At least 67 people are presumed dead after an Army Black Hawk helicopter collided with a regional passenger jet attempting to land at Reagan National on Wednesday night. After colliding, both aircraft plunged into the dark and frigid Potomac River. This is what we know so far:

The aircraft collided over the Potomac River

D.C. has restricted and congested airspace

Those who fly in and out of Reagan Washington National Airport – a scant three miles from the White House and two miles from the Pentagon – say it’s a complicated airspace to navigate. The sky around the busy airport serving the nation’s heavily populated capital is hemmed in by restrictions imposed due to national security and other concerns. What’s left is shared by commercial, civilian and military aircraft.

The Federal Aviation Administration usually requires aircraft to fly above the Potomac River, which shrinks air traffic volume to a narrow corridor. Aircraft must maintain specified altitudes to reduce noise levels in residential and tourist areas, including the National Mall.

The Black Hawk helicopter left Davison Army Airfield at Fort Belvoir in Virginia, about 15 miles southwest of the airport, authorities said Thursday. The helicopter was on a “routine annual re-training of night flights on a standard corridor for a continuity of government mission” when it crashed, Secretary of Defense Pete Hegseth said in a video statement posted to X on Thursday morning.

American Airlines Flight 5342 departed Wichita, Kansas, on Wednesday afternoon for a flight to Reagan National.

The crew was “fairly experienced,” and was conducting a night evaluation, Hegseth said. Night goggles were on board. Hegseth says that “a mistake was made” and that there was “some sort of an elevation issue.”

The helicopter may not have been equipped with ADS-B

In-flight aircraft are tracked by air traffic control and other aircraft with special communications systems:

ADS-B: Automatic Dependent Surveillance Broadcast is a primary system that automatically transmits an aircraft’s position, using GPS-based latitude and longitude.

Mode S: Mode Select is a secondary surveillance and communication system, an aircraft transponder that sends data to air traffic controllers and other aircraft.

MLAT: Known as multilateration, this system applies mathmatical calculations to Mode S signals to triangulate an aircraft’s position.

ADSBexchange was able to triangulate the flight path of the helicopter using multilateration (MLAT). FlightRadar24 describes MLAT as a flight tracking technique that determines aircraft position by measuring the time difference of arrival (TDOA) of signals sent from the aircraft to multiple ground stations.According to Dan Streufert, founder and president of flight tracking network ADSBexchange, the passenger jet was broadcasting ADS-B data, which details an aircraft’s position, including GPS-based latitude and longitude. However, the helicopter was not broadcasting ADS-B and was only broadcasting Mode S, a secondary surveillance and communication system.

Strufert says that because MLAT is a function of the receiver network it was possible to triangulate an approximate flight path even without ADS-B data. While FlightRadar24’s network did not have enough nearby receivers, the ADSBexchange network had at least 4-6 in the area. The path appears a bit jagged due to the lack of precision.

Streufert says that it appears that the Black Hawk may not have been equipped with ADS-B at all — however it did have a transponder, and would have been visible on air traffic control radar in either case. Without ADS-B broadcasting, the helicopter would not have been “invisible”, but would be “less visible” to the jet.

The jet would have been equipped with Traffic Alert and Collision Avoidance System (TCAS), which, at a higher altitude would have likely prevented this collision. Streufert adds that “TCAS would have issued a traffic advisory under most circumstances, even without ADSB. However, TCAS is inhibited at low altitudes around airports to avoid nuisance/false alarms due to aircraft taxiing, etc.”Why would the helicopter not have had ADS-B? Streufert says that security and cost issues are often cited, though he thinks integration with the civilian air traffic system would argue in favor of installing the equipment.

“While civilian aircraft are required to be equipped in most US airspace, the FAA does not have the ability to mandate this for military aircraft. The military has been slow to deploy ADSB, especially on it’s smaller aircraft”, said Streufert.

Air Traffic Control was in contact with jet, attempted to contact helicopter

Squeezing into Ronald Reagan Washington National Airport is a choreographed dance for experienced pilots to navigate a smaller-than-standard and complex runway configuration. Add on top of that the prohibited airspace around the White House and Capitol complex known as P-56 and it requires special training and focus, said University of North Dakota Aviation Professor Jim Higgins, who flew into the airport as a commercial pilot.

“As someone who’s flown that approach, your focus and attention is almost entirely on the landing zone, completing the landing, rolling out and bringing the aircraft to a stop,” Higgins said. “You are not expecting a piece of traffic or helicopter or plane so close to you and so close to the ground.”

Higgins said one plausible theory has emerged that air traffic controls instructed the UH-60 Black Hawk to maintain separation from the regional jet and those pilots identified the incorrect aircraft.

Higgins said pilot proficiency of the regional air carrier or military would be deeply examined along with all the communications and actions by the National Transportation Safety Board. The NTSB will also examine whether the helicopter was communicating with traffic controllers on a different radio frequency that the commercial pilots.

The Bombardier regional jet was equipped with a traffic alert and collision avoidance system aimed at alerting pilots of a hazard, but so close to the airport and ground, Higgins said the system often “declutters” the clusters of planes on the radar system and goes into “inhibit mode” which blocks the alerts in certain landing phases.

“Obviously something went catastrophically wrong,” Higgins said.

Audio recordings from air traffic controllers captured dramatic communications before and after the collision.

LiveATC.net, a respected source for in-flight recording, recorded the moments before and after the crash, according to reporting from Reuters. The audio details communications between air traffic controllers and other aircraft, as well as the the final communication attempt with the three Army crew members in the helicopter – call sign PAT25 – before the collision with the jet.

Timeline of aircraft collision

Times are Eastern and approximate:

• 6:18 p.m.: Flight 5342 departs Dwight D. Eisenhower National Airport in Wichita.
• 8:45 p.m.: Flight 5342 approaches Ronald Reagan Washington National Airport in Virginia.
• 8:46 p.m.: Air traffic controllers ask Flight 5342 to land on Runway 33. Pilots acknowledge.
• 8:47 p.m.: Air traffic controllers attempt to contact the helicopter pilot seconds before the two aircraft collide.
• 8:48 p.m.: Control tower sounds alarm to alert responders to crash.
• 8:53 p.m.: Washington Metro police receive calls about the crash.
• 9:30 p.m.: FAA and law enforcement confirm aircraft collision and crash.
• 10:30 p.m.: National Transportation Safety Board investigators arrive on scene.
• 11:30 p.m.: At least 18 bodies reported recovered.

SOURCE USA TODAY Network reporting and research; Reuters

Contributing: Nick Penzenstadler and Jim Sergent, USA TODAY.