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4 Answers

Two ILS approaches to the same runway with different minimums

Asked by: 2532 views Instrument Rating

Hello!

There are two ILS approaches for Pangborn Memorial (KEAT) which are related to the same runway: ILS Y RWY 12 and ILS Z RWY 12. Both charts are quite similar:

  1. DME is required;
  2. IAF/IF and the altitude between them 9400 are the same;
  3. the GS angle is the same 3.6 degrees.

But their minimums are different: 1555-1 for the "Z" chart and 2433-4 for the "Y" one. Also, they have different missed approach routes. 

So, here is the question. Why do the charts have different minimums? KEAT is placed in the mountains. I guess it might be that the crew must be learned in order to make the "Z" approach but there is no "Authorization required" statement on the charts.

Thank you in advance.

P.S. I haven't mentioned it before. I'm not a real pilot. I'm just interested in aviation very much.

4 Answers

  1. Best Answer


    KDS on Nov 13, 2019

    I believe the answer lies in the climb requirement for the ILS Z RWY 12 approach. That approach requires minimum climb of 420′ per NM as opposed to the standard 200′ per NM. Take a look at the differences in the missed approach procedures.

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  2. Marat on Nov 13, 2019

    KDS, thank you very much! Yeah, it seems like the best reason! I haven’t seen that requirement 420’/nm. I should be more attentive :))

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  3. Gregory on Nov 13, 2019

    Just so you understand the “why” of this minimum climb gradient, from an operational perspective:

    There’s terrain at the end of the runway that you’ll need to outclimb on the missed approach. If your aircraft is performant enough to do 420 feet-per-nautical-mile, you can accept the lower minimums of the ILS Zulu approach. You’ll have further to climb on the missed approach because of the lower approach minimums, so the approach is ensuring your have the performance to execute that climb.

    Remember that aircraft measure climbs in feet-per-minute, versus feet-per-nautical-mile, so there’s some quick arithmetic needed. If you’re flying a Cessna 172 at 90 knots, you can multiply by 1.5 to convert feet-per-NM to feet-per-min. Here’s why.

    Remember that 60 knots is 60 nautical miles per hour, or 1 mile per minute. Similarly 90 knots is 1.5 miles per minute and 120 knots is 2 miles per minute. So 420 (feet / nautical-mile) * 1.5 (nautical-mile / minute) = 630 (feet / minute).

    The Cessna 172 I fly can climb at ~700 feet/minute so I could accept this approach.

    You should do flight training! Take a “discovery flight” with an instructor at your nearest small airport; it’s a typical introductory flight where the instructor takes off and lets you fly the plane under their supervision. You’ll never regret it.

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  4. Marat on Nov 13, 2019

    Gregory, thank you very much! Highly appreciate your answer.
    Despite the fact that I’m not a real pilot, yeah, I can understand “why” those climb gradients exist. And I know the difference between vertical speed (ft/min) and climb gradient (ft/nm). Anyway, thanx again.

    You’ve mentioned about the C172 which can climb about 700 ft/min. But there is an important moment. The higher an airport is placed, the lower the airplane’s performance. So it would be great idea to assess the ability to fly with 420ft/nm gradient in the high-mountain areas. The elevation of KEAT is 1249′. I believe it’s not too high even for a C172 :)) I’m sure you know all that stuff, of course :))

    Also, thank you for your advice!

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