GPS approach with the GTN 650

[DXB]

7 hours ago, mike_elliott said:

The GTN commands the GPSS, not the other way around. This error is in the GTN's algorithms. I cannot fathom why heading vs track might be used in the calculation, but that could account for it. Just guessing of course, its on Garmin to resolve.

My rate based autopilot is not always spot on in holding the GTN-derived GPSS course it receives, particularly in turbulence.  I wonder if the GTN simply adjusted the entry at the last minute based on such an error - i.e. it goes with where you actually are, not where it is commanding your autopilot to be.  

[midlifeflyer]

5 minutes ago, DXB said:

 

I have another burning question for you regarding how I'm tripped up on VTF with the GTN 650, but I'll maybe start a new thread rather than continuing to hijack this one.  

The AIM specifically advises not to load VTF initially. The reason is the loss of waypoints prior to the FAF. Although the GTN avoids much of that issue, it's still good SOP.

[mike_elliott]

1 hour ago, DXB said:

My rate based autopilot is not always spot on in holding the GTN-derived GPSS course it receives, particularly in turbulence.  I wonder if the GTN simply adjusted the entry at the last minute based on such an error - i.e. it goes with where you actually are, not where it is commanding your autopilot to be.  

Very plausible that it is recalculating based on "new" data of the "new" inbound course. Mr Lawler could be definitive here I am sure.

[Jim Peace]

On 6/23/2018 at 3:15 PM, DXB said:

Were you told to join at NENNO after flying direct from RBV by any chance? 

 

I was southwest of NEENO told to fly direct to it and cleared for approach.  clearly a teardrop from my view.

[Jim Peace]

3 hours ago, teejayevans said:

The GTN gives annunciations of what it’s going to do about 8 seconds before it does any turns, FWIW.

Yes and it told me it was going to do a teardrop....it did not......

[PTK]

This is a good theoretical discussion. Practically speaking however, no one cares how we enter a hold. All atc cares about is that we follow the hold instructions and execute proper turns staying on the protected side.

In my view teardrop and parallel are really the same animal and GPS obviously makes it very easy. But without GPS sa I do prefer the parallel over the drop. I find it easier to know where I am in relation to the inbound  leg.

[Cris]

Anomoly Solved! In reading the Garmin GTN 625/635/650 Pilots Gide I found the following under 6.5 “Approaches with Procedure Turns:” The procedure turn is stored as one of the legs of the approach. For this reason, the GTN 6xx unit requires no special operations from the pilot- other than flying the procedure itself- beyond what is required for any type of approach. Roll (GPSS) Steering is provided to aircraft with compatible autopilots.

Note: The steering provided for the procedure turn does not guarantee that the aircraft will stay within charted procedure boundaries. As such the crew will need to ensure that the approach is flown within the confines of the charted procedure.

Thus it is clear that Garmin anticipates that the guidance for the procedure turn provided may at times be incorrect relative to the charted procedure. Furthermore Garmin expects that the pilot will take over from the autopilot and ignore the guidance to keep the aircraft within the charted and protected airspace. Guess we are back to monitoring the approach like all good Pilots should do! 

Edited June 25, 2018 by Cris

[carusoam]

Great detective work, Cris!

+1 for the posts that define the lessor protected side... that goes with the protected side...

The old conversations about protected and non-protected side were simple and easy to use, but not quite as accurate as GPS navigation can be...

Thanks for sharing the details...

Great memories....

Best regards,

-a-

[Marauder]

Anomoly Solved! In reading the Garmin GTN 625/635/650 Pilots Gide I found the following under 6.5 “Approaches with Procedure Turns:” The procedure turn is stored as one of the legs of the approach. For this reason, the GTN 6xx unit requires no special operations from the pilot- other than flying the procedure itself- beyond what is required for any type of approach. Roll (GPSS) Steering is provided to aircraft with compatible autopilots.
Note: The steering provided for the procedure turn does not guarantee that the aircraft will stay within charted procedure boundaries. As such the crew will need to ensure that the approach is flown within the confines of the charted procedure.
Thus it is clear that Garmin anticipates that the guidance for the procedure turn provided may at times be incorrect relative to the charted procedure. Furthermore Garmin expects that the pilot will take over from the autopilot and ignore the guidance to keep the aircraft within the charted and protected airspace. Guess we are back to monitoring the approach like all good Pilots should do! 

Does this mean we can’t be “Children of the Magneta Line” anymore and actually have to think again?


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