No
3
Electrical
Power: AC and DC
Inertial Nav: Schuler Error
VOR: Bearing Information
Avionics Shops: Getting Started
GPS: Pseudolites
Electrical
Power
AC and DC.
Q.
What kind of power does an airplane use, AC
or DC?
A.
Light aircraft usually generate DC (direct
current) . Early airplanes produced 12
volts DC with a battery-generator combination. The
generator was replaced with the lighter, more
efficient alternator about 30 years ago but
voltage remained at 12 VDC.
During the 1970's,
manufacturers redesigned the electrical
systems of light aircraft,
raising that voltage to 28 VDC.
Higher voltage causes less electrical loss
(through heating in copper wires), allowing
lighter and thinner cables to run through the
aircraft.
12 and 28 VDC are not practical
for large transport aircraft, like the Boeing
727. Wiring
runs are so long that much power would be
lost in the copper cables. To solve this,
large aircraft not only generate much higher
voltages, but generate them as AC, alternating
current. The most widely used standard is
115 volts at 400 Hz. You will notice that
this is as high as the 115-volt current in your
home, but the frequency is 400 Hz, rather than 60
Hz. This high frequency is another way to
reduce the size and weight of aircraft
components. Another advantage of AC is that
alternating voltage is easily stepped up or down
through transformers.
Large aircraft, however, also need
28 VDC to recharge their storage batteries (used
for starting and emergency back-up power)
. This 28 VDC is obtained by
stepping down and rectifying the 115 volt output
of the aircraft alternators.
Thus,
to sum up the answer:
12 VDC - Old, light aircraft
28 VDC -
Recent light aircraft and medium-size aircraft
such as such as medium twins.
28 VDC and 115 VAC
@ 400 Hz - Large transport and heavy
aircraft.
Incidentally, you will
often see "12 VDC" written as
anything from "12 to 14 VDC",
and "28 VDC as anything from "24 to
28 VDC".
Inertial
Systems
Schuler Error
Q.
We know that the stable platform in a
conventional gyro suffers from Schuler errors.
Does a ring laser gyro in a strapdown system also
suffer from the same error? (From a
university in the
UK.)
A.
The basic principles affecting conventional
platform and strapdown systems are the
same.In the strapdown system, however, we use the
gyro platform calculation for the direction
of the cosine matrix between the body frame and
navigational frame. The matrix is actually the
analytical image of the gyroplatform. This
is the reason why the error of a strapdown
system has a similar Schuler oscillation as in a
platform system.
(Note: This question was answered by
Prof. Oleg Salychev, author of
"Inertial Systems in Navigation and
Geophysics." More information on the book is
available at:
Inertial
Systems in Navigation
VOR
Bearing Information
Q. How does a VOR
provide bearing information? (From a trainee pilot
in Mumbai, India.)
A.
A VOR station is like a lighthouse, sweeping
a narrow beam around in a
circle. Let's say you are flying east of the
VOR station. At some instant, the
beam will illuminate your airplane. Moments
later, as the beam
continues to rotate, it will light up an
airplane south of the station, then
west, and so on. This radio beam is known as
the "variable" signal.
To make sense of this information,
and determine which direction
the station lies, we need one more piece of data.
Where was the beam when it
began rotating? The is done with a second
signal.
Imagine that atop the same lighthouse we
place a large lamp that throws
light in every direction, illuminating all
airplanes on the horizon at once.
However, that lamp stays dark until an exact
moment occurs; when the
rotating beam passes exactly through magnetic
north. At that instant, the
lamp flashes briefly, telling every aircraft in
all directions to: "START
COUNTING!"
Now the VOR receiver on each airplane
has everything it needs. First, it
marks the time it received the magnetic north
signal. If it's the airplane to the east of
the
VOR, an instant later it receives the rotating
signal, the airplane south of
the VOR receives it the next instant, the airplane
in the west even later,
and so on. Therefore, simple arithmetic performed
in the receiver can
measure the time difference between the narrow
rotating signal and
the all-directional northerly beam (known as the
"reference" signal).
Because the variable beam is known
to rotate 30 times per second, the elapsed
time measured between the two signals can
be indicated as degrees of the compass.
Avionics
Shop
Getting Started
Q.
Where is the best place for information about
starting an avionics shop? (From the Caribbean.)
A.
My answer will be in terms of the U.S.
and FAA, but
virtually all should apply to your situation in
the Bahamas. To start an
avionics shop you need three basic building
blocks: training, test equipment
and maintenance manuals.
Training: The most
thorough approach is to attend one of the two-year
technical schools which teach avionics. (A list of
these schools is elsewhere on this website.)
Well-known schools include Embry-Riddle in
Florida, Parks College in Missouri, Spartan
in
Oklahoma and Pittsburgh Aero in Pittsburgh.
Avionics technicians are generally
classified in two categories; the
"bench" technician, who opens the
malfunctioning radio and troubleshoots
down to the printed-circuit board or component
level. He or she may also
be called upon to go out to the airplane and
troubleshoot problems in
connectors, wiring, antennas and other
systems. The bench technician must
have a deep knowledge of how circuits operate, how
to read a schematic and
how to use test equipment. The avionics
schools are the best way to acquire
the background for this level of work.
