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Collision risk demands rapid roll-out of new TCAS: analysis

European researchers are advising an aggressive transition to a new version of the traffic collision-avoidance system (TCAS) that aims to address two critical weaknesses in the current standard.

TCAS II version 7.1 features a modification, known as CP112E, which can resolve geometries similar to the "vertical chase" scenario that led to the mid-air collision over Uberlingen, Germany in July 2002.

Both aircraft - a Tupolev Tu-154 and a Boeing 757 - descended and collided when the Tu-154 crew failed to obey a TCAS "climb" instruction.

This scenario, known as an SA01 event, can result in the two aircraft staying at the same altitude and inhibit an automatic reversal of the TCAS instructions to the crew.

In a newly published analysis of TCAS 7.1 regulatory decision criteria, undertaken through Eurocontrol's Safety Issue Rectification Extension Plus  (SIRE+) project, researchers identified eight other SA01 occurrences in Europe over a recent five-year period.

"Each of these events resulted in severe losses of separation where collision was only avoided by chance," it says. Change CP112E will ease the trigger threshold of TCAS reversal during conflicts in which the aircraft remain with 100ft (30m) vertically of one another.

The second modification, CP115, solves a second weakness by replacing TCAS's ambiguous "adjust vertical speed" instruction with a simpler order to "level off", preventing pilots' inadvertently responding incorrectly and making the conflict worse.

Without the TCAS upgrade, says the SIRE+ analysis, the weaknesses in version 7.0 mean aircraft in European airspace face a risk equivalent to a mid-air collision every three years: "This exceeds the tolerable rate for catastrophic events related to equipment hazards by a factor of more than 25."

Revision to the TCAS 7.1 standard will increase the collision-risk interval to 12 years, assuming full fleet carriage.

US RTCA standards committee members approved the revised minimum operational performance standards in June and European approval from corresponding organisation EUROCAE is expected shortly.

After examining 15 potential implementation schedules - beginning in 2009, 2010 and 2011 - the study has determined that an "aggressive" fleet-wide roll-out, based on progressive forward-fit and retrofit, would present the best opportunity to reduce collision risk. It says transition should be initiated "as rapidly as possible", adding that even a two-year delay would result in "serious debasement" of the benefits.

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