Garmin GDL 39 pressure alt has me on the moon

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George Chityat

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george99
Just got a brand new GDL39 3D.
I am using it in my car a bit to play around with it.
First day worked fine, but now it has my pressure altitude literally over a billion ft.
I could hardly find anything about this on google. From what I read it has an internal barometer, but I don't know where on the unit. I don't see anything about recalibrating. Practically nothing about this.
Any idea how to fix this?
 
Ummm, the GDL 39 3D does not have a barometer so you aren't seeing pressure altitude. It does have a built-in AHRS but the airspeed and altitude data shown on the PFD display are both GPS derived. My guess is your problem is related to not having enough satellite fixes (4 IIRC) for a valid altitude display.
 
According to multiple aviation websites/forums, and Garmin user manual, it does have a pressure altitude sensor. I have the 3d model, maybe that's the difference.
In the ios Garmin pilot app, I don't see pressure altitude indicated anywhere, however, the altitudes of the traffic that it shows are tens of thousands of feet below me. I am in florida where the highest elevation is a speed bump. So nobody is below me when I am on the ground.

I tried using the GDL39 with foreflight and it works. Foreflight is showing the the gdl39 is specifically reporting a pressure altitude in the trillions. The number is so long it is cut off, so I can't even read it.

Furthermore the GDL 39 has the pressurized cabin on/off switch. So how would it know without either having it's own barometer, or relying on the nearest reporting station to get barometric pressure?
 
I too have a GDL 39 3D and see I see no mention of a barometer or a pressurized cabin switch in the manual. Mine is hard wired into my EFIS and I also use it with GarminPilot on an iPad.

Can you provide a link to a site that shows the GDL has a baro sensor? It's not listed on the Garmin site nor in the GDL manual.
 
Look in the manual:
I can't post links since I am a new member.
Page 4, the last red colored warning. It talks about pressure altitude provide by the GDL.
You can also search google for GDL39 pressure altitude sensor, you will see a variety of forums with people mentioning it. Beechtalk.com has a good disussion about it, but you need to be a member to see it.

But the fact alone that it has the setting for a pressurized cabin essentially means that it has a sensor. Otherwise, why would that settting even matter?
 
I've read that warning about the pressurized cabin but I find it odd that the manual nor the Garmin website make any further mention of it. I did read through a bunch of the BeechTalk posts but the info seemed somewhat contradictory and not definitive. At best I'd say if there is a senor, it seems to used primarily for ADS-B traffic.
 
That's exactly what it is used for. But look at the link that RotorDude posted. It is Garmin's website that has the user manual (not the PDF version( that clearly states that it has a PA sensor.
So then back to my original question. Why is it showing me on the moon?
 
I don't know but I did fire off a question to Garmin Aviation support and asked about the sensor. If I get an answer maybe there will be a nugget of info that might help point to a reason.
 
So Garmin responded that yes it does have the sensor (so I stand corrected!!!) and that it is used to aid in displaying the relative altitude of received traffic information. Unfortunately it doesn't answer why yours is reading way off. I wish I had mine here at the house so I could run a test.

Here's their e-mail if you want to ask them direct: aviation.support@garmin.com
 
Wow, they answered you that quickly on a Saturday?
I'll try it now.
Thanks.
 
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