Mixed mode operations play an important role in current day ATM. However, a common understanding seems to be missing. Therefore this paper will first outline the meaning of this term so to have a clear understanding to all. Secondly, the paper will describe the expected future end-state in relation to mixed mode operations. Subsequently, the paper will describe the move from current operations to that expected end-state and describing the relationship with intrinsic and tactical safety.

This paper reviews IFATCA Policy. As IFATCA Policy is rated not as restrictive as desired, new Policy is proposed.

1.1  To increase performance (e.g. safety and efficiency), the Air Traffic Management (ATM) system is subject to continuous development. All these developments could increase the mixed mode environment of the ATM system through either exemptions of non-mandatory equipage.

1.2  IFATCA Policy on mixed mode operations has been accepted in 2003 (WP96):

This Policy has been often quoted over the years when discussing the introduction of new procedures and systems, but has been rated to be not as restrictive as desired. On the other hand, the content of the Policy appears to be very hard to achieve, as it is very hard to produce a system safety analysis (of the whole system) when introducing technology that results in mixed mode operations.

2.1 How to define mixed mode operations?

2.1.1  The most simplified ATM system would contain no exemptions. There would be only one wake turbulence category, one type of surveillance, one type of communication, one standard for navigation, etc. etc. As this is not quite the case, mixed mode operations are the standard method of operation.

2.1.2  Whenever a technological change with exemptions or non-mandatory equipage or procedure is introduced, this results in a mixed mode operation. Over time this has resulted in an enhanced ATM system with many types of mixed mode operations.

Examples of those mixed mode operations are:

• Wake Turbulence Categorisation
• Instrument Flight Rules / Visual Flight Rules
• Operational Air Traffic / General Air Traffic
• Secondary Surveillance Radar (SSR) / Automatic Dependent Surveillance – Broadcast (ADS-B) / Wide Area Multilateration (WAM)
• Reduced Vertical Separation Minima (RVSM) / Non – RVSM
• 8.33 kHz channel spacing
• Navigation standards
• Different datalink technologies

2.1.3  In the near future, the ATM system will be even more fine-tuned. A range of measures will be adopted to enable the passage of more aircraft in a more efficient way.

For example:

• Airborne Separation Assistance System (ASAS)
• Unmanned Aerial Systems (UAS)
• Multiple route structures (2D, 3D and 4D routing)
• Flexible airspace design

2.1.4  As a result of these new developments, the ATM system is becoming more fine- tuned to the needs of airspace users. This ATM system will be able to accommodate all sorts of airspace users with different equipage and different objectives. This capability allows a new technology to be introduced, for which airspace users may not be regulated for this change, but can continue to operate with the existing technology.

2.1.5  Many introductions of mixed mode operations have also resulted in benefits for the individual air traffic controller. An example is the introduction of ADS-B surveillance where only partial radar coverage exists. So mixed mode operations are certainly not always disadvantageous for the air traffic controller.

2.1.6  It is the individual air traffic controller who is often used to mitigate the differences in procedures due to variances in airspace users characteristics and/or the ATM design. These may only be indicated by a single character difference on a strip or in a label. The more the requirement for individual highlights in the ATM system, the higher the chance of confusion or of overlooking its significance.

2.1.7  For air traffic controllers, there will need to be further modal considerations:

• What onboard equipage has the aircraft?
• What route structure is the aircraft flying?
• Who is responsible for separation?
• Is the aircraft manned of unmanned?

Is it possible for the controller to consider these many differences with every decision that has to be made? This answer is only qualified by the extent of the complexity and mirrored workload. The change will become too complex and without the appropriate safety mitigation in place if these components aren’t given appropriate consideration as a whole. The default answer to the question then has to be ‘no’ unless the system fully supports the change.

2.1.8  In many ATM systems over the world, different mixed mode operations have resulted in complexity levels that are bordering on human capabilities. This complexity is the result of progressive addition of new technologies and procedures to a point where the controller (not the system) is required to be part of the mitigation.

