Experiencing LPV approach failures

[bkspero]

I have flown lots of LPV approaches with my GNS-530w (software version 5.xx don't recall exactly), but none in the last 6 months or so. Earlier this week for the first time I tried the RNAV (GPS) 16 at Trenton Mercer, NJ (KTTN) and when I got about 2/3 of the way from HORDE (FAF) to CULEV I got the ABORT APPROACH message and guidance downgraded to terminal mode.

I got a similar message on the VOR - GPS A there shortly after passing ARD (I don't recall its wording exactly, as I was flying it using my Nav2 VOR and not the GPS and didn't pay attention to the warning.

I went up today to give the RNAV-16 another try figuring that the earlier bad results were probably just a satellite position issue and that it would be fine today. But the same thing happened. About 2/3 of the way from HORDE to CULEV it said to ABORT APPROACH and downgraded to terminal mode. I checked RAIM on the 530W shortly after and it got good RAIM.

I then flew the RNAV-10 into Princeton (39N) and it annunciated LNAV+V (there is no LP or LPV on this approach) and the approach went normally.

I recognize that it is normal to get an ABORT instruction on an LP or LPV approach when accuracy is lost inside the FAF. It just seems too much of a coincidence for an LPV approach to fail in this manner twice in a row on different days without some cause other than natural conditions. Trenton NJ is not a hot spot for the government to be testing GPS jamming. And why would the satellites not cooperate twice, after having been well behaved on dozens of LPV approaches in the past?

Could this be a bad antenna connection or a deteriorating GPS antenna? I recently had to have my ELT antenna replaced (121.5 MHz AK-450) and the new antenna is much different than the old one. I've read where ELT interaction can compromise GPS performance. Could the system have been ok with the old ELT antenna but not with the new one?

Finally, my transponder was upgraded from a KT-76a to a 1090es KT-74 a couple of weeks ago. That required a GPS connection. Could it be contributing to the behavior?

Any thoughts would be appreciated. Things work out much better when I to go to the shop with some degree of knowledge.

Thanks, Barry

 

[mkosmo]

It wouldn't give you any LPV indication to begin with if it didn't have sufficient integrity. Recent avionics changes would make me curious, though.

 

[aterpster]

It wouldn't give you any LPV indication to begin with if it didn't have sufficient integrity. Recent avionics changes would make me curious, though.

Twice in the same place would point to an avionics issue. As you say, it you get "LPV" when the FAF becomes the active waypoint, the chances of loosing the LPV grade of service from that point to the runway is very small.

I suppose there could be some local interference, but that would affect everyone flying that approach, or so it would seem.

 

[mkosmo]

Twice in the same place would point to an avionics issue. As you say, it you get "LPV" when the FAF becomes the active waypoint, the chances of loosing the LPV grade of service from that point to the runway is very small.

I suppose there could be some local interference, but that would affect everyone flying that approach, or so it would seem.

I wonder if it's local plus environmental. Any other LPVs work?

 

[bkspero]

I wonder if it's local plus environmental. Any other LPVs work?

I haven't tried any others recently. I'll do that next. Maybe fly up to KABE to get away from the area. I guess it's possible that there is an issue with this approach. 16/34 is the shorter runway with the less common wind alignment, and when wind is a factor it is typically out of the north putting 34 in play. So 16 doesn't get alot of traffic.

 

[Sundancer]

Ferried our airplane to Trenton for an interior estimate this spring, and our 530 experienced the same thing.

 

[bkspero]

Ferried our airplane to Trenton for an interior estimate this spring, and our 530 experienced the same thing.

Sorry that it happened to you, but it's helpful to me. Do you recall the approach? I was planning to check the RNAV34 there next, but if your bad experience was on other than the RNAV16, then I should probably go to a different airport.

 

[Sundancer]

Sorry that it happened to you, but it's helpful to me. Do you recall the approach? I was planning to check the RNAV34 there next, but if your bad experience was on other than the RNAV16, then I should probably go to a different airport.

Just checked my log book - it was June 17 thus year; I'm pretty sure the approach was to 06, but my handwriting is awful. Looking at the airport diagram makes me think it was 06', anyway.

