AOPA Pilot Magazine - April 2015 - 80

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ON INSTRUMENTS

ILS basics
Riding the pipe to 200 and a half
BY THOMAS A. HORNE

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* Learn all pvt & comm maneuvers
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* Avoid the accidental stall or spin
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* Never fear a crosswind again
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* And much more!
508 full color pages
1,250 pictures and illustrations
Book - $59.95/eBook - $44.95
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TO MANY INSTRUMENT- rated pilots,
the Instrument Landing System (ILS)
approach rates as the gold standard. ILS
approaches have been reliably providing precision guidance to hundreds of
runways around the world for the past
70-odd years, so when the weather turns
seriously rotten it's the approach that
pilots are most apt to request. That's reason enough for instrument-rated pilots
to stay proficient flying ILSs, and for the
noninstrument-rated to gain a working
knowledge of the basics-just in case.
The big value of the most common
Category I ILS approach is its ability to
provide both lateral and vertical guidance to runways experiencing weather as
low as 200-foot ceilings and/or one-halfmile visibilities. (Actually, the visibility
is measured by runway visual range-
RVR-transmissometers, which measure
visibility along a portion of the runway's
length.) However, those minimums are
generalities. Nearby terrain or obstacles
can push minimums higher, for example.
GUIDANCE AND COVERAGE

The ILS uses paired frequencies to
provide an exact approach path for alignment with the runway centerline, as well
as precise descent guidance along that
path. The localizer part of the ILS signal gives left-right steering commands
so you can stay on the signal's centerline;
the glideslope signal gives similar up and
down commands. The pilot-and/or the
autopilot-responds to these commands
by observing course deviation indicator
(CDI) deflections and making small heading and pitch corrections so as to keep
both localizer and glide slope needles
centered. Fly the centered needles "down
the pipe," and you'll find the runway dead

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80 | AOPA PILOT April 2015

ahead, assuming a no-wind condition.
Large crab angles of the sort needed to
compensate for healthy crosswinds may
require that you look to one side or the
other to spot the runway. That's well
worth remembering on windy nights, or
whenever visibilities are restricted by
fog, rain, or other restrictions.
Localizer and glideslope signals have
limited ranges. At most, reliable signals
extend as far as 18 nautical miles or so,
but that's only for localizer guidance
within 10 degrees of the course centerline. Closer to the runway-out to about
10 nautical miles from the runway-signals are good within 35 degrees of the
course centerline. What does this mean?
It means that if you are disoriented and
outside these coverage areas, you won't
have reliable ILS signals. Or, in the worst
case, you may be tricked by false signal
"lobes" that mimic inbound-path CDI
indications.
INITIAL PROCEDURES

Like all IFR procedures, the key to shooting a good ILS begins with preparation,
anticipation, and staying ahead of the
airplane. Once you've received your
approach clearance, tune in the ILS frequency, identify it, check that the marker
beacon lights are working, and get ready
to intercept the localizer. Often, in areas
where there's adequate radar coverage,
the usual drill is for ATC to vector you to
the localizer, at a point somewhere outside the final approach fix. (In nonradar
and/or nontowered environments, "full"
procedures can be the rule, leaving you to
navigate to the ILS' via feeder and initial
fixes on your own.)
Take a good look at the approach
chart, noting everything-but paying
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AOPA Pilot Magazine - April 2015

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