![]() ![]() LNAV/VNAV approaches have the same lateral OEA and lateral CDI scaling as LNAV approaches, minimums as low as 250 feet, but have a unique OCS that is partially sloping and partially flat. LNAV/VNAV is the non-WAAS approach with vertical guidance, meaning it has protection between decision altitude and the runway through a Glidepath Qualification Surface. Within an LNAV final segment, full-scale lateral deflection of your CDI is at 0.3nm, half of the primary area, so there’s a lot of leeway. Lacking vertical guidance, it has a flat OCS with a standard ROC of 250 feet (plus any adjustments). The LNAV approach is the most common GPS approach, and the only one whose minima apply to circling. For en route, terminal and non-precision final segments, ROC is constant throughout the OEA, resulting in a flat OCS. This is the case for approaches with vertical guidance. The OCS can get lower as you get closer to the runway and towards the center of the approach course, thus giving it a three-dimensional shape. The result of providing the ROC within the OEA creates the obstacle clearance surface (OCS). However, the intermediate segment ROC is only 500 feet and has an OEA that tapers from the initial width to the final OEA width. For instance, initial segments have a ROC of 1,000, a primary width of 2 nm from the centerline and a secondary width of 1 nm. (RNP approaches, beyond our scope here, only have primary areas.) The primary area is considered the normal course width, with the secondary area protecting for “bad days.” Within the secondary areas, ROC typically tapers to zero at the outer edge.ĭifferent approach segments have differing ROC values and OEA dimensions, as do the various final segment types (LNAV, LP, etc.). The OEA for each segment of an approach is further divided into a primary area down the middle of the approach course and identical but mirrored secondary areas on the left and right of the primary area. 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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