14.38 When using an approach plate, how do you fly an "arc D15"?

The first place you'll go on a DME arc is to the IAF (Initial Approach Fix).
This will be labled (IAF) most likely next to a certain radial of the VOR
beacon you're using for the arc. In a small prop a/c, anticipate the turn to
the arc by .5nm. Heavy jets (an other props), will need more room for turning
but I've not seen an arc for an airport that could be landed on by a jet
anyway! If your on the 74 radial for a 10nm DME arc, fly inobund heading
(recip) 254 until you reach 10.5 DME of the arc VOR. Then you'd turn right 80
deg. to a heading of 334. After the turn, you should be on a tangent to the arc
at a distance of about 10nm. rotate your OBS 10 deg. opposite the direction of
to the first 90 deg. turn (in this case, 64 deg.)

Note that the CDI needle will start on the same side as the station and move
to the opposite side as you pass the radial. In this case, left to right as you
pass radial 64. When the CDI needle centers, turn left 10 deg. to a heading of
324 deg. Reset the OBS to read (-10) 54 deg. With no wind, the DME should read
10nm after each turn.

If your DME is increasing, you are being blown away from the station. To
counteract this, turn sooner or add a few degrees to your 10 to correct the
wind. If the DME is decreasing, the wind is blowing you towars the station.
Change your OBS for 20 deg. but turn after the normal 10 deg. This makes your
tangent longer.

Lead the inbound course (final course to the runway) by 5 deg. For example,
if the inbound course is 146, set the OBS to 151 after passing the 334 radial
to lead the inobund course. Turn inbound when the needle centers. After the
turn, set OBS to 146 and track that course inobund.

BTW, I am using VOR/DME RWY 15 approach plate for Johnstown-Cambria County,
Johnstown, Pennsyvania for the examples. I'm sure yours will differ!.

Noel Benford

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Rather than a true arc, it is flown as a series of small straight lines, each
with a slight heading change to stay near the arc, usually within a couple of
10ths of a mile. Since you are looking at charts, you know where you are
entering and what the approximate initial heading should be. You just have to
practice until you can fly a bit (you're now .1 or .2 wide of the arc), change
heading 4 or 5 degrees (or a touch more) toward the inside of the arc,
hopefully enough to get to .1 to .2 inside the arc, then you drift back out to
another .1 to .2 outside, etc. until intercepting the desired course.

Larry Nussbaumer

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A geometrical solution could be this: flight straight for D/10, where D is the
required distance from the radioaid, then turn 5.3 degrees toward the inside.
This way you fly APPROXIMATELY a polygon with about 70 sides, close enough to a
circle. Check the DME reading all the time, because it is only an approximated
formula. If you start a side at the right distance and perpendicular to the
turn center, and you fly straight, you should never be more than 1% far (on the
external side) from the desired arc. Pay attention that D/10 is not measured by
the DME: you have to use time and speed!

You could also use a VOR to check your heading. YOu must fly all the time 90
degrees with respect to the radial you are on, but this requires to move
continuously the OBS.

If you would fly a true arc, you should bank the aircraft (in a coordinated
turn) by an angle B=0.00146*V^2/D, where B is the bank angle in degrees, V^2 is
the square of the speed in knots, and D is the required distance in nautical
miles. You can see that the required bank is so small 1 degree, at 100 kt, for
15 nm, that it is impossible to obtain and maintain. So the only solution is a
gentle slalom.

Two word of cautioning: this is true if there is no wind, and more important
don't rely too much on my answers, because now I am in Europe, I forgot my
calculator in U.S. and I am using a slide rule! When I come back I can check if
you want.

Franco

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Nice description, Franco. When figuring bank angles against airspeed in FS it
can be aircraft dependent, unlike real life, as poorly designed aircraft in FS
often don't match up well with each other in turn rate at the same speed.

Larry Nussbaumer