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I was looking at the RNAV (RNP) 6 approach into Gunnison (KGUC) and noticed something quite strange:

KGUC RNAV (RNP) RWY 6 approach

If the image isn't clear enough -- the approach is Not Authorized for approach category A aircraft (which of course includes every last helicopter on the planet). WHY would this be so? Is there some operational or technical reason a slow-moving category A aircraft or helicopter can't fly this approach, even though their avionics might be perfectly capable of doing so? Or is this some sort of charting error I should bug the FAA about?

Ralph J
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UnrecognizedFallingObject
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  • It's not a charting error, or at least the TERPS form was created with category A NA: https://www.faa.gov/air_traffic/flight_info/aeronav/procedures/application/?event=procedure.results&tab=ndbr&nasrId=GUC#searchResultsTop – NathanG Feb 04 '17 at 04:34
  • @NathanG -- good find – UnrecognizedFallingObject Feb 04 '17 at 04:50
  • Maybe there is a separate procedure designed for cat A aircraft? – 60levelchange Feb 04 '17 at 08:05
  • The aircraft category refers to the flown approach speed (Vref or 1.3Vs0). A pilot may always choose to use a higher category (e.g., a pilot flying a normally Category A aircraft may use the Category B [or C, or any other higher speed] approach minima. @J.Hougaard, there are other approaches, including an ILS to runway 6, which do not have NA for Category A; the RNAV (RNP) 6 approach has the lowest minima. Regardless of category, the flight crew requires authorization to fly the approach. – ammPilot Feb 04 '17 at 15:09
  • @ammPilot -- is being able to use higher approach Category minima still true for helicopters? – UnrecognizedFallingObject Feb 04 '17 at 17:33
  • RNP is 'Required Navigation Performance' it is highly unlikely that a small aircraft, that has a Vref or 1.3Vs0 of less that 90 kts will have the RNP equipment. If for some bizarre reason they do, they can use the Category B. – JScarry Feb 05 '17 at 02:20
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    @JScarry -- I'm thinking of STOL turboprops and helicopters here, mainly (those would be the most likely critters to be RNP equipped and Cat A) – UnrecognizedFallingObject Feb 05 '17 at 04:31
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    @UnrecognizedFallingObject, following is from the US Aeronautical Information Manual (AIM), section 5-4-7.a:

    "Helicopters may use Category A minima. If it is necessary to operate at a speed in excess of the upper limit of the speed range for an aircraft’s category, the minimums for the higher category must be used."

    Note, though, that this still doesn't explain the lack of Cat A minima when Cat B and Cat C are provided. (NA for Cat D, I understand: nothing Cat D, and no one flying at Cat D speeds on the approach, may fly this approach.)

     – ammPilot Feb 06 '17 at 00:01
  • @ammPilot -- alright, I think it's clear now that helos use whatever minima for the speed they're flying at, just like everybody else. – UnrecognizedFallingObject Feb 06 '17 at 01:21
  • Could it be possible that with cat a speeds, flying a radius-to-fix segment with a constant turn would result in too low bank angles? – Sami Mar 01 '17 at 22:05
  • @Sami -- I'm not sure why having to fly a shallow turn would be a problem for a slow plane? – UnrecognizedFallingObject Mar 02 '17 at 00:12
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    BTW -- I just sent a brief email (with the picture from this post linked) to the AeroNav Products folks (using the contact info found here). We'll see if they get back to me within 7 days! – UnrecognizedFallingObject Mar 02 '17 at 00:26
  • @UnrecognizedFallingObject I don't know either. It was just an idea. I'm not sure if autopilots for example are capable of maintaining a turn with 1-2 degrees bank. – Sami Mar 02 '17 at 07:30
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    @unrecognizedFallingObject I can't wait to compare notes. I used the FAA's online "Aeronautical Data Inquiry" form, instead of email, but it will be interested to see what we each receive and how closely they match up. – Jimmy Mar 04 '17 at 16:12
  • It looks like the AeroNav Services folks didn't meet Ralph's deadline for the bounty...I'll still keep an eye on my email and respond with an answer if they reply, though – UnrecognizedFallingObject Mar 08 '17 at 23:22
  • They did reply (1 day after the bounty deadline expired, but Jimmy takes the cake anyway :) and verified that Jimmy is correct! I'm still not 100% sure what the design logic behind that chunk of 8260.58A is, but I'm seeing evidence that light plane pilots may have to start looking out for Cat A minima adjustments or even NAs on other approaches where they may not expect to find them! – UnrecognizedFallingObject Mar 10 '17 at 00:49

1 Answers1

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Original answer: The speed of Category A aircraft is too low to execute the missed approach. For this particular airport, it's basically a hole in the ground with steep mountains on all sides. To go missed, you gotta get up in a hurry, and the TERPS data probably indicates a minimum speed is needed.

