I agree with your approach but not the math. I think you would need 8 degrees to parallel and 3 degrees to correct for a total of 11. In other words, if I am 8 degrees off, then I am 8 miles off course at 60 miles and 10 miles off course at 75 miles.
On the other hand, if I were 12.5 degrees off, it would be 12.5 miles at 60 and 15.625 at 75 miles.
It's a numerator - denominator issue.
Yup...you're right...I guess I'm one of the 7 out of 4 people who get numbers mixed up sometimes...
acaTerry said:
I'm more of a "get back on track" guy. Take 45 degrees and back on course. Why wait till the VOR? If you're 10 miles off now in just 75 miles you'll be waaaaaaaaaaaay off by the next 200 if your calculation isn't right (which is why you were off course in the first place).
Unfortunately this is a "DR" question, not a "VOR Tracking" question (or at least it has been every other time I've seen it)...You're off course because you don't have VOR radials to track, so you can't just "take 45 degrees" to intercept the course.
Think of it this way...You're flying along under IFR, in the clouds, fat, dumb, and happy. Suddenly you have an electrical failure, and there you sit, in the clouds, without electronic navigation or communication. Sure, you might have your portable GPS with you, but until such time as the FAA decides to REQUIRE portable backups for IFR, many of us are going to be flying without them.
You know your destination is VFR, so you continue on your cleared route by DR under 91.185.
Well, guess what...you didn't fill out a nav log prior to departing, so you really have no clue what the correct WCA is for your trip, so you take a WAG and fly. After about 75 miles, you're in the clear, at night, above a low undercast. You're over familiar territory, and can identify various towns by the pattern of the glow in the clouds below. You determine that your actual course would be "over there" about 10 miles, which means you miscalculated your WCA by 8 degrees. You correct that 8 degrees, but that would put you 10 miles from your destination, so you add another 3 to get you where you want to go.
A simple calculation (assuming you're not one of the 7, like me...but if it were for real, I'd whip out my trusty CR-5 to make the calculations
), and an hour and a half later you see your destination beyond the edge of the clouds, all lit up, right in front of you.
It's one of those techniques that really goes back to when DR was considered a "proper" form of navigation, but it still has some value IMO. Many pilots doubt the accuracy of DR, but when done with proper care, it's fairly accurate. If you can add one intermediate correction fix, you up the accuracy substantially...I've managed to have all my DR errors cancel each other out that way, so that I was less than a degree off course and well within sight of my destination upon arrival.
Fly safe!
David
