What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Altitude encoding problem - MGL Odyssey G2

NM Doug

Well Known Member
We have an MGL Odyssey G2 EFIS, and today approach and then the tower told us our Mode C was intermittent. It displayed our correct altitude sometimes, and then "XXX" on their scope, aka invalid altitude.

What are the possible failure points here? We have the EFIS acting as the encoder, sending the gray codes to our Garmin 320A. We have a Garmin 327 on its way, coincidentally, and we are planning to swap out the 320A, but I doubt the issue lies in the transponder itself.

~ Doug
 
Alt Encoding Issue

My thoughts;

Possible issues
1) Efis is providing the Alt out data intermittently?
2) A loose gray code wire from Efis to Transponder?
3) Transponder is acting up?

Tracking down the problem

1) Does the Efis have any intermittent or flashing data in its altitude display?
Check the Efis recorded internal data (Black Box viewer) to see if Altitude is intermittent.
2) If OK then check gray code wiring between Efis and Transponder? Make sure all are good.
3) Stop by an avionics shop and get there help or borrow a Transponder checker and monitor transponder altitude output.
Borrow a GTX-320 or 327 and swap it with your unit (327 will show altitude on its display which could tell you directly if the Alt output is intermittent), GTX 327 should be directly swap-able with the 320 without touching your tray or wiring.

If its wiring when you get a 327 you could change your Efis to Transponder Alt data wiring from gray code to RS232 or just fix the bad wire.
 
Mode C mystery continues

Thanks for your thoughts, Cecil. ATC is still reporting intermittent mode C, despite what we've tried so far (see interspersed below). I'm going to try to get troubleshooting help from the local avionics shop - I'd welcome theories of what is going on to share with them.

My thoughts;

Possible issues
1) Efis is providing the Alt out data intermittently?
2) A loose gray code wire from Efis to Transponder?
3) Transponder is acting up?

Tracking down the problem

1) Does the Efis have any intermittent or flashing data in its altitude display?
Check the Efis recorded internal data (Black Box viewer) to see if Altitude is intermittent.

These looked OK. The altitude display on the EFIS has been solid throughout, and nothing looked out of the ordinary on the MGL black box viewer.

2) If OK then check gray code wiring between Efis and Transponder? Make sure all are good.

We unplugged the wiring from the back of the EFIS, checked for anything obvious, and sprayed connector cleaner. Same from the inside of the tray, after pulling out the transponder. My helping mechanic reported that wires/pins looked solid.

3) Stop by an avionics shop and get there help or borrow a Transponder checker and monitor transponder altitude output.
Borrow a GTX-320 or 327 and swap it with your unit (327 will show altitude on its display which could tell you directly if the Alt output is intermittent), GTX 327 should be directly swap-able with the 320 without touching your tray or wiring.

If its wiring when you get a 327 you could change your Efis to Transponder Alt data wiring from gray code to RS232 or just fix the bad wire.

On the ground, the altitude is solid with a just-put-in, used GTX 327, and if I start it in setup mode, none of the gray code lines are flickering.

Our VFR transponder check passed OK on the 327. In the air, the 327 displays the correct pressure altitude the whole time...even when ATC reports our Mode C has blinked out (!?). This is the part that really confuses me. ATC has not reported our squawk dropping out, though.

Our Echo ADS-B iPad app in monitor mode is occasionally showing 0 pressure altitude in flight, too - it is set up to wirelessly sniff the squawk and pressure altitude. I noticed a problem with the relative altitudes reported on ForeFlight the same day ATC first alerted me to the intermittent mode C (that was when I had the 320a in).

Thanks
 
I think you may have a transponder problem.
All pointers you mention verify correct operation of the altitude encoder. The transponder will tell you if the altitude drops out at its input.

The way mode A/C works:

Mode A and mode C interrogations are sent by the secondary radar alternately. They are distinguished by the receiving transponder by measuring the time difference of a pulse pair (P1 to P3 timing of either 8 or 21 uS).

Your transponder is seeing the mode A request (Squawk) but on occasion is missing the mode C request (Altitude). This could be due to the transponders internal time base having drifted - this will make it more likely to miss the 21uS pulse for mode C while the shorter mode A may still be in tolerance. On many older transponders the timing is measured with resistor and capacitor delay circuits and these can age.

A transponder test set can pick this issue up - provided it happens when you are busy testing. Most test sets only do a brief interrogation rather than a continuous one so could easily miss an intermittent problem.

The suggestion to try another transponder if available is a good one and might be the easiest way to ensure you are looking in the right place.

Rainier
CEO MGL Avionics
 
The plot thickens

New clue & hypothesis today... NOT what we were expecting to see.

After checking the gray codes from the EFIS to the 327 using the MGL general diagnostic screen and 327 gray code display, we taped one of the static ports and used a manual vacuum pump taped around the other to simulate a higher altitude. The transponder and EFIS both indicated correctly, but we weren't getting a consistent pressure altitude sniffed by the uAvionix - it kept dropping to 0 ft.

In the process of swapping in another transponder for testing, we had coincidentally left the static system at a lower pressure/higher "altitude", and when pushing on the transponder to seat it, the indicated altitude dropped a couple hundred feet and remained at the lower indication after releasing pressure. (!) Further testing showed that pushing on the panel near the airspeed indicator or between the ASI and the altimeter causes this behavior in a repeatable way.

Our current (long shot?) hypothesis is a pressure/vibration activated leak in the ASI in the case or diaphragm between its pitot/dynamic side and the static side. In flight, this could conceivably shoot higher pressure air irregularly into the static system and confuse the encoder and/or transponder.

Our next step is going to be to take out the ASI to inspect or test.
 
Back
Top