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wrote in message ...
and are not assessed with the precision that are accorded IFR altitudes. I'm not sure what you mean by "precision" here. The parameters are clearly specified: 1000' above any obstacle in the designated sector. (The MSA doesn't assure navaid reception, though, so that has to be assessed separately.) The folks who design approach procedures at the FAA use very precise topographical information to design the published segments of an instrument approach procedure. For MSAs, though, they simply use sectionals, which may not provide the required obstacle clearance at all times, simply because sectionals do not have the accuracy that USGS 1:24,000 topos have. Plus, when the FAA assesses the published segments they add 200 feet of assumed adverse obstacle ("AAO") pad, because folks can construct towers, etc, up to 200 feet high without notifying the FAA, unless the towers are within certain distances of an airport. There is no AAO assessment made for MSAs, though. Also, spot elevations on sectionals can be off by a fair abount, without adversly affecting their stated purpose; i.e., VFR navigation charts. And, contours on sectionals are very coarse, although that isn't usually an issue in non-mountainous areas. Hm, so you're saying that the MSA doesn't necessarily provide the obstacle clearance that it's advertised to provide. I hadn't considered that possibility. Are you familiar with any example of an obstacle that's less than 1000' below a current MSA? Thanks, Gary |
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