"NO ALT" Message

Amadeus · #1 08-23-2023, 08:49 AM

My panel is the -12iS dual G3X package with a GPS175 navigator added. Recently, the last 5 or 6 flights, a "NO ALT" message appears occasionally over the transponder code. "NO ALT" is never on for more than 10 seconds at a time and "XPDR" is displayed the rest of the time.

I did pull a PAPR and in the Exceptions the NIC box was red.

Missing Elements has BAlt red with 3.51% fail, Max dT 0:00:11 and MCF 11

Integrity & Accuracy has the NIC box red with 16.92% fail, Max dT 0:00:23, MCF 26, AVG 7.4, Min 0 and Max 9

Are these acceptable error levels?

rcarsey · #2 08-23-2023, 09:07 AM

You should be seeing 0% failures.

The xpdr is getting intermittent barometric data from the G3X.. In the stock configuration, the xpdr gets its ADSB-compliant GPS data over its Serial 3 port from the GPS 20A unit... which you may have rewired now that you have the GPS175 in the mix..

The xpdr also gets ADAHARS data from the GSU25 over its Serial 1 port (pins 9 & 31 at the xpdr and pings 9&10 at the ADAHARS unit under the baggage floor). The xpdr broadcasts your pressure altitude and heading.. and it gets that info directly from the GSU25.

Assuming the G3X is showing good data all the time and no error messages, I'm thinking something is up with that serial line.. intermittent connection, something grounding out etc.

I've attached a report from my plane for a 1 hour flight for your reference. This was at normal altitudes and within normal radar coverage areas in my area.

g3xpert · #3 08-23-2023, 09:09 AM

Quote:

Originally Posted by Amadeus

My panel is the -12iS dual G3X package with a GPS175 navigator added. Recently, the last 5 or 6 flights, a "NO ALT" message appears occasionally over the transponder code. "NO ALT" is never on for more than 10 seconds at a time and "XPDR" is displayed the rest of the time.

I did pull a PAPR and in the Exceptions the NIC box was red.

Missing Elements has BAlt red with 3.51% fail, Max dT 0:00:11 and MCF 11

Integrity & Accuracy has the NIC box red with 16.92% fail, Max dT 0:00:23, MCF 26, AVG 7.4, Min 0 and Max 9

Are these acceptable error levels?

Jayson,

Can you please email us a copy of the complete PAPR report to G3Xpert@Garmin.com?

Thanks,

Justin

Amadeus · #4 08-23-2023, 10:23 AM

Quote:

Originally Posted by g3xpert

Can you please email us a copy of the complete PAPR report to G3Xpert@Garmin.com?

Yes; just sent.

Amadeus · #5 08-31-2023, 11:14 AM

Garmin g3xpert is reviewing flight log data in comparison to the PAPR.

No update as yet.

mburch · #6 08-31-2023, 11:23 AM

Quote:

Originally Posted by rcarsey

Assuming the G3X is showing good data all the time and no error messages, I'm thinking something is up with that serial line.. intermittent connection, something grounding out etc.

Yes, this is the right approach. Be sure to verify both of the RS-232 conductors between the GSU 25 and transponder (transmit and receive) are connected and working; I have seen this same behavior when the RX pin on the GSU 25 is not connected.