19
$\begingroup$

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?

$\endgroup$
  • $\begingroup$ It's not a charting error, or at least the TERPS form was created with category A NA: faa.gov/air_traffic/flight_info/aeronav/procedures/application/… $\endgroup$ – NathanG Feb 4 '17 at 4:34
  • 1
    $\begingroup$ @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) $\endgroup$ – UnrecognizedFallingObject Feb 5 '17 at 4:31
  • 2
    $\begingroup$ @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.) $\endgroup$ – ammPilot Feb 6 '17 at 0:01
  • 6
    $\begingroup$ 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! $\endgroup$ – UnrecognizedFallingObject Mar 2 '17 at 0:26
  • 1
    $\begingroup$ @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. $\endgroup$ – Jimmy Mar 4 '17 at 16:12
8
+100
$\begingroup$

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!

$\endgroup$
  • 4
    $\begingroup$ ...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) $\endgroup$ – UnrecognizedFallingObject Mar 1 '17 at 0:19
  • 3
    $\begingroup$ 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. $\endgroup$ – Ralph J Mar 1 '17 at 20:52
  • 2
    $\begingroup$ 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. $\endgroup$ – Ralph J Mar 4 '17 at 0:44
  • 2
    $\begingroup$ 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) $\endgroup$ – UnrecognizedFallingObject Mar 4 '17 at 5:03
  • 3
    $\begingroup$ 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. $\endgroup$ – Jimmy Mar 4 '17 at 7:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.