2.1.9  In some cases, an introduction of a new technology does not lead to different procedures for the individual air traffic controller and so the controller does not need to consider the differences between these technologies. For example, mixed data link technologies are of no consideration to the individual air traffic controller when the ATM system is provided with dual stack technology and system procedures for interaction regardless of the type of data link. Despite of different technologies, there is only a single operation for the air traffic controller.

2.1.10  Again, if aircraft follow the same precision approach path, regardless of the technology in use (Instrument Landing System (ILS) or satellite signals) they may be equally managed without introducing new considerations to air traffic control operations. But where the difference in navigation capability implies by necessity that the controller must identify and select among a complex set of alternatives, a new window of opportunity for various human related active failures are introduced into the system.

2.1.11  In conclusion, the availability of different modes allows flexibility to the airspace user, but also could make the ATM system more complex. This in turn creates complexity that could lead to higher workload and requires deliberate consideration.

2.1.12  TOC believed a necessary step was to introduce a definition for mixed mode operations. Mixed mode operations can be defined as:

Mixed Mode Operations are ATM Operations that require different procedures due to variances in airspace users characteristics and/or ATM design within the same area of controller responsibility.

2.1.13  Mixed technologies does not necessarily lead to mixed mode operations. The result is ‘mixed mode operations’ only when ATC is expected to apply different procedures as a result of mixed technologies.

2.1.14  Mixed mode operations are sometimes confused with mixed mode runway operations, where a single runway is used for takeoff and departure at the same time. However, mixed mode runway operations are a possible example of mixed mode operations, if by design the ATM system does not adequately support the ATC in its application.

2.2 What is the future (end-state) in relation to mixed mode operations?

2.2.1  The IFATCA Statement on the Future of Global ATM discusses the issue of mixed mode operations:

“Future ATM systems should be designed on the basis that constant incremental improvements will be made, and so the ATM system should be designed for mixed mode operations. This will eventually be a safer and more robust system, but considerable work is required in the design and operation of a mixed mode system. For example, the system may be designed to accommodate only a certain amount of mixed-mode operations which then requires monitoring of the level of mixed-mode operations and procedures that ensure that the level is never exceeded.

However once established, mixed mode ATM will permit aircraft to use improved systems immediately (and not at some future “implementation date”). Also a new ATM service may be provided that, for a fee, assists an aircraft meet a particular performance requirement to use a particular ATM functionality. This example would enable the airspace user to choose whether to retro-fit an aging aircraft or whether it is more cost effective to use the service offered by the ATM service provider until the aircraft is retired from service.”

2.2.2  The IFATCA Statement on the Future of Global ATM clearly encourages a mixed mode environment to benefit the aviation community immediately from new developments. Should a mixed mode environment be unacceptable, incremental changes, historically attuned to the aviation industry would cease. For example, to equip every single aircraft with a new technology is extremely difficult to achieve.

2.2.3  Alternatively, the IFATCA Statement on the Future of Global ATM states that a lot of consideration should be given to the design and operation of a mixed mode ATM system. The most important element in this mixed mode environment is the individual air traffic controller, who has to moderate the mixed mode environment.

2.2.4  ICAO Doc 9854 Global ATM Operational Concept mentions the issue of mixed mode operations as well:

“2.1.6 Airspace user operations refer to the ATM-related aspect of flight operations. Key conceptual changes include:
a) the accommodation of mixed capabilities and worldwide implementation needs will be addressed to enhance safety and efficiency;

and

“2.2.9 Priority for the use of specific airspace will not be constrained by the primary usage or equipage on a routine basis. While it is recognized that airspace designation is useful, it should not be organized in a manner that permanently precludes the possibility of mixed usage/mixed equipage operations.”

ICAO also encourages the accommodation of mixed capabilities in airspace designation. This should not preclude the possibility of mixed usage/mixed equipage operations that could, if not properly supported, lead to mixed mode operations for ATC.

