User Guide

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The DME also displays both groundspeed, in knots, and
time, in minutes, until you’ll pass over the station. Bear in mind,
however, that these figures are only accurate if you’re heading
directly toward or away from the station, as you would be when
flying on an airway. If you’re flying some random course, the
groundspeed and time-to-station displays will be inaccurate. In
the extreme case, if the station is directly off a wingtip, ground-
speed would be zero and time to the station infinite, regardless of
how fast you’re actually flying.
WEATHER RADAR
I’ve often overheard
passengers, as they board
an airplane and see the
radar screen on the panel,
saying, “Oh, we’ve got
weather radar, so we can
fly through thunder-
storms.” Nothing could be
further from the truth: the
whole reason for weather
radar is not to fly through thunderstorms or other severe weather.
In operation, a modern weather radar is very simple. Our
airplane is depicted at the bottom of the screen; the radar scans a
pie-shaped slice of sky, with its outer edge at the range selected
by the pilot. Intermediate range rings and azimuth marks on the
screen help you “eyeball” the position of storms and figure out
how to fly between or around - not through! - them.
All the radar can see is water, in the form of raindrops.
It cannot see clouds as such, and its performance spotting frozen
water (snow or hail) is very poor. Depending on the density of rain
that it sees, it depicts, or “paints,” weather cells in green, yellow,
or red. The assumption, generally a good one, is “the heavier the
rain, the rougher the ride.”
DISTANCE MEASURING EQUIPMENT (DME)
Although it’s grad-
ually being eclipsed (like
most other ground-based
nav aids) by GPS, DME
remains a vital part of the
navigational picture.
Developed in the 1960s
from a military system (still in use) called TACAN, DME provides
the “missing piece” of navigation information not supplied by VOR
or ADF: distance from the station.
It does this by emitting a pulse of radio energy. The DME
ground station receives this pulse and replies to it. By timing how
long it takes to get an answer and calculating in the speed of light
(186,300 miles per second - “it’s not just a good idea, it’s the law!”),
the system determines the range to the station and displays it in
nautical miles and tenths. Almost all DME stations are co-located
with VORs, so by tuning in a single station you can fix your posi-
tion. (Otherwise, you’d have to tune in two different VORs and plot
where the radials crossed.) In fact, the DME has no separate tuning
controls; there’s a pre-programmed relationship between VOR and
DME frequencies, so if you tune your VOR to a given station, the
DME will automatically tune to it as well.
The small knob in the DME indicator selects which of the
two VOR receivers will command its tuning. A center “hold” posi-
tion locks the DME onto its current frequency. This can be very
handy if, for example, you’re shooting an ILS (“they shoot ILSs,
don’t they?”) to an airport that also has a VOR located on the field:
first, tune in the VOR so the DME locks in on its signal. Now, put
the DME in “HOLD” mode; then tune the VOR to the appropriate
ILS frequency. You now have left-right and up-down ILS data dis-
played on your nav indicator or HSI, while the DME reads dis-
tance to the airport. (At some larger airports, the ILS has its own
DME facility, making the hold procedure unnecessary.)
A couple of cautions: since the DME reads actual distance to
the station, what it displays is slant range. Unless you’re flying at
recklessly low altitudes, it’ll never read zero, even if you pass right
over the station; it’ll show your altitude, in nautical miles (1
nm=6078 feet). If you’re close to the station, but at high altitude,
“your mileage may vary.”
Flight Instruction
Flight Instruction