ATC Question

[nickduncs84]

Here's one for the ATC guys, or at least people that know the ins and outs of how it all works. Twice in the past month I've had an ATC at Adelaide Centre call me up saying that I was at the wrong height.

 

The first time, I was tracking coastal along the Adelaide coast within the control zone. The required altitude is at or below 500. Obviously not much room for error, so you're pretty much watching the altimeter the whole time. When he called, he said he had me at 700 feet. on Adelaide QNH I was looking at the altimeter, verified the QNH and the altimeter was reading about 490 feet. So I really had no idea what to say other than. "Confirm QNH currently showing 490 ft". He then came back and said that I was 'just within tolerances' but to carry on.

 

At the time, I didn't think much of it. I know what I was seeing on the altimeter so thought he might of made a mistake and didn't want to elaborate more hence the 'just within tolerances' comment.

 

Then yesterday, I was under the 4,500 step heading east away from the zone. There was a few clouds about so I was up and down a little, but made sure I was on Adelaide QNH and never went above 4,500. Adelaide Centre called me up and after we established communications he made a comment to the effect of 'I have you on SSR varying between 4,000 and 5,000 please confirm ops normal and that you will stay outside class charlie at 4,500.'

 

Note that in both circumstances it was the same controller.

 

So now it's got me thinking. I know that the altimeter is correct for a number of reasons, one of which is that it matches up with the GPS altitude on the Garmin with the QNH set on the GPS. I'm also assuming that the equipment at Adelaide is operating normally. So what is most likely:

 

1) There is a problem with the altitude reporting from the transponder?

 

2) The controller is just letting me know that he sees me. Might explain the second one, but not the first one??

 

And finally, what are you supposed to say when you get a comment from ATC like this and you're looking at information in the cockpit that doesn't match what they are saying?

 

 

[Nobody]

Check (or get whoever does your maintenance to do so) that the transponder encoder is correctly connected to the aircraft static system. It could be that the encoder is using the pressure in the cockpit which is usually lower than the true static which would give a higher reading(first situation) and in thermally conditions would give a fluctuating reading as the gusts pass(your second situation). Some models of transponder allow you to see the altitude that the transponder is outputting. For example refer to page 7 of this. You could compare that to the altimeter reading but note that the transponder is transmitting the altitude with 1013 set as the pressure setting. You can either adjust the altimeter to 1013 to compare or use the approximation that each millibar is 30 feet to convert.

 

I suspect that a number of violations of controlled airspace are due to transponder altitude problems and not true violations.

 

 

[nickduncs84]

Check (or get whoever does your maintenance to do so) that the transponder encoder is correctly connected to the aircraft static system. It could be that the encoder is using the pressure in the cockpit which is usually lower than the true static which would give a higher reading(first situation) and in thermally conditions would give a fluctuating reading as the gusts pass(your second situation). Some models of transponder allow you to see the altitude that the transponder is outputting. For example refer to page 7 of this. You could compare that to the altimeter reading but note that the transponder is transmitting the altitude with 1013 set as the pressure setting. You can either adjust the altimeter to 1013 to compare or use the approximation that each millibar is 30 feet to convert.I suspect that a number of violations of controlled airspace are due to transponder altitude problems and not true violations.

That's interesting. Are you saying that ATC don't have equipment to adjust what is being transmitted at 1013 to local QNH so that what they are looking at on their screen is correct?? Certainly would explain it yesterday at QNH 1027. That's a 420 ft difference, which explains why he said he saw me up around 5,000. Makes sense, but doesn't make sense at the same time!

 

 

[Nobody]

That's interesting. Are you saying that ATC don't have equipment to adjust what is being transmitted at 1013 to local QNH so that what they are looking at on their screen is correct?? Certainly would explain it yesterday at QNH 1027. That's a 420 ft difference, which explains why he said he saw me up around 5,000. Makes sense, but doesn't make sense at the same time!

No that's not what I am saying. The transponder transmits the altitude assuming that the setting is 1013. The screen ATC see does the conversion. If it was based on what setting the pilot was using there would be errors all the time.

 

What I am saying is that some transponders have a readout showing what altitude is being transmitted. To compare that to the altimeter in the aircraft you need to make sure that the altimeter is set to 1013.

 

 

[nickduncs84]

I get it, that makes a lot more sense. The transponder is the original 1979 so no altitude readout. It's starting to sound like there is a problem with it. I might give Adelaide a call next time I'm up and they aren't busy to see if I can do a comparison. Thanks for your help!