Edit: Despite the downvotes and comments stating that my answer was "unlikely", I'm sticking to my guns.

To make sure that I knew what I was talking about, I contacted the FAA's Flight Procedures Standards Branch in OK City and eventually was put in contact with someone who was able to pull up the documentation on this approach.

Listed in the comments of the document he was looking at was this: "Ref 8260.52 CAT A -- final approach speed is too slow for RNP missed approach segment length for obstacle penetration."

Translation: whoever put this approach together did the math that is required, and the formula result was that speeds needed for obstacle clearance are not sufficient when using CAT A speeds.

Note 1: some math and formulas required to build an IAP missed approach procedure can be found in Chapter 4 of 8260.52. (https://www.faa.gov/documentLibrary/media/Order/ND/8260_52.pdf)

Note 2: 8260.52 has since been superseded by 8260.58, which itself has undergone an update and the most current document is 8260.58A.

Gory technical details (or some insight as to why this oddity is the way it is):

The thing that makes this approach special is that not only is it a RNP AR approach, the missed approach requires abnormally precise navigation as well (i.e. RNP values <1.0). The length of this span of the missed approach (from the MAP to TIPOC) is bounded by the results of formula 4-3-8 in 8260.58A, which is dependent on the true airspeed of the slowest aircraft of the approach.

enter image description here
DMASRNP is the max distance allowed for a given true airspeed (category) to return to RNP 1.0 on missed approach. The longer the distance to return to RNP 1.0, the faster the plane needs to be.

In the case of the KGUC RNAV (RNP) 6 -- the MAP to TIPOC span is 6.7NM along track, which is longer than the formula's results no matter which of the three possible RNPs are applied if you are using a category A airspeed (these results are applied to the whole span as per figure 4-3-5).

Why this formula is needed, and applied to the whole span for that matter, is a mystery, though -- 8260.58A gives no design rationale for this limitation, or why it is pegged to airspeed. A rationale is that navigation errors are cumulative though -- the slower you're flying, the more time you have to drift off course before you make it to the next "gate" where a higher RNP prevails.

Other approaches (even in similar terrain and with seemingly similar conditions charted) may not have this peculiar limitation, though -- the KRIL RNAV (RNP) Z 26 approach, for instance, also requires RNP < 1.0 on the missed approach, but for a shorter distance -- a mere 3.5NM from the MAP to TEROE, whereas the limit distance for Category A given the RNPs used in the missed approach there is 4NM.

Moral of the story: pay attention to your approach plates!

