TCAS II V7.1 Intro booklet

Version 7.0. While unnecessary deviations
from ATC clearance continue to be an
important factor in the effectiveness of
TCAS, the number and magnitude of these
excursions have been greatly reduced over
the years since TCAS was introduced.
Inadequate or Improper Coordination
with ATC. Unless RAs are reported by
pilots, most controllers do not have a means
to determine that an RA has been issued.
Thus, an RA is generally unexpected by a
controller and in many cases is a disruption
to his or her workload. This disruption is
due to an aircraft’s unanticipated deviation
from the ATC clearance, use of the ATC
frequency to report an RA, and the
possibility of an induced conflict with a
third aircraft. Pilots sometimes do not report,
or are slow in reporting TCAS-related
clearance deviations to the controller. This
has contributed to situations where
controllers have issued clearances that were
in the opposite sense than that directed by
the RA. The standard ICAO phraseology is
sometimes not used and at times, the
controller does not understand the initial RA
notification from the pilot. This issue has
been effectively addressed by pilot and
controller training programs but deserves
constant attention and continual monitoring.
Aircraft have also been observed making
vertical or horizontal maneuvers based
solely on the information shown on the
traffic display, without visual acquisition by
the flight crew and sometimes contrary to
their existing ATC clearance. Such
maneuvers may not be consistent with
controller plans, can cause a significant
degradation in the level of flight safety and
may be contrary to a limitation contained in
the TCAS Airplane Flight Manual
Supplement. This improper use of the traffic
display has been addressed via pilot training
programs but more emphasis is needed.
Improper Compliance with RAs. Earlier
event reports and analyses of ground
surveillance radar and flight recorder data
indicate that some pilots were not
responding to RAs, delaying response, or
responding in the wrong direction. In some
cases, there was no response or delayed
response when the flight crew did not
visually acquire the intruder, or
misidentified or subsequently lost sight of
the intruder causing potentially hazardous
situations. If the intruder is also TCASequipped,
the RAs are coordinated, and a
non-response or a delayed response by one
aircraft will result in the other aircraft
having to maneuver further to resolve the
RA. In some cases, it may also result in an
aircraft maneuvering in the same vertical
direction resulting in reduced separation. In
addition to noncompliance, there have been
instances observed where pilots responded
in the wrong direction, i.e., the direction
opposite to that indicated by the RA. In
particular, pilots have occasionally increased
their vertical rate in response to an AVSA
RA instead of decreasing it. To minimize
chances of opposite responses to AVSA RA,
Version 7.1 changed “AVSA” RAs to “VSL
0 fpm” or “LOLO” RAs.
Incompatibilities between a TCAS RA and
controller instructions have also contributed
to RA noncompliance resulting in an
accident or serious incident. A contributing
factor in the Ueberlingen Germany mid-air
was the flight crew following the
controller’s instructions instead of the TCAS
RA. TCAS Version 7.1 incorporates a
change to the sense reversal logic that
reduces the risk of an accident like the one
at Ueberlingen.
More recent analyses show that these issues
have been greatly reduced through improved
TCAS logic and pilot and controller training
programs. Most cases of "no response" to an
RA can be attributed to pilots having visual
contact with the intruder or being on parallel
approaches to runways during VFR
operations and visual separation procedures.
Wrong direction responses, though now
rarely reported, must always be avoided. In
summary, the safety benefits provided by
TCAS decrease significantly when pilots do
44