VNAV, Autopilot coupling, LPV and Avidyne IFD

[362PC]

I am deep in to my IFR training and having issues with getting LPV to reliably arm and activate. All cases so far are in VMC practice situations but if they were in IMC I would be less than thrilled with the performance. Lots of questions in this one...

From the IAF I have GPSS active with ALT hold at the required altitude. About 2 miles from the FAF I get the LPV annunciation on the Avidyne IFD and visibly see the glide slope advisor alive next to the HSI. If I then press the VNAV button sometimes I get the LPV AP annunciation, and sometimes is sticks to ALT mode and won't switch over, keeps trying to climb to the previous fix's altitude and results in a missed approach.

Should I be using VNAV prior to IAF and not using the ALT hold for step downs?
When can I arm VNAV into approach mode to get the LPV transition ahead of FAF?
The Avidyne IFD is showing GPS->LPV annunciation showing it is ready for the switch.
Is there required proximity to the glide slope or max deviation that will keep it from activating?
Must be below glide slope to capture? if above won't capture at all?

I am hoping to get a more specific step by step process for how to ensure this works every time.

 

[CanardMulti]

Skyview will only capture GS from below, so if approach control keeps you high for too long and you are forced to intercept the GS from above, that intercept will have to be hand flown.

To get the FD or autopilot to arm for VNAV, you need to be on the last leg before the FAF. Just because you can SEE the GS displayed, you still have to be on that last leg in order to successfully arm. I have an IFD540 so I cannot say for certain if that's a Skyview or an Avidyne constraint - or both.

Just personal technique: Even if I am on that last leg and I know the system will accept arming, I do NOT arm VNAV until ATC actually clears me for the approach.

Ken

 

[362PC]

Skyview will only capture GS from below, so if approach control keeps you high for too long and you are forced to intercept the GS from above, that intercept will have to be hand flown.

To get the FD or autopilot to arm for VNAV, you need to be on the last leg before the FAF. Just because you can SEE the GS displayed, you still have to be on that last leg in order to successfully arm. I have an IFD540 so I cannot say for certain if that's a Skyview or an Avidyne constraint - or both.

Just personal technique: Even if I am on that last leg and I know the system will accept arming, I do NOT arm VNAV until ATC actually clears me for the approach.

Ken

Thanks Ken, I am using the IFD540 as well. Knowing that the Dynon AP won't capture from above makes more sense. I will do a few more test flights on arming sooner.