Jimmy
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    ...I don't see how having a higher airspeed on approach would help you with your sustained missed approach climb rate (especially if you're in a helo, where you can climb quite steeply at low airspeeds if you so desire AIUI) – UnrecognizedFallingObject Mar 01 '17 at 00:19
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    If you have a STOL turboprop that can fly slow & climb like a rocket, then the missed approach gradient should be achievable. And if the gradient is beyond the aircraft's capabilities, then THAT fact would prohibit the aircraft from flying the approach, not its Cat A approach speed. I found nothing in TERPS that gives a maximum assumed climb capability for Cat A aircraft, so this answer seems to lack credibility. – Ralph J Mar 01 '17 at 20:52
  • @ralphJ, I added sources and references to hopefully get my credibility back. :) These were provided by an FAA employee who was looking at the original source document for this approach. – Jimmy Mar 03 '17 at 23:55
  • Also.. I was looking at NathanG's source link (https://www.faa.gov/air_traffic/flight_info/aeronav/procedures/application/?event=procedure.results&tab=ndbr&nasrId=GUC#searchResultsTop), and I could not find what the FAA employee was referring to, but I'm not positive we were looking at the same document. – Jimmy Mar 04 '17 at 00:04
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    The two docs (linked + updated) are COOL documents (in a really, really nerdy sort of way!), but I'm not finding any formula where LOW airspeed creates an issue. High speed, sure -- and Cat D+E are NA in that approach -- not surprising. But if you meet the GRADIENT, why does your (low) speed matter? High speed can spit you out wide on a turning (RF) segment, but low speed doesn't drop you inside -- you're just at a lower bank angle to maintain track. So I guess I'm still not seeing it, but I'm willing to be shown what I missed where a low speed disqualifies during missed approach. – Ralph J Mar 04 '17 at 00:44
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    I'm not sure what the FAA bloke was looking at, but the explanation still isn't making sense to me (see the KRIL RNAV (RNP) Z RWY 26 for a comparison point) – UnrecognizedFallingObject Mar 04 '17 at 05:03
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    In addition to the verbal confirmation from the FAA (that my answer was spot on, regardless if we understand the logic or not), I've also put in a formal "Aeronautical Data Inquiry" about this chart. Not sure how long that takes to get a response, but I'll post the information I receive when I do. – Jimmy Mar 04 '17 at 07:52
  • I just heard back from the AeroNav folks, and lo and behold, your answer is correct! – UnrecognizedFallingObject Mar 09 '17 at 23:59
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    Quoting the email I got back from the FAA: "CATEGORY A NA: CAT A FAS AIRSPEED TOO SLOW TO SUPPORT DMASRNP DISTANCE NEEDED MISSED APPROACH OBSTRUCTIONS. MAXIMUM DISTANCE MISSED APPROACH FOR RNP (DMASRNP) CALCULATED lAW AFS-400 MEMO "CORRECTION TO ORDER 8260.52 , U.S. STANDARD FOR RNP SAAAR", DATED MAR 14, 2008." – UnrecognizedFallingObject Mar 10 '17 at 00:02
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    Reviewing 8260.58A, what they're referring to is the maximum length of the reduced-RNP missed approach legs on the procedure, which is computed using the missed approach KTAS for the slowest CAT that uses the approach (from formula 1-2-7, using the DA for the altitude and final KIAS for that CAT) plugged into formula 4-3-8 along with the limiting RNP 0.3 from the MAP to POSLY yields 2.96NM, which while longer than that limiting leg, is shorter than the sum of the RNP < 1.0 missed approach legs (see figure 4-3-5). – UnrecognizedFallingObject Mar 10 '17 at 00:29
  • Likewise, if you use the RNP 0.44 from the leg from POSLY to JENRO or the RNP 0.51 for the leg from JENRO to TIPOC, you still get a value that limits the sum of the length of the RNP < 1.0 missed approach legs to shorter than the total of 6.7NM you have to fly the missed approach with RNP < 1.0. I suspect this is because of error accumulations being proportional to time, not distance, and a slow Cat A aircraft has too many chances to drift off course and smack into a rock if they tried to fly the approach...or is there some other explanation for the formula 4-3-8 limit? – UnrecognizedFallingObject Mar 10 '17 at 00:32
  • Well, I guess that answers two questions: Which response will be faster?Your email response was faster than my online "data inquiry" form response (I still haven't heard back). And that pasted text is pretty much what the FAA guy on the other end of the phone told me, and which I then mangled up in translation when attempting to post that answer here. Thanks for posting what they sent. – Jimmy Mar 10 '17 at 01:54
  • Doing a similar analysis (for comparison purposes) for the KRIL RNAV (RNP) Z 26 approach (with a DA a couple thousand feet lower that shaves a few knots off the TAS of the aircraft, but with the limiting leg being the RNP 0.4 SIYOP-PUVOY, yields a value of 4NM, and you get to RNP 1.0 at TEROE after flying 3.5NM of that missed approach, overall. – UnrecognizedFallingObject Mar 11 '17 at 16:39
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    @unrecognizedFallingObject , I got the following response back on my FAA query: “Category A NA because CAT A FAS airspeed too slow to support DMASRNP distance needed to avoid missed approach obstructions.” (DMASRNP = The maximum Distance that the <1.0 RNP value may be extended into the missed approach surface (MAS)). – Jimmy Mar 13 '17 at 21:23
  • @Jimmy -- yeah, that's pretty much spot on to what I got back -- do you want to edit my commentary into your answer, would you rather I do that, or would you rather I post my commentary/analysis as an answer unto itself? – UnrecognizedFallingObject Mar 13 '17 at 22:03
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    @unrecognizedFallingObject, I think it makes sense to roll it into my answer. Feel free to do the edits. After looking at the actual process for this answer (and the Seattle one), I realize I have much less confidence in my understanding of the whole process. – Jimmy Mar 14 '17 at 02:30