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Late turn after GPS waypoint

I noticed that when I use the GPS overlay for the ILS27 approach at EDVK from the North – out of LUXUX – the A/P turns very late. To the radar controller it looks like I’m going through the localizer and obviously they call me.

Might that be an issue with the encoding in the database? I would assume so. I’d like to understand.

Obviously, the solution is to switch early to the localizer as nav source instead of staying with the GPS overlay. Still I’d like to know what’s going on behind the scenes.

Frequent travels around Europe

You are not supposed to fly an ILS with GPS overlay, the Garmin even warns you when you set it up like this.

Your AP can automatically switch from GPS to NAV, this is what you should use then flying it by AP.

What Achim said.

You fly towards the Localizer on GPS (in NAV mode) – and before reaching it you switch the source from GPS to VLOC. For a good reason the DFC100 has eliminated the “GPSS” button, so with your R9 you simply fly NAV mode for all of it and only switch the source.

I stopped using the automatic switching. I do it all manual, because that’s safer, and this way i check that the Final Course is set right.

i remember that some waypoints are encoded as fly-by and others as fly-over waypoints. I would assume that the last one before it has to turn it encoded as fly-over. Therefor the turns happens too late.

That would make it a job for Jeppesen I presume?

And yes, the R9 also gives the warning about the GPS overlay.

Frequent travels around Europe

In general fly-by waypoints are preferred because they require the least protected area. The FAF and MAPT are however always fly-over as well as one point on the first segment of the missed approach.


NAV mode on most autopilots does not include turn anticipation. With roll steering, fly by waypoints will use turn anticipation.

KUZA, United States

With the DFC90 and DFC100 (ad GFC700) the autopilot wil behave exactly the same in NAV and in GPSS mode. That’s why there is no GPSS mode on the DFC100 (and the DFC90 only has it because it’s a retrofit for the S-TEC and the customers “expected” it. All the A/Ps get their data frome the AHRS’ in the PFD and that is essentially the same as traditional “roll steering”.

I have discussed this with the developers of the Avidyne autopilots and it was them who told me this.

True, but unfortunately this is not true for MOST autopilots. The KFC225/325 also have roll steering in NAV mode when the CDI source is GPS and the GPS supports roll steering. The KLN94 and KFC225 also support an analog form of roll steering, but IMHO it is worthless.

KUZA, United States

Yes, i understand what you are saying! I must say i am very pleased with the DFC90 … i have it two years and the plane flies like on rails …

The reality however is that most autopilots manage to follow the programmed route very closely indeed. When the GPS activates the new leg, the EHSI course pointer flips around to it and the aircraft turns. If you have a mechanical HSI then you turn the CP manually.

I don’t have roll steering on my KFC225 (and have never seen the special KLN94-KFC225 mode actually working) but it follows anything, even with a 90 deg bend, very accurately. Later in this writeup I have some tracking plots.

Shoreham EGKA, United Kingdom
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