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is the only equipment configuration fully
compliant with ICAO’s ACAS II
Standards And Recommended Practices
and, therefore, the version required to
meet the ACAS II mandate in ECAC
Member States. This change introduced a
host of improvements to the performance
and operational efficiency of TCAS, with
the most significant enhancements being:
• Coordinated RA sense reversals in
TCAS-TCAS conflicts.
• Replacement of the previous version’s
100-ft vertical tracker with a new 25-ft
tracker, allowing Version 7.0 to produce
significantly better estimates of the
vertical rate for Mode S-equipped
intruder aircraft.
For its part, ATC
has a fundamental
obligation to
maintain adequate
separation between
aircraft and thus to
prevent collisions
EUROCONTROL / IFATCA 2008: a collaborative approach to the future > Designing the network
65
Commercial pilot Edward Downs looks at the progress being made on Airborne
Collision Avoidance Systems
Avoiding the issue
▼
• The criteria for removing a TA have been
made more stringent, to reduce the
likelihood that TCAS will issue multiple
TAs against the same target.
• The TA altitude threshold has been
reduced from 1,200ft to 850ft for
altitudes between 30,000ft and 42,000ft.
This change supports Reduced Vertical
Separation Minima (RVSM), where,
without this change, many ‘nuisance’
TAs would be generated, thus reducing
the overall effectiveness of the system.
• A new multi-aircraft logic, which is
much more versatile and robust than the
old version, allowing ‘Increase Rate’ RAs
to deal with worsening situations.
So far, so good, but, unfortunately, the
implementation of TCAS II Version 7.0
has not eradicated the threat of mid-air
collisions entirely. Both aircraft involved in
the accident over Überlingen on 1 July
2002 had TCAS II Version 7.0 equipment
and both crews had received the required
training. Nonetheless, the crash
investigation revealed deficiencies in both
ATM and the aircrew’s response to the
subsequent RA. As a result of the
Überlingen accident, it was recommended
that PANS-OPS Doc 8168 be amended to
reinforce that TCAS RAs should take
precedence over ATC instructions and
pilots are to obey RAs until the aircraft is
66
EUROCONTROL / IFATCA 2008: a collaborative approach to the future > Designing the network
The crash investigation revealed deficiencies in both ATM and
the aircrew’s response to the subsequent Resolution Advisories
clear of conflict. However, Doc 8168 also
states that should the pilot consider that,
by following TCAS instructions, the
subsequent manoeuvre would jeopardise
the safety of the aircraft (eg, Enhanced
Ground Proximity Warning System), he
may ignore the RA.
Operationally, the implementation of
ACAS on civilian aircraft has been a great
success, and the introduction of TCAS II
Version 7.0 was unquestionably a
significant step forward over the earlier
Version 6.04 in making the system more
‘user friendly’ in the increasingly crowded
skies over Europe. However, there is scope
for more training for both pilots and ATC
operators. For example, in RVSM airspace,
there are still a significant number of TAs
issued due to the high rates of climb and
descent employed by aircrews – good
airmanship should dictate more moderate
rates of change in areas of high traffic
density; when a TA is generated against
lower traffic, particularly during a rapid
descent from medium level, this should
automatically prompt a reduction in rate
from the crew – not always the case,
resulting in an ‘adjust vertical speed’ RA.
Similarly, ATC should be prepared to
give an indication of required
climb/descent rates against possible
conflicting traffic to minimise potential
for a TCAS alert. While the change to
Version 7.0 reduced the number of
‘nuisance’ TAs, the aforementioned pilotinduced
TA/RAs can mitigate this
enhancement by similarly reducing pilot
confidence in the system through nuisance
warnings. For the system to be most
effective, crews must have the utmost
confidence in its integrity.
So, where does the current ACAS II
standard fit in with future ATM plans? As
alluded to earlier, there is still some work
to be done in terms of integrating ACAS
II more completely into the ATM system.
In this respect, more performance data
　
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