azure
Final Approach
Thanks.
Now, does anyone know why the LNAV/VNAV DA is significantly lower than the LPV DA on the RNAV 16 X at KRNO? Inquiring minds and all...
LPV DA NA - Can I fly this approach and to what minimums?
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Now, does anyone know why the LNAV/VNAV DA is significantly lower than the LPV DA on the RNAV 16 X at KRNO? Inquiring minds and all...
John Collins
En-Route
Anytime you have two different specifications that use different surfaces, there will always be the possibility that an obstacle affects one specification differently than the other which results in a different DA. In the case of the LPV, there is a single sloped surface and with an LNAV/VNAV there is a sloped surface down to a point and then a level surface. This makes close in obstacles get treated differently. In these DA inversion situations with obstacle penetrations cause the DA to be adjusted upwards, two similar methods are used, but they are not precisely the same and in the case of the LNAV/VNAV, some obstacles that
Here are some excerpts from the TERPS 8260.58B which is used for RNP procedures:
3-3-1. General. An LNAV/VNAV approach is a vertically-guided approach procedure using baro-VNAV or WAAS VNAV for the vertical guidance. Obstacle evaluation is based on the LNAV OEA dimensions, level surface OCS based on ROC, and a sloping baro-VNAV OCS.
This shows the two LNAV/VNAV OCS that are used:
This shows how the LNAV/VNAV has the DA adjusted for obstacles that penetrate the two surfaces. Note that two methods are used depending on where the obstacle is.
Here is the LPV single sloped OCS:
Here is the simpler but different method that the LPV uses to adjust for obstacle penetrations.
Here are some excerpts from the TERPS 8260.58B which is used for RNP procedures:
3-3-1. General. An LNAV/VNAV approach is a vertically-guided approach procedure using baro-VNAV or WAAS VNAV for the vertical guidance. Obstacle evaluation is based on the LNAV OEA dimensions, level surface OCS based on ROC, and a sloping baro-VNAV OCS.
This shows the two LNAV/VNAV OCS that are used:
This shows how the LNAV/VNAV has the DA adjusted for obstacles that penetrate the two surfaces. Note that two methods are used depending on where the obstacle is.
Here is the LPV single sloped OCS:
Here is the simpler but different method that the LPV uses to adjust for obstacle penetrations.
John Collins
En-Route
Note the TERPS wording "An LNAV/VNAV approach is a vertically-guided approach procedure using baro-VNAV or WAAS VNAV for the vertical guidance. Obstacle evaluation is based on the LNAV OEA dimensions, level surface OCS based on ROC, and a sloping baro-VNAV OCS". Either vertical guidance may be used by the pilot if they have the capability and the chart does not exclude WAAS and a NOTAM is not issued. It does not matter how much higher or lower the DA of the LNAV/VNAV is with respect to a co-charted LPV DA.
aterpster
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aterpster
That seems to be confirmed by:
ILS OR LOC RWY 17, AMDT 7B...
S-ILS 17 DA NA ALL CATS. 11 AUG 12:55 2021 UNTIL 11 AUG 12:55 2023 ESTIMATED.
CREATED: 11 AUG 12:55 2021
Wow, it's estimated to be in effect for TWO YEARS!
I had a bit of spare time and sort of learned a little bit on how to interpret the CIFP database. If I read things correctly, turns out there's 3760 approaches with both LPV and LNAV/VNAV. I poked around a few of them and discovered this interesting one:
The ILS, RNAV-Z and RNAV-Y all have the same 19283 revision (year 2019, 283rd day). They all have the same 10OCT19 amendment date. However, the RNAV-Y has a LNAV/VNAV DA 200 feet lower than the LPV DA.
Not unusual.p when they don't know when it will be "fixed" or if there is going to be an amendment to the procedure.
azure
Final Approach
Very interesting. Two RNAV approaches along the exact same approach course with the same glide path and TCH, but with vastly different minimums. How can the algorithm to determine the vertically guided DAs and the non-precision MDAs through analysis of obstructions along the final approach path generate two distinct sets of minimums for the exact same approach path? What "tweak" to the assumptions is used to generate the higher minimums?
