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Assessment of Pilot Compliance with TCAS RA Using Radar Data

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Category: Safety Nets Safety Nets
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The development and implementation of the Traffic alert and Collison Avoidance System (TCAS) was driven by aviation accidents. When there is a risk of collision, TCAS will issue a Resolution Advisory (RA) telling pilots how to change the vertical rate to avoid a collision, so a prompt and accurate pilot response to all RAs is particularly important. Late or incorrect responses may degrade safety.

The Network Manager implemented a dedicated risk identification and monitoring process and identified the Top 5 Operational Safety Hazards. One of these hazards is TCAS RA not followed.

About the study

In 2020, a study has been carried out to support the TCAS RA not followed operational risk in order to provide operational data assessing pilot responses to TCAS RA, as well as TCAS operational mode and serviceability. To conduct this study, Mode S radar recordings from core European airspace were collected for the period of one year and analysed to assess if pilots are responding to TCAS RA as required.

The analysis covered 1184 RAs of over 8 second duration, i.e. the duration long enough to give the pilot a chance to respond. Nominally, pursuant to ICAO provision, a response to an initial RA is expected within 5 seconds.

The pilot compliance criteria were based on the Guidance Material jointly developed by IATA and EUROCONTROL. Following the Guidance Material, each RA has been classified as ‘Followed’ (the pilot’s reaction was correct and the anticipated vertical rate was achieved); ‘Not Followed – too weak’ (the vertical rate was not sufficient); ‘Opposite’ (the RA was flown in the opposite vertical); ‘Excessive’ (the response exceeds the required vertical rate).

Summary of results

The result of the study can be summarised as follows:

  • The overall RA compliance (for all RAs) showed that 38% of RAs were flown with the required vertical rates, while 34% of responses were flown in the opposite direction and 24% were too weak.
  • Opposite responses to RAs (i.e. those response carrying the highest risk) were as high as 40% for Level Off RAs (meaning the pilots increased the vertical rate, rather than reducing it) and 18% for Climb RA.
  • Climb and Descend RAs had the highest proportion of too weak responses (57 and 65% respectively).
  • There was no significant difference in the distribution of pilot responses at various altitude layers.

A subset of the data used in the above-mentioned study was used to assess operations with TCAS out of service or with TCAS in a TA-only mode. On average, 122 flights were conducted daily in core European airspace with TCAS out of service. Fourteen aircraft were observed operating without TCAS for two weeks. Analysis of operations in a TA-only mode recorded on average 50 aircraft a day (i.e. exceeding the time limit allowed by the applicable Minimum Equipment List).

The study did not look into the reasons for non-compliance, which should be researched separately.

The detailed results of the study and explanation of the methodology can be found in this report. The results are also summarised in this presentation.

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