GTN 650 failing at altitude with constant LOI issues

[rwarederek]

Hey all

 

I'm a part of a partnership in a '68 M20F.  We had a GTN 650 installed about two and a half years ago and from install time till about last March, the unit has been rock solid.  We're having serious LOI issues with the unit and can't seem to figure out what the heck is going on.  When the GPS unit is turned on on the ground, it captures satellite signals and everything is golden; however, the GPS unit will fail at seemingly random times and never re-acquire satellites until we're on the ground.  Usually what happens is after takeoff, and after some random time (sometimes after a couple minutes while still climbing, sometimes during cruise) the GPS unit displays that big red "X", indicates a LOI and does not acquire satellite signals until on the ground (usually about when we we're about ready to talk to the ground controllers).  The issue has been presenting itself on almost every single flight since March.

 

Our avionics shop is stumped and is convinced the issue is unrelated to the install or the equipment.  They have performed the following with no effect:

 - Antenna swapped out

 - Coax ends re-crimped

 - All connections verified and tested

 - Ground tested with the engine running and not running

 - GTN 650 replaced with a refurbished unit

 

Also maybe relevant to note is that ALL on-board GPS receivers (iPad, iPhone) continue to receive GPS signals and track the airplane without any issue - even when the 650 has failed.  We have also tried to fly with all on-board devices disabled (iPhone, iPad, etc) with no effect. 

 

The avionics show has been unable to re-create the issue on the ground which obviously has made this whole process much harder.  At this point I'm at my wits end with the unit and am reaching out to see if anyone out there has any clue as to what may be going on or what we should instruct another shop to check.  Thanks in advance!!!

 

Derek

[aerohawk]

any other equipment installed or modified recently? Do you ever transmit on 121.5. How far is the gps ant from other ANT?

[ArtVandelay]

On the ground, I would try fiddling with the GPS antenna (push, bang on it little), so the same from the inside. If that doesn't cause a problem, ask the avionics shop to wire up a temporary antenna, lay it on top of your glareshield, go flying, if problem disappears, you know its something with antenna or its wiring.

One more thing, I had similar problems with my radio, turned out to be a bad circuit breaker: it passed continuity test but was not passing current.

[M016576]

When I had the nefarious failed WAAS antenna that you read about in the other forums with my G430W, it bled out a signal that prevented any of my GPS's from receiving a signal. Of course it manifested itself as I was arcing to an ILS final in hard IMC.

It sounds to me like this may possibly be software related, or perhaps a line is receiving interference. Any new gadgets or gizmos on or in your plane lately (a new cell phone? A new Adsb receiver, new iPad)? Also, have you tried updating the units software?

[Marauder]

Hey all

I'm a part of a partnership in a '68 M20F. We had a GTN 650 installed about two and a half years ago and from install time till about last March, the unit has been rock solid. We're having serious LOI issues with the unit and can't seem to figure out what the heck is going on. When the GPS unit is turned on on the ground, it captures satellite signals and everything is golden; however, the GPS unit will fail at seemingly random times and never re-acquire satellites until we're on the ground. Usually what happens is after takeoff, and after some random time (sometimes after a couple minutes while still climbing, sometimes during cruise) the GPS unit displays that big red "X", indicates a LOI and does not acquire satellite signals until on the ground (usually about when we we're about ready to talk to the ground controllers). The issue has been presenting itself on almost every single flight since March.

Our avionics shop is stumped and is convinced the issue is unrelated to the install or the equipment. They have performed the following with no effect:

- Antenna swapped out

- Coax ends re-crimped

- All connections verified and tested

- Ground tested with the engine running and not running

- GTN 650 replaced with a refurbished unit

Also maybe relevant to note is that ALL on-board GPS receivers (iPad, iPhone) continue to receive GPS signals and track the airplane without any issue - even when the 650 has failed. We have also tried to fly with all on-board devices disabled (iPhone, iPad, etc) with no effect.

