Safe Handling of TCAS Alerts

Flight crews reacted correctly to a TCAS Resolution Advisory (RA) in only 58.7% of cases according to a recent study published by Eurocontrol(*) in April 2021. In 29.8% of cases, the flight crew reacted by modifying the aircraft trajectory but did not reach the expected target. In 11.5% of cases, the flight crew did not react, or they reacted excessively and sometimes had the opposite reaction to what the RA requested.

Flight crews must always remember that a prompt and accurate response to TCAS RAs is important to maintain the highest level of safety.

The Eurocontrol study observed a low level of compliance with “Climb” or “Descend” RAs: only 33.7% of the “climb” and “descend” RAs were correctly followed, 44.2% did not reach the expected target, and 22.1% were not flown correctly.

The TCAS warning logic uses the current trajectory of the aircraft to predict collision threats. This does not anticipate the expected level off when the aircraft finishes its climb or descent and reaches its expected flight level. If another aircraft approaches on an adjacent flight level, it can trigger a “level off” RA, or a “level off” with a corresponding “climb” or “descend” RA (fig.2). This is more likely to occur in congested airspace.

(fig.2) Example of an RA triggered when reaching a cleared flight level, near an aircraft flying on an adjacent flight level

“Level off” RAs made up 65% of all observed RAs in the Eurocontrol Study. ICAO recommends that the flight crew manually select a lower vertical speed when approaching the target flight level to prevent the triggering of “level off” RAs. This solution is effective but not always optimal, because it requires intervention by the flight crew. This is why Airbus developed the TCAS Alert Prevention (TCAP) function.

The TCAP function is an improved _ALT*– altitude capture law for the flight guidance computers. When a TCAS Traffic Advisory (TA) is triggered for an aircraft that is almost at its target flight level, the _ALT*– guidance mode will engage earlier and use a reduced vertical speed to reach the target altitude. This will prevent the need for an RA (fig.3). If the _ALT*– guidance mode is already engaged when the TA is triggered, the TCAP function will further reduce the _ALT*– vertical speed to prevent the RA.

The TCAP function automatically reduces the vertical speed in the case of a TA, therefore, it is not necessary for the flight crew to manually reduce the vertical speed when approaching their target flight level. If an Operator has a mixed fleet configuration of aircraft with and without the TCAP function activated, the flight crew can still manually select a lower vertical speed when approaching the target flight level on any aircraft depending on the Operator’s policy.

(fig.3) Principle of the TCAP function

Sometimes the flight crew does not comply with RAs as expected or performs a maneuver that is too weak or excessive (fig.4). To help the flight crew perform optimum maneuvers in accordance with the RA, Airbus developed the AP/FD TCAS function.

(fig.4) Response to TCAS RAs are not always optimum

The AP/FD TCAS function is an additional flight guidance mode of the autoflight system available for A320, A330, A350 and A380 aircraft. This function enables flight crews to either keep the autopilot ON to automatically follow the RA, or to manually perform the RA maneuver using FD guidance (fig.5): ① When a TA is triggered, the –TCAS– mode is armed. ② When a RA is triggered, the –TCAS– mode is engaged and targets a vertical speed 200 ft/min inside the RA green band. ③ When the aircraft is clear of conflict, the TCAS mode then reverts to a –V/S +/-1000– vertical speed mode toward the initially selected altitude.

(fig.5) AP/FD TCAS ensures an optimum handling of the TCAS RAs

The AP/FD TCAS function assists flight crews to:

• Correctly respond to the RA and in a timely manner
• Perform a maneuver only to the extent necessary
• Perform a maneuver with a moderate load factor to ensure passenger comfort and to reduce the risk of injury.
• Prevent the triggering of TCAS alerts on other aircraft.

Possible reversion to the standard procedure

If the flight crew prefers to follow the RA using the standard TCAS warning procedure, they can revert to it at any time. However, an Airbus analysis of more than 130 000 flights performed by A350 and A380 aircraft confirms the confidence of flight crews in the AP/FD TCAS function: in 91% of the RA situations, the flight crew kept the autopilot ON.

An analysis of the in-service fleet shows that only slightly more than 30% of A320 family and A330 aircraft have the AP/FD TCAS function activated. The analysis also shows that a number of A320 family and A330 aircraft have all the system prerequisites installed and only need to activate the function to be able to use it. Operators should contact their Airbus Customer Support Director for information on the service bulletin about activating the AP/FD TCAS function on A320 family and A330 aircraft.

Despite the possibility of retrofitting the AP/FD TCAS function on in-service A320 and A330 aircraft, some Operators may not be able to equip a full fleet with AP/FD TCAS. This should not stop Operators from activating the function on their capable aircraft, because operations with a mixed fleet are possible.

When operating a mixed fleet, flight crews can easily check the Aircraft Configuration Summary (ACS) table in the QRH during cockpit preparation. This will tell them if the aircraft has the AP/FD TCAS function activated.

In addition, flight crews must know how to apply both TCAS warning procedures (with and without AP/FD TCAS function), because even with the AP/FD TCAS activated, they will need to apply the standard TCAS warning procedure if the AP/FD TCAS function is inoperative.

Both procedures are described in the FCOM and QRH Memory Items part [MEM] SURV / TCAS WARNING.

Step 1: If a TA is triggered, select the appropriate procedure to apply

• If –TCAS– is NOT displayed on the FMA, it is not available or not installed. If a TA is triggered, the PF announces “TCAS, I have control” and must be prepared to apply the standard TCAS warning procedure.
• If –TCAS– is displayed as armed on the FMA, the flight crew can use the AP/FD TCAS function in automatic or manual flight. In this case, the PF announces “TCAS blue”, ensures that the autothrust is engaged, and prepares to either fly the potential RA maneuver using autopilot or manually fly the RA with FD guidance.

Step 2: Use the appropriate level of automation to follow the RA

Depending on the availability of the –TCAS– mode, the PF follows the RA with the appropriate level of automation:

• If the –TCAS– mode is not available or does not engage
• when the RA is triggered, the PF disconnects both the AP and the FD and flies the green on the V/S scale as per the standard TCAS warning procedure.
• If the –TCAS– mode engages when the RA is triggered, the PF can either leave the AP ON and monitor that the V/S reaches the green zone on the PFD, or manually follow the RA using FD guidance. The FD bars will automatically appear when the TCAS mode engages if they were previously switched off.

Step 3: Return to the initial trajectory when clear of conflict

When the aircraft is clear of conflict:

• If the RA was flown manually, the PF may adjust both the lateral and vertical trajectory to resume normal navigation in accordance with ATC instructions and may re-engage the AP and FD.
• If the RA was flown in –TCAS– mode, the flight guidance reverts to –V/S +/-1000– mode when clear of conflict to go back to the selected altitude. The PF should then engage an appropriate vertical mode, or adjust the V/S target, in accordance with ATC instructions.

The Airbus Flight Crew Training Standards (FCTS) manuals provide recommendations for both type rating and recurrent training programs on how to train flight crews to apply the TCAS procedures. The manuals contain recommendations to define the training objectives in line with ICAO (doc 9995 Manual of Evidence-Based Training) and IATA recommendations (Evidence-Based Training Implementation Guide). FCTS manuals are available for download from the AirbusWorld portal.

During the type rating and recurrent training courses, flight crews should be trained on how to apply the TCAS procedure both with and without the AP/FD TCAS function. This is to ensure that the flight crew will react correctly to a TCAS warning if the AP/FD TCAS function is not available or if it is inoperative on their aircraft.

Evidence-Based Training (EBT) programs recommend that the TCAS procedure training should be performed at least one time every three years.