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IFD5XX/4XX FMS Logic vs. GNS5XX/4XX for Activating an Approach

The Avidyne flight plan model is different than that of a Garmin.  Garmin treats approaches as special entities.  Avidyne treats them just like any other leg in a flight plan.  The Avidyne system doesn't have the concept of "loading" an approach.  You simply insert an approach into the flight plan just like you would insert a waypoint or an airway.  When an approach is inserted into the flight plan, if the leg before the approach happens to end at the same fix where the approach begins, then there is no gap before the approach.  If not, the approach will be preceded by a gap, indicating to the pilot that he is required to tell the FMS how to get from the last "enroute" waypoint to the first point in the approach.  Most often, this is done by simply closing the gap, thus creating a direct leg between the two.  If the gap is not closed, then when the aircraft reaches the preceding fix, the FMS will continue to guide the aircraft outbound from the fix on the same course it was on inbound to the fix.  That will continue until the pilot takes an overt action to either activate a different leg or go direct-to a waypoint.

If an approach is inserted into the flight plan and either there was no gap or the gap was closed, then the FMS will sequence onto the approach without any pilot action.  There is no need to "activate" the approach.  If, however, a gap existed and was not closed, that's when the pilot needs to provide instructions to the FMS about how to proceed onto the approach.  The "Activate Approach" button is nothing more than a shortcut to close the gap (note that the shortcut behaves slightly differently for a VTF approach because of the leg type at the beginning of that kind of approach).

For example:

Flying from KSEG > ETX > KXLL you obtain a clearance to fly the RNAV7 approach into KXLL with DOSTR intersection. You are enroute to ETX and then receive the RNAV7 clearance with DOSTR as the intersection. The proper way to enter that into the FMS is to insert RNAV7. then tell the FMS to DIRECT-TO DOSTR (IAF). Looking at the FMS> FPL Tab and seeing how the flight plan is presented will give the pilot an understanding of what to expect in flight. 

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  • 14-Nov-2019
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