No
4
Subjects:
Databuses: Fiber
Optics
Interference: Alternator and DME
Instruments: Compass Problem in
Convair
Transponders: Mode C Code
Training: Instructor Qualifications
Lightning: Strikes on Aircraft
Databuses
Fiber Optics
Q. Are
any fiber optic databuses certified for aircraft?
A. Fiber optic
databuses for aircraft are still in an early
stage. The airlines. for example, are
defining FDDI---the Fiber Optic Digital Data
Interface. SAE has a committee to
standardize HSRB (High Speed Data Bus). The
military databus based on copper wire,
MIL-STD-1553, has already defined a fiber optic
equivalent known as MIL-STD-1773. Widespread use
of fiber optic buses, which is sure to happen,
must await several developments; completely
new aircraft designs and simpler methods for
installing fiber optic connectors in the field.
Interference
Alternator and ADF
Q. I have a King KN85 ADF which works very well
when the alternator in my Warrior is turned
off. The alternator brushes were cleaned,
diodes checked, shielding installed on the field
wiring, a new capacitor installed on the
alternator. There is a Hysonic filter on the
firewall. Several people have said it is not
worth trying, but IFR GPS is expensive.
A. Those are
certainly good steps to take. Be absolutely
sure, also, that the case of the alternator makes
good contact with ground through its mounting, and
there is no corrosion to prevent clean
metal-to-metal contact. The same applies to
your capacitor, filter and shielded wire.
Another item to check is the engine
cowllng. Unless it makes good contact with
ground (usually through the hold-downs) it
might even pick up noise and re-radiate it to the
ADF antenna. Clean up any grease or corrosion to
eliminate this possibility.
There
is a good possibility that much of the problem is
in the ADF antenna system. Unless it is working at
high efficiency, the receiver is very susceptible
to noise. An ADF loop antenna often lies at
the bottom of the belly where it collects
gunk and other foreign matter that corrode the
connector pins. This should be the first step in
troubleshooting: remove the connector and shine up
the pins. The best material for this
is "crocus cloth" which contains a
jeweler's rouge that removes rust without
scratching the metal. Also, the female part
of the connector should be sprayed with
contact cleaner. Examine the length of
antenna cable for kinks, cuts, breaks or
abrasions---which steal signal strength. Inspect
the cable where it enters the back of the radio,
too. A thorough clean-up or replacement of ADF
cable and connectors often restores
performance.
Compass
Convair
Compass Problem
Q. I have a problem with the No. 2 compass on
a Convair 580. Whenever I select No. 2 nav
on either RMI, I get a No. 2 compass flag. The
wires check OK. (From an FBO in Ft.
Lauderdale, FL.)
A. There are several possibilities. You
said you checked the wires and found no
problem. But did you also measure the
wires for shorts to ground? There is a
possibility that your continuity checks between
wires and connectors were OK, but did not account
for shorts to ground.
Does the airplane have two nav receivers which can
be swapped? If so, exchanging their
positions would show whether the trouble is
following one receiver. This could tell
whether the receiver is loading down the
system, causing the flag to drop into view.
You may have a problem at the junction box,
which brings together all the
associated wiring on terminal strips. It is
a common problem that a meter placed on these
terminals will indicate proper continuity, but the
terminal will not pass the signal. Check
these terminals in the junction box to be sure
they are clean and tight.
Transponders
Mode C
Code
Q. An FAA Airworthiness Directive says
"...any Mode C transponder with a single
Gilham code input." What is a "Gilham"
code?
A.
The Gilham code is the system to encode altitude
information from the aircraft's altitude sensing
devices (either a blind encoder or digitizing
altimeter). This is sent to the transponder, where
it is transmitted as a Mode C reply----thus
informing Air Traffic Control of the aircraft's
altitude. When an aircraft receives an
interrogation from ATC surveillance radar, it
replies with a train of 12 pulses, each a zero or
one. Controlled by the altitude encoder, this
creates a possible 4096 number combinations.
Actually, altitude is encoded only every 100
feet, which provides 1,290 different
altitudes between -1000 ft and 127,000 ft.
Since airplanes rarely fly
underground, why report -1000 feet?
The reason is that the encoder is adjusted
to pressure altitude (referenced to 29.92
inches of mercury) and not height above ground or
a local altimeter setting. This also
explains a widely held misconception by pilots
that when they adjust their altimeter, they are
also affecting the altitude reported to ATC.
The encoder is fixed on 29.92 inches (or 1013
millibars) regardless of the local baro pressure.
When the signal reaches ATC, it is corrected to
local pressure, thus eliminating a potential
source of pilot
error.
Incidentally, the FAA uses the
term "Gilham," but in the
avionics industry is it more frequently
called the "Gray" code.
Training
Instructor
Qualifications
Q.What
are the qualifications for teaching
avionics/electrical systems besides having the
curriculum approved by the FAA?
A. According
to the Federal Air Regulations (Part 147, Aviation
Maintenance Technician Schools) a school must
maintain a list of current instructors with their
ratings and certificates. The rules
are not more specific, except to say
instructors must hold ratings the FAA
Administrator determines are necessary to
provide adequate
instruction. Practically
speaking, these rules apply mainly to
instructors teaching airframe and powerplant
mechanics and these instructors are expected to
have A&P ratings.
However, the FAA makes an
exception when it comes to specialized subjects
such as electricity, physics and
other areas. Here the rules say that such
teachers do not need a mechanics license.
Although the regulations don't spell it out, we
can assume this includes the highly
specialized subject of avionics, which is
often taught at A&P schools.
Thus, the real-world answer to
your question is that a Part 147 school will
examine a prospective teacher's credentials
and decide on the individual's qualifications to
do the job. There is no other formal FAA
process.
Lightning
Strikes
on Aircraft
Q.
What protects an airplane from lightning?
A.
On the average,
an airliner is struck by lightning once a
year. However, there are almost no proven
cases where lightning has brought down an
airplane. There are several reasons.
One is that most aircraft are protected
by their aluminum skin which blocks the
lightning stroke. On newer airplanes, which
have large areas of plastic skin (composites), a
mesh of metal wires is embedded to provide similar
protection.
Another factor is that
lightning isn't really attracted to an
airplane but strikes it on the way to somewhere
else. Most lightning is caused by large
voltage differentials (plus and minus
charges) which exist from cloud-to-cloud
and from cloud to ground.
Lightning occurs when that voltage breaks
down (ionizes) the air between these
areas. An airplane in flight
develops only a minor charge and is
not very electrically attractive.
Airplanes do suffer lightning
damage, but they are rarely fatal. After a
strike, small burn pits or tiny holes are
often observed in the skin of the airframe.
Compasses and other instruments are
sometimes affected and need repair.
One item that does need
lightning protection is the radome, or
covering, for the weather radar in the nose
of the airplane. Because it is made of
plastic, it is fitted with "diverter
strips"---metal runners which fan out from
the nose . This helps lead the electrical charge
away from the radome to the metal airframe.
In recent years, as airplanes
acquired digital circuits, they must meet much
higher standards of protection against
outside electrical charges. This is
because digital signals are much lower in level
than the old analog ones and are more easily
upset. Thus, new aircraft use more shielding,
bonding, filtering and other measures to reduce
the effects of "EMI":---electromagnetic
interference. Lightning fits in that
category.
A most awesome video was made
when an F-4 Phantom fighter was sent directly into
a violent thunderstorm on a research flight,
with an on-board camera aimed at the
tail. Lightning could be seen striking the
tail again and again. But the airplane
landed safely with no more than the minor effects
described above.
