VOR DME Approach [Archive]

TeeS

9th Apr 2022, 15:01

Hi FD, I absolutely agee that those are step down fixes. If you note the mast elevation 1074ft just prior to the D3.8 fix, add 247ft gives 1321ft, round that up to the next 10ft gives 1330ft which is the OCA until you pass D3.8. The later step down fixes are calculated in exactly the same way.

Now, lets pretend that that step down fix was provided by an NDB, the Final Approach Fix was provided by some other means, maybe a VOR (unlikely but stick with me) and all other step down fixes were DME based.

You could now publish two MDA(H)s:
with DME 650’(625’)
without DME 1330’(1305’)
However, if you have not got DME available for the approach that Boyington is asking about, then you can’t fly the approach as DME is required and if you do have DME, why would you ever stop the descent at 1330ft or 780’. Hence, why publish MDA for 2.8 D and 3.8D?

Now don’t get me started on why Jeppesen mention CDFA on their plates!! Time for another thread.
Cheers
Tees

FlightDetent

9th Apr 2022, 15:21

No issues with any of the above.

Just that I avoided speculating why it needs to be charted in such a peculiar manner.

It's not the only approach or the the first time I see one published this way, with conditional OCA(H)s.

InSoMnIaC

9th Apr 2022, 20:51

I believe the lower MDAs are available only if CDFA is followed all the way to the Stated DME distances. Eg if you choose to dive and drive to 1330ft after 7.5DME then you are not allowed to rejoin the 3.26deg profile afterwards and continue to a lower MDA. They are effectively making CDFA a requirement beyond the FAF if you want to benefit from a lower MDA.

FlightDetent

10th Apr 2022, 01:21

OCAs are not dependent of flying technique.
CDFA by definition of FA only applies after FAF.

Caution, terminology matters.

stilton

10th Apr 2022, 05:00

What is CDFA ?

Capn Bloggs

10th Apr 2022, 05:49

What is CDFA ?
Constant Descent Final Approach.

I believe the lower MDAs are available only if CDFA is followed all the way to the Stated DME distances.

CDFA by definition of FA only applies after FAF.

I have never heard of a regulatory requirement to fly a CDFA to permit use of a lower minima. This is not a "vertical guidance" approach (such as a RNP LNAV/VNAV or LPV); it is an NPA/2D approach.

The Indians must have something specific/unique in their AIP that allows for lower minima if a CDFA is followed; using what, I wonder? VNAV?

Don't be misled by the BBB DME/Altitude table which is the profile reference nut not official, hard-coded SDFxs.
That is, I assume, on the state plate. It is the profile that, if followed, will keep you clear of all limiting steps. Nothing to be "misled" about it.

FlightDetent

10th Apr 2022, 06:52

The Indians must have something specific/unique in their AIP that allows for lower minima if a CDFA is followed; using what, I wonder? VNAV?Repeating this nonsense occludes things.

It's not hard.

FlightDetent

10th Apr 2022, 06:53

. Nothing to be "misled" about it. Lost in the translation.

Let me ask, have you seen guys check the ALT/Dist table on a VNAV approach? Do you?

Capn Bloggs

10th Apr 2022, 07:32

Let me ask, have you seen guys check the ALT/Dist table on a VNAV approach? Do you?
Yes mate, we have been doing that stuff, using 3x and had profiles on our plates, before you started flying. I will go so far as to say the world copied us in that regard.

Repeating this nonsense occludes things.

It's not hard.
You're speaking in riddles. Of course we "check" the profile table as we go down an approach (which we do in VNAV, despite the fact that this is a NPA). But that is a far cry from "requiring it to be followed" to allow a lower minima.

As I said, as far as I am aware, there has never been any requirement, on a 2D approach, which this obviously is, to fly a CDFA to allow use of a lower minima. If you have an ICAO reference, or indeed any regulatory reference that has such a procedure/policy, please provide it. In other words, explain the regulatory requirement/reference for your claim that "CDFA by definition of FA only applies after FAF."

TeeS

10th Apr 2022, 07:50

Sorry Capn but I think FD is technically correct in that PANS-OPS does define a CDFA as a method for flying the final approach segment, that does not mean of course that you couldn't intercept that flight path angle prior to the final approach fix, as I believe modern commercial fixed wing operations would try to do (I normally reside in Rotorheads, sorry).
Cheers
TeeS

Continuous descent final approach (CDFA). A technique, consistent with stabilized approach procedures, for flying
the final approach segment (FAS) of an instrument non-precision approach (NPA) procedure as a continuous
descent, without level-off, from an altitude/height at or above the final approach fix altitude/height to a point
approximately 15 m (50 ft) above the landing runway threshold or the point where the flare manoeuvre begins for
the type of aircraft flown; for the FAS of an NPA procedure followed by a circling approach, the CDFA technique
applies until circling approach minima (circling OCA/H) or visual flight manoeuvre altitude/height are reached.

