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Visual Descent Point & descent rate more than -1000fpm

Asked by: 431 views Instrument Rating

AIM states that VDP is only advisory in nature but when I looked up TERPS, it seemed like they put in OCS to evaluate obstacles in the area, I know AIM is non-regulatory and sometimes does not match wording with official FAA docs, I want some clarification on this one.

Also, I have read from book that descent rate more than -1000fpm is not recommended, even saw check instructors putting lots of emphasis on that for pass fail criteria. Where did this originate from?


1 Answers

  1. Russ Roslewski on Apr 13, 2017

    It’s important to understand what a VDP is, before going on to understand why it’s “advisory” for many operations.

    The VDP is a simple calculation of where the glidepath from the runway (3 degrees usually, but may be slightly steeper) intercepts the MDA. So, if you see the runway and meet the requirement to descend from the MDA, you will have a 3 degree glidepath if you start down from the VDP. That’s all it is – an advisory.

    If you wait until after the VDP to descend, you will have a steeper than 3 degree descent. This is not necessarily bad, for many small aircraft a 3 degree glidepath is really pretty shallow. Remember, the requirement for descending below MDA is that descent “can be made at a normal rate of descent using normal maneuvers” (91.175). A 5 degree glidepath is perfectly “normal” in a 172, for example. So starting down after the VDP is fine as long as you, the PIC, deem it safe.

    Starting down before the VDP is probably not a great idea, because a descent shallower than 3 degrees isn’t a “normal” descent in most aircraft. Furthermore, there may be obstacles down there.

    There are many reason why a VDP may not be published, you seem to be reading TERPS anyway, so I’m sure you can find those.

    As for the 1000 fpm maximum descent rate, I assume you’re talking about when on final. There’s no magic to the 1000 fpm number, it’s just a good, round number on the gauge. If you’re flying a 3 degree glidepath, you’d have to be going 200 knots in order for 1000 fpm to be required to hold the glidepath. Not many aircraft of any type come down final above 200 knots, and the ones that do certainly require special training. So, for the rest of us, 1000 fpm is a good limit – if we need more than that to correct for being high on the glideslope, we have probably already flown such a bad approach that we should just go missed instead.

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