Transponder Issue “Pressure Alt”

[WAFI]

I’m looking for some suggestions as to why my Stratus Transponder is not transmitting my altitude to ATC. Actually it probably isn’t receiving pressure altitude data anymore. I’ve never had this issue before I did my panel refit. Where does a Stratus get is pressure altitude from? 

Before show Press ALT & After show flight ID and my tail #

[ArtVandelay]

Altitude encoder, follow the static lines.

[WAFI]

1 hour ago, ArtVandelay said:

Altitude encoder, follow the static lines.

I don't think i have a altitude encoder. Maybe I do... Does the Status get is altitude direct from the encoder? Can the transponder get its altitude from somewhere else?

[N201MKTurbo]

On my Avidyne transponder, you just press the FUNC button until you get the display you want. If you do this what does the altitude display?

Im not familiar with that transponder, but most modern transponders can get its altitude from just about any source. It can be a grey code encoder or a serial encoder. The Garmin transponders have a built in serial encoder.

If it isn’t actually receiving a valid altitude signal, it could be a bad encoder or a bad configuration.

[LANCECASPER]

Trig makes transponders that they market under their own name and some for other re-sellers, such as Avidyne (AXP340, AXP322 remote) Bendix King (KT74), Appareo (Stratus).

For some reason the Appareo Stratus seems to be the least reliable of the ones they make. I hear of a lot of failures. It's also got the highest retail price of the Trig transponders.

[WAFI]

Yesterday flying back to the airport ATC asked me to reset my transponder and swak altitude. I did then ATC said that i popped up and when away again. I was on short final doing this so I proceeded to land and that's where im at now.

[N201MKTurbo]

34 minutes ago, WAFI said:

Yesterday flying back to the airport ATC asked me to reset my transponder and swak altitude. I did then ATC said that i popped up and when away again. I was on short final doing this so I proceeded to land and that's where im at now.

The tower here says that about my Cessna. I had it had it checked twice by the avionics shop. They couldn't find anything wrong, but said the tower has a lousy receiver.

[Ragsf15e]

5 hours ago, N201MKTurbo said:

On my Avidyne transponder, you just press the FUNC button until you get the display you want. If you do this what does the altitude display?

Im not familiar with that transponder, but most modern transponders can get its altitude from just about any source. It can be a grey code encoder or a serial encoder. The Garmin transponders have a built in serial encoder.

If it isn’t actually receiving a valid altitude signal, it could be a bad encoder or a bad configuration.

I have a gtx-345, and I didn’t think it has a built in encoder?  When installed, they asked if i wanted to keep using the old grey box encoder or the G5s I also installed (i opted for the g5).  I guess I thought the Garmin transponder gets the altitude from the PFD.  Possibly I’m mixing up the terms though?

So what other avionics does the OP have, and what is supplying the altitude input to the transponder?

[N201MKTurbo]

17 minutes ago, Ragsf15e said:

I have a gtx-345, and I didn’t think it has a built in encoder?  When installed, they asked if i wanted to keep using the old grey box encoder or the G5s I also installed (i opted for the g5).  I guess I thought the Garmin transponder gets the altitude from the PFD.  Possibly I’m mixing up the terms though?

So what other avionics does the OP have, and what is supplying the altitude input to the transponder?

Yes, you can use the G5. When I installed my avionics, the G5 specifically said you couldn’t use it. Now you can. I wired it up anyway, so all I have to do is change a few configuration settings and I can remove my encoder.

[Shadrach]

3 hours ago, WAFI said:

Yesterday flying back to the airport ATC asked me to reset my transponder and swak altitude. I did then ATC said that i popped up and when away again. I was on short final doing this so I proceeded to land and that's where im at now.

Happens to me frequently in the DC area. I have a Garmin 335 that is only a few years old. My 335 sources altitude information from a blind encoder mounted behind the panel. I’m betting that your Stratus is the same.

[jaylw314]

9 hours ago, WAFI said:

I’m looking for some suggestions as to why my Stratus Transponder is not transmitting my altitude to ATC. Actually it probably isn’t receiving pressure altitude data anymore. I’ve never had this issue before I did my panel refit. Where does a Stratus get is pressure altitude from? 

Before show Press ALT & After show flight ID and my tail #

Does your J have an encoding altimeter?  I'm not sure how many J's came from the factory with that, I suspect the ones with BK autopilots did.  It's easy to tell, the altimeter says "ENCODING" in big yellow letters on the instrument dial  

[jaylw314]

7 hours ago, N201MKTurbo said:

On my Avidyne transponder, you just press the FUNC button until you get the display you want. If you do this what does the altitude display?

Im not familiar with that transponder, but most modern transponders can get its altitude from just about any source. It can be a grey code encoder or a serial encoder. The Garmin transponders have a built in serial encoder.

If it isn’t actually receiving a valid altitude signal, it could be a bad encoder or a bad configuration.

