Primarily for general aviation discussion, but other aviation topics are also welcome.
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By Flying_john
FLYER Club Member  FLYER Club Member
#1838225
Ask your LAE or Inspector to bring his pitot static test set to the airfield and check altimeters and transponders to a known good calibrated piece of equipment and make sure you have an external static source unaffected by turbulent airflows.
By rainbow462
#1838227
Thanks for the replies

I'm observing the FL from the transponder screen. Also when I check back FR24 it matches what I saw in flight. Finally using getyourwings software and a polotaware base station I can it is receiving the ADBS signal and the altitude is significantly out e.g. -600'

I'm assuming the TRT800H has a built in encoder.
The static port at the back is not connected to the altimeter/external port and is just is open in the back of the dashboard
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By Jonzarno
#1838249
I’m not sure that the conversion is always infallibly accurate.

I have repeatedly, but not on every flight, had a problem when flying the ILS into one particular airport in Germany (only there and nowhere else!) of ATC telling me that I am 200 feet below my actual altitude based on my mode S read out.

I stress that this does not happen in any other sector, and I have confirmed that my transponder encoder is properly calibrated and both my altimeters show that I am at my assigned altitude. It has happened often enough on the sector for me to point this out to controllers each time it happens.
#1838256
rainbow462 wrote:Thanks for the replies

I'm observing the FL from the transponder screen. Also when I check back FR24 it matches what I saw in flight. Finally using getyourwings software and a polotaware base station I can it is receiving the ADBS signal and the altitude is significantly out e.g. -600'

I'm assuming the TRT800H has a built in encoder.
The static port at the back is not connected to the altimeter/external port and is just is open in the back of the dashboard


It will not work properly unless plumbed to the same static pressure as the altimeter.

I would suggest you get the system checked over and sorted properly before it gets you into trouble.

On a similar note, there are likely a bunch of the new crop of carry-on EC devices with similar cockpit static/ altimeter static pressure discrepancies.
By PeteM
#1838310
I've been down this route recently.

Transponder and altimeter disagree.

Wander around the hangar and discover that my altimeter disagrees by about 3-5 millibars (although a lot more for one!).

Nice new altimeter. Transponder still disagrees - although by somewhat less! Check all the static lines, renew a couple of bits of brittle plastic tubing - still not quite right - recalibrate blind encoder. Bit shocked that without the external static, the altitude error is over 300ft!

All agree! (well generally the digitalisation does mean they are not quite 'in step'). But the surprise was this all happened over less than a year - surprisingly quickly for instruments to drift.

(and yes the LAA do not work to ICAO tolerances) which started this whole business off after a new radio!)
By Boxkite
#1869478
rainbow462 wrote:Thanks for the replies

I'm observing the FL from the transponder screen. Also when I check back FR24 it matches what I saw in flight. Finally using getyourwings software and a polotaware base station I can it is receiving the ADBS signal and the altitude is significantly out e.g. -600'

I'm assuming the TRT800H has a built in encoder.
The static port at the back is not connected to the altimeter/external port and is just is open in the back of the dashboard

A bit late to this but I had an issue with my TRT800H recently (though nothing to do with any fault on the unit itself). After climbing to 6000ft, and after descending to 1600ft, it was still indicating FL034 (which worried the local ATC), and was showing FL019 on landing at near sea level. I have no idea of whether it kept up with the ascent rate or whether it achieved an output of the max altitude I reached (I wasn't up there for longer than a minute). I assumed a blockage and pulled off the static tube from the back of the transponder, by that time it was still showing an output of FL009 and pulling the pipe made no difference. I have since removed the transponder and found a tiny piece of a translucent film and a tiny amount of waxy looking residue (yes, like earwax) blocking the port at the back of the unit.
I have now cleaned it out and hopefully it's cured (not yet refitted).
By SteveX
#1869488
Out of interest what are the consequences of setting the altimeter to exact airfield altitude before departure and ignoring the QNH the tower/fiso says it is? So if setting the altimeter to their alleged qnh shows the alt is 3hp out, which would you set?

What consequences if any of then using that same hpa difference throughout the flight, eg an airport states 1015 but you set 1012 knowing your alt shows the exact real alt when set 3hp difference?
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By Dave W
FLYER Club Member  FLYER Club Member
#1869491
The published airfield elevation is based on a specific reference point on the field. If you are not at that point, setting the altimeter based on that number can be incorrect - sometimes significangly so at some airfields.

Use the ATS provided setting.
By Boxkite
#1869494
SteveX wrote:Out of interest what are the consequences of setting the altimeter to exact airfield altitude before departure and ignoring the QNH the tower/fiso says it is? So if setting the altimeter to their alleged qnh shows the alt is 3hp out, which would you set?

What consequences if any of then using that same hpa difference throughout the flight, eg an airport states 1015 but you set 1012 knowing your alt shows the exact real alt when set 3hp difference?

I am not sure how this question applies to transponders, but in your case, especially with a rented aircraft that you cannot re-zero the scale on, I would carry on applying your reckoned correction. But beware, just because it's out on the ground doesn't mean it's out at altitude. Without a second altimeter, or a test set, you can only compare it with (ideally) at least two GPS sources (using different means of GPS signal, not two tablets using the same PilotAware for example).