As they flew south along the Potomac River on the gusty night of Jan. 29, the crew aboard an Army Black Hawk helicopter attempted to execute a common aviation practice. It would play a role in ending their lives.

Shortly after the Black Hawk passed over Washington’s most famous array of cherry trees, an air traffic controller at nearby Ronald Reagan National Airport alerted the crew to a regional passenger jet in its vicinity. The crew acknowledged seeing traffic nearby.

One of the pilots then asked for permission to employ a practice called “visual separation.” That allows a pilot to take control of navigating around other aircraft, rather than relying on the controller for guidance.

“Visual separation approved,” the controller replied.

The request to fly under those rules is granted routinely in airspace overseen by controllers. Most of the time, visual separation is executed without note. But when mishandled, it can also create a deadly risk — one that aviation experts have warned about for years.

On Jan. 29, the Black Hawk crew did not execute visual separation effectively. The pilots either did not detect the specific passenger jet the controller had flagged, or could not pivot to a safer position. Instead, one second before 8:48 p.m., the helicopter slammed into American Airlines Flight 5342, which was carrying 64 people to Washington from Wichita, Kan., killing everyone aboard both aircraft in a fiery explosion that lit the night sky over the river.

One error did not cause the worst domestic crash in the United States in nearly a quarter-century. Modern aviation is designed to have redundancies and safeguards that prevent a misstep, or even several missteps, from being catastrophic. On Jan. 29, that system collapsed.