![]() Thus, the minimum altitudes of an approach are designed to meet the ROC within the OEA. The OEA defines the lateral boundaries of the protected airspace for an approach. Next, the obstacle evaluation area (OEA) tells you where you must have that clearance. So, ROC tells you how much clearance you need. When certain criteria are met, such as the final segment being longer than 6nm or a remote altimeter being used, ROC may be increased beyond the standard. As you might have guessed, this is the minimum vertical separation between a given position and the nearest obstacle below. The fundamental concept behind instrument approaches is the required obstacle clearance (ROC). In order to fully understand the differences between the different types of GPS-based approaches, we must first delve into some TERPS concepts, perhaps a bit deeper than you might wish. The last major standards overhaul was in 2007 with the 8260.54A order, and the recent 8260.58 order didn’t change much. Since each type of GPS minima is evaluated independently, these instances indicate that there’s more going on in these procedures than meets the eye.Ĭurrently available GPS procedures may have been developed under one of several obstacle evaluation standards since they first started appearing in 1994. There are quite a few procedures that have lower non-precision LNAV minimums than vertically guided LNAV/VNAV minimums, and even some that have lower non-WAAS LNAV/VNAV minimums than WAAS LPV minimums. These are usually safe assumptions, but occasionally you’ll come across a procedure that turns them on their head. After all, that’s how the magic box picks the type of service to provide. We tend to mentally order the types with WAAS as better than non-WAAS, and vertical guidance better than those without. When we think of GPS-based instrument approaches, we usually lump all the approach minima into a single “GPS approach” bucket.
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