In its certification, Airbus was required to prove that its A350 aircraft can be evacuated in less than 90 seconds. In Tuesday’s collision in Japan, the last crew member escaped the aircraft after 18 minutes—but there were still no casualties.

The sizable discrepancy, for the moment, represents a puzzle for the industry: Does the safe and deliberate evacuation represent a triumph of new aircraft designs and improved procedures? Or was it a one-off, a fortunate confluence of events that doubles as a cautionary tale showing how difficult it is to quickly evacuate modern aircraft?