Transponder FL fault following PAW and ADSB install.

[rainbow462]

Just wanting to see if anyone has any ideas on the following issue.....
- C42 with funke TRT800H transponder (recently updated by funke) 
- Fitted PAW rosetta with external aerials and RS232 cable to transponder for ADBS out.   
- PAW is seeing plenty of contacts, so all good.
- ADSB is broadcasting position correctly as checked via a basestation and getyourwings software.   

However, the transponder FL reading is now behaving very strangely.   Sometimes it is correct and on a recent flight it started correct (vs Altimeter at 1013) on the ground, still behaved at varying heights for next 10 mins and then started to read about 1,500' lower than altitude.    Looking back at a few recent  flight tracks on flight radar also show this trend of reporting 1,500' lower than actual altitude.  On landing the error then varies and can even change while taxying.   Very odd. 

Any ideas as to what might be the issue or what to try gratefully received.

Thanks

[steveu]

For those of us not Funke owners, where is the Funke getting its altitude information from?

Is it an internal altimeter? If so, can we check piping for static port?

If something else is supplying data to the Funke, is the cable and power supply to the external alti reliable?

[Ian Melville]

I would disconnect the RS232 cable and test again. Having said that, any data from the serial feed should only be used on the ADSB transmission, which you say is correct?

The TRT800H has an internal pressure sensor for the flight level encoder used by Mode C and Mode S. The static tube should be connected to the aircraft static system. If the Altimeter is behaving correctly then look at the tube that was added to 'Tee' off the static to the transponder. Is there a low point that is collecting moisture?

[rainbow462]

Thanks for the reply Ian.

Would it still need connecting to the static pressure system in an unpressurised cockpit like a C42.  The funke manual just refers to having a silicon tube attached to the rear static port.

Thanks

[exfirepro]

rainbow 462,

What settings do you have the USB Port in your Rosetta set to?

Regards

Peter

[Ian Melville]

Would it still need connecting to the static pressure system in an unpressurised cockpit like a C42.  The funke manual just refers to having a silicon tube attached to the rear static port.

Depends on the aircraft. Some aircraft can get away with it, but yes, it is best to connect to the static system to get an more accurate reading. Cockpit air pressure can change with speed and factors like having a DV panel open. Static system minimises that variable.
The Funke manual is indeed very light on specifics regarding the static system, or what you should connect the tube to. If they really thought that it was OK at cockpit pressure, they would have dispensed with the port to which the silicon connects and saved a few bob.

[rainbow462]

Hi Peter

usb port 1 = Transponder Funke - 4,800

rainbow 462,

What settings do you have the USB Port in your Rosetta set to?

Regards

Peter