Capn Bloggs

10th Apr 2022, 08:37

TeeS, that's fine, we all know what it is, "A technique...". I'm asking about the regulations that allow (or disallow) a lower minima if using this "technique". For example, there are very clear requirements for flying vertical paths on an LNAV/VNAV or LPV, for our country, a couple being +/-75ft at and after the FAF, have a VNAV display and be Baro-VNAV approved. What are the actual requirements for flying that Indian CDFA, apart from just saying "if you do a CDFA you can use the lower MDA".

In other words, what special requirements apply to this apparent bog-standard approach that contains a couple of step-down fixes? What are the actual requirements for this so-called CDFA as opposed to every other NP approach with obstacles and during we fly on a continuous descent down finals, missing the steps?

FlightDetent

10th Apr 2022, 08:50

The question and approach has nothing to do with CDFA.

It's great you watch the profile chart on VNAV. Next question - do you have to i.e. is that a system & certification requirement?

Try not to embarrass yourself any deeper, playing the ball would be a start.

peeush

10th Apr 2022, 09:09

Hi Boyington,

It appears to be related with resolution of cockpit display and range accuracy of on board DME. Provided the cockpit display resolution is 0.2 or less, lowest of published minima may be accessed if the range accuracy so permits. Wherever the measured distance affects its accuracy (e.g. +/- 3% of DME measured range), inherent errors must be factored along side display resolution capabilities for check altitudes. It's possibly for this reason that difference MDAs are stated with/without D2.8 or D3.8 or both. For e.g a display resolution of D0.5 would not show D2.8 or D3.8. A range accuracy of 0.2nm at 4nm with display resolution of 0.2 or less gives access to MDA of 780' (755') with D3.8. Unless, the range accuracy or display capability at shorter ranges changes, MDA of 650' should also be accessible in this case.
The crew may therefore check accuracy range and cockpit display resolution of DME to determine applicable minima on this approach plate.
That's the best I could think.

Cheers

TeeS

10th Apr 2022, 10:16

I'll go back to my original comments:

Firstly, I think the original AIP plate has been poorly/incorrectly drawn because all of the approach fixes are designated by DME, so if you can identify the step down fix at 3.8D, it is also available at 2.8D and even at the missed approach point at 1.0D; there is no need for MDAs for some of the SDFs because absolutely everyone is going to continue the approach to 2.8D! It can't be argued that you could brief different MDAs depending on where on the approach the DME fails because DME is required for the approach therefore, if the DME fails you have to go around unless visual.

Secondly, the AIP plate (https://aim-india.aai.aero/eaip-v2-03-2022/index-en-GB.html) does not mention CDFA which is absolutely correct - CDFA is an operational consideration and the operator is required to calculate a 'derived DA(H) from the OCA(H) at which their pilots will commence a go-around if not visual, this will prevent the aircraft from descending below the OCA during the go-around. Jeppesen seem to insist on mentioning CDFA on 2-D approaches thus leaving a trail of confusion across the World.

I'm not a fixed wing pilot (well I did have a CPL(A) IR once!) but I'm well aware that CDFA is absolutely sensible (and generally mandated) for large/fast commercial fixed wing aircraft and probably smaller fixed wing as well; in the helicopter world, I strongly believe that the second PANS-OPS option 2, which is a constant angle descent but maintaining MDA to the missed approach point, is the better option. That doesn't really matter but that CDFA statement on the Jeppesen plate makes so many helicopter pilots think that they must fly a CDFA because the plate says so.

I suspect the following paragraph from PANS-OPS Vol II may be the reason the procedure designer added the extra minima but really it just produces an excess of confusion in this case.

2.7.3.2 The use of the stepdown fix in the final approach segment shall be limited to aircraft capable of
simultaneous reception of the flight track and a crossing indication unless otherwise specified. Where a stepdown fix is
used in the final approach segment, an OCA/H shall be specified both with and without the stepdown fix. Where a
stepdown fix is used in the final approach segment of an RNP procedure, OCA/H is specified only for the case with
stepdown fix.

