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Terrain, Terrain, Pull Up, Pull Up!

Amadeus

Amadeus

Well Known Member
My RV-12iS has the dual G3X VFR package that has had the GPS20A removed and replaced with the GPS175 in March 2023. Last fall, shortly after a take off in VFR conditions, the G3X map screen and PFD background displayed in the color red and the audio alert of "terrain, terrain, pull up, pull up" began repeating itself. Having flown from that airport multiple times and also being able to see clearly on a CAVU day I knew there was nothing in my path at my altitude and continued my flight. Once at cruising altitude my pilot/passenger and I began to assess why this may have occurred and came up with a WAG (wild assumption guess). We were attending a fly-in event that had many aircraft departing once festivities were over. I had just started up and the GPS175 was still in TERR TEST mode when the marshaller began directing me to pull into the line of traffic moving toward the runway. My temperatures were in the green so I began movement. We both figured that moving the airplane while the TERR TEST was still being conducted confused the navigator as to actual location so I made mental note to always wait for the test to complete before beginning to taxi.

Waiting for TERR TEST to complete before movement seemed to do the trick for quite a few months. Then in March of this year I was departing another familiar airport on a VFR day when the same "terrain, terrain, pull up, pull up" event again occurred despite waiting. I wrote it off as a one time thing before it once again occurred leaving an unfamiliar airport (but in a very flat farmland area on the Illinois/Indiana state line) in the path of totality for the April 8 eclipse. Because this was once again taking off from an event when everyone else was I wondered if the close proximity to other navigators could have an input to the problem; after all, two of the three data points are leaving crowded events. However it happened again just last weekend at a small uncrowded airport.

Fortunately all these occurrences were in VFR conditions. Until I figure it out I will not be taking off into IFR conditions. Has anyone else experienced this navigator behavior? I'm stumped on where to begin my research.
 
Your alert audio will be coming from one of your GDU displays, not your IFR navigator.

You can adjust the sensitivity of terrain alerts using the menu options on the Terrain page; they default to the most sensitive settings.
 
mburch said:
Your alert audio will be coming from one of your GDU displays, not your IFR navigator.

You can adjust the sensitivity of terrain alerts using the menu options on the Terrain page; they default to the most sensitive settings.
Click to expand...
Thank you, I will look at those setting options.

The GDU displays are receiving GPS data externally (the GPS175). Would this have any bearing on perhaps stemming from the navigator unit?
 
mburch said:
They will use their own GPS data for terrain alerting, unless something is amiss with your GPS installation.
Click to expand...
Perhaps something is amiss. I do get the "message" on the GDU that "GPS is from an external source" once everything is powered up and operating.
 
Amadeus said:
Perhaps something is amiss. I do get the "message" on the GDU that "GPS is from an external source" once everything is powered up and operating.
Click to expand...
Yes, this is indicative of a problem with the GPS receiver, antenna, or reception to your GDU displays. I would address this before further flight as it is not a normal condition.
 
Walt said:
Huh? The G3X requires a GPS antenna and the GPS175 requires its own antenna, neither has a built in antenna.
Perhaps a review of the associated manuals would help you.
Click to expand...
It isn't built in to the GDU. THIS is the antenna.
 
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I see that I can choose between "external" and "internal" antenna in the G3X settings. Once the GPS175 is booted up and running I get the message on the G3X GDU that the GPS source is from the "external" antenna (meaning the GPS175). If I just click MENU and select INTERNAL as the GPS source would this solve the issue or would it mean that the G3X is now ignoring the GPS175 for everything including the flight plan, approach, etc.?
 
Amadeus said:
I see that I can choose between "external" and "internal" antenna in the G3X settings. Once the GPS175 is booted up and running I get the message on the G3X GDU that the GPS source is from the "external" antenna (meaning the GPS175). If I just click MENU and select INTERNAL as the GPS source would this solve the issue or would it mean that the G3X is now ignoring the GPS175 for everything including the flight plan, approach, etc.?
Click to expand...
Don't confuse GPS navigation with GPS reception. The setting you are looking at affects the flight plan only, not the GDU's GPS receiver. Ignore this and focus on the issues with your GDU's GPS reception. Make sure the GPS antenna configuration is correct, and make sure your GDU can achieve a valid GPS fix. If you don't know how to do this, Garmin product support can help you.
 
I get the alert when I'm going into a local, private, grass strip. It is very annoying and I can't understand why my GTN 650xi doesn't understand it is an airport.
 