The avionics show has been unable to re-create the issue on the ground which obviously has made this whole process much harder. At this point I'm at my wits end with the unit and am reaching out to see if anyone out there has any clue as to what may be going on or what we should instruct another shop to check. Thanks in advance!!!

Derek

any other equipment installed or modified recently? Do you ever transmit on 121.5. How far is the gps ant from other ANT?

On the thread I created concerning an open squelch issue on my 650, I found a recently installed JPI 900 is the root of the problem. I would suspect any work done since the problem began.

If nothing was done, I would have the shop check the GTN rack to make sure nothing is wrong with the tray itself. We all have vibration in our planes, perhaps the connectors are worn from constant vibration. Keep us posted!

Sent from my iPad using Tapatalk

[Cruiser]

did you turn the unit off and then back on in flight?

[rwarederek]

Hey all

 

Thanks for the all the comments, I'll try to address as many as I can here and update you all on the progress as well:

 

- New stuff to the airplane:  Nothing new in the panel except a new USB cigarette adapter to replace an older one (it's been in the plane for quite a while now though).  This was removed during a failure in flight with no positive results (thinking about it, I may to fly with this off from startup to shutdown to test it again).  The only airplane related thing was a new battery within a couple months before the GPS started exhibiting issues.

 

- New stuff in the airplane:  We have iPhone 6's and I guess those are relatively new.  We all have iPads that we velcro to the yoke when we fly (has been that way for years) and all maintain the same hardware since the GPS was first installed.  We tried flying with the phones and all peripherals completely off with no positive results.

 

- GTN software:  Updated by the avionics shop to the latest release

 

- Turning the unit on and off:  That was tried as well when the signal was lost in flight - power cycling had no effect

 

- Playing with the antenna on the ground:  The shop said they covered the antenna to induce a failure but every time they uncovered it, it re-acquired signals...on the ground

 

Attached is a screenshot of an inflight failure with the GPS status page.  Note it shows the satellite constellation but no satellites at all.  I have seen this exact problem myself and have also seen the satellite bars (predominantly in grey during a failure).

 

Does anyone have any ideas why the 650 will acquire signals on the ground but not at altitude?  That's the part that bugs me the most since there really should be no change in configuration to the airplane environment aside from the landing gear.

 

 

 

Thanks all!

Derek

[N601RX]

Try turning other stuff in the plane off such as other radios, transponder, alternator, possibility even one mag at a time. Its most likely either a loose connection on the gps which the avionics shop seems to have done a good job on or outside interference.

[Marauder]

Try turning other stuff in the plane off such as other radios, transponder, alternator, possibility even one mag at a time. Its most likely either a loose connection on the gps which the avionics shop seems to have done a good job on or outside interference.

That was exactly what I did to narrow my issue with the GTN to the JPI. Is the USB one of those that replace the cigarette lighter? I would definitely make sure that is off before I started.

There is a possibility you got a bad replaced antenna. Considering how much was replaced it has to be something environmental in the plane or like I mentioned above, a weird problem with the rack.

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[Marauder]

Also, I know that you said other GPS products were working, have you run a RAIM test on the GTN while you are experiencing this issue? Curious what it is saying.

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[M016576]

Hey all

Thanks for the all the comments, I'll try to address as many as I can here and update you all on the progress as well:

- New stuff to the airplane: Nothing new in the panel except a new USB cigarette adapter to replace an older one (it's been in the plane for quite a while now though). This was removed during a failure in flight with no positive results (thinking about it, I may to fly with this off from startup to shutdown to test it again). The only airplane related thing was a new battery within a couple months before the GPS started exhibiting issues.

- New stuff in the airplane: We have iPhone 6's and I guess those are relatively new. We all have iPads that we velcro to the yoke when we fly (has been that way for years) and all maintain the same hardware since the GPS was first installed. We tried flying with the phones and all peripherals completely off with no positive results.

