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TCAS II Operations in European RVSM Airspace

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Category: Safety Nets Safety Nets
Content source: Skybrary Skybrary
Content control: Eurocontrol Eurocontrol


Before Reduced Vertical Separation Minima (RVSM) operations commenced in European airspace, work was carried out to ensure that Airborne Collision Avoidance System (ACAS) II would be compatible with RVSM and would deliver the expected safety benefit. This work was followed by further studies which confirmed that TCAS II would provide the expected additional safety benefit.

Following more than four years experience of TCAS II operations in European RVSM airspace, a further safety study has been completed. This study has confirmed that TCAS II provides substantial safety benefit in European RVSM airspace. (See conclusions below).

Typical scenarios considered in these studies are described briefly below to illustrate the range of encounter studied. Fuller discussion of each scenario is contained in EUROCONTROL ACAS Bulletin 8.


Typical Scenarios

ATC Error. The controller clears an aircraft to descend through an other aircraft's level. This triggers coordinated TCAS RAs in both aircraft.

Initial ATC Error results in a Multiple Aircraft Encounter. The controller issues a descent clearance to an aircraft which brings it into conflict with an aircraft below. Coordinated TCAS RAs are received by both aircraft and the descending aircraft begins to climb, bringing it into conflict with an aircraft above.

Incorrect Altitude Reporting. Due to an equipment malfunction, the altitude reports from an aircraft in level flight indicate frequent abnormal 100-foot “jumps”. As this aircraft is reporting altitude in 25-foot increments, the TCAS on an aircraft 1000 ft below is tracking some high vertical rate variations for the aircraft above. This triggers a "Descend" RA which brings the aircraft into conflict with traffic below, resulting in coordinated TCAS RAs for both aircraft.

RA due to High Vertical Rate. An aircraft is descending at a high vertical rate to level 1000 ft above opposite direction traffic. Both aircraft receive co-ordinated TCAS RAs.

Opposite Manoeuvre to RA to follow ATC Clearance. An aircraft is erroneously cleared to descend, bringing it into conflict with an aircraft 1000 ft below. Both aircraft receive TCAS RAs, but the descending pilot decides not to respond to the "Climb" RA.

Limited Aircraft Performance. Due to performance limitations an aircraft is unable to climb at the required climb rate, bringing it into conflict with other traffic.


Conclusions

  • The recent safety study mentioned above concludes that TCAS II provides substantial safety benefits in the European RVSM airspace. This has been confirmed by operational monitoring.
  • Prompt and accurate pilot response to all RAs is key to achieve maximum safety benefits in all airspace, including European RVSM airspace. This will also minimise the risk of a domino effect with a third aircraft at an adjacent RVSM flight level.
  • “Climb” RAs should be complied with even at high altitudes, albeit that aircraft performance might dictate a climb rate lower than required by the RA.
  • Multiple aircraft encounters in RVSM airspace are rare. However, TCAS II provides significant safety benefits in these encounters.
  • As can happen at lower altitudes, RAs can be triggered in 1000 ft level-off geometries due to high vertical rate or as a result of poor altitude reporting or altitude keeping. It is recommended that the

vertical rate should be reduced to less than 1000 fpm in the last 1000 ft before reaching the cleared flight level. This will reduce the number of RAs generated in such geometries.

  • Finally, aircraft operators are reminded that accurate altitude reporting is a fundamental requirement for effective TCAS safety

performance.


Further Reading

EUROCONTROL

ACAS Bulletin 8;

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