Hangar 1271 said:
I get the alert when I'm going into a local, private, grass strip. It is very annoying and I can't understand why my GTN 650xi doesn't understand it is an airport.
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Terain alerts are generated by the G3X (I believe) and have nothing to do with the GTN or it’s databases…..
 
The G3X antenna is not certified. It cannot see as much of the sky. The G3X internal GPS is not WAAS either. Depending on where the antenna is mounted, it can be blocked by the fuselage or something and it is not getting any help from ground stations for error correction and altitude/position lock. The internal GPS in the G3X is likely taking priority over the external. External certified systems are designed to be used standalone so you can pre-load flight plans and approaches without them changing the nav on the G3X display until one is ready. If the G3X does not have a view of the sky or it is getting conflicting info from the certified unit, it will take the most risky situation as priority to call out. I think that is what you were experiencing.

My theory is that when you had the GPS20A the G3X was taking it as a priority. The 20A is a position only system and WAAS certified. The G3X was designed to work with the 20A and was likely ignoring the internal source if there was a conflict. There is a ranking and checksum system in the software. The software developers decide what GPS position takes precedence. Only way to override them is to remove a source that is causing problems.

I believe you can disable the internal G3X GPS completely in the config or unplug the antenna. When the internal system is out the G3X will only be able to rely on the certified GPS position coming in on the 429 serial bus. I recall settings having a setting to set the priority source as well. Try flying off of the certified GPS only to see if it you have any issues. If all works on the certified then you may want to change your G3X antenna to a fuselage mount to give it a better view of the sky or just rely on the external GPS source.
 
Ironflight said:
Terain alerts are generated by the G3X (I believe) and have nothing to do with the GTN or it’s databases…..
Click to expand...
As an owner/operator/installer of GTN650xi - I can tell you that the GTNxi DOES do Terrain Awareness, DOES use it's own single external antenna & internal database.
Warnings ARE displayed on GTNxi screens & are output as Voice warning to EFIS GDU and/or audio panel.
Please review Pilots guide subsection section on Terrain Awareness in section 5 Hazard Awareness (see front page attached).
You CAN select minimum runway conditions (type & length) that are acceptable and many many more things
It's hard to follow this thread - looks like several drifts & avionics suits being discussed....
Just adding known factual info.... sorry if someone considers this yet another drift.
I just checked GPS175 Pilots Guide:

GPS 175 | Garmin Customer Support

Garmin Support Center is where you will find answers to frequently asked questions and resources to help with all of your Garmin products.
support.garmin.com support.garmin.com
It has the SAME functions as the GTNxi
Hope this helps.
 

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AZSteve053 said:
As an owner/operator/installer of GTN650xi - I can tell you that the GTNxi DOES do Terrain Awareness, DOES use it's own single external antenna & internal database.
Warnings ARE displayed on GTNxi screens & are output as Voice warning to EFIS GDU and/or audio panel.
Please review Pilots guide subsection section on Terrain Awareness in section 5 Hazard Awareness (see front page attached).
You CAN select minimum runway conditions (type & length) that are acceptable and many many more things
It's hard to follow this thread - looks like several drifts & avionics suits being discussed....
Just adding known factual info.... sorry if someone considers this yet another drift.
I just checked GPS175 Pilots Guide:

GPS 175 | Garmin Customer Support

Garmin Support Center is where you will find answers to frequently asked questions and resources to help with all of your Garmin products.
support.garmin.com support.garmin.com
It has the SAME functions as the GTNxi
Hope this helps.
Click to expand...
Everything that you say about the GTN is true….but the OP was talking about getting terrain alerts from his G3X, and those are generated from within that unit, not from the GTN. You are correct that this thread has many tangential comments that don;t apply to the origins of the post…..
 
Ironflight said:
Everything that you say about the GTN is true….but the OP was talking about getting terrain alerts from his G3X, and those are generated from within that unit, not from the GTN. You are correct that this thread has many tangential comments that don;t apply to the origins of the post…..
Click to expand...
I am in violent agreement with you Paul. It's IMPORTANT that the OP understands that originally the Terrain warnings came from his VFR GDU & internal GPS & database, when the GPS175 was added in lieu of the GPS-20A he added a 2nd source of Terrain warning (the GPS175). He needs to enable One & ONLY one of the Terrain warning sources... and be sure to put all of his warning & mode choices in the selected Terrain warning source.
 
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