Blogarithms
Doug Kaye's thoughts on web services, web hosting and managed services.
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Instrument Approaches. A non-pilot friend asked, "What's a circle-to-land?" The purpose of an instrument approach is to get an airplane that's flying in conditions of limited visibility to a position from which the pilot can land "visually"--by looking out the window instead of by reference to instruments. As Philip Greenspun explained, there are two general categories of approaches: precision and non-precision. The former have "glideslope" guidance, so the airplane can proceed at a constant rate of descent. The latter instead has one or more "step-downs" in which the pilot descends to lower altitudes as he crosses navigational positions.
Regardless of the type of the approach, there's another variation having to do with how well the airplane is aligned with the desired runway at the time the pilot reaches visual conditions. In some cases, due to terrain, an approach gets the airplane to the airport, but at such an angle (more than 30 degrees, I think) that maneuvering is required to land. In other cases--even though the airplane may be aligned with a runway--the wind conditions may be such that the pilot wants to use a different runway.
If the pilot will land with less than a 30-degree course change, the approach is referred to as a "straght-in." More than 30 degrees of turns, and it's called a "circle-to-land" approach. Because turning requires geater tolerances and altitudes (particularly at slow speeds and/or during limited visibility), the instrument approach procedures require better weather ("higher minimums") for circle-to-land maneuvers than for straight-ins.
Even though the procedures and minima are designed to account for the increased complexity and risk, there are still more accidents during circle-to-land approaches than during straight-ins. I believe that some airlines don't permit their pilots to perform circle-to-lands, and many non-airline pilots use "personal minima" that exceed those published by the FAA when making such approaches. Hope the explanation helps.
Posted Wednesday, April 16, 2003 9:24:34 PM 
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Staying Alive in Small Airplanes. I've never met Philip Greenspun, so he doesn't know that the uniqueness and quality of his book, Philip and Alex's Guide to Web Publishing (Morgan Kaufmann; 1999), was such an inspiration when I wrote two books of my own.
In his weblog, Philip argues the dangers of non-precision instrument approaches in small airplanes. As a fellow pilot, I suggested in a comment that his criticism should perhaps be directed to the circle-to-land maneuver that potentially terminates any instrument approach, precision or non-precision. IMHO and that of others, it's a circle-to-land in marginal condition that presents the greatest danger, not the choice of a non-precision versus a precision approach.
If you're not a pilot, don't be swayed by the terms "precision" and "non-precision". While not altogether inaccurate, the names imply more than the truth. Humorous aside: I can recall one time hearing a non-pilot TV reporter refer to an "ILS" (instrument-landing system) approach as an "eyeless approach."
Posted Wednesday, April 16, 2003 7:24:18 PM
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