VOR DME Approach
 

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....17715b09f.jpeg
in why it is done. Thanks

General advice first, for any procedure/charting queries always check the AIP.

Without too much detail, the OCA/Hs here have limiting obstacles. Step-down fixes are put in place to protect you and provide lower MDA - but only if you can observe them and comply.

Don't be misled by the BBB DME/Altitude table which is the profile reference nut not official, hard-coded SDFxs.

Hi Boyington
It just looks wrong to me, if you can receive the DME at 9.5NM and 3.8NM then I can't see why you wouldn't receive it at 2.8NM and 1NM. As FlightDetent hints, I think they are just step down fixes with confusing extra information.
Cheers
TeeS

I hint otherwise. The MDA ergo OCA are different because of a critical obstacle.

If the obstacle is protected by step-down fix, and you had passed it, the MDA is calculated differently, becoming lower.

There is two questions.
- What is it and how does it work
- Why is it designed this way.

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

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.

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.

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

Caution, terminology matters.

What is CDFA ?

Constant Descent Final Approach.

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?

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.

Repeating this nonsense occludes things.

It's not hard.

Lost in the translation.

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.

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."

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.

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?

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.

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

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 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

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

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

The approach is not complicated, but this thread is.

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

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....b1d457058f.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.

It looks like we need an Indian...

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

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

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.

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”...

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

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.

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....30da4abe77.jpg

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).

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

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.

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

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.

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

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...