Background

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

TCAS RA not followed by the pilot has been identified as one of the Top 5 Operational Safety Hazards in Europe by the Network Manager (EUROCONTROL) within the scope of their annual operational risk identification and monitoring process.

Methodology of pilot compliance with TCAS RAs assessment

RAs are rare events, but when they occur, the situation may be critical, thus correct, and immediate flight crew action is required, unless it would jeopardize safety of the aircraft. Any delayed or incorrect flight crew response may negate the effectiveness of the RA, their actions will be the most important single factor affecting the performance of the TCAS system. 

If the pilots decide not to respond to an RA, they not only negate the safety benefits provided by their own TCAS system, but also jeopardize the safety of all other aircraft involved in the encounter. 

In order to support aircraft operators and ANSPs in their investigations to determine whether the response to TCAS RA was timely and correct, a Pilot TCAS RA compliance tool has been made available. 

The tool calculates and grades (based on the IATA/EUROCONTROL Guidance Material Method B) the RA response by the flight crew. The following pilot response categories are used:

•    Following: the pilot’s reaction is consistent with a manoeuvre towards the required vertical rate, and with the anticipated acceleration and reaction time (including the required vertical rate being achieved);

•    Weak Response: the pilot has made an adjustment in vertical speed in the required direction, but insufficient in terms of vertical speed or acceleration to fulfil the requirement;

•    No Response: any change in the vertical speed is within the measurement noise and therefore no response is registered;

•    Opposite: the change in vertical speed performed by the pilot is in the opposite vertical sense compared to the instruction generated by the TCAS;

•    Excessive: the response exceeds the required vertical rate.

The above-mentioned response categories are assigned considering the following:

•    The pilot has to process and respond to the RA and is allowed a time budget of 5 seconds to start the required manoeuvre (as per the standard pilot response as defined in the ICAO ACAS Manual); 

•    The pilot is expected to maintain a vertical acceleration of 0.25g;

•    The vertical rates provided in the radar recordings are subject to noise / altitude quantisation and tracker lag; therefore to take account of this imperfect data a vertical rate tolerance of ±300 ft/min. is allowed for in the assessment of pilot compliance.

The assessment method compares the achieved vertical rate with certain pre-defined thresholds to determine the compliance. The thresholds are dynamic (varying with the time of evaluation) and take account of a response time and an acceleration phase as defined in the standard pilot response model. 

More information about the assessment of pilot compliance with TCAS RAs and the method used can be found in the IATA/EUROCONTROL Guidance Material. (Note: The Guidance Material covers two Methods – A & B. The tool herewith covers only Method B, as it is more sophisticated and nuanced).

The Pilot TCAS RA Compliance Tool

Limitations & Conditions

The RA Compliance Tool is intended for post-event operational evaluations of pilot compliance to ACAS Resolution Advisories. Neither the tool nor any of its code is to be used in any operational system.

The RA Compliance Tool supports the evaluation of the following airborne collision avoidance systems:

•    TCAS v7.0

•    TCAS v7.1 

•    ACAS Xa CP1

No other systems are supported.

The RA Compliance Tool relies on accurate input data, specifically vertical rate measurements/estimates and timing data. It is the responsibility of the user of the Tool to ensure the data is sufficiently accurate and reliable.

The tool is provided free of charge and may be used solely for the user’s own purposes and may not be modified or distributed to third parties.

IATA and EUROCONTROL do not accept any liability for any inaccuracies in the results presented by this tool, nor recommend any specific course of action to be taken as a result of evaluations using this tool.

IATA and EUROCONTROL do not give warranties of any kind and shall in no event be liable for damages in connection with the use of this tool.

How to use the tool?

1.    Confirm that you have read and understood the Disclaimer.

2.    From the pull-down menu select the collision avoidance system that the aircraft was equipped with.

3.    Select the RA sense.

4.    Select the RA strength (type). 

5.    Enter the aircraft vertical rate in fpm (ft/min.) at the time of RA. Precede the number with a minus sign (–) for descent rates.

6.    If the RA requires a specific vertical rate (e.g., a Maintain Vertical Speed RA) enter the required rate in fpm. Otherwise, leave it blank.

7.    In the evaluation window, enter the time at which the compliance should be assessed, in seconds after the time when the RA was issued.

8.    Enter the vertical rate in fpm which was achieved at the time which is assessed. Precede the number with a minus sign (–) for descent rates. Do not evaluate the pilot response at the beginning of the RA. Start your evaluation at 6 seconds in order to achieve an objective view of the pilot response. 

9.    Optional: enter data for up to two subsequent times.

10.    Press “Evaluate Initial RA!” button. The evaluation category will be shown at the bottom of the window.

11.    Optional: repeat the steps described above for a subsequent RA.

12.    The evaluation results can be saved as a PDF file – select “Download as PDF” button.

For illustration purposes, a generic IVSI will show aircraft’s vertical speed at the time of the RA and indicate red and green bands corresponding to the RA. The corresponding TCAS aural annunciation is shown underneath the IVSI.

Only single-threat RAs can be evaluated using the tool.

Press the “Reset” button before starting a new evaluation to clear all the previous entries.

Launch the Pilot TCAS RA compliance tool

Nominal response delay

The nominal response delay and nominal acceleration constitute the “standard pilot response” which is implicit in the collision avoidance algorithms:

• for an initial corrective RA, the pilot is assumed to initiate a response to the RA after a delay of 5 sec. and to accelerate the aircraft to the required vertical rate with an acceleration of 0.25g;
• for a subsequent corrective RA, the pilot is assumed to initiate a response to the changed RA after a delay of 2.5 sec. and to accelerate the aircraft to the required vertical rate with either:
  • an acceleration of 0.25g, for ‘normal’ RAs; or
  • an acceleration of 0.35g, for ‘exigent’ RAs (‘reverse-rate’ RAs and ‘increase-rate’ RAs).

    Note: The value of g (the acceleration due to gravity) is taken as 9.8 m/s2 as per the ICAO standard atmosphere model.

Related Articles

•    Assessment of Pilot Compliance with TCAS RA Using Radar Data

•    Airborne Collision Avoidance System (ACAS)

Further Reading

•    Guidance Material: Performance assessment of pilot compliance with Traffic Alert and Collision Avoidance System (TCAS) advisories using Flight Data Monitoring (FDM), IATA and EUROCONTROL, 3rd Edition, January 2022 (note: downlink from IATA website, file size 99 MB).

•    The Assessment of Pilot Compliance with TCAS RAs, TCAS Mode Selection and Serviceability Using ATC Radar Data, EUROCONTROL, 2nd ed., 10 February 2022.

•    Traffic Alert and Collision Avoidance System (TCAS) - Selected Statistical and Performance Data in Core European Space, EUROCONTROL, 15 February 2021.

•    HindSight 31 - Assessment of pilot compliance with TCAS RA

•    EUROCONTROL ACAS Guide