GPS approach downgrade

[Let'sgoflying!]

Yesterday I was flying into an airport in which the ceiling continued to lower until at approach time, it was at LPV minimums for ceiling (vis was 7).
That's cool I thought; might make it in.
Passing the IAF, had LPV illuminated then soon after got a warning about the approach being downgraded to LNAV which meant higher ceiling minimums and yes, a missed in actual.

I've been looking around to find out what can cause this downgrade to occur and have found nothing.
Nothing in AIM, using their search tool.
The unit displayed "Raim Available" prior to commencing approach.
There is no notam.
I have done this approach many times before using LPV all the way down.
Hoping it's not our equipment.

 

[FORANE]

Solar storm.

 

[KE5BM]

What FORANE said...we're in the middle of a massive geomagnetic storm courtesy of that little fireball that gives us life. The likes of which haven't been seen since 2005. Pretty much anything that uses a radio signal is degraded is some way. Hell, last night the UPS battery in my office started clicking on and off. There was enough interference in the power lines that the UPS was reading it as a power outage.

 

[chemgeek]

Most likely a side effect of the current G5 geomagnetic storm, but if it persists, it is possible that the vertical guidance board has failed. This happened to my 430W and it took a while for me to realize it was not environmental factors, but the unit itself that had partially failed. (It would happily fly LNAV approaches without any other flags or warnings.)

 

[asicer]

Most likely a side effect of the current G5 geomagnetic storm, but if it persists, it is possible that the vertical guidance board has failed.

Is a G5 geomagnetic storm capable of damaging the vertical guidance board?

 

[Let'sgoflying!]

jeez, battling the weather down to mins with an autopilot acting up, passengers breathing down my neck and now I gotta worry about geomagnetic storms making me go missed and scrambling for an alternate!
Time to write that last chapter for E. Gann lol!

 

[KE5BM]

Is a G5 geomagnetic storm capable of damaging the vertical guidance board?

Not likely. chemgeek is suggesting that there's a coincidental failure in the aircraft at the same time a major geomagnetic storm is causing problems to otherwise perfectly-functioning equipment.

 

[chemgeek]

Is a G5 geomagnetic storm capable of damaging the vertical guidance board?

No. But a vertical guidance board failure will have the same symptoms. LPVs will automatically downgrade to LNAV without any other clues as to why.

 

[Skymac]

I had an approach today act goofy too. LPV never downgraded but glide slope never popped up on the G600, tried reactivating the approach a couple of times but no joy. Loaded the visual approach and boom, glide slope popped up.

 

[John Collins]

I had an approach today act goofy too. LPV never downgraded but glide slope never popped up on the G600, tried reactivating the approach a couple of times but no joy. Loaded the visual approach and boom, glide slope popped up.

I have seen that occur when VLOC is set on the GPS, but the CDI/PFD is set to GPS.

 

[John Collins]

Yesterday I was flying into an airport in which the ceiling continued to lower until at approach time, it was at LPV minimums for ceiling (vis was 7).
That's cool I thought; might make it in.
Passing the IAF, had LPV illuminated then soon after got a warning about the approach being downgraded to LNAV which meant higher ceiling minimums and yes, a missed in actual.

I've been looking around to find out what can cause this downgrade to occur and have found nothing.
Nothing in AIM, using their search tool.
The unit displayed "Raim Available" prior to commencing approach.
There is no notam.
I have done this approach many times before using LPV all the way down.
Hoping it's not our equipment.

Undoubtedly the Solar Storm. LPV, LP, and WAAS vertical guidance was wiped out for the entire US CONUS, Canada, and Alaska for about 8 hours on the date of your flight. If you look at the LPV annunciation, it would most likely have been amber rather than its normal green color on the GPS. Downgrades happen when you are 1 minute from the FAF. You can also check the satellite page and if the VFOM figure is above 40 feet, don't expect LPV200 (LPV200 is the service when the DH is below 250 feet) and don't expect LPV service (DH 250 feet or higher) when VFOM is 60+ feet.

 

[40YearDream]

Yesterday I was flying into an airport in which the ceiling continued to lower until at approach time, it was at LPV minimums for ceiling (vis was 7).
That's cool I thought; might make it in.
Passing the IAF, had LPV illuminated then soon after got a warning about the approach being downgraded to LNAV which meant higher ceiling minimums and yes, a missed in actual.

I've been looking around to find out what can cause this downgrade to occur and have found nothing.
Nothing in AIM, using their search tool.
The unit displayed "Raim Available" prior to commencing approach.
There is no notam.
I have done this approach many times before using LPV all the way down.
Hoping it's not our equipment.

Had the same thing happen - I'm assuming it was the solar storm, that took out some of the GPS signals. I had the glide slope come and go twice on the same (normally LPV) approach. The same equipment worked fine the next day.

 

[Let'sgoflying!]

Had the same thing happen - I'm assuming it was the solar storm, that took out some of the GPS signals. I had the glide slope come and go twice on the same (normally LPV) approach. The same equipment worked fine the next day.

I was surprised that it wasn’t till near the FAF that I learned there was a problem.
I always check “RAIM available”.
Maybe that’s not testing what I think it’s testing.

 

[John Collins]

I was surprised that it wasn’t till near the FAF that I learned there was a problem.
I always check “RAIM available”.
Maybe that’s not testing what I think it’s testing.

By design, downgrades occur when you are 1 minute out from the FAF. RAIM available will not have anything to do with WAAS service because WAAS does not use RAIM. Also RAIM available means that the algorithm can be performed in the GPS, that is different than the result of whether or not the RAIM determined integrity is acceptable or not for the operation being conducted. Assuming RAIM was available, meaning that the integrity can be determined, during the outage of LPV vertical and LP service that was recently experienced, since RAIM would only generate an alert if the LNAV approach integrity dropped below 0.3 NM (556 meters), no alert would have occurred. All during the outage, LNAV service was available. The criteria for LPV200 is vertical 35 meters and lateral 40 meters. Both of these limits were exceeded in the Solar storm, but the 556 meters required for LNAV was solid.

RAIM is not used for a WAAS GPS when inside a WAAS SV, so can usually be ignored unless there is a WAAS Outage NOTAM issued. Since 2007, there was only one such NOTAM issued for a 4 hour period when the WAAS system failed during a software update. As long as WAAS is available, RAIM is only relevant to non WAAS GPS systems. There is always a distinction between WAAS being available and the integrity values it provides either being suitable or not for a given PBN specification. WAAS integrity is broadcast from the three WAAS satellites in geostationary orbits with the SV covering the entire hemisphere over US CONUS, Canada, Alaska, Hawaii, Mexico, the Caribbean, and points south. See this for the limits of the WAAS footprint. https://www.nstb.tc.faa.gov/DisplayGEOStatus.htm