Cheers
TeeS

Capn Bloggs

10th Apr 2022, 10:26

Thank you TeeS, that explains it. Jeppesen strikes again.

Capt Fathom

10th Apr 2022, 11:25

Never occurred to me that a VOR/DME approach could be so complicated!

172_driver

10th Apr 2022, 19:06

Never occurred to me that a VOR/DME approach could be so complicated!

The approach is not complicated, but this thread is.

TeeS

10th Apr 2022, 21:32

Sorry Capt. and 172
The aim wasn't to make a simple VOR/DME approach complicated; however, I looked at the approach plate and was confused, it sounds like Boyington was confused, that is why he asked the question. We have now discussed and pondered the intention of the plate for a day or so. We may have come to a conclusion, it may be correct or it may be incorrect!
The question is, how long would a pilot have to brief on the plate having just diverted towards that airfield for the first time in their career, if that MDA(H) table was unnecessarily complicated would deciphering it have interfered with more important tasks?
Cheers
TeeS

oggers

10th Apr 2022, 22:54

2.7.3.2...Where a stepdown fix is used in the final approach segment, an OCA/H shall be specified both with and without the stepdown fix...

That is the definitive answer to the question posed in post #1. PANSOPS says it, therefore it is.

Capn Bloggs

11th Apr 2022, 01:46

That is the definitive answer to the question posed in post #1. PANSOPS says it, therefore it is.
I don't believe it does. That's just a statement of fact. I'm sure the OP (and me too), is interested in not only the "why" but the operational implications of the multiple MDAs. How do I use that chart in the cockpit to do my approach, what minima is actually applicable to me and what special requirements, over and above the normal NPA procedures of not busting any steps, are applicable, if any?

The thread hasn't answered that question.

An example in my area; a similar approach with a limiting step in the FAS. No separate OCA/MDA.

https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/562x794/ypkg_vor11_7263a22aac1256d72d3151ebf51686b1d457058f.jpg

Perhaps, that Indian chart means "if you comply-with/miss/clear SDF1, you can use 780, and if you comply-with/miss/clear SDF2, you can use 650."?

I can't find anything in the Indian AIP nor a 2014 copy of Pansops to explain the use of the different MDAs.

rudestuff

11th Apr 2022, 04:55

It looks like we need an Indian...

Rico_Corp

11th Apr 2022, 05:16

To me this looks like database coding determines which minima is used:

1. If the coding is FD32 > MD32 then MDA is 1330 ft
2. If the coding is FD32 > 38VOR > MD32 then MDA is 780 ft
3. If the coding is FD32 > 28VOR > MD32 then MDA is 650 ft

Capn Bloggs

11th Apr 2022, 05:39

What about the case where you're not using an FMS?

TeeS

11th Apr 2022, 06:02

That is the definitive answer to the question posed in post #1. PANSOPS says it, therefore it is.

Sorry Oggers, I don't believe that is quite correct:

3. STATUS
Procedures for Air Navigation Services (PANS) do not have the same status as Standards and Recommended Practices.
While the latter are adopted by Council in pursuance of Article 37 of the Convention and are subject to the full
procedure of Article 90, PANS are approved by Council and are recommended to Contracting States for worldwide
application.

FullWings

11th Apr 2022, 06:24

I don't believe it does. That's just a statement of fact. I'm sure the OP (and me too), is interested in not only the "why" but the operational implications of the multiple MDAs. How do I use that chart in the cockpit to do my approach, what minima is actually applicable to me and what special requirements, over and above the normal NPA procedures of not busting any steps, are applicable, if any?
Exactly.

From initial inspection, I can’t see why if you respect the crossing altitudes, be it by traditional or electronic means, you can’t use the lowest minima. The approach is depicted in a very similar way in Lido. It reminds me of when you sometimes have VNAV and LNAV/VNAV minima that are different: if you can use both, you can pick the lowest, which could be either of them due to various technicalities in the way they are calculated.

The “how” is just as important, if not more so, than the “why”...

Rico_Corp

11th Apr 2022, 07:12

What about the case where you're not using an FMS?

Non-CDFA minima 1330 ft applies (w/o D2.8 & D3.8 CDFA).

FullWings

11th Apr 2022, 08:16

What’s the connection between FMS use and CDFA? We fly all NPAs as CDFA, whether database or basic modes with fix altitudes, and use the published MDA as a DA.

