Should we believe unverified MODE C??

[karrank]

Another thread reminded me of a difference between the way US and our ATC treat unverified transponder levels. In the US they just read the level off their radar screen for 1200 squarks and base services on that, if they have 500' separation they do nothing.

Here we are required to take action (such as pass traffic) unless MORE THAN 2000' is between whatever shows for the unverified 1200 and one of our 'real' aircraft, does anybody else think this is ridiculous overservicing? (Trust the transponder Luke). I mean, if it's wrong it is usually only a couple of hundred feet, but could be thousands. I hear the "what the f.." in the acknowledgements of traffic 1,500FT away and wonder???

Additionally, since an incident 60NM north of Melbourne we have learnt we can't trust pilots to actually turn the knob to ALT, so assume anybody not squarking altitude could be in E or above 10,000FT, and base services on this. Is this overservicing?

[Capt Claret]

karrank

I can recall an incident a few years ago in the Top End where a scenic flight pilot was unable to grasp the TN ATCO's attempt at humor, when she asked him if he was flying through Katherine Gorge. The TXPDR altitude readout indicated that the aircraft was subterranian!

I've had two TA's in the last 6 months, where traffic showed on the TCAS <1000' above and dead ahead, both were in radar covered CTA. In both cases ATC assure me the closest aircraft were > 3000' BELOW!

To answer your question I don't think we should believe unverrified levels.

[ferris]

Just yesterday, we had a radar calibration aircraft :eek: as he climbed thru FL300 transponder showed slow descent. This aircraft had more bells and whistles than you could poke a stick at, yet still managed an incorrect modeC. It worked fine until FL300, but not above.
That said, there would have to be a risk model out there that when you plug the data into (known transponder errors per hour flown) showed what sort of risk you are looking at. It's just money these days, as to whether we are overservicing that risk. Mostly it boils down to a regulator having the balls to show us where the line in the sand is ie. grasping the nettle and giving you the legislative back-up instead of the usual arse-covering stuff that results in the coalface being totally risk-averse. They do it in other parts of the world- you gave the example of the States, where they take modeC as gospel, other places have notified tolerances (such as allowing 2000' on an unverified target in oz).
Personal opinion; disregarding (and seperating from) any unverified modeC is overservicing. There needs to be discretion based on the phase of flight eg 2 aircraft below 10000' allow 2000' tolerance, above 10000' (ie the verified aircraft is above 10000') maybe 5000' tolerance, or 3000' with traffic. Above FL150 disregard any NMC (just basing these ideas on assumptions that pressurised a/c will have modeC selected etc huhhmmmmm). I just think that seperating guys at FLs from unverified modeCs showing low altitudes is mismanaging resources. Therefor Australia seems to have a good balance.

We have no discretion here. Then again, we also have very few unverified squawks (shoot first, verify later) ;)
Don't you get quizzed by the TCAS watchers' about NMCs anyway?

[DirectAnywhere]

No. Why do you think ATC verifies it in the first place? And, I would imagine, at every frequency change thereafter in radar CTA.

[SM4 Pirate]

Karrank; don't believe everything you hear or read... Yanks do put faith into the Mode C of 1200, but the capture all is merging traget procedure/advice. They are happy not to give traffic if the blips will not merge, but will shout out traffic if minimum exists and the paints will overlay on the scope. The minimum is up to the ATC... Some use 3500 foot with unverified targets; it is true some do use 1500 as enough..., but with only 500ft definately shouting out if there is a true collision risk, not a I'm going to lose 5 NM risk...

[divingduck]

I don't think that it's too bad using the 2000 feet as a traffic guide.
That said, ther are many occasions here in the ME where transponders just go nuts... I remember having to vector several aircraft around an almost stationary squark at Fujeirah..it was showing Fl225 (so naturally assumed a Yank P3..but thats another story) right on the departure radial from Dubai and SHarjah...after about 20 minutes of such muppetry we decided to investigate. Result? The aircraft was doing circuits at Fuje.. transponder was just a whisker out.

Capt Claret....i was also on duty the other nght when a GulfAir A332 was adament that he had traffic on his TCAS at F330 (it was showing minus 7000 and he was at F400). The only problem was that there was nothing there...I mean nothing. Closest aircraft to him was a B737 at F280 about 12-15 nm south. So it seems that the TCAS are not infallible either? (still rather have them than not). Do you get many "false positives"?

[Whiskey226]

Not trying to ruffle any feathers here, but is the ADS-B system any more reliable/accurate?
I haven't heard any feedback from the trial but would be interested if anybody has.

[karrank]

I had an Ansett B767 TCAS RA half way across the Bight once, with no conflicting (known) traffic, so the odd ghostie at low level is no surprise. TCAS uses MODE C, (I believe,) so your box and our box should read the same for the same target.

It is always curious to me that our radar is deemed the final authority here, is it possible particular SSR/TSPR units don't like talking to each other, with no detectable faults in equipment?

ADS/B just tells us where the aircraft's navigation system thinks it is. You may be confused about whether you are tracking for CANTY or CANDY, but the raw L/L we (and other's CDTI) get will not lie. Haven't heard about any problems, but our propaganda sites generally only spread good news.

[Sunfish]

I think I understand that the altitude information comes from a separate pressure transducer and not the altimeter. If this is the case, I am assuming that an area QNH is applied automatically electronically to arrive at an altitude.

I've noticed that Radar Melbourne generally paints me +- 200 feet . Should we all do a "transponder check" regularly or wait till we are hear our altitude being misreported before we act? I'd rather know that its reporting altitude correctly at the start of a flight rather than find out the hard way by giving ATC grief.

To put it another way, is there some way we can do away with the phrase "Altitude Unverified" completely?