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VOR DME Approach

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VOR DME Approach

Old 12th Apr 2022, 02:02
  #41 (permalink)  
 
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Originally Posted by swh View Post
They are step down fixes, you cannot fly below those altitudes at those distances, if you do go around. If you want to know why have a look for “VABB Aerodrome Obstacle Chart Type-A RWY 32” on the inter web. Some thought needs to be made on how you are going to fly the approach now that you know what the chart is saying. They are charted this way because blindly using VNAV/LNAV may not guarantee obstacle clearance. If you blindly fly a 3.3 degree FPA or a VNAV path that would be rounded to 3.3 degrees you could fly below the step down fixes which compromises the designed obstacle clearance.
That applies generically to every instrument approach: don’t fly below hard altitudes at fixes. If it is coded correctly in the database, then the automation (monitored) should respect these altitudes; if you’re using basic modes you make sure you cross these fixes at or above. Why should this particular NPA be “special” and what’s the proposed difference in technique between flying this and a VOR/DME anywhere else?
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Old 12th Apr 2022, 03:57
  #42 (permalink)  
 
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Originally Posted by FullWings View Post
That applies generically to every instrument approach: don’t fly below hard altitudes at fixes. If it is coded correctly in the database, then the automation (monitored) should respect these altitudes; if you’re using basic modes you make sure you cross these fixes at or above. Why should this particular NPA be “special” and what’s the proposed difference in technique between flying this and a VOR/DME anywhere else?
Flying the approach conventionally without the FMS, at DME fixes you sometimes land up high or low and then correct the descent for the next DME. In this procedure the DME fixes are hard safety altitudes you cannot go below. So you will have treat them as minimums and that cannot be progressively done all the way to the lowest minimum of 2.8DME. so if the fixes are not coded in the FMS then the highest minimum will apply whether you use the FMS or fly with manual vertical guidance.
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Old 12th Apr 2022, 07:54
  #43 (permalink)  
 
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Originally Posted by Vilas
Flying the approach conventionally without the FMS, at DME fixes you sometimes land up high or low and then correct the descent for the next DME. In this procedure the DME fixes are hard safety altitudes you cannot go below. So you will have treat them as minimums and that cannot be progressively done all the way to the lowest minimum of 2.8DME. so if the fixes are not coded in the FMS then the highest minimum will apply whether you use the FMS or fly with manual vertical guidance.
Only if that is what your SOP says. Surely you are not suggesting you step down fix in the altitude selector and treat each as an MDA until you are past it?

In any case, that is what the dist/altitude table is for. Follow that, either using FMS VNAV if approved, or manually, and you will miss all the steps.

Which brings us back to the original question. Why are there 3 MDAs and when would you use each one?



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Old 12th Apr 2022, 09:26
  #44 (permalink)  
 
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Originally Posted by Capn Bloggs View Post
Only if that is what your SOP says. Surely you are not suggesting you step down fix in the altitude selector and treat each as an MDA until you are past it?
Boeing FCOM SP - Instrument Approach Using V/S

Approximately 2 NM before the final approach fix, set the first intermediate altitude constraint or MDA(H).
Set the MCP altitude to the nearest 100 foot increment at or below each intermediate altitude constraint.
When the current constraint is assured, set the next constraint before ALT is engaged to achieve a continuous descent path.
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Old 12th Apr 2022, 09:49
  #45 (permalink)  
 
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Originally Posted by vilas View Post
Flying the approach conventionally without the FMS, at DME fixes you sometimes land up high or low and then correct the descent for the next DME. In this procedure the DME fixes are hard safety altitudes you cannot go below. So you will have treat them as minimums and that cannot be progressively done all the way to the lowest minimum of 2.8DME. so if the fixes are not coded in the FMS then the highest minimum will apply whether you use the FMS or fly with manual vertical guidance.
My CAA’s standards doc. for the IR Skills Test states: ‘Not below’ minima (from FAF altitude down to MDA/H) vertical tolerance -0’, so this is well established and part of normal operations. If you think you might go below a hard altitude approaching a fix, you reduce the rate-of-descent so you don’t.