It was real basically, VFR, just some puffy and slightly building cumulus to punch through, ceiling was not a factor, as I recall, so losing the 530 wan't a big deal.

Just struck me as odd that we had about the same experience, same airport, same GPS box. . .other than that one time, I haven't had an issue eith our 530.

 

[bkspero]

Just checked my log book - it was June 17 thus year; I'm pretty sure the approach was to 06, but my handwriting is awful. Looking at the airport diagram makes me think it was 06', anyway.

It was real basically, VFR, just some puffy and slightly building cumulus to punch through, ceiling was not a factor, as I recall, so losing the 530 wan't a big deal.

Just struck me as odd that we had about the same experience, same airport, same GPS box. . .other than that one time, I haven't had an issue eith our 530.

Ok, that tells me that if there is a local issue, it may be more widespread than Rwy 16, and that I should probably do my next test elsewhere. Probably not even KPNE, as the approach to Rwy 24 at KPNE is the same airspace as Rwy 06 approach to KTTN. I'll try KABE.

 

[John Collins]

Did you use direct-to at any time during the approach? This will cause the message you saw if you do this to HORDE or to CULAV.

 

[bkspero]

Did you use direct-to at any time during the approach? This will cause the message you saw if you do this to HORDE or to CULAV.

John, thanks for the input.

The first time it happened I with AMIDY as the IAF and then simulated getting vectors to final so that I intersected the approach course about a mile inside of AMIDY. To do that I hit Direct-Direct-Enter with HORDE selected.

The second time I flew the full approach starting from east of the approach course with AMIDY as the IAF, a teardrop into the hold, and then inbound from there. No Direct-To.

Did I do it wrong the first time? And also, why would doing a direct-to HORDE or CULEV cause the an ABORT message minutes later? Or are you saying that it would give the ABORT message as soon as one tries a direct-to to the FAF or a waypoint inside of the FAF? Because even in the first case, it was minutes between pressing D>D>enter and the ABORT message.

Barry

 

[bkspero]

I flew the RNAV-31 into Lehigh Valley Intl (PA, KABE) today and got an ABORT message shortly after crossing BEHEM (FAF) and before reaching ASOMY. I set it up and flew it using Vectors to final from northeast of the approach course and was vectored to intersect a couple of miles outside of BEHEM. I had LPV indication well before intersecting the final approach course and it remained until the messages started.

There may have been some differences, but I can't be certain because I was watching the GPS screen more closely in this approach compared to the prior 2 at Trenton. I initially got an INTEG alert and a downgrade to LNAV shortly after starting the descent from 3000 ft after BEHEM (even though it was inside of the FAF it did not immediately give an ABORT). I continued the approach as an LNAV and shortly before ASOMY it gave a MSG alert. When I accessed the message, the ABORT message showed up (in the 2 approaches at Trenton the ABORT message showed up on the screen without me pressing the MSG button...but that may only have been because I responded more quickly to the indications at KABE than I did at KTTN).

One other thing I forgot, I followed up with an RNAV-07 approach into 47N. This approach does not have LNAV minimums. I got LNAV+V annunciation and it flew normally to the MDA.

So, it doesn't look like this is a local issue at Trenton. What else could it be?

 

[mkosmo]

The integrity flag would have me thinking local interference. Any avionics shops nearby to take a look?

 

[bkspero]

The integrity flag would have me thinking local interference. Any avionics shops nearby to take a look?

Our local shop is certificated for Avionics, but this has the earmarks of becoming a swap new parts fix instead of a test and diagnose fix. I am hoping to get a better handle on things to test so I can encourage those tests before we start replacing parts (or worse, sending my 530w out for a fixed price $1100 service where Garmin tells me that it is fine).

I will call Garmin tomorrow.

 

[chemgeek]

I had a failure mode in my gns-430 where the vertical guidance circuit failed. Everything else worked OK. Downgraded every approach to LPV only. Garmin replaced the vertical nav circuit and everything returned to normal.

 

[fiveoboy01]

I had a failure mode in my gns-430 where the vertical guidance circuit failed. Everything else worked OK. Downgraded every approach to LPV only. Garmin replaced the vertical nav circuit and everything returned to normal.

