The DME has been acting up on today’s flight. Now it’s doing it again.
Annoying. But you won’t squawk this glitch to maintenance. The cause is sitting right next to you, in the person of your instrument flight instructor, who can make any and all equipment within and attached to your aircraft fail at the most awkward moments.
Sometimes it’s to see how long it takes you to recognize a problem; sometimes to probe your ability to make necessary adjustments, as on approaches. That’s what he has in mind today as you receive clearance to fly a full VOR-A approach to Purdue University Airport in Lafayette, Indiana.
It would have been a real workload-reducer to fly it with DME, ticking off fixes and sparing the need to time the leg from the final approach fix, BATLE, to the missed approach point.
Well, if he wants you to fly it the old-fashioned way, so be it.
Inspecting the plate again, you realize that there is more to this mock mechanical mayhem than just having to run the old stopwatch drill. On the approach planview is a note: “ADF or DME REQUIRED.”
Why? To identify the FAF. Although the Boiler VOR is an initial approach fix, a nuance of the IAP is that the FAF is seven nautical miles beyond, on the Boiler 143-degree radial. Without DME, the only way to pinpoint fixes along the approach course is to identify bearings referenced to the Earle LOM. BATLE is the intersection with the 239-degree magnetic bearing to that NDB. (Oddly, the old ADF in the aircraft never fails when the CFII is aboard, even though he is just about the only person who ever turns it on anymore.)
Been awhile? Then some review is in order; don't just give up and call the miss when the DME dies. Suppose you are flying the final approach course with a 10-degree correction for a southwest wind. What bearing-pointer indication will represent BATLE?
You could do the math on the fly, adding an 86-degree relative bearing to your wind-corrected heading of 153 degrees. Or you could use the work-saving method your CFII showed you long ago. Rotate the ADF card by hand so that your magnetic heading, 153 degrees, is at the 12-oclok position. When the ADF needle points right to 239 degrees, you’re at BATLE.
Then remember to start timing to MAFFY, the MAP.