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IFD 540 LPV Unavailable

majuro15

Well Known Member
Hi Everyone,

While flying a few RNAV approaches, I've had a fault come up saying LPV Unavailable, Use LNAV MDA (or DH) followed with a note that says GPS is not in SBAS. I then lose glide path guidance.

I'm thinking what this means is the GPS has lost WAAS and the accuracy level needed to navigate the LPV approach. Am I correct in this assumption?

Now, why? I have my antenna (both WAAS and AFS for my EFIS) under my cowl. I am thinking they are blocked by the skybolt flanges since I have the antenna only 1" from the firewall. My guess is they are able to get good signal through the cowl itself but the metal of the brackets and receptacles are blocking the signal. I have found that during steep left turns, I lose GPS signal as well (the antenna are on the left side above the engine mount.

I know others have had issues with antenna under the cowl and some have not. I'm going to extend the shelf forward to provide a better path for GPS signal and see if that helps.

Just curious if my guess on what is happening is correct.

Thanks!
 
Hi Everyone,

I'm thinking what this means is the GPS has lost WAAS and the accuracy level needed to navigate the LPV approach. Am I correct in this assumption?

Just curious if my guess on what is happening is correct.

Thanks!

Not necessarily loss of waas signal. You may have just stumbled into a situation with poor satellite geometry, signal interference, etc.

Now I will say up front I am not a fan of engine-compartment GPS antennas if used for IFR. So, sorry if this sounds harsh, I just do not understand the logic or motivation of engine compartment mountings for IFR critical equipment.
1. Technically, it?s illegal. (Your Op Limits almost certainly say you need to equip per 91.205 for IFR, and 91.205 says ?approved gps?, and so-far the only approved gps units carry a TSO, and part of the TSO is the antenna and its TSO installation on the top, external skin....)
2. Heat is the enemy of electronics. Most modern gps antennas have electronics in them.
3. IFR is serious business. Without extensive testing it?s not possible to say how good or not the reception is. Even if it?s only degraded by 1%, or 0.1%, is this worthwhile to avoid the external antenna?
4. You lose signal in a turn! This is not acceptable for IFR.
 
Not necessarily loss of waas signal. You may have just stumbled into a situation with poor satellite geometry, signal interference, etc.

Now I will say up front I am not a fan of engine-compartment GPS antennas if used for IFR. So, sorry if this sounds harsh, I just do not understand the logic or motivation of engine compartment mountings for IFR critical equipment.
1. Technically, it?s illegal. (Your Op Limits almost certainly say you need to equip per 91.205 for IFR, and 91.205 says ?approved gps?, and so-far the only approved gps units carry a TSO, and part of the TSO is the antenna and its TSO installation on the top, external skin....)
2. Heat is the enemy of electronics. Most modern gps antennas have electronics in them.
3. IFR is serious business. Without extensive testing it?s not possible to say how good or not the reception is. Even if it?s only degraded by 1%, or 0.1%, is this worthwhile to avoid the external antenna?
4. You lose signal in a turn! This is not acceptable for IFR.

Well Said.
 
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