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.

“Multiple layers of safety precautions failed that night,” said Katie Thomson, the Federal Aviation Administration’s deputy administrator under President Joseph R. Biden Jr.

The New York Times examined public records and interviewed more than 50 aviation experts and officials, including some with extensive knowledge of the events, to piece together the most complete understanding yet of factors that contributed to the crash.

Up to now attention has focused on the Black Hawk’s altitude, which was too high and placed the helicopter directly in the jet’s landing path at National Airport. But The Times found new details that show that the failures were far more complex than previously known.

The helicopter crew appeared to have made more than one mistake. Not only was the Black Hawk flying too high, but in the final seconds before the crash, its pilot failed to heed a directive from her co-pilot, an Army flight instructor, to change course.

Radio communications, the tried-and-true means of interaction between controllers and pilots, also broke down. Some of the controller’s instructions were “stepped on” — meaning that they cut out when the helicopter crew pressed a microphone to speak — and important information likely went unheard.

Technology on the Black Hawk that would have allowed controllers to better track the helicopter was turned off. Doing so was Army protocol, meant to allow the pilots to practice secretly whisking away a senior government official in an emergency. But at least some experts believe that turning off the system deprived everyone involved of another safeguard.

The controller also could have done more.

Though he had delegated the prime responsibility for evading other air traffic to the Black Hawk crew under visual separation, he continued to monitor the helicopter, as his job required. Yet he did not issue clear, urgent instructions to the Black Hawk to avert the crash, aviation experts say.

These lapses happened against the backdrop of systemic deficiencies in U.S. aviation. The F.A.A. has struggled for years with low staffing among controllers, and the National Airport tower has been no exception. At the time of the crash, for reasons that remain murky, a single controller was working both helicopter traffic and commercial runway traffic — jobs that would typically be done by two controllers.

The F.A.A. said in a statement that it could not discuss “any aspect” of a continuing investigation led by the National Transportation Safety Board.

Brig. Gen. Matthew Braman, the Army’s director of aviation, said, “I think what we’ll find in the end is there were multiple things that, had any one of them changed, it could have well changed the outcome of that evening.” He, too, deferred detailed questions about the investigation to the N.T.S.B., adding that the Army was conducting its own reviews of the accident.

Investigators from the N.T.S.B. will issue their final report on the causes of the crash by early 2026.

In the meantime, data recently analyzed by the board revealed that National Airport was the site of at least one near collision between an airplane and a helicopter each month from 2011 to 2024. Two-thirds of the incidents occurred at night, and more than half may have involved helicopters flying above their maximum designated altitude.

Given those findings, the F.A.A. recently banned most helicopter flights along a portion of the route the Black Hawk used.

And, critically, the F.A.A. has also vastly limited the use of visual separation.

The maneuver is primarily used by pilots flying helicopters and smaller aircraft, and is used less frequently for commercial jets. When using visual separation, pilots take responsibility for noticing and steering clear of neighboring air traffic if certain conditions, like good visibility, are met. It has long been viewed in the industry as essential to keeping traffic moving.

But the occasional difficulty for pilots to see and avoid nearby air traffic has also been implicated in at least 40 fatal collisions since 2010, according to the N.T.S.B. It has led to stern safety warnings to pilots from both the F.A.A. and the N.T.S.B.

Human error, blind spots not evident from a cockpit and environmental conditions “leave even the most diligent pilot vulnerable to the threat of a midair collision with an unseen aircraft” under this maneuver, the N.T.S.B. wrote in a safety bulletin published in 2016.

The practice of allowing pilots to navigate around traffic on their own “has long been seen as a flawed concept but a necessary one,” said Jeff Guzzetti, a former accident investigator for both the F.A.A. and the N.T.S.B. “But it has been linked to a number of deadly midair incidents throughout the years.”