Capn Bloggs

11th Apr 2022, 08:56

Non-CDFA minima 1330 ft applies (w/o D2.8 & D3.8 CDFA).
There's no such thing as a CDFA minima as far as I am aware (that's what I'm asking about: why the SDFs affect the MDA and what are the rules for using them). The words "CDFA" are a Jeppesen addition. They are not on the AIP India plate:

https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/789x1108/vabb_vor32_9ecd44be7cc1198e20a8b8c545758430da4abe77.jpg

172_driver

11th Apr 2022, 09:29

Sorry Capt. and 172
The aim wasn't to make a simple VOR/DME approach complicated; however, I looked at the approach plate and was confused,

Agree that discussion shouldn't be discouraged. To me this thread is going in circles.

Brief and use MDA 650. Stranger to me is that it's mentioned as VOR-appr. (and not VOR/DME).

TeeS

11th Apr 2022, 09:37

What’s the connection between FMS use and CDFA? We fly all NPAs as CDFA, whether database or basic modes with fix altitudes, and use the published MDA as a DA.
Hi FullWings
If you use the published MDA as a DA then you will have exceeded your minima. PANS-OPS again:

1.8.2.3 If the visual references required to land have not been acquired when the aircraft is approaching the MDA/H, the vertical (climbing) portion of the missed approach shall be initiated at an altitude above the minimum descent altitude/height (MDA/H) sufficient to prevent the aircraft from descending through the MDA/H. At no time is the aircraft to be flown in level flight at or near the MDA/H. Any turns on the missed approach shall not begin until the aircraft reaches the missed approach point (MAPt). Likewise, if the aircraft reaches the MAPt before descending to near the MDA/H, the missed approach shall be initiated at the MAPt.

1.8.2.4 An increment for the MDA/H may be prescribed by the operator to determine the altitude/height at which the vertical portion of the missed approach shall be initiated in order to prevent descent below the MDA/H. In such cases, there is no need to increase the RVR or visibility requirements for the approach. The RVR and/or visibility published for the original MDA/H should be used.

Cheers
TeeS

Capn Bloggs

11th Apr 2022, 09:53

Stranger to me is that it's mentioned as VOR-appr. (and not VOR/DME).
The naming convention changed a couple of years ago, removing the "DME" part. I think it was to standardise with the FMS names for approaches.

TeeS

11th Apr 2022, 09:55

Agree that discussion shouldn't be discouraged. To me this thread is going in circles.

Brief and use MDA 650. Stranger to me is that it's mentioned as VOR-appr. (and not VOR/DME).

I agree the circles 172; however, the Mumbai plate is absolutely correct to call the approach VOR RWY 32, the UK insist on doing it not in accordance with PANS-OPS but that is their prerogative! I hate to say it but here is PANS-OPS again:

9.5.2 Procedure chart identification
9.5.2.1 General. The chart identification for procedures requiring ground-based navaids shall only contain the
name describing the type of radio navigation aid providing the final approach lateral guidance. Precision approach
systems such as ILS or MLS shall be identified by the system name (ILS, MLS, etc.). If two radio navigation aids are
used for final approach lateral guidance, the chart identification shall only include the last radio navigation aid used.
For example:
if an NDB is used as the final approach fix and a VOR is used as the last navaid on the final approach to runway 06,
the procedure shall be identified as VOR Rwy 06. If a VOR is used for the initial approach followed by a final
approach to Rwy 24 using an NDB, the procedure shall be identified as NDB Rwy 24.
Note.— For chart identification of procedures supporting PBN, refer to Part III, Section 5, Chapter 1.
9.5.2.2 If additional navigation aids or equipment are required for the approach procedure, associated additional
equipment requirements shall be specified on the plan view of the chart, but not in the the chart identification.
9.5.2.2.1 The equipment requirements mentioned on the plan view refer only to the equipment on board the
aircraft necessary to conduct the procedure in normal mode (i.e. not for backup). For example:
“ADF required ” on a VOR approach.
“Dual ADF required”, on an NDB approach where two ADFs are required to fly the final approach segment.
“DME required” on a VOR approach.
9.5.2.2.2 Equipment that is required in the corresponding airspace may be mentioned as equipment requirements.

Cheers
TeeS

swh

11th Apr 2022, 12:51

On some VOR DME Charts three different MDAs are mentioned for different DME distances, Can someone please explain. Please see the attachment.

in why it is done. Thanks

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.