Even more interesting is that the ILS has the same IAF, HILPT and FAF as the two RNAV. The difference is the missed approach path, where the ILS curves back to the SGD VORTAC and then to BURDE. The RNAV continues straight ahead but the missed approach for RNAV-Z to 1L at KAPC requires a 420ft/nm climb angle to 2200MSL.
If you are flying a fully laden C172 and the localizer is out, then you'd best choose the RNAV-Y. But then the question becomes whether you use the LPV or go 200 feet lower on the LNAV/VNAV.
azure
Final Approach
The steeper climb gradient on the missed is, of course, to be expected since the RNAV Z gets you so much lower and closer in. And the chart note, I guess, also explains why they created the RNAV Y with its higher minimums, to establish an approach that aircraft incapable of such a steep climb gradient could fly using that same missed approach. I'm just curious about HOW the algorithm works to specifically establish 1252 and 1051 as the DAs for the approach with the shallower missed. Is the lower DA (LNAV/VNAV in this case) the lowest you can go on the approach and still be able to fly the missed at the standard climb gradient (200 ft/nm)?
Part of the reason may have to do with on an approach with a glide path, the procedure designers (I believe) make an allowance for the aircraft to descend below the DA as the pilot is deciding to go missed, powering up, pitching up and cleaning up. For the non-precision approach, the aircraft would be at the DA established at a constant altitude and fly the missed from there. Maybe Wally or somebody else more knowledgeable than I am (wouldn't take much to be more knowledgeable than me!) will chime in again.
azure
Final Approach
I've been digging through a number of internet sources, including Order 8260.50 (FAA's LPV Approach Procedure Construction Criteria, but might be superseded as it is from 2002) and I think I see another, simpler way the algorithm for determining the LPV and LNAV/VNAV DAs might generate a lower DA for LNAV/VNAV than for LPV.
It seems (from the IFR magazine article I linked to earlier) that in the simplest case there is one controlling obstacle for LNAV, LPV, and LNAV/VNAV minimums. The LNAV OCS is a horizontally flat surface that passes through the top of the controlling obstacle. The LNAV MDA is then 250 feet above the LNAV OCS. The LPV and LNAV/VNAV OCSes are sloping surfaces, except that the LNAV/VNAV OCS has a horizontally flat section close in to the airport (it's not clear to me where the transition between sloping and flat sections occurs; maybe it is at a specific height above either ground or the ASBL). Where the LNAV OCS intersects the sloping part of the LNAV/VNAV OCS determines the decision point for the LNAV/VNAV approach (i.e., the DA is the altitude of the glide path at the point directly above the intersection). The LPV DA is determined similarly. But the LPV OCS is angled more shallowly than the glide path because the sensitivity increases as you approach the runway threshold, while the sloping part of the LNAV/VNAV OCS is steeper than the glide path. I've never seen a diagram of the two OCSes together but it stands to reason that they must cross somewhere. Then if the controlling obstacle is inside the point where they cross, the LPV decision point will be closer in and thus the LPV DA will be lower than the LNAV/VNAV DA. If the controlling obstacle is outside the crossing point, then the LNAV/VNAV DA will be lower. The controlling obstacle is probably normally closer in where the two OCSes are lower, but in cases where it is further out, both DAs will be higher and it might be that the relative geometry of the two surfaces gets you a lower DA for LNAV/VNAV - and the further out it is, the greater the difference between the two DAs will naturally be.
This is mostly guesswork and is probably wrong, but I thought I would throw it out there. TERPS experts, feel free to tear it to pieces.