Do you mean downgraded to LNAV?

LPV isn't a downgrade, it's the best GPS approach available in terms of the lowest DA..

 

[mkosmo]

Do you mean downgraded to LNAV?

LPV isn't a downgrade, it's the best GPS approach available in terms of the lowest DA..

Maybe he meant LP or LNAV?

 

[Henning]

It's not just the FAA, military, or terrorist that can jam GPS, and it doesn't necessarily even have to be intentional. There is plenty of industrial equipment that if the shielding goes bad play all sorts of havoc.

 

[aterpster]

It's not just the FAA, military, or terrorist that can jam GPS, and it doesn't necessarily even have to be intentional. There is plenty of industrial equipment that if the shielding goes bad play all sorts of havoc.

Back the late 50s, early 60s, it wasn't unusual to be flying an ILS with the NARCO trash and have an FM station bleed through on the LOC frequency. Another one with that trash was a centered GS needle with no flag, but it was dead. Saw that far too many times.

 

[bkspero]

My shop had no insight beyond calling Garmin. Garmin suggested that we turn off any SBAS types other than WAAS in the last page of the AUX menu. It made things worse. I lost GPS position before even getting established on a feeder route. Even after turning them all back on and rebooting, performance seemed even worse than it had been the last few days. It lost LPV integrity outside of the FAF. It did downgrade to LNAV for a while, then went to enroute with an INTEG alert, which cleared in a few minutes and went to terminal mode. Then I went back and tried the approach again and it kept LPV lock until past the FAF (at which point I broke off the approach to stay out of the Delta).

Unless someone here has a better suggestion I guess it's time to bring it into the shop and have them start with checking the connections.

 

[mkosmo]

My shop had no insight beyond calling Garmin. Garmin suggested that we turn off any SBAS types other than WAAS in the last page of the AUX menu. It made things worse. I lost GPS position before even getting established on a feeder route. Even after turning them all back on and rebooting, performance seemed even worse than it had been the last few days. It lost LPV integrity outside of the FAF. It did downgrade to LNAV for a while, then went to enroute with an INTEG alert, which cleared in a few minutes and went to terminal mode. Then I went back and tried the approach again and it kept LPV lock until past the FAF (at which point I broke off the approach to stay out of the Delta).

Unless someone here has a better suggestion I guess it's time to bring it into the shop and have them start with checking the connections.

When it failed out, did you look at the satellite page by chance? Did it still even show 3D nav?

 

[RotorDude]

Back the late 50s, early 60s, it wasn't unusual to be flying an ILS with the NARCO trash and have an FM station bleed through on the LOC frequency. Another one with that trash was a centered GS needle with no flag, but it was dead. Saw that far too many times.

I've had a couple of those in the early 80's, one on an ILS to minimums. NOT FUN.
(Fortunately the approach was over the ocean.)

 

[bkspero]

When it failed out, did you look at the satellite page by chance? Did it still even show 3D nav?

I did not look at the satellite page, but at the worst today the GPS totally lost position. It displayed something close to "NO GPS POSITION" in large letters across the screen, and my 1090es transponder showed "WARNING No GPS Position Detected". So there wasn't even 2D nav.

On prior days when it lost LPV after the FAF the GPS fell back to TERM mode. Today, when it got far enough to get into an approach, it generally downgraded to ENR (enroute) mode. I did check RAIM once yesterday after it had downgraded to TERM mode and I had flown a few minutes, and the GPS said RAIM was ok.

 

[coloradobluesky]

Do you have a VFR GPS to verify its is or isn't the GPS signal, but is the box?

 

[bkspero]

Do you have a VFR GPS to verify its is or isn't the GPS signal, but is the box?

My yoke mounted tablet operating on its internal GPS did not loose lock during any of this. I don't know how significant that is, though, as it doesn't have to meet Approach Integrity criteria. Just get a plausible solution from enough satellites.

