This article is intended to give an overview of the occurrences where a TCAS RA has not been followed (correctly) by the flight crew. It describes the possible scenarios and the safety barriers used to prevent or mitigate the safety effects of such occurrences.
TCAS is one of the last safety barriers to protect from an accident, the ones after it being pilot visual response (with limited applicability in Instrument Meteorological Conditions (IMC)) and providence (i.e. luck). Therefore, ICAO documents state that all RAs should be followed unless the safety of the aircraft is at risk.
One of the most common reasons for not following a TCAS RA is the visual acquisition of (what is perceived to be) the conflicting traffic. It is worth noting, however, that the flight crew might not be fully aware of the overall picture in complex traffic situations and therefore visual acquisition is not a valid excuse for procedural non-compliance.
A dedicated study was performed in 2017 by the EUROCONTROL Safety Improvement Sub-Group (SISG) after identifying "TCAS RA Not Followed" as one of the Top 5 ATM Operational Safety Priorities. The SISG's analysis examines the operational context (factors that can affect the situation) and establishes a set of generic scenarios that account for different situational developments. While the scenarios are not equally probable (and data collected from both A&I events and a dedicated survey confirm they are not) they do constitute a useful, concise representative subset that substitutes effectively for trying to imagine all possible scenarios. The scenarios are grouped in the following categories and subcategories:
- Lack of pilot detection/awareness of the TCAS RA due to:
- Human-machine interface (Human-Machine Interface (HMI)) issues (i.e., factors concerning the perception and interpretation of the TCAS HMI and alert annunciations);
- Presence of other flight deck signals and alerts, or external distractions, occurring simultaneously with the TCAS RA; and/or
- Flight deck mindset (i.e., any factor concerning the mind-set or expectations of the crew immediately beforehand).
- Misinterpretation of TCAS RA due to:
- Decision not to follow RA due to:
- Other signals and alerts;
- Contradictory ATC instructions (this situation also includes prior ATC instructions being actioned when the RA occurs);
- TCAS credibility issues (e.g. if the RA is perceived as false/unreliable or due to rapid succession of different RAs);
- Aircraft performance issues/concerns (actual or perceived aircraft performance limitations);
- Flight deck mind-set;
- Significant flight conditions (e.g. cockpit warnings, severe weather, terrain, etc.); and/or
- Inadequate TCAS training.
- TCAS RA response not executed correctly due to:
- HMI Issues;
- Late or inadequate pilot response;
- Unintentional pilot response;
- Aircraft performance issues/concerns;
- Flight deck CRM;
- Inadequate TCAS training;
- Flight deck mind-set;
- Duration of RA less than the pilot's reaction time; and/or
- Flight deck control inputs and handling (e.g. inappropriate vertical speed when approaching the selected flight level).
Two major sets of barriers can reduce the risk associated with TCAS RA not followed events:
- Prevention barriers: When deployed and employed correctly, these defences are capable of preventing a TCAS RA not being followed. Three prevention barriers have been identified:
- Following a TCAS TA: TCAS RA Prevention Functionality by Auto Pilot reducing rates of climb/descent approaching selected level.
- Enhanced TCAS training including experience with multiple tracks, conflict-aircraft crossing and reversal RAs; and,
- Training and enforcement of standard operating procedures (SOPs) requiring TCAS RAs to be followed unless the safety of the aircraft would be compromised
- Mitigation barriers: When deployed and employed correctly, these are capable of alerting pilots to a potential conflict during the initial stages of an event involving not following the commands of an activated TCAS RA, in sufficient time to act in order to prevent a collision. One mitigation barrier has been identified:
These barriers are identified in the TCAS-dedicated SISG study and are based on a wide literature search and consultation.
Analysis of the effectiveness of the safety barriers against the generic scenarios has shown that the three prevention barriers cited above achieve limited effectiveness (i.e. they are effective in a small number of situation types). However, the "Training and enforcement of SOPs requiring TCAS RAs to be followed" barrier could prevent the events where pilots detect and interpret the RA but decide not to follow it. Experience has shown that the decision not to follow a TCAS RA is the most common cause for such events. Therefore, this barrier could be beneficial from a practical point of view.
In contrast, the mitigation barrier cited above achieves high effectiveness (i.e. is effective for most of the generic scenarios). However, this capability currently is only available on new models from one manufacturer, with a retrofit being available for older fleet aircraft. For the time being, it is unlikely that this capability will become available from any other manufacturer due to patenting issues.
TCAS RA Not Followed A&I Examples
On 23 February 2018, an Embraer 195LR and an Airbus A320 on SIDs departing Brussels lost separation after the 195 was given a radar heading to resolve a perceived third aircraft conflict which led to loss of separation between the two departing aircraft. STCA and coordinated TCAS RA activations followed but only one TCAS RA was followed and the estimated minimum separation was 400 feet vertically when 1.36 nm apart. The Investigation found that conflict followed an error by an OJTI-supervised trainee controller receiving extended revalidation training despite gaining his licence and having almost 10 years similar experience in Latvia.
On 4 June 2016, a Boeing 737-800 instructed to climb from FL340 to FL380 by the controller of one sector in Bulgarian upper airspace came into sufficiently close proximity to an Airbus A320 under the control of a different sector controller to trigger co-ordinated TCAS RAs. Separation was eventually restored after the 737 followed its RA despite the A320, which had already deviated from its clearance on the basis of a prior TCAS TA without informing ATC, ignoring their RA. The Investigation found that the root cause of the conflict had been inadequate coordination between two vertically separated ATC sectors.
On 10 June 2011 an ATC error put a German Wings A319 and a Hahn Air Raytheon 390 on conflicting tracks over Switzerland and a co-ordinated TCAS RA followed. The aircraft subsequently passed in very close proximity without either sighting the other after the Hahn Air crew, contrary to Company procedures, followed an ATC descent clearance issued during their TCAS ‘Climb’ RA rather than continuing to fly the RA. The Investigation could find no explanation for this action by the experienced crew - both Hahn Air management pilots. The recorded CPA was 0.6 nm horizontally at 50 feet vertically.
On 20 November 2011, a problem in reading the altitude labels on the ATC radar control display led to a Finnair Boeing 757 being cleared to make a descent which brought it into proximity with a Thomas Cook Boeing 757 in day VMC. Co-ordinated TCAS RAs were generated onboard both aircraft but when the Finnair aircraft failed to respond to its Climb RA and continued descent, the other aircraft, which had responded correctly to its initial RA, received a further RA to reverse their descent to a climb. The Finnair aircraft reported retaining visual contact with the other aircraft throughout.
On 7 February 2011 two Air Inuit DHC8s came into head-to-head conflict en route over the eastern shoreline of Hudson Bay in non radar Class A airspace when one of them deviated from its cleared level towards the other which had been assigned the level 1000 feet below. The subsequent investigation found that an inappropriate FD mode had been used to maintain the assigned level of the deviating aircraft and noted deficiencies at the Operator in both TCAS pilot training and aircraft defect reporting as well as a variation in altitude alerting systems fitted to aircraft in the DHC8 fleet.