Fix to fix navigation problem
 

I'm curious to know if there is, and what is, the method to solve fix to fix navigation problems. I found something in Thai but couldn't translate it. Basically an airplane is on a certain radial at a certain distance and needs to figure out what direct heading will take the airplane to a different radial of the same VOR at a different distance. Appreciate any useful replies. Is there a way to do this on paper without looking at a HSI?

Using the math method, I used the pythagorean theorem to find the length in NM of the direct route, then Cosine to find one of the unknown angles. This angle was used to find the 3rd angle (180 degrees in total) and this added or subtracted the recripocal of the radial to find the direct heading. If someone has a shortcut I'd appreciate it, but the above was the only method I could come up with.

Personally, I tend to favor the "direct to" button on my FMS. If that's unavailable, then I figure there's a PTT button on the yoke with which I can request radar vectors.

The only way I've ever done it is looking at an HSI while flying in an airplane. Isn't that the best way to do it since you need to do it while you're actually flying? You can practice on paper to get comfortable with the method, but how often are you going to know the exact fixes you'll be using when you're not in the airplane? When I was going through UPT in the USAF, our instructors would just give us a fix to fly to and we had to figure it out on the spot while flying the jet.

This method is no-wind and the heading you determine must be adjusted for any known winds. In basic terms, you use your HSI as a god's eye view of the situation and picture each fix on the HSI using a relative scale based on their DME. Then you turn to line up the fixes vertically so your heading is toward the destination fix. I'll assume you have some kind of bearing pointer available on your HSI or RMI to locate the radial you're currently on.

Example: You're on the 120/30 and you want to go to the 030/60 and you're currently heading West. You take the fix with the biggest DME and locate that on the edge of the HSI on the appropriate radial. So, looking at your HSI, the fix you want to go to (030/60) would be around the 4 o'clock position on the HSI at the outer edge. Using that scale, your current position will be on the tail of the bearing pointer half-way between the outer edge and the center since the DME is half of the larger. So, you're halfway from the middle on the 7 o'clock line. Once you have the two fixes located on the HSI, draw an imaginary line between them or even hold a pencil up to line them up. Now move the pencil (without changing its orientation) to the middle of the HSI and read the required heading based on the direction you need to turn (in this case a right turn). My guess would be the heading would be in the ballpark of North, no wind (I'm laying in bed and don't feel like drawing it out).

It's good to update as you fly toward the fix in case your first heading was a little off or winds are a factor. It's best to make your updates at whole number DMEs that are multiples of the larger DME (in this case, 10, 15, 20, etc) that make the math to set the relative scale easier.

Use the windface of your whiz wheel. Use the center as the VOR, mark the FROM and TO waypoints on the appropriate radials, using a distance scale that will accomodate both points but maximize the detail/accuracy. Spin the wheel to place the TO point directly above (vertically) the FROM point, read the no-wind heading under the top index.






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The USAF method as described by @Adlerdriver is the only one I know of. It requires a little practice, but is actually very effective.

Unfortunately, since the FAA no longer does radial/DME fix-to-fix, neither does the USAF, and this technique is no longer taught.

I am all ears to anyone who can tell me where in the real world this example has occurred. I have flown all over South, Central, and North America, radar and non radar, and never been seen it. Not saying it doesn't happen, just saying that I have flown all over South, Central, and North America, radar and non radar, and never been seen it.

Actually, used it a lot in the USAF, never in civilian flying.

They don't even teach it in UPT anymore? What's Check Section got to say about that? No more students going crazy trying to perfect fix-to-fix NAV problems in a 38.

GF

I HATED PTP problems!
Thought they was going to down me a few times :(
Much easier in my fleet aircraft with a planview HSI with selectable ranges and the ability to display courselines.

Never been seen it? :D

I've only seen it once since the Air Force. It was probably 7 or 8 years ago, in the B-727, heading somewhere in the Northeast, maybe in the neighborhood of Pennsylvania. We were cleared to a radial/DME fix off of a VOR, the FO was confused by the clearance and wasn't sure if we could comply, I assured him we could, we hit the fix within 0.2 NM, and then held.

I had a T-37 student who struggled with Fix-to-Fixes, just had a hard time conceptualizing the process. He managed to consistently apply a method to accomplish the task, but probably never understood it. On his Instrument Check ride, he successfully accomplished the task early in the profile (when the task was assigned by the Check Pilot), then was unexpectedly assigned another fix-to-fix by ATC near the end of the profile. The grading philosophy was "first or worst", so if he aced the first and blew the second, he'd still bust the checkride. He tried to persuade ATC to send him direct to the VOR instead of a radial/DME fix, but ATC would not relent. Finally he asked for a vector. ATC obliged. He passed the checkride.

That's the "real world" solution these days. If your airplane isn't RNAV equipped, it's still vector equipped. ;)






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