 

 

[ayavner]

Good thread, I wonder how many of us are flying around out there in a similar state.

 

 

[KRviator]

Don't forget you don't need to be under a CTA step. If the base of the control zone is 500, you can cruise along quite happily at 500', just the same as if the base of C over E is 8500 and you're at 8500, you're in Class E.

 

 

[rhysmcc]

Basically there is 200ft tolerance, if you come up at 800ft when you should be at 500ft you'll be queried and if you report your altitude at 500ft then you'll be asked to turn off your Mode C as it's giving errors. Sometimes they may let you know when it's 200ft out as a friendly suggestion to get it checked.

 

Best of luck finding the problem and I'm sure ATC would be more then happy to provide read out checks when not busy.

 

 

[facthunter]

I was always taught to have a 200' below designated step as traffic can be at the step height in controlled airspace. Below transition height you should be on area QNH either on forecast or from the ATIS. NO? Nev

 

 

[Ultralights]

get the transponder checked and calibrated if need be. they should be done regularly, and are for IFR category aircraft, i had a similar story but in Sydney control zone, and the erroneous transponder data caused an advisory warning to a B737 on approach to Sydney, i was at 500ft tracking under the approach path of runway 16 at Sydney, about 5 miles from the end of the runway, but the transponder readout said i was at 2000ft, right in front of the B737 on finals. my response to ATC was to confirm i am at 500ft, and i had the 737 visual, the 737 crew confirmed to the tower they had me visual as well at what appeared to be 500 ft. i told the tower i will turn off the transponder if they wanted me to, they asked me to turn it off, and not fly in CTA until it was checked.

 

Once back at bankstown, i found out the aircraft was recently painted, and the transponder antenna removed and the hole masked over for painting. when the paint was done, the transponder antenna was reinstalled without the masking tape being removed. we found this out after we removed the antenna on my return, and found the hole full of chewed up masking tape.

 

sure enough, once the tape was removed, calibration found the transponder was fine.

 

in your case, i would check the transponder is plumbed to an external static source. and if so, its not leaking, as this to can cause some errors in reporting altitudes.

 

 

[KRviator]

I was always taught to have a 200' below designated step as traffic can be at the step height in controlled airspace. Below transition height you should be on area QNH either on forecast or from the ATIS. NO? Nev

Traffic cannot be at the step height, as it is not in controlled airspace at that height if you're in G. Well,if the overlying airspace is C and oure in E then he could be there I suppose, but then he's not being separated from VFR traffic.Where two layers of airspace adjunct vertically, you are always in the "lower" class (not the physically lower) of airspace. Ie C/E, you're in E. E/G, you're in G. AIP ENR 1.4 1.1.7 refers.

 

 

[ben87r]

Below transition height you should be on area QNH either on forecast or from the ATIS. NO? Nev

Yes! Pet hate of mine, I know guys out here who cruise around on Terminal all day long, update from aerodromes they transit past, all because they don't want to bother (couldn't be bothered?) getting Area off Brisbane. Their idea is that it is more accurate, which it is but its no good you're the only one using it!! The difference between Area and Terminal this morning was 4 hectopascals or 120' before any instrument errors.

 

Hanging around YPAD tho ild be on Terminal inside CTA tho.

 

Alright sorry for the rant, back to topic!

 

 

[KRviator]

Yes! Pet hate of mine, I know guys out here who cruise around on Terminal all day long, update from aerodromes they transit past, all because they don't want to bother (couldn't be bothered?) getting Area off Brisbane.

Maybe because that is what the AIP says to do? Use a QNH setting from a reference source within 100Nm, if not available, use area.If you blast off into the briny blue and don't touch the subscale again, then you're a goose though, I agree...

 

 

[ben87r]

Not going to get into that too much but the intent is 100nm from departure aerodrome. Cruising along at A095 surrounded by IFR traffic is not a good idea. Like I said, no point being the only one with an accurate QNH.

 

 

[Patrick Normoyle]

AD APP equipment takes the area QNH below A100 or YPAD QNH or above A100 it uses 1013.2, when you set QNH to the same as everyone else in the similar area, you are all using the same height datum or reference point, and if you are on different settings the information is useless. Between VFR and IFR. In CTR we can use 500ft verticals separation, if your transponder is out by 200 and the other guy is out by 200ft, there is a minimum of 100ft. The 200ft is the maximum tolerance for a mode C transponder, if you have a faulty transponder and also set it incorrectly this can potentially make a greater error.

