Primarily for general aviation discussion, but other aviation topics are also welcome.
By patowalker
#1719953
Google ICAO Doc 4444 and look up chapter '8.5.5 Level information based on the use of pressure-altitude information'.

If you have an LAA aircraft, you can forget about that, because Francis will only allow 50'. :)
By Nomad63
#1719958
I have a Trig Transponder that I inadvertantly entered and messed with the pressure calibration, I managed to re calibrate it by putting a tee in the static line and pulling like bu99ery till my altimeter read the various calibration points and matched the trig to the same points, it worked perfectly and was never more than 50 feet out afterwards.
When I say pull like bu99ery I mean it, IIRC 3000ft required all my strength and I thought the syringe would implode.

P.S. You have of course to plug the static ports first
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By BEX
#1719992
ICAO 4444 section 8.5.5 was listed as a reference for the "200ft" tolerance in transponder output.

Sadly that isn't quite correct as it relates to information "displayed to the controller", which isn't the same as the output from the transponder as it also takes into account any errors that may be present in the ATC side of the system.

The output from the transponder should be much less than 200ft in error to meet this requirement.

I believe (but can't find the reference just now) that EASA mandate that the tollerance should be around 125ft. Hence my question.

BEX
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By T67M
#1720004
it also takes into account any errors that may be present in the ATC side of the system.


The errors in the ATC side of the system are effectively zero - it just has to add the offset of the day, which is a mathematically perfect operation, based on the measured Qnh which has a miniscule error (typically 15'). The reference to Doc 4444 is correct, the transponder is only required to output is flight level to within 200ft.

The reference to 125' comes from the annual test requirement for transponders which, when you allow for all of the tolerances, the biggest one being that most transponders and encoders only send heights in 100ft steps, means that testing to 125' guarantees performance to 200'. The test requirement is not the functional requirement, and should not be mistaken as such.
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By T67M
#1720005
Farmstripflyer wrote: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!


A friend of mine had a transponder without a connection to the static line. He was chased by the CAA for multiple airspace infringements (I believe three digits) because his transponder was over-reading based on cabin pressure rather than static pressure. Of course, ground testing of the transponder showed no fault because at zero airspeed, static pressure equals cabin pressure. Now that the CAA has a zero tolerance policy on infringements, what worked six years ago is no longer deemed acceptable.

[edited to fix two typos]
Last edited by T67M on Sat Sep 14, 2019 10:20 pm, edited 1 time in total.
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By Talkdownman
#1720010
ATC may 'consider' an aircraft to be 'at' an assigned level provided that the Mode C readout indicates 200 feet or less from that level. If the transponder output tolerance is also 200ft, that would mean that two aircraft which are provided with standard vertical separation using assigned levels 1000 feet apart could be as close as 200 feet... :shock:
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By PaulSS
#1720012
that would mean that two aircraft which are provided with standard vertical separation using assigned levels 1000 feet apart could be indicating as close as 200 feet... :shock:


Word added as we're discussing transponder tolerances, not altimeters.
By rainbow462
#1838215
old thread I know but hoping "@Farmstripflyer" is still active and how the story ended as I have a similar issue. Just fitted PAW and RS232 cable to funke TRT800H transponder. PAW all fine and ADSB position reporting correct but transponder FL now all over the place with errors as much as 1,500' Conclusion to story or any similar experiences/ideas gratefully received. Many thanks
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By leemoore1966
#1838220
rainbow462 wrote:old thread I know but hoping "@Farmstripflyer" is still active and how the story ended as I have a similar issue. Just fitted PAW and RS232 cable to funke TRT800H transponder. PAW all fine and ADSB position reporting correct but transponder FL now all over the place with errors as much as 1,500' Conclusion to story or any similar experiences/ideas gratefully received. Many thanks


Hi rainbow462
how are you observing the FL ?
is this from FR24, or an ADSB receiver ?

The altitude emitted from your transponder is the barometric altitude, not the GPS reference.
The only thing we have seen of being in error (with regard to Funke) is either the track or speed, Not altitude or position report.

Thx
Lee
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By alexbrett2
#1838221
I presume you've checked for something silly like the static line (assuming you're using the built-in encoder rather than a separate one) being disturbed / disconnected when connecting up the RS232 link?
By rainbow462
#1838222
how are you observing the FL ?
is this from FR24, or an ADSB receiver ?

Thanks for replies

I'm observing the FL from the funke display. This is corroborated via FR24 when I play it back later. I've also used the Getyourwings software to check ADSB output and this is also showing errors e.g. -600'

The transponder has never been connected to the altimeter external static tube and is just the stub coming out of the back of the funke