The place for technical discussions about GA and flying.
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By rf3flyer
#1527789
I heard yesterday from an ATC person that for the purposes of traffic deconfliction a Mode C return from an 'unverified' source (not sure unverified was the actual term used but clearly referred to a technical standard) cannot be used. It can be seen but cannot be used for deconfliction.
Airliners and such have 'verified' (see above) sources and are therefore relied upon, but the nice Mode S box in your RV is not.
The Mode A is accepted as is the Mode S data but not the Mode C…unless ATC are in radio contact with the a/c and can have the pilot confirm his altitude!

Is this true? It seems absurd to me.
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By T67M
FLYER Club Member  FLYER Club Member
#1527790
Mostly correct. ATC cannot take any height readout (Mode C or Mode S) as correct without checking with the pilot first. Altitude encoders are often in error by more than 200'.
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By JonathanB
FLYER Club Member  FLYER Club Member
#1527794
Verifying a Mode C readout is as simple as an aircraft calling in with their altitude and ATC checking that it is within limits on the radar. This can happen once a descrete Mode A code is allocated and seen to be selected by the anticipated radar return (known as validating the Mode A squawk code) - if requested then a "squawk ident" may be part of thus process. If a radar return that a unit/controller is not working is wearing another unit's squawk that is known to be capable of validating Mode A codes and verifying Mode C altitudes, then you can assume the Mode C has indeed been verified.

MATS Part 1 has all the in depth details!
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By T67M
FLYER Club Member  FLYER Club Member
#1527795
rf3flyer wrote:
T67M wrote:...ATC cannot take any height readout (Mode C or Mode S) .

I understood that Mode S did not transmit height information, only Mode C did that. Not so?

There are lots of variants of Mode S. Some only transmit the aircraft registration, but many add all sorts of information including pressure altitude, GPS position, track, heading, ground speed, airspeed, autopilot status, selected heading, selected altitude, ... The list is huge.
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By Irv Lee
FLYER Club Member  FLYER Club Member
#1527815
Sadly the article (not mine) in Flight Safety on the importance of mode C as a safety net attracted one or two letters from transponder users saying "don't understand your banter" (my paraphrase). They apparently had used a barman or barista for education when they had had transponders fitted. I have been using "Mode C (Alt)" or vice versa in articles or postings since.
Ps from years of experience of sitting in many syndicate aircraft, (including those of a forumite or two), unless the transponder is showing ridiculous flight levels, I would say if a read out is a few hundred feet out against pilot reported level, it is much more likely to be an accurate transponder and a shagged out altimeter that they can not be r-sed to (pay for a) fix, yet still you get "he said I was at 2600' and I was definitely below 2500 on the qnh". (Chances are: "Oh no you weren't" -your altimeter said you were below 2.5k, not quite the same thing!)
T67M, FrankS liked this
#1527825
T67M wrote:There are lots of variants of Mode S. Some only transmit the aircraft registration, but many add all sorts of information including pressure altitude, GPS position, track, heading, ground speed, airspeed, autopilot status, selected heading, selected altitude, ... The list is huge.

I'm sure it is but perhaps not huge for the average light GA fit that most might be familiar with. And the same ATC person also listed the very few chunks of data returned by such devices and I'm fairly sure altitude wasn't mentioned, though i may have missed it.

I also seem to recall a previous thread on this forum which insisted that it was Mode C that handled the altitude, but I cannot remember which thread that was and a search isn't very helpful.

It was notable that of the many people who heard the ATC talk and whose aircraft were fitted with transponders none had any idea of the 'uselessness' of an unvalidated Mode C return. The consequence for deconfliction being that a SSR return with an unvalidated Mode C sterilised the same 78.5 square miles of airspace (5 mile radius) as a non transponding primary return!
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By flybymike
FLYER Club Member  FLYER Club Member
#1527827
VFR/IFR separation not required in CAS but I think that otherwise, it's reasonably safe to assume that any mode C readout in CAS will be routinely verified as a matter of course.
Last edited by flybymike on Sat Apr 01, 2017 2:31 pm, edited 1 time in total.
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By flybymike
FLYER Club Member  FLYER Club Member
#1527829
Ps from years of experience of sitting in many syndicate aircraft, (including those of a forumite or two), unless the transponder is showing ridiculous flight levels, I would say if a read out is a few hundred feet out against pilot reported level, it is much more likely to be an accurate transponder and a shagged out altimeter that they can not be r-sed to (pay for a) fix, yet still you get "he said I was at 2600' and I was definitely below 2500 on the qnh". (Chances are: "Oh no you weren't" -your altimeter said you were below 2.5k, no

