On Fri, 27 Feb 2004 07:29:56 GMT, Judah wrote:
Hi Ron,
I would love to hear more about that technique. Now that you mention it,
I could guess that it would resemble one of the techniques for flying a
DME arc - flying perpendicular to radials from the VORs on the
intersection, twisting the OBSes 10 degrees at a time until catching
either one of the two airways or the intersection itself. Or maybe just
dead reckoning it and waiting for the needles to swing, and following
whichever airway you hit first in...
Still, to my recollection, my instructor actually referred to some
regulation that prohibited it. IIRC, the topic came up in a conversation
about using an intersection as the first point in a flight plan...
Still, if I'm off-track on the technique, I would love to hear more.
Thanks!
Judah
I'd love to know what that regulation is. I've never heard of it but am
willing to learn. Perhaps you could ask your instructor? My surmise is
that it may be his own (mis)interpretation of the regulations.
In any event, rather than re-invent the wheel, I'll post copies of two
previous posts on this topic, in this newsgroup, and I'm sure you could
find more explanations by using Google. The key is visualization. Ideally
a slaved RMI would be the instrument to have, but you can do it with
VOR/DME and other combinations of instruments:
============================
Explanation 1 (with RMI):
It turns out to be quite straightforward if you have a slaved RMI. The
technique uses the visual picture of the instrument to "plot" the
solution to the triangle.
This is easily demonstrated with an RMI in front of you, but I'll try to
explain in words. First, lets explain how it works if the DME from the
present position happens to be the same as the DME to the intersection.
(This isn't how you really do it, but it makes the explanation easier).
Imagine that the centre of the RMI represents the VOR station. Imagine
that the airplane is attached to the tail of the RMI pointer. The tail
half of the RMI needle represents the track from present position to the
VOR. Now imagine a line from the center of the instrument out on the
desired fix radial. Because we assumed that the distance from us to the
station was the same as the distance from the station to the fix, we
can see that the fix is on the rim of the RMI. Hold a pencil up to the
RMI between the tail of the needle and where the imaginary line crosses
the rim of the instrument. That's the third side of the triangle, so
turn the airplane until the pencil is vertical (+/- drift). Now you
are on track to the intersection.
Of course in the real world you need to allow for different DME's, but
the principle is the same. First figure out which is the bigger DME
distance, the current indicated value or the DME at the desired fix.
Define the radius of the RMI instrument is that distance. Work out the
approximate proportion of the instrument face that corresponds to the
smaller of the two distances.
Now when we "plot" a position either for the airplane or the fix on
the face of the instrument, adjust the distance from the center of the
instrument that you hold the appropriate end of the pencil so that you
make the correct shape triangle.
(My apologies for this relatively poor explanation, but it's hard
without visual aids).
Once you are on-course for the intersection, refining your heading is
made easy by the fact that it's going to be straight up the face of
the instrument, so with a little practice you can correct by eye without
needing to hold a pencil up against the RMI.
And it is possible to achieve surprising accuracy with this method -
you can hit within 0.1 nm of the fix fairly easily. Good enough for
government work.
Andy Davis
=========================================
Explanation 2 (without RMI):
Sigh. Not really. There are a number of ways of navigating direct
to a fix, using an HSI/RMI and DME , a TACAN, or a VOR and DME. In
fact, any one who carries an E6-B in his shirt pocket could go
direct to a fix using the above reference navigation instruments.
The technique is found in the Air Force AFM 51-37, which is the old
manual. I understand that they have updated and changed the manual
number. I'm sure any AF pilot can give us the current manual
reference, and if anyone could rip off a new manual for me, I'd
appreciate it!
The following factors must be understood:
1. The station is located at the center of the compass card.
2. The compass rose simulates the radials around the station.
3. The aircraft position is visualized along the reciprocal
(radial) of the bearing pointer.
4. The position with the greater range is established at the outer
edge of the compass card.
5. The position with the lessor range is established at a point
proportional to the distance represented by the outer edge of the
card.
In other words, if the outer edge of the compass card were a
hundred miles, 50 miles would be half the distance to the outer
edge of the card, 25 miles would be 1/4 distance, etc., etc.
6. Determine the no wind heading direct to the fix by connecting
the aircraft position and the fix position with an imaginary line,
i.e.., connect the dots.
7. Establish another line in the same direction, parallel to the
original line through the center of the compass card.
8. Read the no wind heading to the fix under the lubber line.
9. Apply any known wind correction.
The distance to the desired fix can be estimated since the distance
between the aircraft position and the fix is proportional to the
distance established from the center to the outer edge of the
compass card.
The same problem can be more accurately solved using an E6-B. You
DO carry one right? Anyway, flip it over to the wind side, imagine
that the center grommet is the station, and plot out your aircraft
position and the fix position, draw or imagine a straight line
between the fixes, rotate the card so the line is parallel to the
direction your are fling, then read the no wind, direct heading
under the arrow.
To see if you have the concept, grab an E6-b and plot out this
problem.
1. You are on the 180 degree radial at 50 miles.
2. The airport/fix is on the 270 degree radial at 25 miles.
What is the no wind heading direct to the fix/airport?
Have fun, hope this technique helps and gives you something to do
on those long IFR/VFR cross countries.
Norm (Melick)
===================================
Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)
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