KingAir1978

11th Apr 2022, 19:50

On some VOR DME Charts three different MDAs are mentioned for different DME distances, Can someone please explain. Please see the attachment.
https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/1500x2000/img_816521df01d1_1_fba929fb22605e3e6a8d7b3a9994c1b17715b09f. jpeg
in why it is done. Thanks

I am not familiar with the potential differences between PANS-OPS and DGCAA. The plate, however, has the PANS-OPS logo printed in the left margin, suggesting that the plate would comply with PANS-OPS regs. I am not sure, but would these minima not refer to failure of DME cases? IAW if your DME fails after you have commenced your final approach (indicated by the Maltese cross) you can go down to 1330', because obstacle clearance is guaranteed to that altitude. If your DME fails after you have passed 3.8 DME you can continue to 780', because obstacle clearance is guaranteed to that altitude. IF your DME fails after 2.8 DME you can continue to 650', because obstacle clearance is guaranteed to that altitude?

TeeS

11th Apr 2022, 21:02

I am not familiar with the potential differences between PANS-OPS and DGCAA. The plate, however, has the PANS-OPS logo printed in the left margin, suggesting that the plate would comply with PANS-OPS regs. I am not sure, but would these minima not refer to failure of DME cases? IAW if your DME fails after you have commenced your final approach (indicated by the Maltese cross) you can go down to 1330', because obstacle clearance is guaranteed to that altitude. If your DME fails after you have passed 3.8 DME you can continue to 780', because obstacle clearance is guaranteed to that altitude. IF your DME fails after 2.8 DME you can continue to 650', because obstacle clearance is guaranteed to that altitude?
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

FullWings

12th Apr 2022, 01:48

Hi FullWings
If you use the published MDA as a DA then you will have exceeded your minima. PANS-OPS again:

1.8.2.3 If the visual references required to land have not been acquired when the aircraft is approaching the MDA/H, the vertical (climbing) portion of the missed approach shall be initiated at an altitude above the minimum descent altitude/height (MDA/H) sufficient to prevent the aircraft from descending through the MDA/H. At no time is the aircraft to be flown in level flight at or near the MDA/H. Any turns on the missed approach shall not begin until the aircraft reaches the missed approach point (MAPt). Likewise, if the aircraft reaches the MAPt before descending to near the MDA/H, the missed approach shall be initiated at the MAPt.

1.8.2.4 An increment for the MDA/H may be prescribed by the operator to determine the altitude/height at which the vertical portion of the missed approach shall be initiated in order to prevent descent below the MDA/H. In such cases, there is no need to increase the RVR or visibility requirements for the approach. The RVR and/or visibility published for the original MDA/H should be used.

Cheers
TeeS
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...

FullWings

12th Apr 2022, 02:02

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?

vilas

12th Apr 2022, 03:57

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.

Capn Bloggs

12th Apr 2022, 07:54

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?

mbcxharm

12th Apr 2022, 09:26

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.

FullWings

12th Apr 2022, 09:49

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.

Capn Bloggs

12th Apr 2022, 13:57

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...

vilas

12th Apr 2022, 14:51

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.

FullWings

12th Apr 2022, 16:08

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!

FlightDetent

12th Apr 2022, 22:05

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.

TeeS

12th Apr 2022, 23:49

Please can you explain the geometry Flight Detent, I'm genuinely trying to get my head around that one.
Thanks
TeeS

Capn Bloggs

13th Apr 2022, 01:29

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.

vilas

13th Apr 2022, 03:44

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.

Clop_Clop

13th Apr 2022, 04:01

InSoMnIaC

13th Apr 2022, 21:20

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)

KingAir1978

13th Apr 2022, 23:49

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.

common toad

15th Apr 2022, 11:17

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.

swh

16th Apr 2022, 11:12

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.

Clop_Clop

16th Apr 2022, 12:07

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.

vilas

16th Apr 2022, 14:00

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.

FlightDetent

16th Apr 2022, 15:03

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-reports/aviation/2015/a15h0002/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/record.php?id=20150304-0

swh

18th Apr 2022, 07:55

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.

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."

Capn Bloggs

18th Apr 2022, 13:10

SWH, all that is irrelevant because it doesn't explain why there are 3 different MDAs on that approach.

STBYRUD

18th Apr 2022, 14:08

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.

BizJetJock

18th Apr 2022, 14:54

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!

swh

18th Apr 2022, 17:00

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

FlightDetent

18th Apr 2022, 19:14

almost.

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

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/403x264/mdas_c2ab5bd476c098ba91c7b2abf8fa6381074eb35c.png

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.