Two Departures, Six Minutes Apart

At 6:39 p.m. Eastern Standard Time, a CRJ700 regional jet departed Wichita under cool, dry conditions with 60 passengers, two pilots and two flight attendants on board. It was operated by American Airlines’s subsidiary carrier, PSA Airlines, and the direct route to National Airport had started the previous January.

Capt. Jonathan J. Campos, a 34-year-old raised in Brooklyn who had wanted to fly since an early age, was the pilot. Sam Lilley, a 28-year-old former marketer whose father had been an Army Black Hawk pilot, was the first officer.

National Airport is one of only five airports in the United States that the F.A.A. designates as complex because of high density.

It is one of 57 airports in the United States that has a special-qualification designation from the F.A.A., according to an agency document reviewed by The Times. Nearly all of the remaining airports, such as those in Durango, Colo., or Missoula, Mont., are included because of hazardous mountainous terrain that pilots must navigate during takeoffs and landings, or because they are smaller airports without radar or a control tower.

“You have to have an aggressive defensive posture coming into DCA,” said Dennis Tajer, a spokesman for the Allied Pilots Association who is also an American Airlines pilot, using the identifier for National Airport. “You have to take your A-game and add a plus to it.”

Six minutes after Flight 5342 departed, the Black Hawk took off from Davison Army Airfield, at Fort Belvoir, Va., about 20 miles southwest of Washington.

The crew was ordered to fly about 40 miles north of the base to a suburb near Gaithersburg, Md., where it would turn around and head back to Virginia.

The crew’s mission was to conduct an annual evaluation of Capt. Rebecca M. Lobach, who joined the Army in 2019, to ensure that her helicopter piloting skills were up to par.

That night, her assignment was to navigate the conditions of a scenario in which members of Congress or other senior government officials might need to be carried out of the nation’s capital during an attack.

Captain Lobach was the highest-ranking soldier on the helicopter, but Chief Warrant Officer 2 Andrew Loyd Eaves, who was acting as her instructor, had flown more than twice as many hours over time.

A third crew member, Staff Sgt. Ryan Austin O’Hara, whose job was to help with equipment and other technical issues, sat in the back.

Captain Lobach, who was sitting in the front left seat, was initially handling the radio communications. To her right working the controls was Warrant Officer Eaves, a former Navy petty officer who joined the Army, according to his brother Forrest Eaves, because it would train him and permit him to fly helicopters.

Investigators believe Captain Lobach and Warrant Officer Eaves were wearing night-vision goggles, which were required attire for this type of evaluation. Goggles allow exponentially enhanced visibility of nearby people and objects, which is helpful at night in complex surroundings. But urban lights can also become cripplingly bright, according to military pilots.

Despite differences in rank and the delineation of duties, all three Black Hawk crew members bore responsibility for searching the sky for other aircraft and helping to stay clear of them.

A Blip Every Five to 12 Seconds

Sitting in the control hub of the National Airport tower that night, an air traffic controller watched the lighted dots on the radar scope in front of him.

His colleagues’ air-traffic instructions punctuated the ambient noise as he directed aircraft by radio. A little after 8:30 p.m., an Army helicopter, known in the tower as a “P.A.T.” for priority air transport, made contact with him.

The controller had worked for the F.A.A. for about a decade in two smaller air-traffic control centers, but had been stationed at National Airport for about two years, according to government employee filings. The controller, whom The Times is not identifying because his name has not been publicly revealed as part of the investigation, did not respond to requests for comment.

Like his colleagues in the tower, he typically worked one control duty at a time, such as directing just helicopters, or only handling airplanes on taxiways. He worked about seven hours that day, according to a government document reviewed by The Times. The F.A.A. says all controllers get required breaks.

But after a co-worker left the control hub at 3:40 p.m., some controllers began to assume combined duties. The controller who ended up directing the Black Hawk took over combined duties at roughly 7 p.m., according to the government document. An N.T.S.B. spokesman declined to confirm how long the controller operated in both roles.