The procedure under discussion is not available without DME, so the fixes will always be there to check. If you don’t have an FMS but do have a VOR receiver with DME, you should be able to use the lowest minima for your aircraft category? An FMS 3D approach has different tolerances.
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Old 12th Apr 2022, 13:57
  #46 (permalink)  
 
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Set the MCP altitude to the nearest 100 foot increment at or below each intermediate altitude constraint.
When the current constraint is assured, set the next constraint before ALT is engaged to achieve a continuous descent path.
So in the space of ~1nm and less than 500ft to the MDA, you have to reset the altitude selector from 130ft above the MDA to the MDA (650, if you can set it). It appears to me that the Americans still don't have the distance/altitude profile table on their FAA approach charts so one is forced into this altitude selector catch-basket fiddle at every limiting step on the way down final. Getting off topic though...
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Old 12th Apr 2022, 14:51
  #47 (permalink)  
 
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Originally Posted by Capn Bloggs View Post
Only if that is what your SOP says. Surely you are not suggesting you step down fix in the altitude selector and treat each as an MDA until you are past it?

In any case, that is what the dist/altitude table is for. Follow that, either using FMS VNAV if approved, or manually, and you will miss all the steps.

Which brings us back to the original question. Why are there 3 MDAs and when would you use each one?
The final segment from FAF is supposed to have 246ft obstacle clearance but here 1330ft at 7.5 DME there's nothing. It's hard altitude. So in a conventional descent how will you ensure exact 1330ft. or above?Therefore this becomes the minimum for non FMS coded approach. Because even 10ft below will compromise obstacle clearance. Only the two subsequent minimas are coded only then the lower minimas can be used. That's how it's used by local carriers.
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Old 12th Apr 2022, 16:08
  #48 (permalink)  
 
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Interesting. I think that it is actually the opposite way round: using raw data and VS/FPA, you can ensure that you are at or above any hard heights by adjusting the vertical profile; if you’re using VNAV (3D) then up to 75’ deviation is allowable and there is no way of controlling that short of using another mode and/or taking the autopilot out, which is contrary to many carriers' SOPs for this type of approach.

All this could be made moot by coding a straight-in RNP procedure like they’ve done with all the other runways. It would even avoid flying directly over the obstacle!
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Old 12th Apr 2022, 22:05
  #49 (permalink)  

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Originally Posted by FullWings View Post
We have authorisation in our FOMM to treat MDA as a DA with no increment on a CDFA; I don’t think we’re the only ones...
No, you are not. The safety case can be explained with simple geometry, that's why it is approved.

However on PPRuNe not allowed to discuss lest the wrath of the Oztronauts is unleashed, mate.

Especially those who think altitude table distance table should be mirrored on alt selector.
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Old 12th Apr 2022, 23:49
  #50 (permalink)  
 
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Please can you explain the geometry Flight Detent, I'm genuinely trying to get my head around that one.
Thanks
TeeS
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Old 13th Apr 2022, 01:29
  #51 (permalink)  
 
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Originally Posted by Flight Detent
Especially those who think altitude table distance table should be mirrored on alt selector.
That's not me. I say set the real MDA (650) and fly down the profile. Been doing it for years.

Vilas, if you're worried about clipping a step, fly the profile + 100ft. Better than resetting the altitude selector three times inside the FAF.
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Old 13th Apr 2022, 03:44
  #52 (permalink)  
 
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Originally Posted by Capn Bloggs View Post
That's not me. I say set the real MDA (650) and fly down the profile. Been doing it for years.

