Unverified Mode C Altitude -what are your rules?

[Acker Demick]

I am a PPL operating in the UK, in an aircraft equipped with a Mode C transponder. I would be grateful if an ATCO could explain your rules regarding the use of unverified altitude readouts. For example, if I overfly a regional airport, well above your airspace in the open FIR, while squawking Mode C, but I don't call you for FIS, are you required to ignore my altitude readout and act on the worst-case assumption that I am really barging through your zone at circuit height?
I would like to know so that I don't unwittingly cause uneccessary aggro; generally I dont bother with FIS unless I need to know something specific (like a danger area status) - I can get the QNH from volmet!

Thanks,

AD

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If God had meant us to fly he would have given us more money

[Chilli Monster]

If you're indicating well above then they will give traffic information on you (Traffic in the overhead not talking to this unit, indicating FL........) but the standard rules for unverified mode 'C' are that they will avoid you by 5000ft vertically. If you are VFR (I take it you are) then not your problem

BUT

If you are going to do this then it would be wise to find out where this airfields holds are. You DO stand the risk of going through a bunch of IFR's in the hold doing this. If that's the case then a call would be advisable.

Do you have anywhere in particular in mind, to clarify the position better?

CM

[5milesbaby]

To add to Chills, it is correct VFR is just given info, but a/c getting RAS (not RIS, traffic info only) are bigger problems. An unverified Mode C is on an a/c not in contact with radar control or, one that is but that has not had its level confirmed. Other a/c on RAS have to be given either 5000ft or vectored all over trying to get 5 miles (baby), not an easy task when other a/c intentions are not known, they usually turn just at the wrong moment

[passepartout]

The above posts are completley correct,one thing I would add working at a unit which has class E airspace around it ,is that please,please call us, you will almost certainly get a better service than an fis, it wont cost you a penny and it helps you and me out no end. You try putting something on a 10 mile final around unknown traffic whilst trying to take 5miles or 5000 feet!!!

[HEATHROW DIRECTOR]

I'm inclined to say, if you're not working an ATC unit then switch off mode C. I had a very bad fright yesterday afternoon whilst vectoring traffic for the 09L ILS at Heathrow. An aircraft appeared at 3000 ft on a heading to take it through our ILS at about 10nm. Luckily the inbounds I was working were coming down to 4000 so I could maintain vertical. We quickly checked with Farnborough, which was working the aircraft, and found that it had a transponder fault and was really at around 1000 ft. I have seen similar "light a/c transponder problems" on numerous occasions and such a fault could lead the pilot into some trouble if we believed that he was infringing controlled airspace.

I'm also a little dubious about the conspicuity code system - I KNOW it lets us see uncontrolled traffic but sometimes there are so many 7000 squawks around that we have trouble seeing our own traffic!

[Acker Demick]

Thanks for your replies guys - I'll make sure to call. It makes me wonder how units cope that have "stubs" of controlled airspace along their approach paths that dont extend to ground level (e.g. Bristol, Bournemouth, Southampton and many others). You must get loads of VFR traffic passing under the stubs, some of it non-radio - do you have to abort ILS approaches each time someone passes under the stub without contacting you?

[RogerOut]

I agree with comments above, however I thought the new separation standards within UK when providing a RAS were to "seek to achieve" 5nm (or more!) or 3,000', the emphasis on 3,000'. IS this the whole of the UK?



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RogerOut
I Keep Mine Hidden

[Chilli Monster]

Acker

The class 'D' stubs that you speak of are actually there to protect IFR traffic on approaches. If you were to pass under it and your mode 'c' indicated that you were below it then that's good enough. If it indicated that you were inside though that would be cause for concern.

I think the best example of this has to be Stansted. I don't know of anyone who calls them as they transit under the stub because it is probably the most pointless exercise known to man. Just fly at 1400' underneath it and you're not causing anyone any heartache.

Bear in mind though flying under CAS is totally different to your original question about flying over a regional - my original answer then still applies :)

CM

[Warped Factor]

RogerOut,

I was under the same impression as you re the new rules.

WF.