- GTN software: Updated by the avionics shop to the latest release

- Turning the unit on and off: That was tried as well when the signal was lost in flight - power cycling had no effect

- Playing with the antenna on the ground: The shop said they covered the antenna to induce a failure but every time they uncovered it, it re-acquired signals...on the ground

Attached is a screenshot of an inflight failure with the GPS status page. Note it shows the satellite constellation but no satellites at all. I have seen this exact problem myself and have also seen the satellite bars (predominantly in grey during a failure).

Does anyone have any ideas why the 650 will acquire signals on the ground but not at altitude? That's the part that bugs me the most since there really should be no change in configuration to the airplane environment aside from the landing gear.

Thanks all!

Derek

The GPS signal from the satellites is extremely low power- so low, in fact, that a signal message is below the atmospheric noise threshold. The way that the signals are acquired is by stacking up all the noise over time- the actual noise cancels itself out (as its random), the GPS signals stack atop one another, thus building a signal. to have the constellation loaded but not acquire a signal is almost 100% an antenna issue (edit- after ruling out software bugs) of some form- whether that be interference causing blanking, noise in the line or a physical problem with the antenna itself.

Realize the above description is an over-simplification, but is "good enough" to demonstrate the point...

[ArtVandelay]

Once it loses the satellites, does it re-acquire them, or at least some of them before losing them again?

[rwarederek]

When the unit loses the satellites, it enters LOI mode and does not display anything in gray.

The unit sometimes will start to re-acquire satellites and will display them in gray - at this point the unit enters DR (Dead-reckoning) mode and will provide some nav guidance.

 

The unit will typically remain in either LOI or DR mode until we turn off the runway on the ground.

 

A couple words about DR mode for those that may not have experienced it during an in-flight failure of the GPS.  If you read the Garmin manual, it states that under DR mode, navigational guidance is only recommended as a last resort (meaning, if you have other nav available, use them instead!).  According to the manual, any changes in true airspeed, altitude, winds aloft, etc will produce significant variances in the level of accuracy.  I can tell you from experience that this is absolutely true and that you probably don't want to rely on DR mode very long as it will produce extremely misleading information.  For example, one of our partners was on a flight to the Los Angeles area and experienced a LOI.  After some time without any satellites, the unit switched into DR mode and for a period of about 15 minutes, all Track information seemed correct.  After a small altitude change of ~1000 feet and some more time in level, unaccelerated flight, the Track information became extremely inaccurate (> 30 degrees off).

[rwarederek]

Thanks all for the input - I think the next step is to get another antenna (a verified working one!) to rule that out.  

[Cruiser]

It sound like a loose or shorted antenna connection. The plane is not vibrating on the ground like it does in the air. The random occurrence also suggests a connection problem. 

 

I know it is not a GPS but I had a random transmit/receive problem with my #1 COM that was totally unpredictable but always worked on the ground. It turned out to be a broken center pin inside the end connector on the antenna cable that plugged into the tray. 

 

You have a connection problem aggravated in flight.

[chrisk]

When I first had my GTN650 installed, I had a similar issue.  It worked fine on the ground, but not in the air.   In the air it would lose the GPS signal.   Since it was a new install, it went back to the shop.  They checked all the wiring, then replaced the antenna, and finally replaced the unit.  Replacing the unit did the trick.  Let us known how it turns out.

[Piloto]

It appears that the problem may be related to engine/alternator rpm. Next time while in-flight pull the alternator field CB to see if the 650 will reacquire the GPS signal. It could also be a cooling related problem. Check the seating of the 650 into the tray. The unit may be seating on the instrument panel instead that on the tray. This will cause intermittent problems.

 

José

[M016576]

When the unit loses the satellites, it enters LOI mode and does not display anything in gray.

The unit sometimes will start to re-acquire satellites and will display them in gray - at this point the unit enters DR (Dead-reckoning) mode and will provide some nav guidance.

 

The unit will typically remain in either LOI or DR mode until we turn off the runway on the ground.

