I have a dual G5, GFC500 and IFD540 setup. 

I picked up the plane about a week ago after the GFC500 installation. 

So far everything works great except LPV approaches. ILS works great. 

I noticed today that when I loaded the LPV, I could not arm the APR mode on GFC500. 

It remained in HDG and ALT mode. I did see however megenta vertical and horizontal guidence on G5. 

I just could not engage APR mode on the autopilot.

I am quite confused about the "Approach roll steer" setting in IFD. Some say it needs to be DISABLED some say ENABLED. Mine is set to DISABLED at the moment. What exactly it should be set to? 

Could this config be the reason why I could not engage APR in LPV?

Baris 

You probably want "Approach Roll Steer" enabled.  The setting allows the GPS to continue to send out "steering commands" to the autopilot in approach mode instead of the simple left/right of a CDI needle.

In my case with it disabled I lost GPS Steering on my autopilot as soon as the IFD switched to LPV (or VLOC) on the approach.  With GPS Roll Steering enabled I was able to use the Steering mode from the GPS to track the inbound course. 

Steering mode if I understand correctly, the GPS will look at the desired ground track and the current ground track and tell your Autopilot which way to turn to keep them aligned, kinda like being in heading mode and moving the heading bug back and forth to maintain a specific ground track.  If you don't have GPS Steering you are relying on the left right indication of a CDI (Autopilots monitor the same CDI output as the VOR indicator) and will slowly adjust their left right in response to the CDI needle and wind correction.

Hi I have the same equipment setup and can confrim that if you dissable rool steer you will not be able to arm app mode. I had tried this to deal with a 1-2 dot offest error on LPV approaches on last 1000ft AGL.  Good luck. kz  

I have not yet tried an LPV approach. Does it really always fly it 1 dot or so off the centerline? 

This has been my general experience as well.

That‘s an interesting comment. Given the GFC500 being the only modern digital full (VHF&GPS) a/p for PA28/C172 class aircraft, it would seem to be in Avidyne‘s interest to make this combo work well. Otherwise it is just another argument for a GTN over an IFD.

With approach roll steering enabled, it flew down to minimums with horizontal deviation stayed between 0-100feet along the whole approach. It was a windy day with 30 knots from the left. So I am very satisfified with the performance. 

It took me a while to figure this out, but you can readily confirm by using intentional gentle left rudder pressure for a minute or so, then right rudder pressure and note the commensurate course deviation that is caused since the GFC is trying to fly the poorly yaw trimmed aircraft straight.

I went for 6 LPV approaches yesterday. Dampening out the descend yaw with slight left rudder helped GFC500 nail the approaches dead on centerline. I don't know why the deviation is much more aggressive in LPV compared to ILS but with the ball coordinated, IFD/GFC500 flies the ILS/LPC approaches extremely well. 

out of curiosity what are you using to display the course line?  eg digital/mechanical HSI? mechanical VOR indicator?

Something you can try, put the IFD in configuration mode and on one of the configuration pages there is the option to display/hide the to/from flag and set the CDI needle centered and see what you are actually seeing.

-PA