Traffic Collision Avoidance System (TCAS)
Dave Jesse on June 6, 2018

The TCAS system is designed to avoid traffic collisions between two aircraft. At first a Traffic Advisory (TA) warning is given to alert the crews of approaching aircraft. Then, if the separation continues to reduce the two aircraft exchange position, direction and speed information and, in a clever piece of negotiation, agree which aircraft needs to climb and which to descend to avoid a collision. The crew in the two aircraft are then presented with Resolution Advisory (RA) fly up or fly down commands, and by following these the crews will ensure adequate separation.

The question then arises, in a monitoring world, how do we know what happened, and if the crew reactions were correct. Here are the KPVs we use to monitor this system:

TCAS TA Warning Duration

This is simply the duration for which the TCAS TA Warning was active. In practice, one second warnings are commonplace around airports, hence there is a minimum 2 second threshold built into the algorithm.

TCAS RA Warning Duration

Astonishingly, this is the duration for which the TCAS RA Warning was active. The 2 second threshold is retained to avoid spurious triggering which is seen on some types.

TCAS RA Reaction Delay

This measures the time taken for the pilot to react, determined by the onset of the first major change in normal acceleration after the RA started. A change of more htan +/- 0.15g is needed, and then the truck and trailer algorithm is used to find the point of inflexion. It is the time from the RA alert to this point that is used as the reaction time.

TCAS RA Initial Reaction Strength

The pilot will cause a pitch up or down, and this results in a change in acceleration on the aircraft. If he reacts sharply, this acceleration comes on more quickly, and so we measure the strength of the first reaction to the RA, in g per second.

There have been a significant number of cases where the crew reacted, but moved the controls in the wrong direction. The reason for this is being investigated by other organisations involved in human factors and this is one of the drivers towards automating the aircraft response to RAs. This KPV is positive if the reaction is in the same sense as the Resolution Advisory (up for up or down for down) but negative in sign if the action is in the opposite direction to the RA.

TCAS RA To AP Disengaged Duration

Here we are concerned with the time taken between the RA coming on and the pilot disengaging the autopilot. It is normal practice for the crew to disengage the autopilot following an RA alert, but in some cases the crews have forgotten to do so, and the autopilot may “fight” the pilot’s actions and thereby reduce the effectiveness of the system.

TCAS Failure Duration

If the TCAS system should report a failure, this is recorded and can be used to alert the maintenance crews accordingly.


So a simple system gives rise to a number of monitoring parameters. Of course, on almost all flights the TCAS does not come on at all, but when it does, we can now check that the crew acted as they had been trained.