Garmin 430W losing the WAAS on the LPV

alfadog

alfadog

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alfadog
On Sat, I split a club 172 with a friend doing his currency requirements and me getting going again with my IR.

On the LPV he was well high of the GS, like almost busting it high, and suddenly the GS snapped centered. The pilot apparently did not notice that it had snapped centered and thought he was on the GS. I had noticed the snap and mentioned it to him. There was now a red flag in the 106A near the GS indicator but neither of us could remember if it had been there before (blind leading the blind, LOL). After going missed, I saw that the 430W now said LNAV rather that LPV in the info box and knew we should have reverted the approach to LNAV or gone missed when we lost the GS.

GMN_GI_106A.jpg


When I first started my instrument rating in the club 172 with another instructor than I use now, we had the same thing happen on the same approach (KTMB 9L). In this case, I was flying and noticed the LPV to LNAV change in the info box. Asked the instructor what to do and he was baffled (LOL, more of the same but less forgivable).

Anyway, lesson learned for me, and for my friend too, but how common is this loss of WAAS signal integrity mid-approach? I have flown that approach at most a dozen times and lost it twice. Same airplane which may also be a factor.
 
If the VPL integrity is below the required 35 meters for this particular approach, the approach will downgrade to LNAV. The point at which it downgrades is one minute prior to the FAF at GABEC. Prior to reaching the point one minute prior to GABEC, it is likely that the LPV indication on the GNS430W which is normally green will be yellow, indicating that integrity is an issue. This could be caused by jamming from the ground. I would be suspicious if you pass over a truck stop or brake station as truckers have been known to jam their own GPS units so the boss can't tell where they are.

If you look on the satellite page, there is a parameter VFOM that gives you a good idea if the approach might be downgraded. If it is above 40 feet, it is likely you will be downgraded to LNAV on a LPV approach with a DH of 200 feet. If the LPV has a DH of 250 feet or higher, a VFOM above 60 will likely result in a downgrade.

Other things to check, were there any NOTAMS for GPS interference testing by the military?
 
I've never lost integrity on any of the LPV approaches I've flown in Colorado. (I've flown quite a few because I find them so darn enjoyable)

It sounds like either there is an environmental issue or a jamming source near that field (no bueno).
 
COFlyBoy said:
I've never lost integrity on any of the LPV approaches I've flown in Colorado. (I've flown quite a few because I find them so darn enjoyable)

It sounds like either there is an environmental issue or a jamming source near that field (no bueno).
Click to expand...

Check this interference out scheduled in your neck of the woods on July 11.

interference near colorado.jpg
 
Raytheon and LockMart have competing GPS jamming systems. They're both running regular tests here in the States that cover enormous distances from the transmitters. NOTAMs all the time.
 
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