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3 Answers

Controlling obstruction

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General Aviation

What is a controlling obstruction? It's mentioned in the AFD. I did try to find it in the FAR AIM but no luck! Is it measured from MSL or AGL.?

3 Answers

  1. John D Collins on Aug 29, 2014

    In the context of the AFD, it is a word such as Trees or Tower. It is an indication of what obstacles or terrain feature are of note. I am sure there is a specific criteria, but I don’t know it offhand. IFR departure procedures provide more detailed information such as the distance from the departure threshold, height above ground, and distance off the runway centerline.

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  2. Russ Roslewski on Sep 02, 2014

    Controlling obstruction, in the IFR approach and departure world, is quite simply the obstruction that results in the highest minimum descent altitude, decision altitude, or (for departures) the highest required climb rate or ceiling/visibility.

    Think of it this way – you’re coming in to land at an airport, and there is a 50-ft tree at 1/4 mile final. You notice there is also a 40-ft tree at 1/2 mile final. That 40-ft tree is pretty much irrelevant – it’s the 50-ft tree you need to worry about, if you make it over the 50-ft tree you will of necessity have made it over the 40-ft tree. Same theory for departures – a lower obstacle that is farther away is less of a concern.

    But it’s not always just the taller obstacle. For departures, clearly a 50-ft tree 1/4 mile from the departure end of the runway is a concern, whereas a 200-ft tree 3 miles from the departure end is not.

    In the A/FD, it will simply say something like “tree”, with no height data. If you want or need more specific information, you’ll need to refer to the instrument departure for that airport, which will list any pertinent obstructions.

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  3. psequeira on Sep 12, 2014

    Thank you John and Russ

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