USMCFLYR

Just read this and thought it might make for interesting discussion (or worthless trivia)

Question:
Can a VOR based instrument approach procedure utilize a DME arc which tranverses an area in which the VOR radials are unusable?

(too draw a mental picture)
An approach using ABC VOR starts from the north and arcs east to the 090 radial; but the VOR is unusable between 030-060 degs within its' service volume. Is it a good approach.

USMCFLYR

Cubdriver

Since there is no penalty for guessing I am going to say yes it is valid, as long as the IAF & FAF are defined by the appropriate radials, and the DME works through the appropriate range of the arc which it would have to anyway being non-directional. In practice though I doubt I would fly the approach in IMC because of the loss of situational awareness you would suffer on the dead section.

EasternATC

I'll say no. DME is imprecise and useless without azimuth.

captain152

This could be argued both ways pretty easily. But I would say yes simply because you're not using the azimuth off the VOR while you're in the arc.. just the DME from it. As long as that is working when you intercept the azimuth inbound you'd be good.

That being said I would NOT do this in hard IMC!! lol

ERJF15

Good question. What if you're using the gps/fms to fly the approach? At my company, if you're flying a VOR approach, the PF willl use gps nav and the PM will use conventional nav.

So, I say yes.

brianb

Do you lose the "ident" portion of said VOR in the dead areas? If you do, I would say NO!!!!

USMCFLYR

I waited a little while to see if any would venture a guess. thanks to those wold gave it a shot. Overall I'd say good questions AND responses.

brianb - excellent question about the ID and something that is not addressed in the particular passage from the book. I thikn that in the context of this question we are only talking about an interference with the radials for whatever reason - not a loss of Ident - but it is a question that I will pose for clarification.

ERJF15 - this question is probably posed from the perspective that it is flight checked from "...a minimally qualified sole pilot flying an aircraft with basic IFR instrumentation in IMC using standard navigation charting."
If you are dual piloted and using a GPS as primary while backing it up with raw data on the other side - the intent is probably diferent.

EasternATC - DME is as precise as the approach designed is required to be when TERP'ed and honestly I'm not sure I watched the radials tick by the last time I was on an arc other than the..........................

...and this is where Cub and captain152 came to the closest "book" answer.
(tip of the hat to you)

If I come across any other trivia I think would be intersting in my reading I'll be sure to tease the mind some more
Thanks for playing!

USMCFLYR

USMCFLYR

Edit:

brianb - asked another and the answer was that if the DME ident was not available then the procedure would not pass inspection, but if only the VOR ident could not be received then the paragraph quoted above would apply and it could still be considered in tolerance.

As an aside - it is no longer in the book as such, but most still apply the 5 degs of either side of those radials listed above as areas that need to be within tolerance (so the IAF radial +/- 5 degress, the lead radial +/- 5 degrees, etc...)

USMCFLYR

KC10 FATboy

Stupid question. In your example, you have an obstacle south of the lead and inbound radial. So you are established on the arc. You enter the area of unusable service area for the VOR bearing. You truck along, then suddenly smack into the obstacle because while you were in the unusable service area, the VOR bearing failed, and you didn't pick up the lead or inbound radial and proceeded south of course.

Question, is it possible for the VOR bearing to fail and not the DME?

USMCFLYR

Part of that sounds like more of a TERPsters question specifically regarding the obstacle clearances, but as far as the last part of your question; YES. From my understanding it is possible for the VOR bearing to fail (or at least the IDENT portion) and not the DME.

USMCFLYR