Single Person Operations – Four Eyes Principle (4EP)

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Single Person Operations – Four Eyes Principle (4EP)

44TH ANNUAL CONFERENCE, Melbourne, Australia, 17-22 April 2005

WP No. 162

Single Person Operations – Four Eyes Principle (4EP)

Presented by PLC


1.1. Discussions on Single Person Operations at the Hong Kong Conference 2004 brought up another aspect of staffing: the often used term for it is Four Eyes Principle (4EP).

1.2. PLC and corresponding members searched within ICAO, IFATCA and Eurocontrol documents. This principle has never been defined.



Single Person Operations (SPO) can be defined as operations where an operational ATC unit is providing service with only one appropriately qualified controller on duty.

Four Eyes Principle (4EP) can be defined as the situation where an active controller is accompanied by another appropriately qualified controller whose function includes an element of safety net by monitoring the same working area as the active controller does.

Example: a planner controller next to an executive controller on the same sector, approach unit or tower.

2.1.1. Employers are using different labour practices in their efforts to increase the number of aircraft movements handled per person and/or reduce the number of staff required to run the system.

Examples: in Contract Towers in the USA and at ACC-sectors in Germany SPO is introduced to reduce staffing costs.

2.1.2. Because these practices may have potential to adversely effect the safety of the system, their implementation and effect must be closely monitored.

2.1.3. It should be noted that this Working Paper reflects the realities of the ATCO’s working environment, namely that 4EP is not available on a regular basis.

2.1.4. Up till 2005 there is no definition on Four Eyes Principle.

2.1.5. Recent incidents and accidents have shown that there is a lack of 4EP at many ATC working places and that it is a serious element in safety risks. Examples: Überlingen accident, several cases of Level Busts, Tahiti incident.

2.1.6. Often Single Person Operations (SPO) are part of historically implemented staffing procedures. There seems to be a lack of awareness of the effects of the lack of a seconder available to the ATCO in safety management, both on regulator’s and provider’s, as well as on controller’s side.

2.2. Effects on the ATS system

2.2.1. Because workload may increase unexpectedly or uncontrollably, back-up staff should be available on call-in within reasonable time limits to ensure the safe continuation of the service. But it should be noted that in case of an aircraft emergency or any other incident/accident it is unlikely that staff could be called in quickly enough to assist in the increased workload in case of SPO. This is a risk that ANS providers and their customers must be willing to take if they use SPO. At the same time it is a strong argument against the use of SPO.

2.2.2. Studies on Safety Cultures in Organisations do show that safety thinking should be integrated in all actions taken by an Air Navigation Service provider (ANSP). In a safetycritical working environment, redundancy in system components is a requirement, including the human side of the system.

2.3. Effects on ATCOs

2.3.1. Fatigue influences the controller’s abilities and leads to decreased alertness and low motivation. A seconder functions may help to mitigate the effects of stress and fatigue that the ATCO may be experiencing.

2.3.2. Immediate relieve should be available in case of an incident/accident.

2.3.3. Relative information on the resulting effects of SPO can be found in the following areas of the IFATCA Manual:

Page 4112 paragraph 1.6 – Single Persons Operations (SPO)

1.6.1 Rostering Single Person Operations (SPO) shall be avoided. In the unlikely event of unavoidable SPO appropriate measurements shall be taken to ensure that the SPO-situation will be alleviated as soon as possible. Until such time measures shall be taken to mitigate all impacts of SPO such as: traffic regulation, provide breaks, informing neighbouring ATC units. Procedures shall be in place to implement such measures in an efficient way, not increasing the workload of the ATCO.

1.6.2 The use of single controller shifts should be strongly discouraged by MA’s, both through their providers as their regulators.

1.6.3 When providers choose to use SPO, they must bear the responsibility for the resulting risk(s) to the system. The ATCO must not be held liable for incidents or accidents resulting from the use of SPO.

(Geneva 01.C.3 amended Hong Kong 04.C.10)


Page 4223 paragraph 2.5 – Fatigue in ATC

2.5.3. The provision of a satisfactory working environment, appropriate rostering, rest periods, facilities, use of overtime, relief controllers and education in human factors shall be agreed with the air traffic controllers involved. Attention must be given to individual differences, age and gender (Toulouse 98.C.5).

2.5.4. In exercising the responsibilities of designing of duty rosters (page 4131, para 3.1.4 refers), management shall be responsible for providing physical arrangements (relief controllers and adapted rest area) and sufficient break periods for controllers to try to maintain their daily eating habits regardless of which shift they are working. Such physical arrangements and sufficient break periods shall be provided to allow for strategic naps during night shifts (Toulouse 98.C.5).

2.5.5. Management shall approve the implementation of strategic naps as an effective way of improving alertness and anchoring the circadian rhythms of controllers during night shift. A strategic nap is defined as a short period of sleep taken at specific times during a night shift. Recommended duration of a strategic nap varies from maximum 20 minutes for a nap early in the night to maximum 50 minutes late in the night (after 4 am) (Toulouse 98.C.5).

2.5.6. Management shall, in close co-ordination with the air traffic controllers involved, carefully consider staffing levels during night shifts. For those controllers who have very heavy traffic loads during the night shifts, additional relief should be considered as an appropriate countermeasure to sleepiness and fatigue in order to increase the safety margins, and to reduce subsequent daytime sleepiness (Toulouse 98.C.5).



3.1. 4EP improves redundancy in the human element of the ANS system. This can lead to a higher safety level of the whole system.

3.2. 4EP may decrease the level of stress and fatigue under which an ATCO is operating. It is a safety net should the active ATCO be distracted.

3.3. The minimum level of staffing should, for these reasons already, be more than one ATCO.

3.4. The 4EP should be strongly encouraged by MA’s, both through their provider as their regulators.


4.1. It is recommended to insert in the IFATCA Manual on page 4113 new paragraphs 1.7, 1.7.1, and 1.7.3 and to renumber existing Para 1.7. as 1.8.

1.7 Four Eyes Principle

1.7.1. Another aspect of SPO is the lack of the Four Eyes Principle. Definition: Four Eyes Principle (4EP) can be defined as the situation where an active controller is accompanied by another appropriately qualified controller whose function includes that of safety net by monitoring the same working area as the active controller.

1.6.3. Implementation of 4EP shall be strongly encouraged by MA’s, both through ANSPs and regulators.

1.6.4. Individual ATCOs shall not be held liable for incidents or accidents resulting solely or in part from the non-implementation of the 4EP safety net.


Fatigue in Air Traffic Controllers: Literature Review Transport Canada, Transportation Development Centre, 2000.

Focus on Aviation Safety – Prof. Patrick Hudson, Leiden University, 2002. – BFU/Überlingen accident 2002. Investigation Report AX001-1-2/02 May 2004

Tahiti incident 2003

Single Person Operations – Paul Neering, IFATCA Circular June 2003, The Controller 2003.

WP C.5.18 Hong Kong 2004. Safety by the EB of IFATCA.

Last Update: September 29, 2020  

March 26, 2020   1267   Jean-Francois Lepage    2005    

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