[Chilli Monster]

WF

What's a Radar? ;)
(1 down, 7 to go)

CM

[Acker Demick]

OK - I'm trying to piece together info from the replies above, and from the other thread on "London TMA VFR traffic", but I am still slightly confused. The event that prompted my original post occurred last weekend. I was sight-seeing in a C150 at FL100 over the Bristol channel, well clear of controlled airspace, squawking 7000 mode C, and monitoring, but not talking to, Bristol's LARS requency (lovely day, vis limited only by curvature of the Earth :) ). As I passed S to N while just W of Bristol's Westerly stub, I heard the controller warn someone on an ILS approach that he may need to abort because of unverified traffic at FL100 - obviously yours truely. I made a quick call to verify my altitude, and all was well. From your replies I assume that the a/c was above FL50, so the controller couldn't get 5000ft separation. So far so good, BUT, what about traffic that might be Moda A or primary contact only which passes below CAS under an ILS approach? Despite the lack of altitude info, I gather that if you think such traffic is probably, judged from its behaviour, below CAS (e.g. a class D stub, or the London TMA) then you don't attempt a 5000ft separation? Somone said on the other thread that vertical separations with unverified traffic in the London TMA could be as little as 500ft. Would the Bristol controller have been unusually cautious about my presence because a very slow target at FL100 is a rare event, and therefore likely to be an a/c at lower alt with a faulty transponder?

[arrow2]

HEATHROW DIRECTOR Can you clarify - I thought that you had the facility to filter out unwanted 7000 codes? Second, I seem to remember reading somewhere that if you had Mode C capability you should turn it on when squawking 7000 - not sure if this was in AIP or not - am I dreaming?

arrow2

[Warped Factor]

AD,

I'm not totally au fait with Bristol's airspace, but if the IFR inbound was outside Controlled Airspace, which is quite likely given the size of the CTZ/CTA, and under a RAS then ATC would have to provide the 5 miles or 5,000ft ( 3miles/3,000ft under the new rules?) from unknown traffic like yourself.

Regarding the inside/outside CAS scenario as happens with traffic operating below the London TMA. The rules differ here depending on the classification of the airspace.

For Class A, such as the LTMA, as ATC should know about all aircraft operating within the CAS, we are allowed to assume that all the unknown traffic is operating outside (below) the CAS. So we don't pass traffic info or avoiding action to the IFR traffic on all the unknown contacts unless we receive indications that a particular unknown aircraft is lost, r/t failed or we believe it has penetrated CAS.

The rules for Class D are slightly different with regard to unknown aircaft, any Class D specialists want to comment?

WF.



[This message has been edited by Warped Factor (edited 16 January 2001).]

[Warped Factor]

arrow2,

Yes, we tend to filter out the 7000 squawks on the Heathrow positions.

No you're not dreaming, I believe informed opinion and recommended practice is that anyone with Mode C should have it selected whenever the transponder is switched on.

WF.

[Chilli Monster]

Acker

I'm sat here with a Bristol ILS App plate and to be frank I'm just as confused as you.

If you're at FL100 then, even taking into account 5000ft unverified separation (which, it has been now pointed out has been reduced to 3000ft) there is NO reason why the aircraft should have to abort his approach unless Bristol has been moved to a mountain over 2000ft high! The Procedural ILS at Bristol starts at 3000ft, but obviously this was a radar vectored ILS. In view of this the aircraft was probably in the region of 2200ft QNH (Bristol RVA Min alt). Using 5000ft he wouldn't have had to avoid you unless you were indicating FL72, or using 3000ft then FL52.

You had every right to be where you were, doing what you wanted the way I read it. The guy though obviously had his doubts though, as you suggested. The old saying is when in doubt, there is no doubt. Nobody ever got hurt by being over cautious ;)

CM

[Acker Demick]

Thanks to all. A final thought - I think that most PPLs (me included) are unaware of most of this stuff. I have no connection with any flying magazine, but I would guess that Pilot, Flyer or similar would welcome an article on the ATCO's view of the problems of IFR/VFR traffic integration, as long as the tone was GA friendly :)

AD

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If God had meant us to fly he would have given us more money

[cxi]

the bristol controller probably couldn't believe his eyes!! a c150 at fl100! it was probably indicating a ground speed of less than 80 kts and the controller probably didn't think the mode c was right with that sort of speed and therefore the mode c was wrong and the a/c was alot lower! Until you called him the controller was probably treating you as an unkown lost a/c just to be prudent. By the way i'd love to know how long it took to get to fl100!!

[10W]

Lots of bits of the MATS Part 1 (ATC Manual) all mixed above in various posts. Some right, some not so, IMHO ;)

First things first, it depends entirely upon the type of airspace and the service being provided.

As a general rule, when using Mode C to provide separation and one of the aircraft has Mode C which is unverified (i.e not confirmed by the controller), then the minimum separation which can be applied is 5000'. However, when providing a RAS (and only a RAS) to aircraft outside Controlled Airspace (i.e. Class F or G), the vertical separation can be reduced to 3000' based on mode C readouts. The target radar separation remains at 5NM.

