Localizer Directional Aid / Simplified Directional Facility
Both are non-precision and uncommon approaches. You are more likely to find an LDA over an SDF. At the time of this writing, there are only two SDF approaches in the United States (if you’re wondering what information that is okay to let fall out of your brain, you can probably forget about the SDF approach and still survive flying IFR…..probably.)
The localizer type directional aid (LDA) is similar in accuracy to a localizer but is not part of a complete ILS. The LDA course width is between 3° and 6° and thus provides a more precise approach course than an SDF installation. Some LDAs are equipped with a GS. The LDA course is not aligned with the runway, but straight-in minimums may be published where the angle between the runway centerline and the LDA course does not exceed 30°. If this angle exceeds 30°, only circling minimums are published.
To sum it up…..it’s a localizer approach, that isn’t aligned with the runway very well. It’s not a precision approach, and if you encounter one, it’s due to terrain or airspace restrictions that won’t allow for a straight in approach.
The simplified directional facility (SDF) provides a final approach course similar to the ILS localizer. The SDF course may or may not be aligned with the runway and the course may be wider than a standard ILS localizer, resulting in less precision. Usable off-course indications are limited to 35° either side of the course centerline. Instrument indications in the area between 35° and 90° from the course centerline are not controlled and should be disregarded. The SDF must provide signals sufficient to allow satisfactory operation of a typical aircraft installation within a sector which extends from the center of the SDF antenna system to distances of 18 NM covering a sector 10° either side of centerline up to an angle 7° above the horizontal. The angle of convergence of the final approach course and the extended runway centerline must not exceed 30°. Pilots should note this angle since the approach course originates at the antenna site, and an approach continued beyond the runway threshold would lead the aircraft to the SDF offset position rather than along the runway centerline. The course width of the SDF signal emitted from the transmitter is fixed at either 6° or 12°, as necessary, to provide maximum flyability and optimum approach course quality.
NTSB Case Study
The pilot reported that he was conducting an approach in IMC. Upon reaching the decision altitude, the front seat passenger reported the runway was in front of the airplane. The pilot looked up from the instruments, but could not see the runway out the windscreen or the side window. Referring back to the flight instruments, the pilot noted that the airplane was level, but he was unable to determine the airplane’s altitude and did not know where the airplane was in relation to the runway. The pilot then applied full engine power and began to climb the airplane; however, the airplane’s right wing impacted a light pole. The airplane immediately pitched nose-down and descended into terrain. The pilot reported that the accident could have been prevented if he had immediately executed a missed approach at the decision altitude.
Probable: Cause: The pilot’s failure to execute the published missed approach procedure in a timely manner, which resulted in collision with a light pole and subsequent impact with terrain.
Pilot: 1,261 Total Time, 40hrs in the last 30 days, 871hrs make and model