"Snowbird" wrote in message
om...
"Dudley Henriques" wrote in message
hlink.net...
you are absolutely right about one VOR radial ident NOT being a
positive"position" fix. True, having a directional bearing from a single
VOR
is better than having nothing at all, but even if only a single VOR is
involved, it is ALWAYS recommended that a SECOND VOR be chosen, and a
cross
check made before considering what you have as a "fix" When discussing a
position FIX, TWO is the magic number, NOT one! The second of the two
can be
DME, an ADF bearing, or a second VOR positive radial ident, but when
discussing a position fix, ESPECIALLY on a student newsgroup, using a
single
VOR radial as an example of a geographical fix is wrong in my opinion,
and I
would urge all students NOT to begin thinking of a positive geographical
fix
on these terms.....PERIOD!!!!
Dudley, I know you're aware of this, but since this is a student
newsgroup I think it should be mentioned that it *is* possible to
obtain a reasonable position estimate using a single VOR.
These days, I suppose a pilot is more likely to be in a cockpit
with 2 GPS and no VOR than to be in a cockpit with no GPS and 1 VOR,
but let's suppose that's the situation. Only 1 VOR station can
be received, for some reason.
Of course you're correct that a single VOR simply places you on a line.
So to determine position, we need additional information.
We all know (or can see, if we think about it) that the distance between
VOR radials varies with distance from the station by a function of 1 in
60. At 60 nm from the station, it's roughly 1 nm between radials,
30 nm it's roughly 0.5 nm between radials. IOW, we use the distance
between radials to estimate our distance from the station.
How to do so practically? We use time and our knowledge of our plane's
approximate groundspeed. One centers a radial, then turns the OBS to
deflect the needle 10 degrees, notes the time, and flies until the
needle centers. Note the time again.
Let's say one is flying a typical GA plane with a groundspeed of
about 2 nm/min. Let's say it took 3 minutes to fly 10 degrees.
We calculate our distance as 60 * (2 nm/min * 3 min)/10
or 36 nm from the VOR.
To boil it down:
*know your airplane's approx. speed in nm/min
*calculate 6 * speed in nm/min * time in min to fly 10 deg
Of course, for accuracy, one would need to correct for wind
either by estimating groundspeed, or by flying in both directions
and averaging the time.
I mention it because I was taught this technique as a student, but
many don't seem to be. As I said, Dudley, I'm sure you're aware
of it, and I don't mean to contradict what I see as your basic point
that a single VOR radial by itself does not, in fact, give position
but rather only 1/2 of the information needed to derive position.
The practical aspect of the above method is that awareness of
the distance between radials as a function of distance from the
station helps smooth VOR intercepts, something I need to remind
myself of as I use GPS far more often than VOR these days.
Best,
Sydney
What you are saying here is completely accurate. However, if you read back
through Duniho's posts on this thread, there is not ONE instance where he
even comes close to mentioning what you and I, and I assume Duniho knows as
well, is the correct procedure for establishing a reasonable distance check
from a single VOR using the procedure you have outlined. His entire context
deals directly with the premise that a single VOR bearing used alone is
valuable POSITIONAL information. This is not true at all. He does mention
that additional information is "helpful", but he is STRESSING the point that
a single bearing alone is valuable positional information.
The reason I entered a reply was because he was "correcting" someone who had
correctly stated that a single bearing from a VOR was only that....a single
bearing, and NOT a position fix, which is absolutely correct. In fact,
Duniho's opening statement was
"How is this not a position fix?"
As I said, this is a student group. Pilots who post here should be keenly
aware of that. In aviation training, we work fairly hard to get student
pilots to understand that TWO sources are required for position fixes, not
ONE! That second source can indeed be a second VOR bearing, an ADF bearing,
a DME readout, or indeed, a distance check on a single radial as you have
mentioned. If Duniho or anyone else for that matter wants to post here that
a single VOR bearing is valuable positional information, they should include
the need for that second source as PARAMOUNT to obtaining a position fix,
and not simply mention a second source as being desirable, but not
necessary, as did Duniho in his explanations on this thread.
That's my opinion, and as a professional flight instructor, I'll go with
that.
Dudley Henriques
International Fighter Pilots Fellowship
Commercial Pilot/CFI
Retired
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