Not sure how it's possible to deduce whether transponder or altimeter is at fault without a QFE ground check or at least a GPS altitude check, in which case any fault should be apparent to the pilot who could then hardly claim innocent ignorance.
Is altimeter accuracy not verified for an ARC/Cof A renewal?
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By JonathanB
FLYER Club Member  FLYER Club Member
#1527831
It's not useless. You need to read through Part 10 of Chapter 6 of MATS Part 1 for the full chapter and verse, but...

Basically if the intentions of the other traffic are not known then you need to apply 5000ft vertical separation inside CAS or 3000ft outside CAS if on a Deconfliction service. If the Mode A code indicates that the Mode C altitude has not been verified (it's a conspicuity code like 7000 for example) then you mustn't let the blips merge, but you are effectively still able to use the Mode C data to provide reduced horizontal separation. If the aircraft was primary only, has no Mode C/Altitude or is squawking 0000 then you'd have to keep standard horizontal separation regardless (normally 5nm).
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By Irv Lee
FLYER Club Member  FLYER Club Member
#1527833
FBM: annual: clearly not in 100% of cases (in my observational experience), but no doubt it is on a tick list with a tick on it somewhere, and perhaps that is all that anyone other than controllers with alerts going off are worried about.
#1527855
OK, to be clear, personally I'm not looking for a deconfliction service in my little VFR, fun aeroplane but I may very well be of interest to ATC who are handling a CAT flight which is receiving a deconfliction service. I don't have a transponder so at best I'm a primary return, but if I did have one, mode A, C or S, altitude unvalidated, it was said that ATC would have to put a ring around my return, radius 5 miles! Same thing with an unvalidated Mode C return. They went on to mention an instance of an aircraft nearby, in class G airspace, not talking to them, flying aerobatics and that 5 mile radius 'bubble' which, they said, they had to apply required the holding off and orbiting of 3 inbound CAT flights.

Incidentally, CAP493 is not a fast read at 460 or so pages but chapter 6 part 10 is about vertical separation.
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By JonathanB
FLYER Club Member  FLYER Club Member
#1527858
There seems to be some confusion about this. If you are outside controlled airspace and aredisplaying an unvalidated Mode C (or S) altitude then they can route traffic that they are working on a Deconfliction service either keeping 5 miles away if within 3000ft vertically or if more than 3000ft vertically then they can route their traffic closer as long as the radar returns don't merge. So yes there is a large buffer that essentially gives a 5nm radius bubble which is up to 6000ft tall with the unverified return in the middle. The primary reason is that it's not know how accurate your Mode C is or what your intentions are. Within controlled airspace the vertical buffer increases to 5000ft from the unknown traffic (i.e. an infringer).

If you're a primary return only, then they would have to apply the 5nm radius bubble at all levels outside controlled airspace. In this instance it would be assumed that you are below the base of any controlled airspace unless the airspace extended to the surface.

All this separation gets much reduced as soon as you are talking to an ATC unit (not necessarily the same unit, as co-ordination can be effected) as your Mode C can be validated (if you have it) or at least your altitude and intentions ascertained - you become known traffic.

These are minimum separation standards and so if it is known that someone is doing aeros then it might be prudent for ATC to increase separation to account for anything unexpected.
#1538683
Your unvalidated mode C is still of value to a conflicting aircraft fitted with TCAS. Which includes airliners, Bizjets, some military aircraft and quite alot of GA machines. SO even if the controller is obliged to disregard it, TCAS will still use it.

Airliners mode C altitude is still validated in the same way as light aircraft. On departure with certain Towers, or first contact with a radar or Area controller "passing altitude" is reported, or as others mentioned squawk ident and altitude is confirmed.