Lanes narrow ahead

But aside from meeting the generous standards in the Instrument Rating Airman Certification Standards for lateral guidance (“maintain a course allowing no more than three-quarters-scale deflection of the course deviation indicator (CDI)”), most of us rarely learn or think about the horizontal dimensions of the lanes we travel in along the final approach course, probably because those values are not published on the charts we use in flight.

The Instrument Flying Handbook explainings that during an RNAV (GPS) approach to an LNAV MDA, the sensitivity of the CDI changes from 1 nautical mile to 0.3 nm either side of the centerline along the final approach segment. Similarly, a localizer course narrows from about 300 feet per dot of deflection near the final approach fix to about 100 feet per dot at the typical location of the DA for an ILS. The Instrument Procedures Handbook adds details about how procedure designers use obstacle clearance surfaces (OCS) and obstacle evaluation areas (OEA) to establish the minimum altitudes for segments of departures and approaches.

Deeper appreciation of the importance of staying in your lane, especially when flying approaches, comes from two FAA orders known as TERPS. Order 8260.3 United States Standard for Terminal Instrument Procedures covers approaches based on navaids—VORs, localizers, and NDBs. The companion Order 8260.58 United States Standard for Performance Based Navigation (PBN) Instrument Procedure Design describes the details for RNAV and RNP procedures, which for most of us means GPS-based procedures. You don’t need to memorize the criteria established in these orders to pass an FAA practical test, but a review of some details can help you better understand why minimum altitudes vary—and why keeping the CDI centered is important as you descend toward a runway.

For a VOR approach, the lateral dimensions of a path that clears obstacles at the MDA along the final approach segment (FAS) assume that your VOR receiver (tested every 30 days per FAR 91.171) may be as much as 6 degrees off. With the navaid at the airport, the primary obstacle evaluation area along final tapers from 3 nm either side of the centerline (6 nm total) at the beginning of the final approach segment to 2 nm total width at the navaid. Obstacles and terrain within that large wedge drive the establishment of the stepdown altitudes and the MDA for a particular approach.

The primary obstacle evaluation area along final for a 2D (nonprecision) RNAV (GPS) approach to an LNAV MDA is a much narrower path, only 0.6 nm wide on either side of the centerline. Obstacles that intrude into the wedge for a VOR approach to a particular runway may lie outside the lateral limits for a corresponding GPS-based approach and therefore allow a lower MDA—assuming you don’t stray outside the lines.

The lateral dimensions of the obstacle evaluation area for 3D (precision) approaches with approved vertical guidance (usually an ILS glideslope or LPV glidepath to a DA) are funnels that narrow from a total of 8,546 feet (1.4 nm) either side of the centerline at the final approach fix to just 1,000 feet left or right at the threshold. Note that designers use primary and secondary obstacle evaluation areas when checking whether obstacles intrude on approach paths. The farther you wander from the centerline, the greater the risk that you’ll encounter obstacles. Remain within the primary obstacle evaluation area for LPV minimums, however, and you can descend to a lower DA while remaining clear of obstacles in the broader, constant-width obstacle evaluation area standard for a 2D approach to an MDA.

Procedure designers now use the same basic obstacle clearance paths for ILS and RNAV (GPS) approaches with LPV minimums. But the DA along an LPV glidepath may not get you as low as an ILS glideslope because other factors, such as the presence of an approach lighting system.

The obstacle evaluation area for an approach with localizer performance (LP) minimums is also a funnel, but it is not as narrow as that for an approach to a DA. Even when advisory (+V) vertical guidance is available, an approach with LP minimums remains a 2D procedure to an MDA.

The illustrations that show the lateral dimensions of final approach courses emphasize why you must verify the annunciations on your HSI or navigator as the final approach fix becomes the active waypoint. These displays typically show “LNAV,” “LP,” “LPV,” “VOR,” or “LOC” (check the guides for the avionics in your panel) to confirm that the CDI is set to the proper scale as you continue toward the runway. If, for example, the error-checking algorithm in a WAAS-capable GPS navigator determines that LPV precision isn’t available, you’re limited to the MDA associated with the wider LNAV lane to the runway and you must observe stepdowns and stop your descent accordingly.

Bruce Williams is a CFII and specializes in IFR training and instruction in aircraft with advanced avionics. He owns a Beechcraft Bonanza A36.