What can be done to avoid these RAs?

Controllers and pilots consider that too many RAs are generated in 1000 ft level-off encounters. Some
solutions can be envisaged to avoid these RAs, or at least to reduce their number.

Rule making

In order to increase safety and to minimise the likelihood of Two core area European States have published regulations in
RAs in 1000 ft level-off encounters, it is proposed that aircraft their AIPs, which require the vertical rate, in the last 1000 ft
have a reduced vertical rate when approaching their before level-off at the cleared altitude, to be below 1500 fpm.
cleared level. Recommendations or rules already exist. This can be expected to improve the compatibility of TCAS II
with ATC, and bring improvements in safety.
The EUROCONTROL ACAS Programme recommends that
pilots climb or descend at a rate less than 1000 fpm in the last In addition, a proposal for a recommendation to reduce the
1000 ft to level-off. The EUROCONTROL RVSM Programme vertical rate to less than 1500 fpm in the last 1000 ft before
also recommends a similar rate for RVSM operations. level-off at the cleared altitude, is under discussion within
ICAO for inclusion in Annex 6, Aircraft operations.

Near/Medium term: procedure modifications Long term: technical modifications

Two solutions could be adopted to improve operations in locations where Potential solutions include a modification of
RA “Hotspots” have been identified: aircraft autoflight systems and a TCAS logic
modification:
• To increase the vertical separation between aircraft to 2000 ft in
• Modification of the altitude capture laws of
specific cases (e.g. between arrivals and departures).
the autopilot by an earlier reduction of
• To avoid simultaneous horizontal and vertical convergence of vertical rate. This would reduce the
aircraft by modifying either the horizontal route or the vertical trajectory. probability to RAs during level-off. Although
this solution will require a lengthy
These proposals, which could be implemented in a relatively short term,
development and certification process, it is
are also likely to provide improvements in safety.
expected to provide significant contribution
Two procedure modifications in line with these proposals have already been to safety.
implemented by one ANSP to address identified RA “Hotspots”. Neither of
these procedure modifications have had any significant effect on capacity. • Radical redesign of the TCAS logic to use
own aircraft selected flight level. This would
• 2000 ft vertical separation is now applied between Geneva arrivals and require a lengthy development and
Lyon departures. certification process. Unlike the other
proposed solutions, overall ATM safety
• In Paris TMA, the MOSUD arrival descent point from FL140 to FL120 is would not be improved.
delayed by 4 NM on a tactical basis. Thus RAs are avoided with the
departures climbing to FL110. As a bonus, a STCA “Hotspot” has also These modifications must be viewed as long
been suppressed. term solutions.

Conclusion

Despite several specific features, TCAS II still generates some This issue involves all ATM actors:
RAs in 1000 ft level-off encounters, which can be perceived as
operationally unnecessary. This perception results from the • Pilots: TCAS II is an effective protection in the event of
level busts: follow the RA! Where possible, the vertical
fact that 1000 ft is the standard ATC vertical separation
rate should be reduced in the last 1000 ft before level-off.
applied between aircraft.
Some of these RAs are necessary, particularly in the case • Aircraft Operators: Where feasible, operational
procedures should be implemented requiring a vertical
of level busts, which are not infrequent events. Therefore,
rate <1500 fpm in the last 1000 ft from a cleared altitude.
pilots must follow all RAs.
RAs in 1000 ft level-off encounters are generally due to high or • Aircraft Manufacturers: Autoflight system designs should
take into account TCAS performance when determining
very high vertical rates. Therefore, it can be easily appreciated
vertical rates for altitude capture.
that these RAs contribute to the prevention of some level
busts where there would be a risk of collision. These RAs are • Controllers: It should be noted that these RAs are
justified from a TCAS standpoint, and are not false alerts. justified from a TCAS standpoint. Traffic information may
improve the pilots’ situational awareness.
Where 1000 ft level-off RAs are recurrent, it could serve to
highlight a potential safety issue in ATM design, or • Aviation Authorities and Service Providers: Airspace
procedures. design and procedures should take into account any
potential safety issues highlighted by TCAS II monitoring.

Contact: John Law This is one of a series of ACAS Bulletins planned to

March 2003 Eurocontrol ACAS Programme