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"Skip Hold" disables Auto Enable Missed IFD540

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C88atapult View Drop Down
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    Posted: 09 Jul 2022 at 1:55pm
*** I have Auto-Enable Missed ON and have flown the following approach and gone missed with no problems before***

1) The Skip Hold LSK disables the Auto Enable Missed function in both the real IFD540 and the trainer. I discovered this in IMC at the MAP.  I am really disturbed to see this behavior is not documented anywhere. I'm not talking about "Sequence Leg" when the active leg terminates at an altitude.  I'm talking about NO GUIDANCE being provided after flying past the MAP, other than seeing the mileage start to increment.  As I use the map page on final to read the datablocks for TO mileage and NEXT information, I didn't see the "Enable Missed" LSK during final until I switched to FPL on the first leg of the missed because of no guidance, got ready to attempt to sequence the next leg and was greeted with "Activate Missed" instead. Not the best time to wonder WTF the box is doing.

Real World situation recap:
While established inbound on the required HILPT (course 90 deg) for VOR-A at KOKB https://aeronav.faa.gov/d-tpp/2206/05666VA.PDF, I was vectored north for military traffic. Once clear, I was given "Direct OCN".   OCN is the VOR that serves as both the IAF and the FAF as well as the HILPT fix. One cannot program direct to a fix that anchors a Hold, so I was unable to go direct to the FAF as desired.

Thus, I programmed direct to the IAF listing of OCN instead, at the beginning of the procedure.  I was now approaching the VOR from a direction that was NoPT, so I accepted the "Skip Hold" option and was pleased the the final approach segment turned striped. This fits the pilot's guide definition of "Skip Hold": "Displayed when the aircraft is within 5 nm of the FAF and the next leg is a database procedure hold. Pressing the LSK will sequence the active leg past the hold without entering it when the aircraft reaches the FAF. The leg after the hold will become active." 
It does NOT say "you are SOL if you still think the missed approach will auto-enable. Surprise!"

2) I used the trainer to see if this happened in all procedures with a HILPT.  Nope.  The VORDME-A for KRNM https://aeronav.faa.gov/d-tpp/2206/06667VDA.PDF allows me to Delete Hold from any approach direction, including a required PT direction.  I do not get a Delete Hold option at any time during the KOKB approach. Delete Hold has no documentation in the pilot's guide other than to say "this is how you delete a hold." No explanation as to why this is allowed or not in a particular procedure.

The only difference I can see between the two approaches is that KOKB VOR-A has a HILPT fix that serves as both the IAF and the FAF and the hold fix for VORDME-A at KRNM is only the IAF.  If this Skip Hold behavior is a "design feature" then it should be clearly documented and not be left to be a teachable moment at MDA in IMC.

Anybody got any ideas why this would happen or why I can delete a hold on one approach but not another?

Thanks.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AviSteve Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jul 2022 at 1:24pm
The difference is that the KOKB hold at OCN is marked as the FAF.  You cannot delete the FAF in any approach.

The auto-enable missed function is triggered by sequencing the FAF.  Since the FAF was skipped in this case, the FMS didn't automatically enable the missed.  Manually enabling the missed approach is the right answer, but I can understand why you were thinking that it would automatically occur as it does in other cases.  That's behavior that we can modify in a future release.
Steve Lindsley
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FlyingCOham Quote  Post ReplyReply Direct Link To This Post Posted: 12 Jul 2022 at 11:25pm
AviSteve: "That's behavior that we can modify in a future release."

  Anybody want to define when "a future release" might happen?  ( : > )
Jim Patton
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Post Options Post Options   Thanks (0) Thanks(0)   Quote C88atapult Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jul 2022 at 7:57pm
AviSteve:
Thanks for the prompt response. Without knowing how the logic is programmed and why, I'll still weigh in with an opinion :)

Is there a programmatic or regulatory constraint preventing going direct to the FAF in this case?  Since the HILPT fix serves as the IAF and FAF, I wanted to go direct to its "second occurrence", i.e., that of the FAF.  That is not possible, since OCN, the VOR in question, is not listed twice in the approach on the 540 - just once as the IAF.  It is identified as the FAF in the Hold leg, but one cannot go direct to a Hold.

As a waypoint/VOR that serves as both an IAF and a FAF for a procedure turn is listed twice in the approach in the 540, with the Proc Turn as a separate leg in between (KCNY VOR-A), I'm curious as to why the HILPT at KOKB VOR-A  is not represented the same way.

Thanks


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Post Options Post Options   Thanks (0) Thanks(0)   Quote AviSteve Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jul 2022 at 10:47am
Whenever there is a hold in the flight plan, it is preceded by a leg to the holding fix.  When you're initally going to the holding fix, it's the preceding leg that is active.  The hold leg is active only when you're in the hold.  So, if you want to go -D-> the hold, you really are saying that you want to go -D-> the holding fix *and then* hold.  That's why you can't do -D-> the hold leg.  Because the holding fix is identified as the FAF, I can understand why it's compelling to want to use that to go -D-> the FAF, but -D-> the preceding OCN (IAF) leg will do exactly what you want ... especially since you planned to skip the hold anyway.

One possible solution to the problem where the missed approach wasn't automatically enabled would be for the FMS to just mark the first instance of OCN as the FAF if you opt to skip the hold.  Then when the aircraft sequences that leg, the existing logic would enable the missed.  That may or may not be where we end up, but that's my initial thought.

As to why a procedure turn based approach is different than the HILPT based approach, it's because of how the standards define the two kinds of legs.  A procedure turn starts at a fix, does the 45 and then 180 degree turns, but then terminates by intercepting the next leg.  That next leg defines a course into the FAF.  For a HILPT, though, the holding pattern defines the course back to the next fix (which in this case is the FAF).  Therefore, there's no need for a separate leg terminating at the FAF.  The difference manifests itself in the way that the approach is coded in the database and that's why they look different at the user interface.
Steve Lindsley
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Post Options Post Options   Thanks (0) Thanks(0)   Quote C88atapult Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jul 2022 at 3:06pm
Thanks, AviSteve, understood.
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