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I was looking at the RNAV (GPS) approach to runway 10 at Kimball, NE (KIBM):

RNAV (GPS) RWY 10 KIBM

and was left wondering "How on earth could the straight-in visibility minima for approach categories C and D be lower than for approach categories A and B?" Is this merely a charting error, or is there some strange clause in the TERPS that I'm not aware of that makes this possible?

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  • $\begingroup$ Right now it could go either way. My best guess if it's not an error, there're protecting the slower(A&B) guys that could dive to the MDA quicker than the faster guys(C&D), for the obstructions near the approach. For the information, you'd have to hunt down the TERPS design guidlines, and I'm not sure where that is. $\endgroup$ – slookabill Feb 1 '15 at 1:14
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    $\begingroup$ I'm staring at the TERPS tables for NPA visibility -- and they are where this anomaly comes from -- with the 314' HAT, table 3-5a in 8260.3B (the TERPS) calls out 7/8mi visibility, but is only allowed for Cats C and D -- Cats A and B use tables 3-6 and 3-7, which peg the visibility minimum at 1 mile for no approach lights and all HATs from 250' to 750'...but why?! $\endgroup$ – UnrecognizedFallingObject Feb 1 '15 at 2:14
  • $\begingroup$ If you think it's an error, on the digital TERPS website under the FAQ, it does have a contact listed: faa.gov/air_traffic/flight_info/aeronav/faq/#q6a $\endgroup$ – slookabill Feb 1 '15 at 2:28
  • $\begingroup$ @slookabill -- it doesn't look to be a straightforward error as per the TERPS tables I cited; however, I'm still baffled as to why the tables are the way they are... $\endgroup$ – UnrecognizedFallingObject Feb 1 '15 at 2:53
  • $\begingroup$ Without local altimeter Cat C/D leapfrogs A/B to be above 1. Interesting. $\endgroup$ – digitgopher Feb 1 '15 at 21:39
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This is speculation, but in the absence of an obvious technical reason it's at least possible that this is because category C/D pilots have better lobbyists than category A/B ones.

I found an interesting but somewhat dated (1995) document called Establishing Visibility Minimums; the author used to chair the ALPA TERPS committee. It says:

[The TERPS concepts] are a combination of science and politics, with a lot of undocumented historical precedence thrown in for good measure

And:

The political horse-trading that led to the 1976 [TERPS] revision [...]

In other words, we shouldn't necessarily assume that rules and regulations are entirely based on well-defined and documented technical criteria.

The (now outdated) 1976 chart in that document shows a default minimum of 1 mile visibility for all categories (with the single exception of an ILS with DH of 200', which is 3/4) or 1/2 with approach lighting, so a 7/8 minimum is a more recent change. So you could ask, why was this change made? One way would be to find the NPRM or other public records that detail the reasons for the change, but in practice it may be easier to contact the FAA directly and ask them (possibly under the FOIA if the information isn't already public).

One theoretical possibility is that the airline industry successfully lobbied to have 'their' minimums reduced in certain circumstances due to better training, equipment or whatever, and this approach is one of the affected ones. Diverting airline flights is enormously expensive, and the industry has a large incentive to make sure that as many instrument approaches succeed as possible. If lowering the minimums by 1/8 allows even a few flights a day to start an approach that would otherwise be illegal, that could add up to a huge financial saving.

But this is entirely guesswork on my part, I just wanted to make the point that sometimes we look for a technical explanation where the real reason is financial or political. For a definite answer, I would ask the FAA directly.

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As UFO suggested in the comments above, the values come from the Tables in TERPS.

The reason why the visibility is greater for Category A and B is because in the typical "dive and drive" non-precision approach, the slower aircraft would descend to the MDA beyond 10,000ft from the landing threshold point, and therefore before the visual area of the approach. (TERPS Section 3.3.2) Increasing the visibility requirement would allow the slower aircraft to see the airport environment from further away.

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    $\begingroup$ If that were the reason then wouldn't the cat A minimums always be greater than cat C? $\endgroup$ – Pondlife Feb 2 '15 at 17:06

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