Planes have high-tech systems to stop midair crashes. So what went wrong in Washington?

On Wednesday night US time, a passenger jet and US Army helicopter collided at a low altitude near Ronald Reagan Washington National Airport, and crashed into the the Potomac River.

A total of 60 passengers – including US and Russian champion figure skaters – and four crew were on board the American Airlines flight AA5342 from Wichita, Kansas. Three military personnel were in the chopper, which was conducting a routine training flight. Authorities say no one on board either aircraft survived.

This crash comes just over a month after a passenger jet crashed in South Korea – possibly as a result of a bird strike – killing all but two of the 181 people on board. The two incidents have focused attention on aviation safety around the world.

In the case of the most recent tragedy in the US, technology exists that is designed to help pilots avoid midair collisions with other aircraft. It is known as the Traffic Collision Avoidance System – or TCAS.

So how does it work? And why might it have failed to prevent disaster in this case?

What is a TCAS?

A TCAS is an aircraft safety system that monitors the airspace around a plane for other aircraft equipped with transponders. These are devices that listen for and respond to incoming electronic signals.

The system – also sometimes referred to as an ACAS (Airborne Collision Avoidance System) – operates independently of an external air traffic control system. Its purpose is to alert pilots immediately to nearby aircraft and potential midair collisions.

Since the technology was developed in 1974, it has undergone a number of advances.

The first generation technology, known as TCAS I, monitors what’s around an aircraft. It provides information on the bearing and altitude of any nearby aircraft. If there is a risk of collision, it generates what’s known as a “Traffic Advisory” – or TA. When a TA is issued, the pilot is notified of the threat, but must themselves determine the best evasive action to take.

The second generation technology, known as TCAS II, goes a step further: it provides a pilot with specific instructions on how to avoid a collision with a nearby aircraft or conflict with traffic, either by descending, climbing, turning or adjusting their speed.

These newer systems are also able to communicate with each other. This ensures the advice given to each aircraft is coordinated.

Any aircraft used for commercial purposes must be equipped with a TCAS in accordance with international regulations under what’s known as the Chicago Convention. There are specific provisions under the convention for noncommercial aircraft.

Military helicopters are not subject to the provisions of the Chicago Convention (although they are subject to domestic laws and regulations). And there are reports the military helicopter did not have a TCAS system on board.

Limitations of TCAS at low altitudes

Regardless of whether the military helicopter involved in the crash was fitted with a TCAS, the technology still has limitations. In particular, it is inhibited at altitudes below roughly 300 metres.

The last recorded altitude of American Airlines flight AA5342 was roughly 90 metres. The last recorded altitude of the US military helicopter that collided with the plane was roughly 60 metres.

It is not an accident that a TCAS is inhibited at low altitudes. In fact, this is part of the design of the technology.

This is primarily because the system relies on radio altimeter data, which measures altitude and becomes less accurate near the ground. This could potentially result in unreliable collision-avoidance instructions.

Another issue is that an aircraft at such a low altitude cannot descend any further to avoid a collision.

The site of several near misses

Ronald Reagan Washington National Airport is one of the busiest airports in the United States. Commercial, military and private aircraft share very limited airspace and corridors.

It has been the site of several near misses in recent years.

For example, in April 2024, a commercial plane pilot coming into land had to take evasive action to avoid a helicopter that was roughly 100 metres beneath it. In an incident report, the pilot said:

We never received a warning of the traffic from (air traffic control) so we were unaware it was there.

Many people, including Democratic US senator Tim Kaine, pointed to this near miss as evidence of why a plan to allow more flights into Ronald Reagan Washington National Airport should not proceed. Despite this, the plan was approved the following month.

All of this will undoubtedly be examined as part of the investigation by the National Transportation Safety Board into this disaster.

Chrystal Zhang does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.