No, an RNP APCH flown to LNAV minima using a LNAV+V system like Garmin provides is a 3D operation
I guess I should never have mentioned 2D or 3D in the first place. Unfortunately I thought I understood the PBN learning objectives in EASA Annex I to ED Decision 2016/008/R; LO 062 07 05 05 includes:
On the other hand – I have a reference to ICAO Annex 6 implying that an approach with a DA (and supposedly flown using CDFA technique) is 3D, while an approach with an MDA(H) is 2D. So, an NDB approach could be 3D if you had an FMS that somehow could give vertical guidance?
Now I also find a reference to Annex 10 PANS-OPS Vol II stating that an SBAS approach (i.e. LPV) actually could be a precision approach … which FAA reportedly says it cannot …
I guess I should stop making any assumptions that I have any definite understanding on approach classifications, and just wish bookworm and his colleges all the best in the committee.
huv wrote:
State that an RNP APCH to LNAV minima is a non-precision instrument approach procedure designed for 2D approach operations.
Since I also wrote that LO, you got me very worried 
but I think it’s correct. An RNP APCH to LNAV minima is a non-precision instrument approach procedure is indeed designed for 2D approach operations. But it can be flown as a 3D approach operation, which is what you do if you fly it using LNAV+V. The DH vs MDH issue is, unfortunately, even more complicated.
Let me try to write a summary and I’ll try it out here for clarity.
Thanks, @bookworm, looking forward to it.
I get that the obstacle clearance requirements (i.e, the surface below the approach flight path that obstacles must not penetrate) determine the minimum. It is also clear that the MDH for a 2D approach flown with vertical guidance or using CDFA technique or whatever is determined by rules for 2D approaches.
What I cannot comprehend is that in this case the 2D approach, where the approach flight path for obstacle clearance purposes is assumed to be a rapid (vertical?) descent from the last step down fix to MDA and then horizontal flight to the missed approach point, somehow has a hugely lower MDA than the corresponding 3D approach, with exactly the same lateral flight path to exactly the same point. I would understand 20-50ft for dipping through the DH on initiating the go-around, but not 200ft.
So the obstacle clearance surface below the flight path must be quite different for a precision approach.
Hence – some graphs would be nice, maybe there are some pointers to procedure design criteria documents?
Anytime you have different specifications applied to determine the DA or MDA, you can get minimums that don’t follow the normal case where the LPV has the lowest DA, the LNAV/VNAV has the next lowest DA, and the LNAV has the highest MDA. I have examples where the LNAV is the lowest, both in terms of the MDA and visibility requirements. It all depends on where the obstacles are located and the procedure specification being applied. Vertically guided procedures must have the appropriate visibility as measured from the DA location, which often makes the MDA have a lower visibility minimum as one can get closer to the threshold and still land. Also, obsticals in the missed approach segment can have a greater effect on a vertically guided approach as the aircraft is descending at the DA provisions for arresting the descent and beginning the climb are required verses the aircraft must be level or climbing at the MAP for an LNAV procedure as the MDA has a -0 tolerance.
A simple example where the LNAV might result in a lower MDA than a DA is the case where there is an obstacle that once passed, the LNAV permits placing a step down fix to descend to a lower MDA. With a GS, only one angle is permitted and step downs can’t be used to advantage.
huv wrote:
Now I also find a reference to Annex 10 PANS-OPS Vol II stating that an SBAS approach (i.e. LPV) actually could be a precision approach … which FAA reportedly says it cannot …
The FAA wanted to call LPV procedures precision approaches, but ICAO objected claiming that the LPV did not meet all the technical requirements for a precision approach and the APV category was invented. As a practical matter, an LPV works as an equivalent to a Category I ILS approach. This is mostly a terminology issue, but it does affect guidance to controllers in the US. For a precision approach, controllers are required to vector an aircraft to be below the GS. On an RNAV (GPS) approach they are not.
bookworm wrote:
Since the performance requirements (TTA, HAL, continuity) for flying LPV are always more stringent than those for flying a LNAV/VNAV based on SBAS, is there any practical risk in flying an approach to the LNAV/VNAV minima where these are lower than the LPV minima?
I don’t think so. The LPV>= 250 DH has the same VAL of 50 meters as the LNAV/VNAV. In the US, AC 90-105A states:
A.6.4 LNAV/VNAV Line of Minima Qualification.
A.6.4.1 Stand-Alone Systems. Stand-alone TSO-C146 Class 2 or 3 systems meet the aircraft qualification requirements for RNP APCH operations using the LNAV/VNAV line of minima provided that the installations meet at least the performance and functional requirements of this AC.
So at least in the US, I take it to mean that if LPV, LNAV/VNAV, or LNAV minimums are provided on an RNAV (GPS) approach, it is the pilot choice which procedure and minimums they fly to with a TSO C146 Navigator. There are a few procedures in the US territories (Puerto Rico) that are outside of the SBAS service volume and are noted that vertical guidance is not approved via a WAAS GPS and it must be via Baro VNAV.
For a precision approach, controllers are required to vector an aircraft to be below the GS. On an RNAV (GPS) approach they are not.
Does this mean that a “W” GPS will enable LPV GS capture from above, with an autopilot too?
AFAIK that doesn’t work with most autopilots when flying an ILS. The intercept must be done from below.
NCYankee wrote:
So at least in the US, I take it to mean that if LPV, LNAV/VNAV, or LNAV minimums are provided on an RNAV (GPS) approach, it is the pilot choice which procedure and minimums they fly to with a TSO C146 Navigator.
I guess the question is how the pilot can exercise that choice. If the annunciator says “LPV”, am I entitled to say “I’m going to treat this as LNAV/VNAV” or “I’m going to treat this as LNAV+V”? Of course I can disable SBAS to change the annunciator to “LNAV”, but that’s a counterproductive way of lowering my minima, isn’t it?!
bookworm wrote:
I guess the question is how the pilot can exercise that choice. If the annunciator says “LPV”, am I entitled to say “I’m going to treat this as LNAV/VNAV” or “I’m going to treat this as LNAV+V”? Of course I can disable SBAS to change the annunciator to “LNAV”, but that’s a counterproductive way of lowering my minima, isn’t it?!
I interpret that with an LPV annunciation I can chose to fly to any of these minimums:
LPV
LNAV/VNAV
LNAV
With an LNAV/VNAV annunciation, I can chose to fly to any of these minimums:
LNAV/VNAV
LNAV
With an LP annunciation, I can choose to fly any of these minimums:
LP
LNAV
With an LNAV annunciation I can only choose the LNAV minimums
In fact, ATC has no clue what minimums I am flying to, they just know it is an RNAV (GPS) procedure
I wrote a suggested text for the AIM to this effect for my FAA contacts in Flight Standards and although they never adopted it, they agreed with my interpretation, I have not asked the General Counsel for an interpretation and don’t plan to ask, I might not like their answer. I refer to AC 90-105A and AC 90-107 for my guidance along with common sense.
