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Helpful Hints
AWOS Site Preparation: Helpful Hints
- When preparing the concrete foundations, be sure to crown the concrete
slightly so that rainwater doesn't collect and rest against the metal. This
is easy to overlook, but it could shorten the life of an AWOS. As noted
in the site preparation manual, the surfaces of the foundations must be
above grade (not flush) for the same reason.
- When installing the red obstruction light on the tower, mount it so that
the bottom of the fixture rests on the top 'rung' of the tower. If it's
higher, it'll block the wind, affecting the operation of the wind speed
and direction sensors.
- Run the rigid conduit from the obstruction light down the center of one
of the tower faces, not down a leg. If you run the conduit down one of the
tower legs, it will interfere with mounting of the AWOS equipment, which
mounts on the tower legs. Secure the conduit with galvanized conduit clamps,
U-bolts don't work out very well. Ideally, the conduit would be on the inside
of the tower as opposed to the outside face of it, but this isn't terribly
important.
- At the bottom of the tower, the obstruction light conduit can be bent
to go over to the circuit breaker panel, or it may be coupled to a length
of liquidtite. In either case, it should be secured to the concrete. Many
times I've seen the conduit several inches above the concrete, and this
is a tripping hazard. Some FAA
people will overlook this, others won't.
- The visibility mast has a coupling just above the concrete. This is provided
so that the mast can snap off if an airplane should go off the runway. The
thing is to make the coupling good and tight, or the sensor will be able
to rock back and forth. I've seen this many times, don't neglect it. Drill
and tap a hole for a 3/8-16 bolt just above the collar and bolt a ground
lug to the mast.
- The visibility and thunderstorm masts can be galvanized pipe, but unless
you're in a moist, corrosive area it's possible to save money by using rigid
conduit.
- The top end of the visibility mast should be about 7-1/2 to 8 feet above
the surrounding soil. This puts the heads of the installed sensor at the
'standard' 10 feet. If the mast is just a few inches higher than it should
be, it makes servicing much more difficult later on.
- Unless there's a good reason not to, the area around the AWOS sensors
should have a couple of inches of crushed rock. This helps immensely to
keep weeds under control, and in the winter it will be a LOT easier to access
and service the AWOS.
- For future calibration of the Wind Direction sensor, a benchmark should
be placed about 50 to 75 feet either North, South, West, or East of the
tower (based on TRUE North). Directional accuracy is important.
- The sensors and electronic equipment should be safely stored for installation
by an engineer sent by AWI. Please don't open the boxes before he arrives.
- Don't neglect to arrange for a telephone line to be installed at the CDP
location indoors. This should be operational before the AWI engineer arrives.
Also, make sure that the phone number for this line is available, we'll
need to know what it is.
- Some AWOS's will have an optional 2091 Remote Display System. It communicates
with the CDP using a cable that consists of a shielded, twisted pair. We
can supply this if you can't find any locally.
- The AC power at the sensor site should be able to remain between 104 and
126 Volts with load fluctuations up to 40 Amps. If it's a long cable run
from the source of AC power, you may have to use heavier cable or resort
to Buck-and-boost transformers.
- When erecting the tower, it's not important that it has one face in any
particular direction. Just don't erect it and leave it overnight unless
it has a working obstruction light.
- The electrical power conduits must have wiring from the circuit breaker
panel to the sensor locations, and the signal conduits must have pull ropes.
- When installing a lightning rod, use proper clamps. U-bolts aren't good
enough. Be sure to scrape the paint from the tower where it will contact
the lightning rod.
- The indoor equipment (CDP) will require a level space of at least 30"
X 60".
- The correct time to call for the AWI engineer to be on-site is when site
preparation is completed, and AC power and phone service are live. If you
call for our engineer to be onsite before the site is ready and he encounters
any delays, you may be billed for the additional costs. In the site preparation
manual, you'll find a page titled NOTICE TO PROCEED. Make a copy of it,
check all items that are actually completed, sign it, and fax it to our
customer service department (916-928-1165).
- The AWOS will have a VHF voice transmitter and may utilize UHF radios
for data communications between the CDP location and the sensor group. These
require antennas. You will be required to provide a cable path for the antenna
cables and install them, as well as antenna mount(s) and antennas. Please
call us for advice on locating the antennas.
- Site preparation should not begin until the FAA
form "Notice of proposed construction or alteration" has been
approved by the FAA. Ignoring
this could result in dramatically increased costs, as the FAA
could require that the entire sensor site be relocated, or refuse to commission
the system, making it nearly worthless.
- As noted in the site preparation manual, you will have to accurately determine
the elevation at the base of the tower. This is one of the factors
used in calculating the altimeter setting, which is the most critical information
given to the pilot by this system.
- The grounding is described in the site preparation manual. You should
consider this as a minimum, as any inadequacy of the grounding system would
reduce the performance, reliability, and lightning protection of the system.
Should it be impossible to drive a rod ten feet into the ground (11-1/2
feet actually, as the top of the rod should be 18" below grade), there
are alternatives such as burying a flat copper plate or a 4/0 copper cable
in a trench filled with Coke Breeze.
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