GTN 750 xi: Selecting alternate procedure during missed approach

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lucius

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The GTN 750xi allows to load an alternative procedure during a missed approach procedure.

upload_2023-6-10_20-43-28.png

However, in some situations, the “Load Approach” button to load an alternative approach during a missed approach is greyed out. For example, when flying the missing approach for KOLM RNAV GPS 17. See screenshot. This is reproducible in the Garmin PC trainer.

upload_2023-6-10_20-43-28.png

However, once a little bit further into the missed approach procedure, suddenly it is possible to load the alternate procedure. For the missed approach procedure for KOLM RNAV GPS 17, it appears one must pass the first way point CORER, before the alternate procedure can be loaded. Such restriction is not documented in the GTN 750 xi manual.

However, for other procedures, one can select the alternate procedure immediately, for example when flying the missed procedure for SHN RNAV GPS 05.

Why the difference?

Thank you!
 
I haven't had a chance to do a deep dive into this one. My off the top of my head thought is that you can't load the second approach until the MAP hold waypoint is the active waypoint. It actually makes some sense. Is that what you are seeing?
 
No, this is not what I am seeing. I am beyond the MAP. I.e. in the example above I am right before CORER, which is about 3 miles beyond the MAP, or at a location where I would expect the alternate procedure to be loadable. Note, I can load and activate it, but that’s not what I want.
 
lucius said:
No, this is not what I am seeing. I am beyond the MAP. I.e. in the example above I am right before CORER, which is about 3 miles beyond the MAP, or at a location where I would expect the alternate procedure to be loadable. Note, I can load and activate it, but that’s not what I want.
Click to expand...
Not the MAP. The MAP hold waypoint. CETRA would not be the active waypoint 'before CORER"
 
Thank you, @midlifeflyer! I wonder what the rational is. For OLM RNAV 17, CORER is 5 miles out, before CETRA becomes the active waypoint. For approaches where the next waypoint is the MAP hold point, I can load the alternate approach immediately.
 
lucius said:
Thank you, @midlifeflyer! I wonder what the rational is. For OLM RNAV 17, CORER is 5 miles out, before CETRA becomes the active waypoint. For approaches where the next waypoint is the MAP hold point, I can load the alternate approach immediately.
Click to expand...
Guessing...

Garmin built on a system in which you could not load multiple approaches in the same flight plan at all. Going back to at least the 430/530, if you switched approaches once the first was active, the only choice was to activate the new one. And you lost the lost everyhting from the old one.

The new functionality isn't really about loading another approach. It's about not losing the missed from the first one when you do. Although it has other benefits, it's about flying the missed and being in the missed hold for real and setting up your avionics to try again or go to an alternate without losing what you are already flying. So they triggered "load" based on MAHWP being the active waypoint. I guess it's either because they didn't want to mess too with what they had. In the case of this specific approach, with CORER 5 NM away, you are stabilized early, but there will be others where that is not the case. It may just be a matter of functional consistency.

(When the feature came out I wondered whether it was an attempt to copy one of the popular features of the Avidyne IFD, the ability to load multiple approaches from the very beginning.)
 
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