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VOR DME Approach

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VOR DME Approach

Old 18th Apr 2022, 07:55
  #61 (permalink)  

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Originally Posted by Clop_Clop View Post

The issue was about MDAs for this approach and why they have three ones available. Of course need to assume the appch is flown per chart vor cdfa and with dme and also comply with the at or aboves for any fixes there as well... My point was about the 650' (w D2.8) or 780' (w D3.8) minimas earlier.
That is just the way Jepp publishes step down fixes inside the FAF, the charts in the Indian AIP has them as step down fixes. These step down fixes have nothing to do with reported visibility, or missed approach climb gradient. It is for terrain clearance.

"Stepdown Fixes

Occasionally a fix is located on the final approach segment between the FAF and the MAP. This fix is not a final approach fix, but it is called a stepdown fix and is used on nonprecision approaches. When this fix can be identified during the approach, you normally get lower minimums after passing the stepdown fix.

The stepdown fix is used primarily for two reasons. First, there are many cases in which there is a high obstacle in the final approach segment that would cause very high landing minimums. In this case, the FAA will designate a stepdown which is placed beyond the controlling obstruction in the final approach segment. After you have identified and passed the fix, you can descend to the MDA for the airport.

The second reason is when the final approach segment is excessively long, the TERPs criteria requires the MDA to be raised. When the final approach segment exceeds six miles, the MDA is increased at a rate of five feet for each one-tenth of a mile over six miles. When a stepdown fix is incorporated in the final approach segment, the basic obstacle clearance is applied between the stepdown fix and the MAP.

Sometimes, a constant descent rate cannot be made from the FAF down to the runway since a stepdown fix altitude might be higher than the constant descent angle from the FAF to the runway. In these cases the descent rate after the stepdown fix will not exceed 400 feet per nautical mile, or 3.77, and still have straight-in landing minimums."
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Old 18th Apr 2022, 13:10
  #62 (permalink)  
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SWH, all that is irrelevant because it doesn't explain why there are 3 different MDAs on that approach.
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Old 18th Apr 2022, 14:08
  #63 (permalink)  
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Wow, what a thread about a single Jepp approach plate! I agree with the comments above that publishing multiple MDAs depending on which stepdown fixes are used is more than unusual and non standard. Jeppesen just copied the AIP info verbatim, as they unfortunately often do - just for perspective: The corresponding chart from Lido just mentions one MDA, of course the lowest, and in the profile view shows the stepdown fixes as per usual.
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Old 18th Apr 2022, 14:54
  #64 (permalink)  
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And I suspect there is a lot of over thinking going on here. The most likely reason, IMHO, is that the AIP MDAs have not been changed from when the stepdowns were either crossing radials from another beacon or markers, i.e. before the advent of DME!
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Old 18th Apr 2022, 17:00
  #65 (permalink)  

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Originally Posted by Capn Bloggs View Post
SWH, all that is irrelevant because it doesn't explain why there are 3 different MDAs on that approach.
That is just the way Jepp publishes step down fixes inside the FAF, other chart providers only list one MDA. This isn’t the best chart Jepp has done, other providers have Trombay Hill clearly shown on the approach path with a MSA of 2100” shown to the left and below the 3.8 BBB waypoint. It’s a big feature that you fly close to, the Jepp chart gives the impression that it is flat terrain with just one mast.
in the FMC the coding is
FD32 +2600 -3.3
38VOR +1330 -3.3
28VOR +980 -3.3
MD32 350
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Old 18th Apr 2022, 19:14
  #66 (permalink)  

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The AIP chart posted at #32 by Bloggs (2019 edition) has 3 OCA(H)s. Jeppesen only follows that and Lido gents as well.

There is no hidden magic. Why the step-down fixes are there nor what is their effect on MDA and the purpose they serve.

Why the regulator chose to publish the 3 OCA(H)s is the unknown. The first OCA of 1330 can certainly do without (not the SDF, the OCA before it). It's overengineered

Which could lay a foundation for sub-optimal ARINC 424 coding as the database specialist has no way of second-guessing the intention behind an element which is unnecessary from the outset. Then you get various fuzzy / quirky interpretations by onboard installation. Last comes the contemporary pilot who was trained FMS = RNAV = GOOD = NOT MODIFY (does not remember anything more or care, other practical experience largely absent or well-forgotten) and ATTEMPTS to make ad-hoc procedures to tie all the loose ends which are not part of the problem itself.

You've seen it before too.

Last edited by FlightDetent; 18th Apr 2022 at 19:31.
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