Eurocontrol released an Airborne Collision Avoidance System (Tcas in North America) training document entitled "Not so fast" in May, offering pilots fresh insight into how their personal flying habits might be causing some apparently bogus Resolution Advisories (RA) in crowded skies.

The problem stems from the ability of a modern aircraft to maintain significant rates of climb and descent. That performance trait alone does not cause RAs because ATC plans ahead, clearing other traffic out of conflict's way during altitude changes.

But all too often, according to Eurocontrol, pilots are maintaining a high rate of climb or descent until the last possible level-off opportunity. Because computers measure climb/descent rates against the closing rates of opposing traffic, waiting until the last moment essentially fools the technology, often creating an RA because the computer can't predict the pilots' plan to level off abruptly.

Eurocontrol is also concerned that pilots believe they know why they're receiving such RAs and often ignore them as nuisance alerts. "In real time," the document outlines, "pilots cannot and should not assess whether the RA is in fact operationally required." Eurocontrol's message: a second RA could well be missed by pilots trying to think faster than the computers.