V16 Autopilot - Flight Director <> HSI XTE mismatch - GPS approaches

steve m

Member
Joined
Feb 10, 2020
Messages
48
Location
Asheville NC
Shot two practice GPS approaches with safety pilot today in the RV10, first was LPV approach and second was LP +V approach. I am running V16 software and for these two approaches I was hand flying using the Flight Director (FD) and the servo's were NOT engaged. The IFR navigator is Garmin 175. I typically fly approaches at 100 knots, no flaps.

I have not really seen these issues in cruise flight using AP.

First, my approach flying skills are in definite need of improvement and this may be a contributing factor to what I am seeing as I was weaving back and forth across the approach course due to my poor IFR flying skills.

Within about 1/2 mile of the runway the safety pilot is telling me I am way off course. I am tracking rock solid on the FD (it is not commanding any turns) but the XTE on the HSI is showing me off course and visual confirmation out the windshield shows me about 100+ yards left of runway centerline.

I am thinking the response rate for errors on the FD are not happening quick enough...almost like the due to lag time it is getting out of phase or slow to respond

I am going to refly these approaches tomorrow with AP servos engaged and see if I can duplicate the condition.

Doe this sound symptomatic of needing up increase one of the following

- Roll Sensitivity
- Roll Gain
- Roll Gain Limit

- XTE Gain
- XTE Dot Gain



Thx

Steve
 

steve m

Member
Joined
Feb 10, 2020
Messages
48
Location
Asheville NC
I flew the approach again today with the Autopilot flying, both FD and the HSI were centered with no mismatch between data being displayed all the way to the runway.

I am speculating (no data at all to back this up) that the AP control system\algorithms have an easier time guiding the airplane to the runway if the approach is stable (AP Servos flying) vs me hand flying and swerving back\forth the final approach course?

Any thoughts on this theory.... I have no clue how the AP system calculates what to display on the FD.?
 

Stevec

Member
Joined
Jun 24, 2020
Messages
104
Don’t have an answer but reading the manual it seems the system calculates guidance for the flight director and the autopilot follows the flight director. So I would have thought it should not matter for alignment on the runway as the flight director should be guiding you to the centre of the calculated track.
 

steve m

Member
Joined
Feb 10, 2020
Messages
48
Location
Asheville NC
Don’t have an answer but reading the manual it seems the system calculates guidance for the flight director and the autopilot follows the flight director. So I would have thought it should not matter for alignment on the runway as the flight director should be guiding you to the centre of the calculated track.
My understanding is the same as yours. I am wondering\thinking that the system is looking at rate of intercept and many other factors to adjust bank angle, intercept angle, etc....I am thinking this response rate is somehow influenced by the AP settings I have entered?
 
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