That is precisely the way my KT-74 works.  If there is no altitude signal, you just get "FL ---"

[Shadrach]

17 minutes ago, jaylw314 said:

Does your J have an encoding altimeter?  I'm not sure how many J's came from the factory with that, I suspect the ones with BK autopilots did.  It's easy to tell, the altimeter says "ENCODING" in big yellow letters on the instrument dial  

My F had an encoding altimeter. Whether it was factory installed, I don’t know. Why it was installed I can’t  say. We did not have a BK AP. We did have a BK Silver Crown stack installed in the late 80s. Maybe it was installed then.

[WAFI]

3 hours ago, jaylw314 said:

Does your J have an encoding altimeter?  I'm not sure how many J's came from the factory with that, I suspect the ones with BK autopilots did.  It's easy to tell, the altimeter says "ENCODING" in big yellow letters on the instrument dial  

No encoding altimeter but i did found this today—

[WAFI]

After removing and reinstalling my Stratus Transponder and checking all the connections, then powering it back on it show a Press ALT -100ft so, I’m guessing it is getting altitude from somewhere. 

The mode c serializer doesn’t have a static port… Does anyone know how this thing works or is it connected to something else that gives it the altitude????

[jamesm]

On 9/6/2022 at 11:22 AM, N201MKTurbo said:

When I installed my avionics, the G5 specifically said you couldn’t use it. Now you can. I wired it up anyway, so all I have to do is change a few configuration settings and I can remove my encoder.

So what has changed to allow you/us (certified aircraft) to replace separate altitude encoder to use the RS232 output on the G5 ?  (please don't take this a challenge) I would like to do that same thing without have a pitot static test set to verify it and documentation I have been very apprehensive of doing this.  I see that you can set the [RS232] "output format" to [Altitude Encoder] on the G5. Before I noticed this output option on the G5, I was thinking that altitude being sent to ARTCC would corrected for baro  setting rather than 29.92 pressure setting which is what I have been told happens instrument repairing shop.  is there certain setting  you have to do during altimeter / encoder recert  every 2 years or adapters pneumatic/electrical or ???  ?

Thanks,

James

[carusoam]

Expect to find…

Altimeter…

Altitude encoder… (small metal box that communicates raw altitude data…)

All sent to the transponder…

The altitude data is un-adjusted… changing the Khoelsman (?) number won’t change the reading…

It is perma-set to 29.92…

the older BK transponder prior tot he KT74, KT70C was also very digital…. And can show what it is broadcasting for altitude… constantly displayed on its face…

 

Sooooo…. If you set the altimeter’s K window…. To 29.92… see if your transponder shows the same value…?

 

Expect ATC has us using our K-window so we fly on an exact altitude…  they have to handle the differences at their end….  Unit, we reach the magic altitude where we all use 29.92 in the K-window…

Pp rambles only, not a CFI…

Best regards,

-a-

[N201MKTurbo]

3 hours ago, jamesm said:

So what has changed to allow you/us (certified aircraft) to replace separate altitude encoder to use the RS232 output on the G5 ?  (please don't take this a challenge) I would like to do that same thing without have a pitot static test set to verify it and documentation I have been very apprehensive of doing this.  I see that you can set the [RS232] "output format" to [Altitude Encoder] on the G5. Before I noticed this output option on the G5, I was thinking that altitude being sent to ARTCC would corrected for baro  setting rather than 29.92 pressure setting which is what I have been told happens instrument repairing shop.  is there certain setting  you have to do during altimeter / encoder recert  every 2 years or adapters pneumatic/electrical or ???  ?

Thanks,

James

What changed was the wording in the install manual. The software can add the baro correction or not to the serial output. I assume when you configure the output to be an encoder, it outputs pressure altitude.

[jamesm]

So I take it... if the G5 pressure [Altitude Encoder] output was incorrect it would show up on the Transponder IFR cert ?

 

[jamesm]

On 9/6/2022 at 4:49 PM, WAFI said:

No encoding altimeter but i did found this today

My wild guess is that the Northstar LORAN C (which I am guessing that used to be installed) was using the Transponder's gray code pressure Altitude was converted  into the serial format for some input used by the LORAN C or possibly  early model GPS if they made them or perhaps an previous installed avionics equipment. there probably a separate Altitude encode installed at one time. pure speculation on my part.

[N201MKTurbo]

11 minutes ago, jamesm said:

So I take it... if the G5 pressure [Altitude Encoder] output was incorrect it would show up on the Transponder IFR cert ?

 

You can just look at the display on the transponder. You can set your Kollsman window to 29.92 in the G5 and the G5 and transponder should match.

[Ragsf15e]

It’s real nice to have the altitude displayed on the G5 exactly matching the altitude (with baro correction) transmitted through the transponder.  With the old encoders, your altimeter and transponder might not be exact but still within tolerance.  My g5 has been perfect for that.