Such a combination was not unusual, and was approved that evening by a tower supervisor, according to a person briefed on the staffing. But the roles were not typically combined until traffic slowed many hours later, around 9:30 p.m.

Though the reasons why the supervisor combined the duties so early are still not clear, the F.A.A. would later say in an internal report that staffing was “not normal” that evening.

By the time both the Army Black Hawk and Flight 5342 were in radio contact with the controller — starting about 8:43 p.m. — five controllers were working different duties in the control hub of the tower.

In addition to doing two jobs at once, the controller faced another complicating factor that night: He could not watch the helicopter’s movements in real time.

Doing so would have required the use of an aviation broadcasting system called Automatic Dependent Surveillance-Broadcast Out, or ADS-B Out, which reports an aircraft’s position, altitude and speed roughly every second.

But the Black Hawk did not operate with the technology because of the confidentiality of the mission for which the crew was practicing. That is because ADS-B Out positions can be obtained by anyone with an internet connection, making the system a potential risk to national security.

As a result, the controller relied on pings from the helicopter’s transponder to show its changing location, which can take between five and 12 seconds to refresh, according to F.A.A. documents.

Aviation experts said that during that gap, the aircraft could change course or elevation, making for a concerning level of uncertainty.

In a busy airspace, that lapse, said Michael McCormick, a former vice president of the F.A.A. Air Traffic Organization, is “a very long time.”

General Braman said the fact that ADS-B Out was turned off “played no role in this accident” because the transponder meant that the Black Hawk could be seen on the radar and “was never invisible.”

Some federal lawmakers have strongly disagreed. During a contentious Senate hearing on March 27, Chris Rocheleau, the F.A.A.’s acting administrator, announced that the technology would be required on all flights near National Airport — though with some as-yet-undefined exceptions.

Little Margin for Error

Near the end of his shift, the controller handling both helicopters and commercial jets tried to pull off a complicated, and potentially risky, maneuver called a squeeze play.

This is an attempt to keep operations moving efficiently, according to veteran National Airport controllers, by tightly sequencing runway traffic with minimal time between takeoffs or landings.

In this case, the plan was to let one airplane depart from Runway 1 at about 8:47 and let another land on the same runway about a minute later.

Shortly thereafter, the controller needed to bring Flight 5342 in for a landing.

But to fit in the Wichita flight without interrupting the flow of other traffic, the controller made a request that was permissible but atypical, according to the N.T.S.B. He asked to divert its landing to one of the airport’s ancillary runways, a spot normally used by smaller aircraft because of its shorter length.

“Can you take Runway three-three?” the controller asked the pilots.

His request would require Mr. Campos and Mr. Lilley to adjust their route during the final stage of their flight, introducing a wrinkle at the end of a two-hour journey. But commercial pilots train for such maneuvers, and having just passed over Mount Vernon in Virginia, about 10 miles from National Airport, they still had time to make the shift.

After a beat, one of the pilots replied. “Yeah, we can do, uh, three-three,” he said.

The pilots began the process of rerouting the flight to the new runway, which intersected Runway 1 at an acute angle in the middle.

Runway 33 had an additional quirk: a particularly narrow vertical space between the landing slope for a jet and the maximum altitude at which helicopters using a certain route, called Route 4, could fly.

At its highest, near the Potomac’s east bank, the vertical distance between a helicopter and an aircraft en route to landing on Runway 33 would be 75 feet, N.T.S.B. investigators said. But if a helicopter were flying farther from the river’s east bank toward the airport, that distance would be even less.

That is one reason why, after the crash, the N.T.S.B. recommended banning helicopter flights on Route 4 when Runway 33 at National Airport is in use.

Jennifer Homendy, the N.T.S.B. chairwoman, said in a March 11 press briefing that those distances “are insufficient and pose an intolerable risk to aviation safety by increasing the chances of a midair collision at DCA.”

