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I’ll use the KMIA RNAV9 as an example, which has an IAF at waypoint
LLEGG. Say the flight plan contains
V3.PBI and then KMIA. When I enter the
RNAV9 approach, the FMS will insert the legs of the approach, starting with
LLEGG, and precede the approach with a discontinuity. That
discontinuity is the FMS saying, “You need to tell me how you intend to get
from PBI to the IAF.” When you activate
the approach using the button, you’re telling the FMS that “I intend to go
direct from PBI to LLEGG” and the FMS removes the discontinuity. The approach *starts* at LLEGG, so the
leg from PBI to LLEGG is not a part of the approach. As a visual aid, you’ll notice that the procedure
bracket starts just above the second instance of LLEGG.
In earlier versions of the FMS, we showed just one instance
of the leg to the IAF. However, that’s
not an entirely accurate depiction of the plan.
The IAF (the point) is part of the approach but the path to the IAF is
not. To clarify that concept, we
introduced a leg to the IAF fix that is not a part of the procedure and then
the approach starts with a leg actually marked as the IAF. In practice, the second leg just degenerates
once the preceding leg sequences and you end up sequencing to the leg after the
IAF.
The real impetus for us to make the changes was because
things got messy and confusing in the FMS when procedures were deleted. Using the KMIA example again, if you inserted
an approach, closed the gap, and then deleted the approach, what is the desired
result? Because you closed the gap, you effectively
inserted a leg to LLEGG. When you delete
the approach, do you delete that leg as well?
You might say that’s an obvious yes, but what if you had manually
inserted the leg to LLEGG prior to inserting the approach? Do you delete it then?
We think the new solution eliminates that confusion and makes
for stable and predictable FMS behavior.
Also, while I’ll admit that it can seem confusing to have two consecutive
legs with the same waypoint, it actually does a better job of reflecting what’s
actually happening in the flight plan and being consistent with the depiction
you would see on a chart.
P.S. You noticed that when you closed the gap that the IAF
had an altitude constraint and the other did not. That’s because the FMS removed the constraint
from the leg that it used to close the gap.
Operationally, that will have no effect because the FMS will still use
the altitude constraint on the second one to compute TOD. However, if you were to go put a different
altitude constraint on the leg to the IAF waypoint (i.e. not the one marked as
the IAF within the procedure bracket), then the FMS would introduce a
discontinuity between the two to indicate that those two points aren’t the
same. None of this processing has anything
to do with the altitude of the aircraft, though.
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Steve Lindsley
Avidyne Engineering
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