Localizer, LOC BC, LDA, and SDF
Localizer-only approaches, the back-course reversal trap, and the odd cousins -- on the real back-course chart into Santa Ana.
13 min read · Instrument approach systems
After this lesson you can
- Explain why a LOC-only approach is a non-precision approach even though it uses the same signal as an ILS.
- State the technique that avoids reverse sensing on a back-course approach, and explain why it happens.
- Distinguish an LDA and an SDF from a standard localizer by alignment and beam width.
- Read the front-course and back-course values on a real back-course chart and connect them to the runway each serves.
A localizer without its glideslope is still an excellent source of lateral guidance, and the FAA has built an entire family of non-precision approaches around reusing it: on the runway the ILS was built for when the glideslope is down, on the opposite runway using the back side of the same beam, and on runways where a full ILS was never installed at all. Two things are common to the whole family: none of them provide vertical guidance, and one of them can reverse the sense of your needle if you are not ready for it.
This lesson works through LOC-only approaches, the back course and its reverse-sensing trap, and the LDA and SDF variants, all illustrated on a real chart: LOC BC RWY 2L into John Wayne / Orange County (SNA), a back-course approach flown on the reverse side of a localizer whose front course serves the opposite runway.
The rest of this lesson
- 1.LOC only: the ILS final without the glideslope
- 2.The back course and reverse sensing
- 3.Reading it on the chart: LOC BC RWY 2L, Santa Ana
- 4.Two sets of minimums, one chart
- 5.LDA and SDF: the same idea, a different geometry
- 6.Flying any localizer-type approach
These 6 sections, their figures, the checks along the way and the end-of-lesson quiz come with the IFR track. Paid once, yours for good.
Key terms
Try to recall each definition before turning the card.