The second category is the
"installation technician".
He fabricates
sheet metal panels, mounts the supporting radio
trays, installs antennas,
runs cables through the airframe, attaches
connectors and does some limited
checkout with ramp (portable) test equipment. Some
of these skills are
acquired in school, but much must be learned
on-the-job on real airplanes
under the eye of an experienced technician.
As an independent shop owner, you'll
need to know both repair and
installation work.
The second building block is test
equipment. Even in the simplest radio
shop, you will need a variety of test
equipment---for ramp (at the
airplane) and bench servicing. The equipment
is specialized and expensive
and, at the very least, must enable you to
test basic navigation (VOR, ILS,
marker, ADF, DME, ) and communications (VHF
2-way aircraft radio, audio,
etc.), as well as the transponder and encoding
altimeter system. You'll
need a pitot-static test set to perform recurrent
instrument checks. Also
required are interface cables to connect any radio
you'll repair to your
test equipment. Plan on an investment that
could run from $50,000 to
$100,000 (US) to outfit a small shop with test
instruments. Some manuals are
available from manufacturers and some are on the
used market. Plan on
spending $5,000-$10,000 for a basic library for
troubleshooting.
For your shop to be certified by a
government authority, you will need to
demonstrate to the inspector that you have those
basics: training, test
equipment and manuals equal to the work you intend
to perform.
You ask about the business end
of owning a shop, insurance and hidden
pitfalls. First, business conditions have
never been better in the avionics
maintenance business, especially in the U.S.
because of a booming economy.
But that should eventually spread if only because
the number of airplanes in
the transport category will double in the
next ten years.
Avionics shop owners have been
complaining for years that they are
losing business to the avionics manufacturers (who
also offer service), but
we believe the outlook is very
optimistic. It's because the number of
avionics aboard even small aircraft is increasing
rapidly. We are
entering a new generation of "free
flight", datalink, databuses,
integrated displays and other advances, and the
demand for new avionics
should remain healthy.
You ask about insurance, and it is an
important concern. Few avionics
shops have been sued because their work
caused death and injury to airplane owners
or passengers. A greater hazard is having
expensive aircraft in your
hangar and dinging something while towing or
working on it. We know of one
experienced avionics technician who taxiied an
aircraft a few
feet----and sliced through the wing of another
airplane. Another aircraft, while being towed,
struck a wingtip on a hangar door and
incurred $7000 in damage (and it was only to a
navigation light). It's rare, but it happens.
Another pitfall for the new
shop owner is attracting customers. .
Many shops simply
hang out a sign and wait---and wait. The
successful ones have a marketing
plan---even as simple as sending postcards to
every airplane owner for
100
miles around. Pilots will fly long distances
to a good radio shop.
Clearly, the best way to start a shop
is first to become well-trained, then
go to work for an established avionics
facility. Devote several years
to learning this fascinating business---and you
will have all remaining answers you
need to start your own.
One of the best resources at the
outset is your local FAA or civil aviation
authority. If our experience is
any indication, these officials (known as
"avionics inspector" in the
States), will provide a wealth of friendly,
valuable advice on what you need to get started in
your area.
GPS
Pseudolite
Q. What is a pseudolite? (From
a school of civil aviation studies in London)
A.
The word means "pseudosatellite", or
"false" satellite. It is a small
transmitter installed on the ground at an
airport to improve the accuracy of GPS for
precision instrument landings. It can
provide several important functions. GPS
signals for civil use are no longer degraded but
they still cannot provide reliable guidance
for precision instrument landings. One method for
restoring accuracy is "differential
GPS," where a receiver in a known location on
the ground senses the error (the differential),
and transmits that up to the
airplane. The error is subtracted and
accuracy is improved. There are
several methods for transmitting
the correction, and the pseudolite is one.
Another important function of a
pseudolite is improving vertical guidance
(the "glideslope" component of the
Instrument Landing System). A landing
airplane can use differential GPS with great
accuracy to determine its course or track to
the landing point. This is because it will
often be picking up more than a half-dozen
satellites from space, which assures good
"geometry". As you might be aware, if
you are using several stations for navigation, the
best accuracy is when those stations are widely
separated in angle.
However, if the airplane is using GPS for vertical
guidance, GPS geometry is typically
poor---because it's difficult to orbit satellites
below the earth's surface! In other words,
the GPS constellation doesn't provide widely
spaced stations in the vertical plane. This is
where the pseudolite comes in. By planting
one on the airport surface, and having it
transmit GPS signals, researchers have found
a nearly three-fold improvement in accuracy.
There is yet another use. Flying a descent path is
the most critical use for GPS, so the engineers
have come up with a sophisticated solution known
as "kinematic carrier phase tracking".
It uses a small portion of the GPS signal, which
is very accurate, but subject to
"ambiguity" (reading the wrong part of
the signal). The pseudolite provides a signal (of
rapidly changing geometry) which solves that
problem. Although there is
experimentation with pseudolites, the final answer
to precision landings with GPS is not yet here.