With so little margin for error — 75 feet or even less — it would be crucial that the Black Hawk fly below the maximum altitude for the route.

Aboard the Black Hawk that night a curious exchange occurred between the two pilots.

Captain Lobach, who by that point had assumed the controls, announced an altitude of 300 feet, according to cockpit voice recordings. Warrant Officer Eaves then read out an altitude of 400 feet.

The exact time that passed between the statements has not been detailed in N.T.S.B. reports, but records suggest that it was no longer than 39 seconds. And experienced helicopter pilots say that given the ease of mobility in a Black Hawk, the altitude could have changed in fractions of seconds.

But the discrepancy, which neither pilot commented on at the time, was potentially significant.

The F.A.A. mandated an altitude of no higher than 300 feet for that part of the route, meaning that an altitude of 400 feet would have been unacceptable and could have positioned the Black Hawk uncomfortably close to departing or landing airplanes.

By about 8:44 p.m., it seemed to be in a more appropriate spot.

As the helicopter approached the Key Bridge, from which it would fly south along the river, Warrant Officer Eaves stated that it was at 300 feet and descending to 200 feet — necessary because the maximum height for its route closer to the airport had dropped to 200 feet.

But even as it reached that juncture, Warrant Officer Eaves evidently felt obligated to repeat his instruction: The Black Hawk was at 300 feet, he said, and needed to descend.

Captain Lobach said she would. But two and a half minutes later, the Black Hawk still was above 200 feet — a dangerously high level.

‘Threading the Needle’

Seconds after the Black Hawk crossed over the Tidal Basin, a shallow lake near the Washington Monument ringed by cherry trees, the controller informed the Army crew that a regional jet — Flight 5342 — was “circling” to Runway 33.

Aviation experts said that development may have blindsided Captain Lobach.

Though she had flown four or five similar practice rides there over the years, she might have never confronted a landing on Runway 33, because it is used only 4 to 5 percent of the time.

In any case, investigators now believe that the word “circling” was not heard by the Black Hawk crew because one of them was pressing the microphone key to speak when the word came through their radios. If the key is depressed, the pilot can speak but not hear incoming communications.

Around 8:46 p.m., Warrant Officer Eaves responded to whatever he did hear of the circle-landing notification, using the call sign for his own flight: “PAT two-five has traffic in sight. Request visual separation.”

The controller gave his approval.

Visual separation is at the crux of an aviation concept known as see and avoid, which works exactly as it sounds. A pilot is meant to see neighboring air traffic, often without assistance from the controller, and avoid it by either hovering in place until the traffic passes or by flying around it in prescribed ways.

See-and-avoid flying is commonplace in aviation. At many tiny airports, with no controllers, there is no alternative. In busy airspaces, such as parts of National Airport’s, the helicopter’s altitude limits are too low for controllers to easily assist it in maneuvering around obstacles such as ships or tall buildings, while also keeping it clear of air traffic.

The F.A.A. said in its statement that “pilots are responsible for keeping themselves safely separated from other aircraft.”

Nonetheless, even when a helicopter is operating under see-and-avoid rules, if the controller notices it is converging into another aircraft’s path, he or she should — under F.A.A. rules — call out the existence of the nearby traffic and ask the helicopter to affirm that it has the aircraft in sight.

At that point, the helicopter crew should acknowledge that it sees the traffic and can request visual separation — asking permission to stay clear of the nearby aircraft — which the controller can grant or refuse. Or, if the crew says that it does not see the traffic, the controller will likely direct the helicopter to a safer position.

One benefit of the see-and-avoid system is that it can lighten the controller’s workload during busy periods. But see and avoid has proved problematic, even fatal, in recent decades.

In 2019, two airplanes collided above Ketchikan, Alaska, killing six people and injuring 10 others. Three years later, two helicopters collided above San Diego, but there were no casualties. The N.T.S.B. cited failed see-and-avoid efforts in both cases.