 

A couple words about DR mode for those that may not have experienced it during an in-flight failure of the GPS.  If you read the Garmin manual, it states that under DR mode, navigational guidance is only recommended as a last resort (meaning, if you have other nav available, use them instead!).  According to the manual, any changes in true airspeed, altitude, winds aloft, etc will produce significant variances in the level of accuracy.  I can tell you from experience that this is absolutely true and that you probably don't want to rely on DR mode very long as it will produce extremely misleading information.  For example, one of our partners was on a flight to the Los Angeles area and experienced a LOI.  After some time without any satellites, the unit switched into DR mode and for a period of about 15 minutes, all Track information seemed correct.  After a small altitude change of ~1000 feet and some more time in level, unaccelerated flight, the Track information became extremely inaccurate (> 30 degrees off).

About the DR mode- the 750, 650, 530, 430's do not have an internal INS. Without the GPS functioning the nag solution will degrade RAPIDLY as the OP mentions above.....

[rwarederek]

Thanks all for the added input - our starter decided to die so until that gets replaced I won't have an update. Should be able to test out the GTN within the next couple weeks

[JohnB]

Do you have a stratus unit and stratus antenna to go along with your iPad? If you do, should consider Interference from your stratus unit blocking your gps reception. Turning off your ipad/iphone wont stop this interference if your stratus is still on.  This happened to me and after a really expensive change of essentially the things you're doing, nothing changed this until I clearly demonstrated that it was the stratus antenna, and once I changed it, no more intermittent GPS reception.

 

http://mooneyspace.com/topic/13531-garmin-430-reception-problem-possible-stratus-interference/

[rwarederek]

Hey all - starter is in and we were able to test it out a bit more. This whole time we've been flying we've been seeing ~13.8 to 14v

Pulling the CB for the alternator allows the GPS to re-acquire a signal!! However... since we pulled the CB (and subsequently put it back in) for the alternator, we are seeing <12v! Took it to our mechanic and now it would seem that there is something wrong with the wiring to/from the alternator and/or voltage regulator. Alternator seems to be putting out power - regulator seems good too.

We just pulled/tugged on wires, started her up and it showed 13+v. Starting it up a second time and it's back down to 11 ish volts... Hmmmm

[Marauder]

Hey all - starter is in and we were able to test it out a bit more. This whole time we've been flying we've been seeing ~13.8 to 14v

Pulling the CB for the alternator allows the GPS to re-acquire a signal!! However... since we pulled the CB (and subsequently put it back in) for the alternator, we are seeing <12v! Took it to our mechanic and now it would seem that there is something wrong with the wiring to/from the alternator and/or voltage regulator. Alternator seems to be putting out power - regulator seems good too.

We just pulled/tugged on wires, started her up and it showed 13+v. Starting it up a second time and it's back down to 11 ish volts... Hmmmm

Strange for sure...

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[ArtVandelay]

Hey all - starter is in and we were able to test it out a bit more. This whole time we've been flying we've been seeing ~13.8 to 14v

Pulling the CB for the alternator allows the GPS to re-acquire a signal!! However... since we pulled the CB (and subsequently put it back in) for the alternator, we are seeing <12v! Took it to our mechanic and now it would seem that there is something wrong with the wiring to/from the alternator and/or voltage regulator. Alternator seems to be putting out power - regulator seems good too.

We just pulled/tugged on wires, started her up and it showed 13+v. Starting it up a second time and it's back down to 11 ish volts... Hmmmm

I would say it's a bad connection between the voltage regulator and alternator ( I had this problem, turn out to be a bad Alt Field circuit breaker), or it's a problem with the VR itself, especially if it's the mechanical type.

[rwarederek]

Seems like the issue has been resolved! Found a bad crimp on the output side of the alternator. New end fitting is in there now and everything is working. Amazing that the GPS exhibited symptoms months before any actual alternator issues popped up. The signals must be that sensitive.

Derek

[Marauder]

Seems like the issue has been resolved! Found a bad crimp on the output side of the alternator. New end fitting is in there now and everything is working. Amazing that the GPS exhibited symptoms months before any actual alternator issues popped up. The signals must be that sensitive.

Derek

GPS signals are very susceptible to interference.

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