VOR DME Approach

[Boyington]

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

[FlightDetent]

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.

[TeeS]

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

[FlightDetent]

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.

[TeeS]

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]

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]

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]

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

Caution, terminology matters.

[stilton]

What is CDFA ?

[Capn Bloggs]

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]

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]

Originally Posted by Capn Bloggs

. 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]

Originally Posted by Flight Detent

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]

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]

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]

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]

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]

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

[Capn Bloggs]

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

[Capt Fathom]

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