So far so good. Now take the case when the aircraft under the control of ATC is within Controlled Airspace [CAS](Classes A to E inclusive).

In Class A we take no action against unknown traffic (whether it has no Mode C or Mode C indicating it may be within CAS) unless we have information that it is lost, has an RT falure or has made an unauthorised entry into CAS.

Class B, we will ignore the ATC procedures in this thread as it is unlikely that GA will get above FL245. Class C, we don't have none here in the UK !!

Class D, we pass traffic information unless our primary task is likely to be comprised (i.e. subject to workload). If avoiding action is requested by the pilot then this shall be provided to the extent deemed necessary by ATC. If we believe the unknown aircraft is lost, has an RT failure or has made an unauthorised penetration, then we will provide avoiding action.

In Class E, we will pass traffic information and advice on avoiding action.


I would strongly advise against Heathrow Directors not selecting Mode C if you have it available. There are many many more aircraft flying around with correct height readouts than those who do not. Whilst there will be the occasional false alarm, when the event is real it will not only assist ATC in alerting other pilots, but it will also allow commercial aircraft with TCAS to take appropriate action.

In the case of traffic flying above or below CAS (indicated by their Mode C), I would not be seeking to provide 5000' separation against my traffic within CAS. That's because MATS Part 1 deems that aircraft operating in controlled airspace are separated from unknown aircraft flying in adjacent uncontrolled airspace. The proviso of course is that unpredicatable manouevres by the unknown aircraft could quickly erode separation.




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10 West
UK ATC'er
[email protected]

[Warped Factor]

10W,

If and when we filter out the 7000 squawks on the Heathrow positions, that is all we filter, the bit that says 7000. If Mode C height readout is available it will still be shown.

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In Class A we take no action against unknown traffic (whether it has no Mode C or Mode C indicating it may be within CAS) unless we have information that it is lost, has an RT falure or has made an unauthorised entry into CAS.
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It's a brave controller who always interprets that one strictly by the rules :)

WF.

[Acker Demick]

cxi,
climb rate of my C150 is not exactly sparkling - I think it took about half an hour to climb to FL100 - I was probably borderline hypoxic/hypothermic by then so my recollection is hazy ;). That's solo, cold dry day, but full tanks, and lots of fidling with the mixture. Rate of climb at the top was down to about 200ft/min. Probably wouldn't get there at all with a passenger.
Worth the climb for that view though.


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If God had meant us to fly he would have given us more money

[This message has been edited by Acker Demick (edited 17 January 2001).]

[Bern Oulli]

Class D rules, are in practical terms, the same as those for Class A, as regards 7000 squawks passing under or over "my bit" of CAS. If it is reasonable for the 7000 traffic to be there legitimately then assume that is the case and ignore it. Remember that traffic outside CAS is deemed to be separated from traffic inside CAS, and that applies vertically as well as horizintaly.

Regarding C150' rate of climb - back in the good old days of NATS ATC Cadets getting the full PPL treatment + a couple of years refresher flying I remember my instructor telling me to "Climb on that heading", and promptly going to sleep! So I did (The a/c was a 150H) and after about an hour we were at a shade over FL100, nose up, mixture lean, VSI Zero and not a lot on the clock. They spin well from that height though!

Disregard my next transmission.

[bookworm]

<font face="Verdana, Arial, Helvetica" size="2">I would strongly advise against Heathrow Directors not selecting Mode C if you have it available. There are many many more aircraft flying around with correct height readouts than those who do not. Whilst there will be the occasional false alarm, when the event is real it will not only assist ATC in alerting other pilots, but it will also allow commercial aircraft with TCAS to take appropriate action.</font>

I agree strongly (as a pilot) with 10W.

Moreover, there are also many aircraft operating outside controlled airspace with TCAS or relying on radar services. While within class A airspace, it takes a serious mistake from either a pilot or a controller for TCAS to earn its weight (let alone its cost), in class G airspace your transponder may be the only way that a TCAS-equipped aircraft manages to see you and avoid you.

The conspicuity squawk system is not there simply to allow LATCC controllers to check if those aircraft operating outside the system are behaving themselves. If it were, I doubt you'd see very many 7000 squawks at all. :)

[RATBOY]

Acker Demick: C-150 "performance" at altitude is prety amazing, isn't it. Talk about the back side of the curve. In December 1980 a friend and I decided to see how high a C-150 would go. We used a C-150M with tanks about half full, no flight bags or other impedimentia. It took a good 45 minutes but we got it to almost 12,000 ft before the rate of climb went to 0. As I recall that was pretty consistent with the manual.