How airplanes avoid mid-air collisions ?

Airliners are equipped with a device called TCAS to help them avoid mid-air collisions. But as is often the case, even when the technology works, human failure can still occur.

Following on from our article on aircraft tracking and air traffic control, we’ve had a few questions about mid-air collisions. Indeed, if systems such as ADS-C enable aircraft positions to be communicated in real time with such precision, the risk of mid-air collisions should be a thing of the past.

Well, yes…but no.

TCAS in a nutshell

The system used on airliners to avoid mid-air collisions is called TCAS, for Traffic Alert and Collision Avoidance System.

Developed in the 50s, it became widespread in the 70s following a number of accidents in a context of constant air traffic growth.

It is now compulsory on aircraft weighing over 5,700 kg or carrying more than 19 passengers.

TCAS works by relying on the transponders fitted on today’s aircraft, as explained in the article above.

A TCAS-equipped aircraft constantly interrogates the transponders of nearby aircraft, and the latter respond by communicating their position. This position is then reflected on the on-board instruments. In the event of a risk of collision, and if both aircraft are equipped with TCAS, they can initiate a dialogue so that the pilots of both aircraft can take coordinated action.

Please note: TCAS is not a radar. It enhances the latter’s data, but does not replace it.

Let’s go into a little more detail.

How TCAS works to anticipate and avoid collisions

So, as we said, TCAS interrogates the transponders of nearby aircraft, and in response receives their exact location (position, altitude, heading, speed) and displays it on the crew’s flight instruments.

If the aircraft is at a sufficient distance, nothing happens other than the display.

The calculator determines several zones, or rather volumes, around the aircraft, depending on its speed. TCAS does not react in terms of distance, but in terms of time to a potential collision, which makes more sense.

When the second aircraft enters a zone where its trajectory puts it within 40 seconds of a collision, the TCAS issues a Traffic Advisory (TA) alert. It consists of a specific display on the flight instruments and a voice announcement in the cockpit (“Traffic, Traffic”).

At this stage, the TCAS does not go any further, as the situation is considered “normal”. It just calls for vigilance on the part of the pilots, whom it lets manage the situation.

If the second aircraft enters a zone where its trajectory puts it within 25 seconds of collision, and pilot action is imperative to avoid it, the TCAS will issue a “Resolution Advisory” (RA) alert. This alert involves a change in the instrument display (aircraft marked with a red square) and a voice alert giving the pilot instructions on how to avoid a collision (climb, descent…).

Where it gets interesting is if the other aircraft is also equipped with TCAS. In this case, the two systems will collaborate and coordinate to avoid a collision by giving instructions to both pilots, usually by asking one to climb and the other to descend, which will increase the distance between the two aircraft more quickly.

So in theory, a collision could never happen. Theoretically yes, and in fact it almost never happens, but TCAS has its limits.

When the TCAS doesn’t work…

First of all, there are circumstances in which TCAS alerts are deactivated. This is the case, for example, during the landing phase, to avoid false positives, or when the aircraft is close to the ground, so as not to tell the pilot to descend. The same applies when the aircraft is close to its maximum altitude, as it can not climb any higher. All this to avoid inconsistent situations.

Alerts are also deactivated in the event of priority alerts, so that pilots can concentrate on what needs to be resolved first. For example, in the event of a GPWS (ground proximity) or WINDSHEAR (wind shear) alert.

But these are normal and expected cases of alert deactivation, so it’s not a problem.

No, the real limit of TCAS is…human.

Conflicts between TCAS and air traffic control

Sometimes, air traffic control, realizing the danger, gives instructions to pilots to avoid a collision, which can result in the pilot receiving two contradictory instructions at the same time: one from the TCAS and the other from air traffic control.

This is what happened in 2002 in the so-called Überlingen collision.

On July 1, 2002, a Tupolev Tu-154 operating Bashkirian Airlines flight 2937 and a Boeing 757 cargo plane operating DHL flight 611 collided over Germany, killing 71 people and leaving no survivors. Among them were 52 children returning from a trip to Spain won with their school.

After investigation, it was established that the collision was the result, on the one hand, of a lack of rigor on the part of Swiss air traffic control and, on the other, of ambiguity regarding the use of TCAS.

The air traffic controller, despite having only belatedly identified the risk asked the Russian pilot to descend. A few seconds later, TCAS gave him the order to climbwhile asking the DHL pilot to descend. The Tupolev pilot continued to descend, air traffic control maintaining its instruction without knowledge of the TCAS instructions, while the freighter also descended.

The outcome was inevitable: the two aircraft collided, whereas if they had followed TCAS instructions they would have avoided each other.

It’s all very well told in this episode of Air Crash Investigations.

In fact, the accident caused one last death: the air traffic controller was murdered by a Russian father who lost his wife and two children in the crash.

Since then, the rules have been clarified in the event of divergent instructions from TCAS and air traffic control: TCAS instructions take precedence over those of air traffic control.

TCAS not indispensable?

However, TCAS is not indispensable. A TCAS failure in no way prevents an aircraft from flying, as other systems can also be used to avoid collisions.

On the other hand, in areas where air traffic control does not have the material resources to get precise positioning of aircraft, as in central Africa, its use is compulsory.

Bottom line

TCAS enables aircraft to avoid mid-air collisions by detecting high-risk situations and providing pilots with collision-avoidance instructions.

A system that has proved its effectiveness…except when air traffic control gives opposite instructions.

Image : Anynobody, CC BY-SA 3.0, via Wikimedia Commons

Bertrand Duperrin
Bertrand Duperrin
Compulsive traveler, present in the French #avgeek community since the late 2000s and passionate about (long) travel since his youth, Bertrand Duperrin co-founded Travel Guys with Olivier Delestre in March 2015.

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