Peter wrote:
One has to assume the pilot has read the plate.
When applying DME substitution, you can’t just “read the plate”, because the plate assumes you have a DME but you don’t.
There are loads of approaches where you have say a VOR somewhere and it involves a DME distance but the DME is not the one belonging to the VOR.
Yes. But I don’t see your point. You tune and identify each navaid. That’s a completely different issue from selecting the correct navaid from the database when two navaids have the same ID and are located within a few miles of each other.
It is just committee power politics
That (or “job creation”) is your explanation to every decision you don’t agree with, but sometimes there actually is a factual reason for it.
That is BS in all of real life so it is like saying “in theory, bees cannot fly”. Or “this is fine in practice but it will never work in theory” which is a famous cartoon from a certain country.
You (and I) may feel it is BS, but that’s what PANS-OPS says, so that’s what authorities have to apply when they are approving IAPs.
Once again, I don’t agree with the AMC. I understand the reasoning behind it, which is a different thing.
arj1 wrote:
as it is acceptable means of compliance, what stops you from producing your own and using those?
It is NCO, so what is the problem?
Not much, I think. That’s what I’m going to do!
I too would have preferred a little more in the way of substitution, but I do appreciate the little that we get.
As for DME substitution, the accident report of the mentioned accident made it clear – to me, anyway – that this is a potentially serious issue, and I can understand why the rulemakers have been cautious about it.
It is also clear to me that Airborne Again is quite right in that a procedure built around an NDB does not guarantee that the same nominal procedure will fulfill obstacle clearance requirements for an LNAV approach. I have had that discussion several times with procedure designers and some other nerds. This is of course nuts when looking at real life accuracy and flyability of GPS vs ADF, but formalities are not taken lightly when it comes to IFR procedures and obstacle clearance.
huv wrote:
As for DME substitution, the accident report of the mentioned accident made it clear – to me, anyway – that this is a potentially serious issue, and I can understand why the rulemakers have been cautious about it.
I think it is fairly safe if the pilot understands where the DME is located and what is presented as a distance readout on a GPS. But, again, regulator just makes it simpler for the xAA and says “no”. Interestingly, some procedures are not even encoded in the database (for example EGMD DME arc).
huv wrote:
It is also clear to me that Airborne Again is quite right in that a procedure built around an NDB does not guarantee that the same nominal procedure will fulfill obstacle clearance requirements for an LNAV approach. I have had that discussion several times with procedure designers and some other nerds. This is of course nuts when looking at real life accuracy and flyability of GPS vs ADF, but formalities are not taken lightly when it comes to IFR procedures and obstacle clearance.
True, but… what a pain! Although I have to say it is a pain for the UK where RNAV precedure are less common than in continental Europe. But in the UK the rules are different now, so I think it should work.
I’m not sure that there are many procedures in the EASA world that require the use of substitutions and there is no alternative RNAV procedure.
huv wrote:
It is also clear to me that Airborne Again is quite right in that a procedure built around an NDB does not guarantee that the same nominal procedure will fulfill obstacle clearance requirements for an LNAV approach
That is completely wrong for WAAS GPS, the LNAV with a WAAS GPS is functionally equivalent to our typical ILS/LOC guidance on final leg with angular accuracy being within -/-2.5deg near the MDH, now if someone is convinced that LOC/ILS is less precise than NDB, I think I need to go to sleep…obviously, LNAV obstacle area does not care if GPS is WAAS or non-WAAS (or if it’s lateral from FMS IRU/INS), it simply rely on +/-0.3nm lateral obstacles area, the latter may have more obstacles than NDB with +/-5deg on short final especially near the threshold, I get that aspects and some designers may get nerdy about it, with MDH being restricted to NDB guidance only
Now, why I can’t substitute NDB final segment guidance and fly NDB MDH on WAAS GPS? assuming SBAS/RAIM available and I stay within 1/2 CDI scale deflection what are the actual risks?
In practice, the comparison is moot given that a hand flown NDB in 25G35 winds will have it’s RBI wiggling +/-30deg while aircraft wings tilt in turbulence, the GPS tend to give a “clear & robust picture” of what is going on during that final segment
The bottom line, as posted previously, is mandatory equipment carriage, not usage 
So navigation accuracy is meaningless.
The bottom line is everybody who flies an NDB using an ADF tend to do it in good VMC during training (e.g. skill-test, re-validations, renewals), there are empirical evidences that those who fly NDB with ADF in actual IMC down to MDH are sadly no longer with us, either because of normal death (they have done it in the old days) or pure natural selection (one needs a positive level of IQ to stay alive)
The reality is that it’s ‘very unforgiving out there’ to use ADF in real life down to NDB system minima, while GPS is ‘well capable of safe 3D guidance down to MDH’ (actually down to 50ft if you ask my opinion) what few bureaucrats & dinosaurs thinks about it is not an eternal wisdom, after it’s all matter of time before GPS gets approved and/or NDB is removed…
Even in the most restrictive form of NCO.IDE.A.195, I would have imagined the GPS to be ok for use in lieu of carriage of ADF subject to the NDB IAP being available in FMS database and some extras:
- SBAS: PIC is ok to use GPS down to MDH ‘as it is’
- Non-SBAS: AMC for PIC to mitigate the risks or GM to calculate new MAPT/MDH for GPS (better than MAPT from stopwatch on timed NDB)
- Other sorts of LNAV: may need specific NAA approval
Ibra wrote:
those who fly NDB with ADF in actual IMC down to MDH are sadly no longer with us
I did it once at LDSB (NDB RW03 – at that time was RW04) – MDH was (is) 870’ and we popped out from the cloud at 910’ a bit left of runway but still within expected precision. However, the feeling during last minute was kind of not pleasant, especially because METAR obtained 30 min ago was BKN010 and actual weather was OVC009 RA (and worsening). The NDB used for this approach (HUM) is coastal and in the mountainous area, definitely not a good combination. Few years later I flew RNP approach to the same runway in similar conditions and personal feeling was that it was much safer and popping out from cloud at 700’ AGL I was absolutely aligned with runway.
Emir wrote:
personal feeling was that it was much safer
I wonder if it’s just personal feeling? without obstacles in smooth air both NDB & LNAV are supposed to take you to 300ft agl
You could argue that the same hold true on rough & bumpy days with terrain around,
- NDB ADF goes all over the place: it’s normal, staying on average offers better ‘obstacle protection’
- GPS CDI stays where it is as long as you stay where you are but offers better ‘motivation & comfort’
However, such hypothesis can be tested in real life when you become visual near minima:
- On CDI, specially 3D, you know where threshold sits and out of clouds perfectly aligned & established
- On ADF, I tried once for real, I will never try again: it felt like ‘WTF’ after the first attempt of spin recovery
GPS substitution is mostly about feeling: keeping nerves and workload to acceptable level, so that you can effectively keep CDI between your legs with smile, I think it is way more important than what happens when you reach 1/2 CDI scale deflection on LNAV and if that is equivalent to 10deg ADF on NDB all the time…
Peter wrote:
So, as more or less correctly identified in posts above, this substitution is basically worthless.
This is pretty much what substitution is for US. I note that the inclusion of RNP APCH as an alternative, allows the KLN94 to be used for substitution, as the RNAV 1 or RNP 1 requirement was overly restrictive.
