.8 NM.
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