 

[Henning]

My shop had no insight beyond calling Garmin. Garmin suggested that we turn off any SBAS types other than WAAS in the last page of the AUX menu. It made things worse. I lost GPS position before even getting established on a feeder route. Even after turning them all back on and rebooting, performance seemed even worse than it had been the last few days. It lost LPV integrity outside of the FAF. It did downgrade to LNAV for a while, then went to enroute with an INTEG alert, which cleared in a few minutes and went to terminal mode. Then I went back and tried the approach again and it kept LPV lock until past the FAF (at which point I broke off the approach to stay out of the Delta).

Unless someone here has a better suggestion I guess it's time to bring it into the shop and have them start with checking the connections.

Check the antenna, connections, and cable run, for continuity and condition? If it works then doesn't, my suspicions run outside the box rather than inside first.

 

[bkspero]

When it failed out, did you look at the satellite page by chance? Did it still even show 3D nav?

I flew today with the GPS showing the satellite page and recorded the video. Everything worked normally for the ca. 45 minute cross country phase and beginning of the RNAV-26 approach into Lancaster PA (KLNS). Shortly after passing the FAF the satellite signal bars lost their fill in an instant and then the bar outlines depopulated over a few seconds until the graph showed no signal. After a 10-20 seconds the bars began to repopulate and after a minute or 2 the GPS was back in operation in terminal mode.

Snapshots from the video are attached below. The name of each file is the time of the snapshot in hrs-minutes-seconds UTC.

Interestingly, on departure from Lancaster the GPS lost position on the ground for the first time ever. It got position in a normal manner, but about 30 seconds later the satellite signals disappeared as they did on the LPV approach. They repopulated and departure was normal. At about 2500 ft (2000 ft AGL) the bars were lost again. It repopulated normally and it worked normally for the flight home.

So now the pattern is that is has failed 7 out of 7 LPV approaches, it has worked properly on 4 out of 4 LNAV+V approaches, it has worked normally departing, enroute, and arriving (other than the LPV approaches) for probably 6 hrs of flying except for the 2 incidents departing KLNS.

With the photos the shop is now willing to get more deeply involved troubleshooting connections.

 

[Henning]

My bet is now a firmware/software issue, but I wouldn't be surprised at a bad antenna either.

 

[mkosmo]

My bet is now a firmware/software issue, but I wouldn't be surprised at a bad antenna either.

I'd think that, too, but only on lpv? That's what has me thinking strange.

What software version is running?

 

[bkspero]

I'd think that, too, but only on lpv? That's what has me thinking strange.

What software version is running?

The boot screen says Main Software Version 5.1 and GPS Software Version 5.0.

Why are you and Henning leaning towards a software glitch? This software version is nearly 2 years old. If it had issues with LPV approaches, I would think that it would be well known by now.

 

[bkspero]

One other odd thing about this morning's experience that may or may not be related to my situation. As I was experiencing my signal failure inside of the FAF, a friend was behind me near the IAF and he experienced a GPS signal loss as well. He said that his 530w had never lost satellite signal before.

Is there any way my plane could be transmitting an interfering signal that could cause a problem a couple of miles away?

 

[mkosmo]

One other odd thing about this morning's experience that may or may not be related to my situation. As I was experiencing my signal failure inside of the FAF, a friend was behind me near the IAF and he experienced a GPS signal loss as well. He said that his 530w had never lost satellite signal before.

Is there any way my plane could be transmitting an interfering signal that could cause a problem a couple of miles away?

Unless you have a radio breaking all kinds of laws, I sincerely doubt it. I wouldn't investigate it. Did he lose it at the same time or just the same passing vicinity?

 

[bkspero]

Unless you have a radio breaking all kinds of laws, I sincerely doubt it. I wouldn't investigate it. Did he lose it at the same time or just the same passing vicinity?

Best I can tell, it was as at the same time. As for the question of same vicinity, it depends on what you define as aame vicinity. We were both in southeast Pensylvania, but we were a couple of miles apart (in trail him just outside of the IAF and me just inside the FAF). If it was an issue of poor satellite coverage then I would consider us as being in the same vicinity and being affected by the poor coverage. But I checked RAIM predictions online before the flight and on the GPS before starting the approach, and they were gpod. So I don't think it was a satellite issue. So I think I could call us not in the same vicinity.

