Review the Issues Concerning Controller Intervention Buffer

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Review the Issues Concerning Controller Intervention Buffer

40TH ANNUAL CONFERENCE, Geneva, Switzerland, 19-23 March 2001

WP No. 82

Review the Issues Concerning Controller Intervention Buffer


The working paper presented by SC1 at IFATCA 2000 drew on work prepared for RGCSP WG A and the FAA in concluding that Controller Intervention Capability could be defined as:

“ the ability of the ATC system to recognise, specify and deliver corrective action in response to gross errors (with respect to the specified RNP and RCP) in x seconds with a probability of y%”

and that using this definition the value x then provides a definition of the Controller Intervention Buffer (CIB). The working paper also concluded that the concept of “ATC Intervention Buffer” as a fundamental element in the determination of separation required full consideration of technical and human factors under the following headings:

  • Surveillance capability;
  • Communications capability including:
    • System availability and reliability;
    • System integrity;
    • End to end performance;
    • System independence.
  • Sector design including:
    • Volume;
    • Complexity;
    • Traffic density (collision pairs).
  • ATC automation tools available. 

This paper will review some more recent work on the subject and propose policy for inclusion in the IFATCA Manual.


A knowledge of the allowance made (if any) for controller intervention in the formulation of a particular separation standard logically provides the basis by which the applicability of that standard to any given airspace and ATC system may be assessed.

Clearly, any new separation standards should specify comprehensively what performance assumptions have been made about the ATC system in the collision risk assessment.

A review of existing separations in the same context may help to detect deficiencies as well as offering potential improvements in sector design.

A result of this process could well be a list of performance metrics similar to the list in 1.2 above that could be use to assess and monitor the capability and performance of ATC systems. WP/7 presented to RGSP 10 in Montreal in May 2000 proposed an extended methodology for ADS longitudinal separation standards.

This paper includes a reference to a buffer included in the mathematical analysis to allow time for the controller to intervene, convey instructions to the pilot and for the pilot to react and cause the aircraft to achieve a change of trajectory sufficient to ensure that a collision will be averted. No allowance is made here for the controller to formulate the necessary action. Depending on the traffic density and route structure, the time required for this task may not be insignificant. Since the input of aircraft position information is not synchronised to ATC display update in a typical ADS ATC system, there is a need to consider position extrapolation error in the collision risk model. In the case of the model proposed in WP/7 the extrapolation error (at its maximal value) is included with the navigational error, which in turn removes the need to consider the ADS downlink time in the CIB.

This further highlights the complex relationship between navigational performance, communications performance and the performance of the ATC system.

WP/7 Considers three different values for the CIB:

All periodic ADS reports are received and response to CPDLC uplink is received within 3 minutes. Periodic ADS reports are received but response to a CPDLC uplink is not received within 3 minutes in which case an attempt to make contact via HF would be made. A periodic ADS report is lost, and a CPDLC or ADS request for a position update is sent and fails. HF contact would then be established.

In the last two scenarios it is assumed that if HF contact cannot be established then the controller will need to contact one or more other aircraft to ensure separation is maintained. For each of these scenarios values are estimated for a range of parameters relevant to aspects of controller intervention such as screen update, message composition and transit, pilot response and aircraft inertia. These parameters are combined to form the CIB.

Applying these parameters to proposed the collision risk model suggests that a 30nm longitudinal separation standard based on RNP 4 with a maximum ADS periodic reporting interval of 21.5 minutes presents a collision risk (fatal accidents per flight hour) of 4.95 x 10-9 as long as the CIB does not exceed 4 minutes for normal communication means and 10.5 minutes for normal and or other means.

This model makes no specific mention of the time needed for the controller to recognize the ‘blunder’. In fact this allowance is inherent in the two communications scenarios and relies on the standard communications failure response times.

A methodology proposed by the Boeing Airplane Group offers a different view of the Controller Intervention Buffer. This work sees the CIB as being external and additional to published standards stating “The controller must provide buffers larger than actual separation minima in order to minimise the chance of separation infractions.” Most of the important elements of the CIB are recognised in this work but it is assumed that the ATC system and ultimately the controller will dynamically vary the CIB as conditions such as traffic density change.

The question of whether or not to include conflict recognition time in the CIB has been the subject of some debate. The following draft policy includes such an allowance since it is recognized that differing types of surveillance systems will utilize differing conflict recognition methods. Complex software tools such as route probes and STCAs could assist conflict recognition. On the other hand detection of conflict could remain largely procedural as in the model proposed in the RGCSP WP/7.


The two concepts of the Controller Intervention Buffer discussed here show an interesting diversity of approach between suppliers of ATM and aircraft systems and the developers of the standards by which those systems are applied.

This highlights the fact that IFATCA needs to make the controller’s needs known through contact with ATM and aircraft systems developers as well as through its participation in ICAO and other administrative forums.

ATC systems should be developed with the capability to monitor relevant CIC parameters and warn controllers when they fall outside the values used in determining separation standards in use so that alternative standards can be applied.

All of the parameters used to determine the CIB should be part of the documentation of any such separation standard.

It is recommended that:

The Controller Intervention Buffer be defined as:

The time required for the Air Traffic Controller to intervene ensuring that a collision would be averted in the event that a separation standard being applied breaks down. This will include an allowance:

  • to recognize the ‘blunder’
  • to formulate a solution,
  • to convey instructions to the pilot
  • for the pilot to react and cause the aircraft to achieve the required change of trajectory. 

A Controller Intervention Buffer should be included in the development and specification of any separation standard where controller intervention is used as a risk mitigator.


Longitudinal Distance Based Separation Minima in an RNP RNAV Environment: Proposal to Extend the Range of Applicable Minima – RGCSP WG/A WP/8 27/4/98.

Precision Runway Monitor Demonstration Report DOT/FAA/RD-91/5 Feb 1991.

Dr David Anderson – An extended methodology for ADS longitudinal separation standards. RGCSP10 WP/7 May 2000.

Pirrotte, Nakasone, Schwab et al – A proposed Methodology for Operational Enhancement Integration Analysis – ATM Services Boeing Airplane Group. 2000.

Last Update: September 29, 2020  

March 13, 2020   151   Jean-Francois Lepage    2001    

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