It seems (from the IFR magazine article I linked to earlier) that in the simplest case there is one controlling obstacle for LNAV, LPV, and LNAV/VNAV minimums. The LNAV OCS is a horizontally flat surface that passes through the top of the controlling obstacle. The LNAV MDA is then 250 feet above the LNAV OCS. The LPV and LNAV/VNAV OCSes are sloping surfaces, except that the LNAV/VNAV OCS has a horizontally flat section close in to the airport (it's not clear to me where the transition between sloping and flat sections occurs; maybe it is at a specific height above either ground or the ASBL). Where the LNAV OCS intersects the sloping part of the LNAV/VNAV OCS determines the decision point for the LNAV/VNAV approach (i.e., the DA is the altitude of the glide path at the point directly above the intersection). The LPV DA is determined similarly. But the LPV OCS is angled more shallowly than the glide path because the sensitivity increases as you approach the runway threshold, while the sloping part of the LNAV/VNAV OCS is steeper than the glide path. I've never seen a diagram of the two OCSes together but it stands to reason that they must cross somewhere. Then if the controlling obstacle is inside the point where they cross, the LPV decision point will be closer in and thus the LPV DA will be lower than the LNAV/VNAV DA. If the controlling obstacle is outside the crossing point, then the LNAV/VNAV DA will be lower. The controlling obstacle is probably normally closer in where the two OCSes are lower, but in cases where it is further out, both DAs will be higher and it might be that the relative geometry of the two surfaces gets you a lower DA for LNAV/VNAV - and the further out it is, the greater the difference between the two DAs will naturally be.
This is mostly guesswork and is probably wrong, but I thought I would throw it out there. TERPS experts, feel free to tear it to pieces.
RussR
En-Route
8260.50 is ancient in the world of LPV. The criteria has changed multiple times, in fact the .50 has been superseded 4 times now. The current RNAV design manual is the 8260.58B.
With some exceptions, I think most of the rules relatively simple. The explanations can be complex depending how deep you choose to go. That's particularly true when talking about approaches.Is there anything we could do to make flying more complicated? It seems to me that it's not complicated enough.
"The minimums are different." Simple. Just read them off the chart.
"Why are the minimums different?" Your choice how far you want to drill down.
azure
Final Approach
Downloading it now - thanks. Yeah, I thought the .50 might be way obsolete...
aterpster
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When they have no idea when something will be fixed it is always 2 years. That is an fdc notam policy limitation. The system used to be cluttered with temporary notams that were up to 10 years old. If it isn't fixed after 2 years they will issue another 2-year notam.
MauleSkinner
Touchdown! Greaser!
An example in the jet world would be V1…I could probably do an 8-hour dissertation on V1, but operationally you follow the instructions in the AFM. If you haven’t begun to abort by V1, you haven’t got the numbers to stop.
unfortunately, many pilots misunderstand V1 because they’ve got a little too much knowledge that they’ve determined conflicts with the actual definition, and therefore they can abort after V1.
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That makes sense.
Parenthetically, Not a NOTAM, but I get emails on IAP revisions in my area. Got one this morning about an amended IAP chart with an expected publication date in 2030!
luvflyin
Touchdown! Greaser!
Well ya bolded that 30 so obviously not a typo. What airport?
Here's the relevant text from the email. Strangely, there are no production documents on the IFP website (yet) for the amendment.
This is a courtesy notification that an update has been made to the RNAV (GPS) RWY 2 ORIG-C for 2A5, LIBERTY, NC - ABRVT_AMEND - has been updated to Estimated Chart Date: 01/17/2030.
RussR
En-Route
Dates like 2030 don't really mean 2030. What they generally mean is that it hasn't been assigned a real chart date yet, or is being put back into a "holding cell" or that kind of idea.
Every project in the system has to have a chart date. So some distant future chart dates are just used internally as a sort/filter of projects that aren't yet assigned a real one for various reasons.
Unfortunately, that system is the same place that the IFP Gateway gets its information, so you do see things like this sometimes. It's nothing to read anything into.
luvflyin
Touchdown! Greaser!
Oh. That's where I was going to see it. I'm guessing a typo by where ever you got it from.
I kind of figured that.
Is there anything we could do to make flying more complicated? It seems to me that it's not complicated enough.
What I had in mind when I wrote that was all the different kinds of RNAV approach, figuring out when you can fly which type, when you have to get out the manual to find out how to turn off WAAS, and how to figure out whether you should, etc.