One risk is that the pilots will miscalculate which way the other aircraft is moving; another is identifying the wrong aircraft.

John Goglia, a former N.T.S.B. board member, put it plainly: See and avoid assumes that every pilot has sharp vision and can pick out the right aircraft in the direction they have been told to look. But instructions are not always clear, he said. And tools like night-vision goggles can sometimes cloud vision more than clarify it.

Put two planes in roughly the same patch of sky, and even the most attentive pilot might track the wrong one, Mr. Goglia said.

During a recent press briefing on the crash, Sean Duffy, the transportation secretary, criticized the practice of allowing helicopters to use visual separation in confined airspaces like the one near National Airport.

“Having helicopters fly under landing aircraft, and allowing helicopter pilots to say, ‘I’ll maintain visual separation’ — that is not going to happen anymore,” he said. “That is too risky. You’re threading the needle. And it’s going to stop.”

A Last Communication

In the 90 seconds after the air traffic controller granted visual separation to the Black Hawk, the attempted squeeze play started to unfold. At 8:46:48 p.m. the tower cleared a jet for immediate departure off Runway 1.

Then, the Black Hawk, still southbound, passed Hains Point, a park area along the east side of the Potomac, moving it closer to the airport on the opposite bank.

At the same time, Flight 5342 began a leftward turn toward Runway 33. It was flying at about 500 feet and the equivalent of around 153 miles per hour.At 8:47:39 p.m., the controller contacted the helicopter.

“PAT two-five, do you have the CRJ in sight?” he asked, using the abbreviation for the model of Flight 5342’s aircraft.

As he spoke, a conflict alert — which controllers described as a distinctive beeping sound — was audible in the tower behind him, according to the N.T.S.B. report. A warning light, controllers said, would also have been flashing on the radar scope.

Conflict alerts are not rare. Controllers say they can go off numerous times over a long shift, to the point that they risk losing their urgency.

The controller received no response. The helicopter and Flight 5342 were by then about one mile apart.

The controller then issued an instruction to the helicopter crew: Pass behind the airplane.

Cockpit voice recordings indicate that the essence of the controller’s command — to “pass behind” — might not have been heard by the Black Hawk crew, perhaps because of a second bleep-out.

Some former military pilots said that by issuing a proactive command to pass behind the jet, the controller was going above and beyond his obligations, especially under see-and-avoid conditions, and that an experienced Black Hawk crew should have known what to do without help.

Still, some regulators and controllers said that the controller in this case could have done more.

He could have told the Black Hawk crew where Flight 5342 was positioned and which way it was bound. (The F.A.A. manual instructions direct controllers to use the hours of a clock in describing locations.) He could have provided the jet’s distance from the helicopter in nautical miles or feet.

But one thing is critical. When two aircraft are on a collision course, the controller’s top priority must be to warn both sets of pilots.

“Advise the pilots if the targets appear likely to merge,” F.A.A. regulations state.

That did not happen.

Direct, immediate intervention was needed that night. Instead of seeing and avoiding Flight 5342, Captain Lobach continued flying straight at it.

Investigators might never know why. There is no indication that she was suffering from health issues at the time or that a medical event affected her during those final moments aboard the Black Hawk, according to friends and people familiar with the crash investigation, which included autopsies and performance log reviews.

Two seconds after the controller’s cut out instruction about passing behind the jet, Warrant Officer Eaves replied, affirming for the second time that the Black Hawk saw the traffic. “PAT two-five has the aircraft in sight. Request visual separation,” he said.

“Vis sep approved,” the controller replied.

It was their last communication.

The Black Hawk was 15 seconds away from crossing paths with the jet. Warrant Officer Eaves then turned his attention to Captain Lobach.

He told her he believed that air traffic control wanted them to turn left, toward the east river bank.

Turning left would have opened up more space between the helicopter and Flight 5342, which was heading for Runway 33 at an altitude of roughly 300 feet.

Kitty Bennett contributed research.