I was looking at the KSEA ILS 16L approach (depicted below) and was wondering just why the minima are bumped up for Cat A aircraft (64 feet on both DA and HAT, as well as 200 feet of RVR). Thing is, I've already checked Chapter 10 of the TERPS, and nothing in there says anything about approach-category-specific adjustments to ILS minima, nor does the Form 8260 for this approach say anything about the cause. (This also makes the SA CAT I and CAT II/III procedures Cat A NA, which matters for aircraft that have a small aircraft CAT II ILS authorization IAW FSIMS 4.2.3.) Furthermore, neither 16C nor 16R's ILS approaches have this feature, which is even stranger because you'd think that 16L and 16C are close enough together that their ILS OCSes would overlap, and they also use highly similar MAPs (climb to 900 on a heading, then intercept the SEA 161 radial, then proceed to TEBNE along that radial -- the only difference is the initial heading of 160 for 16C vs 165 for 16L).


What is doing this? Is there an obstacle there that is doing this only to small aircraft? Is there some vagary of the TERPS I'm not finding? Is this an error?

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    $\begingroup$ The higher DA will drive the higher required VIS, since you're farther from the runway at 264 ATDZE on glideslope than you are at 200' ATDZE on glideslope. The interesting question is, what drives the higher DA for Cat A... no guess there. Very curious! $\endgroup$
    – Ralph J
    Mar 11, 2017 at 18:02
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    $\begingroup$ I haven't looked at the TERPS for this one (and, following the Gunnison, CO, question, I'm no longer positive that I'd understand what I'm looking for), but in your Q you only mention approach info (ILS OCSes, etc.)... don't forget that missed approach information is also part of the minima determination. Did you continue your TERPS search beyond the approach side, and examine the MAPs as well? I don't know if the MAPs are different for each rwy, but if they go in different directions, then maybe there is an obstacle on the south side somewhere that is more specific to CatA? $\endgroup$
    – Jimmy
    Mar 11, 2017 at 18:41
  • $\begingroup$ @Jimmy TERPS Chapter 10 also covers the ILS MAP as well as the approach itself -- I will admit I mostly skimmed it looking for things that'd change with approach category. The MAP for 16C is very similar to 16L, though, as the only change is the heading of 165 on 16L vs 160 on 16C to intercept the SEA 161 radial to TEBNE. $\endgroup$ Mar 11, 2017 at 18:47
  • $\begingroup$ I'm guessing that it's very closely related to the reasons here: aviation.stackexchange.com/q/35236/69 $\endgroup$
    – Lnafziger
    Mar 12, 2017 at 3:50
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    $\begingroup$ @UnrecognizedFallingObject I get that it doesn't apply, but there are probably similar requirements for other types of approaches and if they have to go missed from a higher altitude then it basically accomplishes a higher climb gradient on the missed. The other example probably just could raise it enough to matter so they just prohibited it while this one just needed a small adjustment. (Again, I'm just guessing but it seems reasonable....) $\endgroup$
    – Lnafziger
    Mar 12, 2017 at 4:17

1 Answer 1


The minima's are usually decided based on the Missed Approach procedure, which one of the criteria being obstacle clearance. I believe the answer lies in your question, its due to the heading course and altitude restriction on the MAP point. RWY 16L needs to climb straight 165 deg which puts you through the obstacle , whereas 16R Missed approach procedure heading can clear the '464' obstacle. . In either case Obstacle driving the change, Having CAT A with the lower speed in the heading of 165 deg it won't get you clear the obstacle. The PBN approaches are even marked higher for all aircraft type which can be inferred from other terminal charts.

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  • $\begingroup$ Yes, but how does speed impact climb performance? $\endgroup$
    – Steve Kuo
    Mar 14, 2017 at 23:32
  • $\begingroup$ Yeah, why would this obstacle only affect Cat A minima? $\endgroup$ Mar 14, 2017 at 23:37
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    $\begingroup$ If you look at the perfomance stats listed here you find that all aircraft with a climb rate of 1000 ft/min or less are all cat A. All cat B aircraft have a climb rate of 1000 ft/min or more. The slower the Vref generally the lower the climb rate. They have to make the mins to be safe for any cat A aircraft that could be equipped. So if somebody shows up in an Aerona Chief that's been fitted with ILS equip they have to be able to clear obstacles with a 360 ft/min climb rate. $\endgroup$
    – TomMcW
    Mar 15, 2017 at 3:47
  • $\begingroup$ @TomMcW clarified the question of why its affecting only CAT A minima. $\endgroup$
    – Karthik
    Mar 16, 2017 at 10:07
  • $\begingroup$ Karthik - Oh, I was responding to UFO @Steve Kuo's question. I forgot to tag them. Also forgot to +1. Done :) $\endgroup$
    – TomMcW
    Mar 16, 2017 at 15:30

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