User Guide

STATION PASSAGE
As you pass the station (right overhead if you’re good or
lucky, to one side or the other if you’re like the rest of us), the nee-
dle will quiver a couple of times and the flag will change from TO,
through its striped “barber pole” or OFF indication, to FROM. If
your course takes you onward without a turn, you don’t have to
do anything else. If you’re changing course over the VOR, set the
OBS to the new radial (since you’re now heading away from the
VOR) and continue using the same heading correction technique.
Remember: with the needle centered and the TO flag in
view, the bearing to the station is at the top of the indicator and
the radial from the station is at the bottom. With the FROM flag in
view, the radial from the station is at the top and the bearing to the
station is at the bottom.
Each dot of deflection indicates a deviation of two degrees.
How much is that in the real world? It depends on how far you
are from the station - after all, the “spokes” are a lot closer togeth-
er near the “hub.” Remember your high school trig? The sine of
one degree is about 1/60 (actually, for those of you working for
extra credit, it’s 0.01745240643728), which gives us the useful
“one in sixty” rule: at 60 nm from the station, one degree equals
about one mile. Thus, if you’re 60 miles out and the needle is
deflected one dot, you’re about two miles off course; at 30 miles,
one dot equals one mile, etc.
The airways depicted on your navigation charts run from one
VOR to another. Typically, you’ll fly FROM the VOR behind you
until you’re about halfway to the next, then retune the nav receiv-
er and fly TO the one ahead. How far out can you receive them?
The FAA has “defined service volumes” for the three classes of
VOR (terminal area, low altitude, and high altitude), but a simple
rule of thumb, if there’s no intervening high terrain, is that every
thousand feet of altitude above the VOR should give you ten nau-
tical miles’ worth of good signal coverage (i.e., at 4000 feet above
the station you should have a good signal at least 40 miles out).
ILS
VOR is used both for enroute navigation and for so-called
“nonprecision” instrument approaches to smaller airports. At larg-
er airports, however, you’ll find a precision approach system
called Instrument Landing System, or ILS. What’s the difference
between precision and nonprecision? Not only is ILS significantly
more accurate than VOR, but in addition to providing left-right
guidance, it also provides vertical guidance along the final
approach glide path. As a result, ILS approaches can be made to
lower weather minimums than nonprecision types - as low as a
ceiling of only 200 feet and visibility of only half a mile, even for
lightplanes, and all the way to touchdown for the latest jets with
fully automatic landing systems.
While ILS uses the same indicator as
VOR, what goes on “behind the scenes”
is quite different (the nav receiver
switches modes automatically when an
ILS frequency is tuned). While a VOR
station provides radials in a full 360-
degree circle around it, the ILS provides
only a single course, aligned exactly with
the centerline of the runway on which it’s
installed. (During ILS use, the OBS knob and compass ring are not
functional; however, it’s a good idea to set in the inbound ILS
course just as a handy reminder.) While the VOR indicator’s full
deflection represents 10 degrees either side of the desired course,
the ILS’s horizontal guidance component, called the localizer, is
much more sensitive: it’s set between three and six degrees,
depending on the runway on which it’s installed, such that at the
runway threshold full deflection equals only 350 feet off the run-
way centerline.
The other major component of the ILS is the glideslope.
Essentially, it’s a localizer “turned on its side” to provide precise
vertical guidance down the glide path (set at 3 degrees above the
horizontal at most installations). It’s even more sensitive than the
localizer; at the runway threshold, full deflection indicates only
about 50 feet above or below the correct glide path.
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Flight Instruction
Flight Instruction