Beyond bare minimums

Digging a bit further, however, can help IFR pilots and instructors understand why those values often vary from familiar standards, such as a DA of 200 feet and visibility of one-half statute mile for a Category 1 (CAT 1) ILS with an obstacle-free final segment, approach lights, and runway markings that identify key features such as the touchdown zone.

A practical, operational grasp of approach minimums doesn’t require a deep dive into publications such as FAA Order 8260.3F, commonly known as TERPS (Standard for Terminal Instrument Procedures), which includes the specifications (and complex trigonometric calculations) that procedure designers use when they create and update instrument procedures, such as departures, holding patterns, and approaches.

But a review of just one table in a reference for folks who build airports—AC 150/5300-13B Airport Design—can help you understand why approach minimums vary. And that background in turn can help you create a plan to complete a smooth landing when the runway environment doesn’t match the standards established for a generic ILS or RNAV (GPS) approach.

The key is Table K-1, Criteria to Support Instrument Flight Procedure Development. Reviewing Table K-1 is similar in practice to checking the inoperative components table in the Terminal Procedures Publication that shows, for example, how you must adjust published minimums if approach lights are out of service. But the data in Table K-1 can prompt you to take a closer look at the approach charts and the chart supplement information for an airport when you see minimums that are higher than normal or otherwise seem odd, especially when obstacles aren’t the obvious culprits.

For example, Table K-1 shows that a runway must be at least 4,200 feet long to support a visibility minimum of less than three-quarters of a mile. The lowest visibility minimum for an approach to a runway 3,200 to 4,199 feet long is three-quarters to one mile. Other features, such as approach and runway edge lights, visual glideslope indicators (VGSI)—a PAPI or VASI—and parallel taxiways also affect how low you can go and the minimum visibility required during an approach to a particular runway. The table also shows that it’s possible to create an approach to a grass or dirt runway.

Suppose your destination is Hobby Field (77S) in Oregon, about 15 nautical miles southeast of Eugene. Hobby has an RNAV (GPS) approach to Runway 16 with LPV and LNAV straight-in minimums. Of course, you can review the approach chart (previous page) to determine that LPV DA is 1,041 feet msl (500 feet agl) and that the visibility requirement is one and three-eighths mile.

A review of just one table...can help you understand why approach minimums vary.But Table K-1 provides more information about why those minimums were established. As noted earlier, because the runway at 77S is less than 3,200 feet long, the minimum visibility must be at least three-quarters mile. Table footnote 5 explains that “runways as short as 2,400 feet (732 m) could support an instrument approach provided the lowest [height above touchdown] is based on clearing any 200-foot (61 m) obstacle within the final approach segment.” Column 1 helps explain that the lack of approach lights at Hobby is another factor that pushes the visibility requirement beyond three-quarters mile.

The introduction to Table K-1 also lists several ways that airport owners and approach designers can make an approach work even when an approach path is not entirely clear of obstacles. For example, the TERPS gurus can establish higher landing minimums and/or steeper glidepath angles, require installation of VGSI, set nonstandard threshold crossing heights, or offset the final approach course.

Memorizing details in TERPS and the 150-series advisory circulars isn’t required to pass the IFR knowledge test, nor will a DPE frown if you can’t re-create Table K-1 from memory during a checkride. But especially as RNAV (GPS) approaches have proliferated at airports that previously may have had only a VOR approach, or no approaches at all, learning about some of the basic criteria that procedures designers work with can help you understand why a particular approach has unusual minimums. That knowledge can prompt you to plan an appropriate approach profile—perhaps a lower airspeed to a short runway—and help prepare you for what you will see when you emerge from the clouds. FT

Bruce Williams is a CFI. Find him at youtube.com/@BruceAirFlying and bruceair.wordpress.com