Vilas, if you're worried about clipping a step, fly the profile + 100ft. Better than resetting the altitude selector three times inside the FAF.
Blogs in Airbus we set the go around altitude not the minimum anyway. And the NPA is flown with FPV to whatever FPA and not with ROD. Well for safety it can be flown with higher FPA than required to the lower minimum. But this approach has complications that need to be understood and taken care off. May be airlines just restrict it to higher minimum if not coded in FMS.
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Old 13th Apr 2022, 04:01
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I'd say use one minima, either 2.8 or 3.8 if vis is more then 3k, if vis is below 3k you need D2.8... Only thing i can think of why use the 3.8 is some better than standard missed approach climb gradients needed below 780', but that is usually depicted on the chart when that's the case...
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Old 13th Apr 2022, 21:20
  #54 (permalink)  
 
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The way I would fly this:

- set the lowest MDA if using FMS vertical guidance, with SDF1 and SDF2 correctly coded. - monitor
- set the lowest MDA if using raw data, ensuring to be at or above the SDFs while
monitoring and correcting the profile all the way down.

In case of a Missed approach - Do not descend below the MDA that is relevant for the segment at the commencement of missed approach. (Eg if the missed approach is commenced between D7.5 and D3.8 - the 1330’ MDA would be applicable)

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Old 13th Apr 2022, 23:49
  #55 (permalink)  
 
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Originally Posted by TeeS View Post
Hi KingAir
That was one of my early thoughts about it but I think there are three problems with looking at it that way:
1. It produces one of the most complex approach briefs ever.
2. Can you continue the approach that requires DME once the DME fails, Note 1 in bold?
3. If you do continue, where is your missed approach point?
Cheers
TeeS
1. Yes...
2. Once you have commenced the final approach, I guess you could. That was the whole point of my previous post, because obstacle clearance is guaranteed.
3. When flying a CDFA you go around once reaching you minima and no contact. The missed approach an be flown without DME.

KA.
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Old 15th Apr 2022, 11:17
  #56 (permalink)  
 
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KingAir1978

Note 1: DME Required. In plain English, if the DME fails (and you have not yet achieved the required visual reference) you cannot continue the approach. It is NOT optional.

I do hope you are not a professional pilot.
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Old 16th Apr 2022, 11:12
  #57 (permalink)  
swh

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Danger

Originally Posted by Clop_Clop View Post
I'd say use one minima, either 2.8 or 3.8 if vis is more then 3k, if vis is below 3k you need D2.8... Only thing i can think of why use the 3.8 is some better than standard missed approach climb gradients needed below 780', but that is usually depicted on the chart when that's the case...
The first step down fix is there because there is a substantial hill just to the left of track rising to the height, the second because there is two high rise buildings under the final course. These step down fixes are coded in the FMS as at or above constraints.

The two step down fixes in this case are required because of the obstacles on final, the only other reason for step down fixes is when the final segment is very long an extra vertical splay needs to be applied, using a step down fix in those cases resets the final segment distance.
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Old 16th Apr 2022, 12:07
  #58 (permalink)  
 
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On some VOR DME Charts three different MDAs are mentioned for different DME distances, Can someone please explain. Please see the attachment.
swh,

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.
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Old 16th Apr 2022, 14:00
  #59 (permalink)  
 
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Originally Posted by KingAir1978 View Post
1. Yes...
2. Once you have commenced the final approach, I guess you could. That was the whole point of my previous post, because obstacle clearance is guaranteed.
3. When flying a CDFA you go around once reaching you minima and no contact. The missed approach an be flown without DME.

KA.
You require DME and ht/dist check also. Air Canada SOP didn't have it they crashed short of RW dispite being on CDFA profile only a parallel little short of the actual.
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Old 16th Apr 2022, 15:03
  #60 (permalink)  

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Originally Posted by vilas View Post
You require DME and ht/dist check also. Air Canada SOP didn't have it they crashed short of RW dispite being on CDFA profile only a parallel little short of the actual.
For those not familiar and the sake of completeness, also because this actually is MDA (OCA) discussion:

That a/c https://www.tsb.gc.ca/eng/rapports-r.../a15h0002.html busted minima by 380 feet without visual reference. If they followed the charted profile (some pilots do with their IR not SOP) they may had just crashed onto the runway instead of ploughing through the ravine. Like these clowns https://aviation-safety.net/database...?id=20150304-0
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