 

If I get an error greater than 200ft I'm required to instruct the pilot to use mode A only, we prefer the transponder on but outside the 200ft tolerance is bad for many reasons TCAS is just one of them, I will suggest that you get it checked before entering controlled airspace again. We can cancel or refuse a clearance to enter or use CTA if your transponder is out of tolerance.

 

I stand to be corrected, but I think if you have and use a transponder IFR or VFR it is a requirement to have it checked for operation and calibration every two years for private operations and less for commercial operations.

 

 

[KRviator]

Clearly I'm not the only one out and about with an accurate QNH, per the AIP if you "know guys around there" that do it as well.

 

I've had a pretty good look through the AIP, and I can't find where it says what it is "intended" to mean. It says use a known QNH within 100nm of your route, if that is not available, use area. That makes the most sense as it means an accurate QNH for local conditions.

 

If you choose not to, well, that's up to you, particularly as you seem to know of the 100nm requirement but ignore it anyway. But if you're that scared of me being on local QNH and you being on area, to use you're example we have a 120' buffer between us vertically for the same altitude enroute.

 

Knowing that, and if you reckon most other pilots are flying on area QNH, I think I'll stick to what I'm doing!

 

 

[frank marriott]

Nick

 

sounds like a RAD43 might be in order for your aircraft

 

 

[ben87r]

Hmm. Like I said I wasn't going to get into this but, here we go.

 

C That makes the most sense as it means an accurate QNH for local conditions.

 

Actually it makes no sense, the whole idea of datum go out the window once there are numerous different datum's

 

If you choose not to, well, that's up to you, particularly as you seem to know of the 100nm requirement but ignore it anyway.

 

Can you show me the CAR that states Terminal is to me used if an aerodrome is within 100NM?

 

But if you're that scared of me being on local QNH and you being on area, to use you're example we have a 120' buffer between us vertically for the same altitude enroute.

 

If you're happy with 120' you have bigger tackle then I do

[facthunter]

Any reference to GPS height has to be out surely for separation from other aircraft.? The actual height is only important if you are clearing obstacles. The main point of ALL being on the same QNH is to provide separation from other aircraft primarily and if you are IFR particularly, it affects your clearance heights and minimums.

 

IF you are at an aerodrome and set the altimeter to read the same as the aerodrome height above MSL (At the ARP) it should read the correct local QNH on the kollsman scale if the instrument is working properly Or 200 ft maximum permissible error which is about 7 millibars ??? (showing my age here.) This is basic altimetry folks. Your altimeter's calibration assumes a standard atmosphere which may be correct or not on the day, But works because everyone is supposed to be using the same reference.. One day we may have something better . using actual heights. Nev

 

 

[Nobody]

Not going to get into that too much but the intent is 100nm from departure aerodrome. Cruising along at A095 surrounded by IFR traffic is not a good idea. Like I said, no point being the only one with an accurate QNH.

ENR 1.7-2 paragraph 2.1.2 is pretty explicit.

 

1. For all operations at or below the Transition Altitude (in the Alti- meter Setting region), the altimeter reference setting will be:
    
   a. the current Local QNH of a station along the route within 100NM of the aircraft; or
    
   b. the current Area Forecast QNH if the current Local QNH is not known.
    
    
    
   

 

 

 

 

[Downunder]

Like these type of threads .....very very interesting.

 

 

[SDQDI]

Like these type of threads .....very very interesting.

Absolutely, it gets the regs out for those of us that don't read through them at night before bed (ok I'll admit I don't know the regs and exemptions back to front).

Personally I use aerodrome alt setting on all my local flights (except the time I forgot to set it, I used area that day:whistling:) and then if on a long trip once 60 or 70 miles from home I'll swap to area. IMHO if below 10000amsl I would have thought most accurate would be the best?

 

 

[Birdseye]

ENR 1.7-2 paragraph 2.1.2 is pretty explicit.

Fact is always better than fiction (BOM forecasts that is). As you say there is no room for interpretation in that paragraph.

 

 

[mothra]

Any reference to GPS height has to be out surely for separation from other aircraft.?

Correct Nev, only barometric altitude info can be used for vertical separation.

 

 

[ben87r]

I would have thought most accurate would be the best?

It would be if everyone was using the same, but, there are large differences in terminal/local especially in the southern states in winter. Area at least gives the guarantees 5HPA of difference before a split happens but as i said it really doesn't matter what is set as long as everyone is on the same DATUM! 100NM is about an hour flight in a jab type, allot happens in an hour but more importantly is ALL IFR traffic will be on area.