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VFR Transponder code trial?

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VFR Transponder code trial?

Old 29th May 2006, 10:25
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Question VFR Transponder code trial?

Greetings to those who know all things re ATC:
Entering Bournemouth zone yesterday for a VFR rejoin, I was given a code to squawk. The radar controller had told someone else that this is an experiment being conducted at a number of airports around the country. I would be interested to know what is the purpose of the said experiment.
With breath bated
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Old 29th May 2006, 14:23
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See AIC 49/2006 (Yellow 204) 27 April 2006
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Old 29th May 2006, 15:55
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I believe that the trial is being conducted at 6 airfields. My understanding is that the purpose of the trial is to create a "known" traffic environment for TCAS.
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Old 29th May 2006, 21:23
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Current policy requires Air Traffic Controllers to instruct aircraft, which are below 3000ft in the vicinity of an aerodrome traffic pattern where transponder operation is not mandatory, to select SSR transponders to STANDBY. This policy originates from a requirement to prevent degradation of ATC facilities, which could have an adverse effect on safe ATC provision.
A negative aspect of this policy, however, is that the benefit which could be provided to aircraft in the aerodrome traffic pattern by the, last-ditch, Airborne Collision Avoidance System (ACAS) is effectively denied. Modern ATC displays have, however, become more capable of dealing with multiple SSR responses and the fitting of ACAS systems is much more widespread and mandated for certain classes of aircraft. Accordingly, the balance of risk is shifting away from protecting ATC systems toward retaining the ‘last-ditch’ collision avoidance function provided by ACAS.
The CAA is considering a change in policy toward widespread SSR use in the vicinity of the aerodrome traffic pattern and, as a precursor activity, the trial is being conducted at a small number of selected aerodromes. The aim of the trial, which is sponsored jointly by the CAA Directorate of Airspace Policy (DAP) and Safety Regulation Group (SRG) is to gather evidence of the relative benefits and drawbacks of such a change in policy.
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Old 29th May 2006, 23:12
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We are one of the airports involved, we've never instructed aircraft joining the circuit or entering the ATZ to select Standby purely because its easier for us to keep track of them on the ATM, it also enables us to give positive traffic information to tcas equipped aircraft.
I suspect that this is the case at the other airfields who have SSR.

To myself and a lot of my colleagues, this is a pointless trial which could have been conducted 4 or 5 years ago by doing exactly what we (and others) have been doing all along, thereby achieving the same result which the CAA hope to achieve now.
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Old 30th May 2006, 06:50
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In accordance with MATS Part 1 Farnborough instruct Blackbushe aircraft to squawk ‘standby’ to de-clutter their radar, but in doing so lose the advantage of TCAS. The CAA ATSSD policy to squawk standby is at variance with CAA GAD policy of squawking Mode A and C where possible in order to retain the benefits of TCAS. The Trial suspends the use of MATS Part 1, Section 1, Chapter 5, Page 7, Paragraph 4.7.1, sub-para b at the trial airfields. Using a discrete squawk such as 7006 means that radar agencies suffering from clutter can now filter out the ATZ traffic 7006 squawks whilst retaining TCAS benefits and 7000 conspicuity in the ‘open FIR’. Farnborough 'pair' the 7006 code to display a 'B'. The trial may reinforce the case for a more general policy change in MATS Part 1 to allow SSR in the traffic pattern at aerodromes generally. The trial is necessary in order to provide confirmation that no significant ATC radar display or TCAS issues would arise from such a change.
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Old 30th May 2006, 15:56
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Slight digression, and before I get too far in, this did happen a good while ago, so hopefully things have changed enough that it can't happen again.

I get VERY uncomfortable when I read about transponder code suppression, and here's why.

As I warned, a long time ago, I was training at Shoreham, and on this occasion, we were doing VOR holds, using Midhurst VOR, at 2400 Ft, just under the base of the TMA at that time, with a radar service from Dunsfold. We were merrily doing our thing, which was slightly harder than usual as it was twin training, and we were doing single engine work, when we suddenly got a call from Dunsfold, with a slightly raised level of decibels and anxiety, "opposite direction traffic range 1 mile height unknown!". We looked out, as we were near enough VFR, to see a large commercial jet at very close to our altitude, and started taking suitable avoiding action. As we did so, it turned, and we saw clearly that it was a Dan Air 1-11. had we been of a mind to, we could have waved it was that close.

Turned out it was at 2500 Ft, the base of the TMA, postioning visually back to LGW from Lasham after maintenance. We spoke with the controller at Dunsfold, who also was not happy at how close it had been, and he made inquiries etc, and subsequently filed the relevant miss paperwork. Transpired that the 1-11 was under Gatwick control, and the reason he'd been given the routing and level he was at was that the LGW controller had supressed the returns from aircraft outside of "his" airspace, so we came within a couple of hundred feet of a mid air with a 1-11 as a result.

You will undertand why I get very uncomfortable when I read about transponder returns being supressed on a screen!!! OK, on the larger aircraft, you've now got TCAS, which would (should) have warned the 1-11 about us, but if I still had the twin, I doubt I would have fitted TCAS in it, for all sorts of reasons, so we still would not know about the other aircraft if the same sceanrio was repeated today.

Thought provoking.
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Old 30th May 2006, 16:49
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Thanks to those who helped answer my original question.
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