Tuesday, August 9, 2011
Making Sense Of The Back-Course Approach
They’re the words every instrument pilot dreads: “Cleared for the back-course approach”
In addition to higher minimums, back- course approaches come with a few other built-in disadvantages besides reverse sensing. Perhaps because the back course is rarely used, airports are reluctant to spend money installing markers or compass locators. Accordingly, you can expect most localizer back-course final approach fixes to utilize intersections with a nearby VOR rather than NDBs or outer markers.
Timing an approach from an intersection can be less precise than from a beacon, depending upon the accuracy of your number-two nav. For that reason, it's perhaps more important to descend to the MDA early and be set up for a miss if the airport doesn't appear in the distance, rather than make a gradual descent to arrive at the missed approach point exactly when the time expires. Also, keep in mind that needle swings with reverse sensing are liable to become very squirrelly as you approach the airport and localizer tolerances become finer.
Standard advice for instrument flight goes double during back-course approaches. Just as with a normal front-course localizer, the trick when off-course is to make a heading change, HOLD IT and watch for a needle reaction before re-correcting; then, correct again if necessary. Don't chase the needle with continuous corrections, or you may wind up prescribing a series of S-turns down final. By reason of the inherent confusion caused by reverse sensing, a back course can get away from you more quickly than you'd believe possible.
To minimize confusion, some instructors suggest making fewer corrections, and then only after major needle deviations. Another trick is to join the BC localizer as far outside the FAF as possible (if you have the option) to help accustom your brain to reverse sensing.
Flying a back-course approach only serves to reinforce the suggestion that pilots should fly specific headings during instrument flight rather than correct "a little to the right (left)." Rather than choose to correct five degrees left, pick a specific heading that's five degrees from your current direction, and think that number.
There's such a thing as a precision back-course approach in the form of a back-course ILS. You may even run across a BC ILS with accompanying DME, or you can sometimes create your own on airports with collocated VORTACs. (Dial up the VOR until distance information appears, select DME, hold and switch to the ILS frequency.)
Back-course approaches typically carry minimums near or right at those of a front course, but a smart pilot will add a fudge factor of at least 100 feet. Trying to fly a glideslope needle normally and a localizer backward is a trick that deserves practice.
Sadly, true practice may be difficult or impossible in the real world. There's frequently no efficient method of practicing back-course approaches, at least not in an actual airplane. As mentioned above, there may not even be a BC approach within several hundred miles of your location, and if there is, ATC may be reluctant to allow you to fly it, as by definition, it will be directly into the flow of traffic.
The only logical alternative is a simulator, and that's a viable method of familiarizing yourself with a back-course approach, certainly one of the most unusual and least popular of IFR- approach procedures.
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