It's almost enough to make me long for the days of NDB approaches!
azure
Final Approach
Somehow I missed this post before, looks like this was going to be a complete explanation for how the LNAV/VNAV DA could end up lower, and even includes some diagrams from the current RNAV approach TERPS order .58B. I really wonder what the rest of this paragraph was going to say?
Either way, it's clear that for the same obstacle to cause the LNAV/VNAV DA to be lower, the LPV OCS must lie below the LNAV/VNAV OCS. Does that mean that the controlling obstacle sticks up into the LPV approach path in such a way that it could be struck by an aircraft flying 2 or 3 dots below the GS, but not to full-scale deflection? Clearly in the case of the KAPC RNAV Y there cannot be any such obstacle since it would preclude the lower minimums on the RNAV Z, so some other consideration must be driving that choice. But I wonder about this in general, as it would obviously make it unsafe in some cases to descend all the way to a lower LNAV/VNAV DA using WAAS vertical guidance, with LPV sensitivity driving the GS CDI.
I've been unable to find a reference for the vertical width of the LPV approach path, so I can't calculate how far above the OCS you would be at FSD.
John Collins
En-Route
The definition of the FSD is not found in TERPS 8260-58B. It is defined in the RTCA DO-229 documents which are in turn the TSO C146 references. The FSD is +/- GPA (glide path angle) divided by 4. At a distance equal to 492/tan(GPA/4) from the glide path intercept point on the runway, the FSD becomes fixed at +/- 150 meters (+/- 492 feet), the FSD becomes a fixed +/- 492 feet. The fixed FSD for a GP angle of 3 degrees occurs at roughly 6 NM from the threshold. Note that the full description of the FSD is in the RTCA DO 229 section that describes LNAV/VNAV and in the LPV section, it simply refers to the LNAV/VNAV section. If you do the math, the switch over from angular to fixed is 492/tan(0.75) =37,584 feet from the GPIP (Glide Path Intercept Point), which is about 1000 feet past the threshold, so calculating the distance from the threshold is (37584 - 1000)/6076 = 6.02 NM. Note the standard ILS GS FSD is +/- 0.7 degrees and the LPV and LNAV/VNAV is very close at +/- 0.75 degrees FSD.
I'm guessing @RussR
I'm sure some people feel that way about the new tech setups, capabilities, and multiple options for accomplishing the same goal, even without dealing with the regulations.What I had in mind when I wrote that was all the different kinds of RNAV approach, figuring out when you can fly which type, when you have to get out the manual to find out how to turn off WAAS, and how to figure out whether you should, etc.
It's almost enough to make me long for the days of NDB approaches!
aterpster
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I'll go one step further: I got my instrument rating in 1958. I stopped flying around 2003. In all those years everything was mostly VOR/LOC/ILS/NDB. Sure, GPS started filtering into the system circa 1997, but I didn't see it. I did fly the 767 for a couple of years 1984-86, but it was DME/DME and there were no RNAV IAPs. Everything was ILS with an occasional VOR IAP.
In all those years I could go from one light airplane to another. By 1970 most of them were VOR/ILS/LOC and some had DME. But, it was all simple and straight-forward. Not know. I may know the G-1000 in Cessna 1234C cold, but not necessarily in Joe's G36 Bonanza. And, if either is a rental the user settings are all over the map. Then, I go to Fred's airplane with the newer Garmin add-ons and I am lost.
dtuuri
Final Approach
At the airlines, you mean. At the corporate level we had RNAV approaches, but you manually set in the waypoints. I remember being hired as a second officer at a major airline and a captain I flew with asked me to explain what "All this 'our nav' stuff is about." After I explained he says, "Shoot, here they won't even let us have a second DME. Afraid we're going to kill ourselves by forgetting which VOR it's toggled to."
I recently got checked out in a Cessna 172. When I tried to adjust the weird-looking attitude indicator the wings wouldn't move. "What are you trying to do?" asks the instructor. "Set the indicator for level," says me. "Oh, it's digital. You're in the wrong menu. I'll print out the
Sheesh.
