The SDF instrument approach is the rarest approach in the USA, with only one example remaining in Morristown, TN. Do you know how to fly it?

An SDF Is Similar To A Localizer

A Simplified Directional Facility Approach (SDF) provides a final approach course similar to a localizer, but with a few distinct differences.

First, an SDF approach is never paired with a glideslope, so it's always a non-precision approach. It's not always perfectly aligned with a runway either, though it's generally within 3 degrees of the runway. However, you will most likely find straight-in minimums published (remember, straight-in minimums can be published when the final approach course is within 30 degrees of the runway).

The SDF is an alternative to a LOC that may be installed at an airport for a variety of reasons, but primarily due to terrain. One of the biggest differences is that, as compared to the 3-6 degree course width of a LOC, the SDF has a course width of 6 degrees or 12 degrees. Usable off-course indications are limited to 35 degrees on either side of the course centerline.

The final approach provides a minimum of 250 feet of obstacle clearance for straight-in approaches within the final approach area. This area is defined by a 6-degree course: 1,000 feet at or abeam the runway threshold expanding to 10 NM from the threshold. If you're flying along a 12 degree SDF course, the same final approach area is significantly larger.

SDF approaches are designed with a maximum descent gradient of 400 feet per NM (roughly a 4-degree descent angle), unless circling-only minimums are authorized.

The Last Of Its Kind: SDF RWY 05 (KMOR)

In 2018, the SDF to RWY 34 at the Marshfield Municipal Airport, Marshfield, WI (KMFI) was decommissioned. This leaves just ONE SDF approach left in the USA, the SDF to Runway 05 at the Moore-Murrell Airport (KMOR), Morristown, Tennessee.

Ok, so you probably won't ever fly this approach. But just for fun, let's take a look at the approach plate. The MOR SDF can be tuned using frequency 109.5, just like you would for any localizer. The straight-in minimums to Runway 05 are 1,860' MSL with 3/4 mile visibility (CAT A/B aircraft). There are also slightly higher circling minimums for the other runway. In multiple places on the chart, you'll see a note that an "ADF is required." This is so you can locate and recognize the IAF, FAF, and MAP.

However, if you don't have an ADF, you can use a suitable RNAV system. It must be an IFR Approved, GPS-based RNAV and can be used to track a VOR or NDB, measure the distance from a VOR, NDB or named fix, and hold at a VOR, NDB or named fix. You cannot use RNAV to substitute for a localizer or SDF. So as long as you have an approved RNAV source, you can still fly this approach without ADF equipment.

Take this quiz: Can You Fly The SDF to Runway 02 at KISW? (Now Decommissioned)

How To Fly An SDF Approach

The approach techniques and procedures for flying an SDF approach mirror those for flying a LOC approach. The course may be wider, resulting in less precision. That means you could be more than 2 times further off-course with a 1-dot deflection on an SDF than while flying a LOC.

Since the SDF may not be perfectly aligned with a runway, you'll need to thoroughly brief where you expect the runway to be when you pop out of the clouds. Have a firm plan for what maneuvers you'll need to execute for a safe, stable descent to the runway.

What's the most interesting or unique instrument approach you've ever flown? Tell us in the comments below.