 

[Henning]

The boot screen says Main Software Version 5.1 and GPS Software Version 5.0.

Why are you and Henning leaning towards a software glitch? This software version is nearly 2 years old. If it had issues with LPV approaches, I would think that it would be well known by now.

"Ghosts in the Machine", spontaneously corrupting files, it happens. It looks like it is getting rebooted. That is where I would investigate next anyway, and if I had a 2 year old load, I would look to do a <format->reload> of the whole thing if that was economically feasible, and get that out of the way.

 

[Henning]

One other odd thing about this morning's experience that may or may not be related to my situation. As I was experiencing my signal failure inside of the FAF, a friend was behind me near the IAF and he experienced a GPS signal loss as well. He said that his 530w had never lost satellite signal before.

Is there any way my plane could be transmitting an interfering signal that could cause a problem a couple of miles away?

No, it is much more likely that you would both be hitting the same external interference. That can also cause the reboot process.

 

[Bravo]

Do you have a "Spot tracking device" or anything similar? I also had this happen consistently and finally figured out it was from the "spot trace" unit. I now turn it off when flying Ifr.

 

[bkspero]

Do you have a "Spot tracking device" or anything similar? I also had this happen consistently and finally figured out it was from the "spot trace" unit. I now turn it off when flying Ifr.

Thanks for the suggestion, but I don't have a Spot unit. Closest I come is a PLB but it is off and in my flight bag. Handheld radio is off, too. ELT (121.5) is not transmitting. Its antenna is located pretty far away from the GPS antenna (about 6 ft). GPS behaves the same whether my cell phone is on or off. So for now I am presuming that none of these are involved.

 

[Henning]

Thanks for the suggestion, but I don't have a Spot unit. Closest I come is a PLB but it is off and in my flight bag. Handheld radio is off, too. ELT (121.5) is not transmitting. Its antenna is located pretty far away from the GPS antenna (about 6 ft). GPS behaves the same whether my cell phone is on or off. So for now I am presuming that none of these are involved.

Something is causing it to get confused, and go back and reinitialize satellite reception using last known good position is what appears to be happening. Does this only happen in one location? It could be external interference.

 

[bkspero]

Something is causing it to get confused, and go back and reinitialize satellite reception using last known good position is what appears to be happening. Does this only happen in one location? It could be external interference.

It has occurred at 3 different Class D airports thus far. One in central New Jersey (KTTN), one in eastern Pensylvania (KABE), and one is south-central PA (KLNS). KTTN and KABE are about 30 nm apart and KLNS is about 75 nm from the other 2. So if its an external interference it has to be pretty widespread.

Until the report of NO GPS POSITION on the ground and then on climb-out at Lancaster (KLNS), it had only happened on LPV approaches at each of those 3 airports (and on every LPV approach in the last 2 weeks after having not flown any LPV approaches in many many months).

BTW, I did a short 1.3 hr breakfast flight today. No approaches. No problems.

 

[rocketflyer84]

It has occurred at 3 different Class D airports thus far. One in central New Jersey (KTTN), one in eastern Pensylvania (KABE), and one is south-central PA (KLNS). KTTN and KABE are about 30 nm apart and KLNS is about 75 nm from the other 2. So if its an external interference it has to be pretty widespread.

Until the report of NO GPS POSITION on the ground and then on climb-out at Lancaster (KLNS), it had only happened on LPV approaches at each of those 3 airports (and on every LPV approach in the last 2 weeks after having not flown any LPV approaches in many many months).

BTW, I did a short 1.3 hr breakfast flight today. No approaches. No problems.

Whatever it is, LPV would always be the first to go if position data starts to go wonky. If you had marginal signal the first thing it's going to do is invalidate the LPV flag although it may still think it can safely provide LNAV guidance. If the issue is worse then it would drop out completely and report no GPS position. Purely based on symptoms this sounds like a hardware issue either with the unit or the antenna/wiring but probably best to have the pros at an avionics shop take a look.

The LPV flag is forward looking (the unit checks predicted signal for the length of your approach) so while you may not get a LPV flag up front, having it drop out mid approach should be an exceedingly rare occurace if the unit operates properly.