Love those long legs: why truck outbound for an hour on an RNAV approach just to reverse course?
The most common reader queries we get at IFR are the "Do I have to?" questions. Just like the battle-hardened freight dogs relish the minutes saved every night by tight vectors to hook localizer and glideslope simultaneously, experienced pilots find ways to safely shortcut procedures they fly often.
Take the RNAV (GPS) X Rwy 1 into Jackson, Wyo., (KJAC). A reader who flys this approach regularly asked if he really had to fly outbound for seven miles in the hold-in-lieu-of procedure turn at HOMVA. Coming from the north, direct HOMVA and cleared for the approach is common. He admitted--names withheld to protect the guilty--that he knows from experience four miles works fine to reverse course. But the chart clearly says to fly out for seven.
This approach provides an easy out if you read it closely: Ask for direct CILEG instead and fly the NoPT transition. That solves the bug simply, but long legs are standard fare at GPS approaches in the mountains. So is there a way to duck the requirement for taking the long way around the block just to turn around?
The FARs are a bit cryptic here. Part 91.181(b) simply says the course to be flown must be "along the direct course between the navigational aids or fixes defining that route." Procedure turns (PTs) kind-of fall into the regulatory abyss here. The AIM leaves no such ambiguity. AIM 5-3-7 (b) says, "... the pilot is expected to hold as depicted on the appropriate chart." And 5-3-7 (j) (5) goes further that for DME / GPS Along Track Distance (ATD) holds, "The end of the outbound leg is determined by the DME or ATD readout."
But the AIM isn't regulatory and perhaps there's some wiggle room that you're just entering the hold as a form of PT, rather than hanging out there. Who among us hasn't, uh, flown a hold entry not exactly as published?
We'll leave the moral quandary on flying full distance outbound up to you, but here's some fuel for the more important safety side of the equation. Two things are happening at this point on the approach. The aircraft is turning around, but at the same time, the GPS is ramping down in scale from one mile full-scale deflection to 0.3 miles. Remember that you can legally hit this hold at speeds up to 230 knots indicated. What early GPS approach designers found was a sharp pilot in a fast bird could turn around so fast in a timed or four-mile-leg hold that their onboard GPS units would go full-scale deflection, even though the pilot was correctly flying inbound, because they were changing scale so quickly.
The solution: Make the legs longer so the scale would change slower. Approach designers have a chart that matches altitudes with hold lengths. At 11,700 feet the hold should be six miles long. But wait: this chart says seven miles. It does and it's wrong. Expect a NOTAM to be issued soon. There's just one more reason not to trust blindly without knowing the why behind it.
With the reason for the six-mile legs, you can make your own call as to how far out to go for a PT when separation isn't an issue. Fly outbound for six miles if you want, but if I was paying for the gas, I'd slow down and turn early. I'd just keep an eye on that GPS to make sure my quick reversal didn't toast my own approach.
Jeff Van West is editor of IFR
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|Title Annotation:||APPROACH CLINIC; area navigation|
|Author:||van West, Jeff|
|Date:||Jan 1, 2008|
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