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RussR
En-Route
I agree. I also learned to fly pre-GPS, and you're absolutely right, jumping from one airplane to another was a pretty simple matter. When I did my instrument rating, the flying club had 4 172s and 2 Arrows that I used for it. All of them were equipped a little differently, but I just hopped between them depending on which one was available. It didn't make much difference, setting up an ILS was pretty much the same regardless.
Now it is definitely more complicated, especially when you add the different autopilots into the mix, which are wired and connected to the avionics in different ways. Any aircraft that has been retrofit with modern avionics is a completely unique system that may or may not behave at all like other variations of that airplane and avionics.
It does keep CFIs like me busy, however. Transition training into new avionics had become a lucrative business.
Fortunately, however, a lot of the fundamentals of glass panels are pretty similar - presentation isn't really that different between a G5 and an Aspen and a G1000. And the actual "flying" hasn't changed much. What has changed is the depth of menus and options and interconnectivity.
In many of them, you don't/can't adjust that anymore.
dtuuri
Final Approach
I just flipped through all 48 pages and still can't tell how or if the pitch attitude can be adjusted. It's an AV-30-C. Do you know?
RussR
En-Route
Not specifically on that model, but on many, it's a installation setting, intended to be done by the avionics shop, not readily user-changeable by the pilot.
Edit - I found the instructions on how to do it in the AV-30C Installation Manual, paragraph 10.2, 3, and 4. under "Install Menu". I believe it's the item referred to as "Pitch Trim". (Which yes, generally has a different meaning to us). As I guessed, it's not intended to be pilot-accessible.
Whether or not it's legal for you adjust it I have no idea. I also know I wouldn't lose sleep over it.
dtuuri
Final Approach
My manual is different, but thanks anyway. Those paragraphs are on Battery Transition Logic (whatever that is.) I'll look for "pitch trim".
This kind of stuff is just evidence the inmates are running the asylum at FAA. They need leadership strong enough and aware enough to demand standardization and simplicity for safety's sake. It's scary that pilots are spending so much time fumbling with gadgetry over fundamental airmanship.
One of the things which gets people moving between a Garmin PFD and a Aspen is, Aspen has not adopted the Garmin pink vs green needles convention. Whether GPS or VLOC, the Aspen needles are always green. Not a big deal if that's what one flies regularly but one more item requiring additional crosscheck when flying an ILS approach.
aterpster
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aterpster
That alone would kill my purchase of Aspen.
RussR
En-Route
This was my reference, it's revision C, I do not know if that's the most current version or not.
https://uavionix.com/downloads/AV-30-C/AV-30-C Installation Manual UAV-1003947-001 Rev C.pdf
dtuuri
Final Approach
Thanks again. My version is "B". I see that version C is a year old, so I bet the CFI printed it from a stored file. Although it's only one month newer than B it looks like a completely different document.
EDIT: Heh, it IS a whole 'nuther manual. Yours is an Installation Manual, like you said, mine's the Pilot's Guide. Frankly, they both suck.
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John Collins
En-Route
What I had in mind when I wrote that was all the different kinds of RNAV approach, figuring out when you can fly which type, when you have to get out the manual to find out how to turn off WAAS, and how to figure out whether you should, etc.
It's almost enough to make me long for the days of NDB approaches!
I don't know of a case where it makes sense to turn off WAAS other than as part of a training exercise. If the GPS is TSO C146, then with an annunciation of LPV, you can fly the procedure to the LPV, LNAV/VNAV or LNAV straight In or circling minimums. If the Annunciation is L/VNAV, then you can fly the procedure to the LNAV/VNAV or LNAV straight in or circling minimums. If the annunciation is LP or LP+V, you can fly to the LP or LNAV straight in or circling minimums. If the annunciation is LNAV or LNAV+V, you can fly to the LNAV straight in or circling minimums. Of course this presumes there are no limitations specified on the approach chart or in a NOTAM and that the charted procedure includes multiple options. The minimums are specified on the chart in a hierarchical order, top to bottom.