Primarily for general aviation discussion, but other aviation topics are also welcome.
By Farmstripflyer
#1719797
I have something strange going on with my transponder and looking for a bit of advice. Panel mount altimeter and Dynon D100 set to correct QNH and both confirm altitude along with Garmin nav and Skydemon. However, Trig TT21 reads 100’ below and ground based radar show me 200’ above aircraft readings! Trig had recent software update prior to installing ADSB using Pilotaware as GPS source. I am planning to send Trig back to factory for checks but could this have anything to do with the Pilot Aware or any other thoughts?
By Lefty
#1719800
The only way radar stations can know your altitude is from the altitude that your transponder sends out. It can’t read your Pilot Aware output.

Where does your transponder get its altitude info? Most transponders have a box called a “blind encoder” - essentially a barometric altimeter without a display , it just gives the measured altitude to your transponder.

Where does your Dynon get its altitude from?
By Lefty
#1719808
CloudHound wrote:And where is the source of air for the encoder? Stubby tube at the back of the panel or T piece into static line?


It should be a T piece into the static line.
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By leemoore1966
#1719811
Farmstripflyer wrote:...Trig TT21 reads 100’ below and ground based radar show me 200’ above aircraft readings! Trig had recent software update prior to installing ADSB using Pilotaware as GPS source. I am planning to send Trig back to factory for checks but could this have anything to do with the Pilot Aware or any other thoughts?

Nah, Pilotaware only provides the GPS coordinates, not the altitude, this comes from the altitude encoder, which I presume is integrated into the trig

Thx
Lee
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By Smaragd
#1719812
Farmstripflyer wrote:I have something strange going on with my transponder and looking for a bit of advice. Panel mount altimeter and Dynon D100 set to correct QNH and both confirm altitude along with Garmin nav and Skydemon. However, Trig TT21 reads 100’ below and ground based radar show me 200’ above aircraft readings! Trig had recent software update prior to installing ADSB using Pilotaware as GPS source. I am planning to send Trig back to factory for checks but could this have anything to do with the Pilot Aware or any other thoughts?


There are several elements here.

1. Your transponder shows a flight level on its display, and outputs that value to ground radar - nothing to do with static sources, in terms of what the radar receives. However ATC will correct for the difference between the relevant QNH and 1013.2, so the altitude reported by ground radar will differ from the FL shown on the transponder when QNH does not equal 1013.2 . Are you allowing for this in your 300' difference between TT21 and ground based radar?

2. Your transponder's accuracy - I believe 200' is allowed , others may have a better figure.

3. When those have been taken into account, then there comes into question the suitability of the static source or sources for altimeter, transponder etc, plus the accuracy of the individual instruments!.
By BehyBill
#1719815
Farmstripflyer wrote:Trig TT21 reads 100’ below and ground based radar show me 200’ above aircraft readings!


I have two barometric altimeters they show 200ft diffs from time to time, kicking the screen I get it reduced to 100ft, maybe ATC should kick their screen as well :lol:
#1719831
Farmstripflyer wrote:Trig TT21 reads 100’ below and ground based radar show me 200’ above aircraft readings!


Unless there's some data link which allows you to set the QNH on the transponder, the transponder will always read a flight level (i.e. altitude with reference to 1013). It will always report a flight level to ATC.

This will be further out from the actual altitude, the further away you get from 1013. Try setting 1013 (remember to make a note of the QNH first so you can go back to it!) and seeing what the transponder reads compared with the altimeter. That should give an indication of how accurate your encoder is.

If both are the same but ATC complain you're not at the correct altitude, then as above, your static source may be the issue.
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By PaulSS
#1719866
No, because
ATC will correct for the difference between the relevant QNH and 1013.2, so the altitude reported by ground radar will differ from the FL shown on the transponder when QNH does not equal 1013.2
#1719868
Thanks for the replies guys, much appreciated. Some interesting points to consider. The static line for the transponder is just a stubby tube coming out of the back, not connected to anything, this however has not been a problem for the last six years! I have removed the transponder head and sent back to Trigg for factory check as everything was fine before the latest software update so hopefully there is something that the guys at Trigg can sort out. I have in fact received a letter from NATS concerning a recent infringement of the Stansted CTA (by 100', right on the edge) which is what has brought the problem to light, so naturally I need to get this sorted out.
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By Paul_Sengupta
#1719871
With an error of just 200ft, you may not have received a report of a clash in the past. With the static port for the transponder/encoder being in the cabin, there could be a problem. The pressure in the cabin is often lower than that outside the aircraft due to suction, thus indicating a higher altitude. If you want an accurate reading, it should be plumbed into the static line which goes to the altimeter.

If you had the transponder you could have done the 1013 check to see what the transponder is showing compared to what the altimeter is showing.
#1719872
I will plumb the transponder into the static line as suggested, (don't know why it was never done in the first place), and see how I get on with my next flight when the transponder comes back from Trig. Yesterday, I asked for an altitude reading while I was still stationary on the ground with the local QNH set and I was indicating 200' above the airfield elevation!
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By PaulSS
#1719894
Yesterday, I asked for an altitude reading while I was still stationary on the ground with the local QNH set and I was indicating 200' above the airfield elevation!


Well that's obviously good news for the infringement investigation if your transponder is over-reading by 200'. It's still within limits but it's good for you because you can tell the feds you were actually 100' below the airspace (hence your 'infringement' by 100').

My only suggestion is to calibrate your transponder. It's a pretty simple process but it does need a decent pressure machine plugged in to do so. Definitely something that you'd need to pay a bloke for to ensure you get proper vertical information fed to your transponder, so you can then tell it what it should be reading. In Oz you have to have your pitot/static system checked every two years, so I used the opportunity to calibrate my mate's TT21. If I can do it anyone can......apart from gaznav (obviously) :D
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By rf3flyer
#1719895
Farmstripflyer wrote:...while I was still stationary on the ground with the local QNH set and I was indicating 200' above the airfield elevation!

I used to have an altimeter that did that.