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altitude and looked to be superimposed on our aircraft.
We got an immediate Traffic Alert followed by a command
to climb at a rate in excess of 2,500 fpm. I disconnected
the autopilot and followed the TCAS resolution. The
target continued to show level at our altitude even as we
climbed to 6,400 feet. We advised Approach Control that
we were responding to a Resolution Advisory (RA). We
were recleared to descend to 4,000 feet and given vectors
to continue the approach. After we were established on
the Glide Slope we got another similar warning with an
aircraft superimposed on our position and again at the
same altitude. This time the command was to descend,
but before I could take any action, the target and warning
disappeared. The flight continued without any further
incidents. We wrote up the TCAS system for maintenance
on arrival.
An aircraft is flying along and there are
no nearby targets on its TCAS display.
Suddenly, the TCAS is issuing rapid-fire
commands, and a red target appears
in the center of the TCAS display. The
red target is only about 0.1 mile away
and co-altitude, or nearly so. After some maneuvering, the
pilots begin to question what they see, since ATC is telling
them there are no aircraft nearby. What is going on?
Any time a flight crew’s TCAS displays a false target right
on top of their own aircraft symbol with an altitude close
to their own ship, the problem usually can be traced to a
failure of the mutual suppression bus.
The suppression bus is a signal line on the airplane
that allows the transponder, TCAS, and navigation
systems to coordinate with each other. When one system
is transmitting, it puts 28 volts DC on the bus to tell
the other equipment not to listen or transmit. If the
transponder and TCAS systems on the same airplane
do not have a suppression bus connection, the aircraft’s
transponder will reply to the TCAS interrogation. TCAS
will see this as a very close transponder, start tracking it,
and immediately will issue a Resolution Advisory (RA).
The suppression bus failure that causes this problem is
usually due to an intermittent connection. The TCAS
tracks all the transponder-equipped aircraft it can “see.”
The TCAS works like a secondary surveillance radar,
scanning 360 degrees in one second, but it does not have
a moving antenna. To establish a track on a target, the
TCAS must see four good replies from a transponder.
On the fifth valid reply, the TCAS will begin to display
the target on the screen. If the target meets the advisory
criteria, TCAS also will issue the advisory. If the target
is a Resolution Advisory (RA), the TCAS must hold the
advisory for five seconds, even if the target is no longer
being seen by the TCAS.
If there is a loose connection on the suppression bus
between the TCAS system and the transponder, and the
transponder only occasionally breaks the connection, the
problem may go unnoticed for some time. For the problem
to be noticeable, the connection must break for about five
seconds continuously or five times in a row just when the
TCAS is interrogating. If TCAS issues an RA, the RA will
be displayed for five seconds, even if an RA condition no
longer exists.
False TCAS Advisories and PEDs
Can Passenger Electronic Devices (PEDs) cause false
TCAS advisories? Although PEDs have been suspected
in many events, experts at FAA’s William J. Hughes
Technical Center feel that PEDs are unlikely sources of
TCAS events, noting that PEDs do not generate signals
of the required complexity and power to effect TCAS.
Most instances of suspected PED-generated false TCAS
alerts reported to ASRS are likely cases of suppression bus
failure.
An incident reported to ASRS by a DC-9 Captain
illustrates how suspected PED use proved not to be a
factor in a false TCAS Resolution Advisory:
■ While level at 6,000 feet...at 250 knots clean, we had just
been handed off to Final Approach when we got a TCAS II
Resolution Advisory (RA) to climb full scale at 6,000 fpm
with no proximate traffic preliminaries. The First Officer
as Pilot Flying immediately began a max performance
climb while I went heads up to acquire the traffic which
had popped up at a 12 o’clock position less than one mile
altitude –03. I did not see any traffic, and by the time I
could get a word in edgewise on the frequency to tell ATC
about the RA, the RA had evaporated with no ‘clear of
conflict’ annunciation. (Initially, the target had appeared
to climb with us, maintaining a –00 altitude differential.)
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