2.3 How do we move from current operations to that expected end-state?

2.3.1  Air traffic controllers are trained to deal with mixed mode operations. This capability has allowed for the transition to environments of complex mixed mode operations. However, the complexity as a result of mixed mode operations has continually increased and appears to soon be on the limits of performance for the individual air traffic controller. In several current ATM systems, any new introduction of a mixed mode could result in an unacceptable situation.

2.3.2  As most new introductions will contain advantages either for the airspace users, the ATM system, or both, the ATM community should find a way to deal with an increased mixed mode environment to realise the potential benefits.

2.3.3  So the question should not be if the ATM system should accommodate mixed mode operations but how it should do so. It should be recognised that there is a limitation to the number of different exemptions in a given airspace and that these will have an impact on safety and capacity.

2.3.4  At the 47th IFATCA conference in Arusha, Tanzania, committee C accepted policy on the professional aspects of the difference between intrinsic and tactical safety in the aerodrome domain. While the direct application from the working paper relates to the aerodrome domain, the principles of ‘intrinsic’ and ‘tactical’ safety also apply to mixed mode operations.

2.3.5  Intrinsic Safety is defined as:

2.3.6  Where the design of the ATM system is not intrinsically safe, safety levels can only be met by introducing procedures requiring human action. This human contribution is known, in this particular case, as tactical safety.

2.3.7  Tactical Safety is defined as:

As mixed mode operations are ATM operations that require different procedures due to variances in airspace users characteristics and/or ATM design within the same area of controller responsibility, these procedures, requiring human action, are a form of tactical safety.

2.3.8  IFATCA Policy (again in relation to the aerodrome/airport) on intrinsic and tactical safety is:

While this policy is relating to the aerodrome domain, TOC believes it is just as applicable to the introduction of new technology and procedures that apply to mixed mode operations.

2.3.9  Operations not dependent of procedures requiring human action are seen as a form of intrinsic safety. Let us take the previous example of two different data link technologies within an Area Of Responsibility (AOR). In this example the ATM system is provided with a dual stack capability and there will be no consideration for ATC (of which data link technology is in use). It is the ATM system itself, which processes the message in the correct format. Despite the different technologies, the system design is intrinsically safe without ‘mixed mode operations’.

2.3.10  Operations dependent of procedures requiring human action are seen as form of tactical safety. In the example above, should the ATM system not be provided with a dual stack, the individual air traffic controller would need to consider which technology was in use and apply different procedures to each. In this scenario the system is subject to a procedure requiring a human action and the system relies on tactical safety.

2.3.11  IFATCA Policy is giving the highest priority to intrinsic safety and so mixed mode operations should be kept to an absolute minimum. Whenever possible, the ATM system should be designed in an intrinsically safe way. This will minimize the amount of mixed mode operations for the individual air traffic controller. History has demonstrated that the controller has the capability to manage mixed mode operations. Mixed mode operations are in use and applied safely by ATC, but it should be recognized that there are limitations. In recent years the demands for efficiency and responsible environmental management has forced the ATM system to continually introduce new technology. In safeguarding this environment for the controller, mixed mode operations need to be mitigated by correct system support, both in the identification and application of these operations.

2.3.12  In the current ATM system, efforts should be undertaken to minimize the amount of mixed mode operations. This should be done by continually reviewing current procedures and introducing intrinsically safe solutions for existing mixed mode operations.

2.3.13  Despite the priority for intrinsically safe solutions, the design of the ATM system cannot be 100% intrinsically safe. It will always remain dependent of tactical safety as well. The total safety of the ATM system is the sum of the intrinsic safety of the ATM system added with the tactical safety of the ATM system- the balance determining that the target level of safety is met.

2.3.14  To benefit to the maximum from new technologies it will sometimes not be possible to design the ATM system in an intrinsically safe way. For example, a higher navigation standard could enable aircraft to fly green approaches. However, if not every single aircraft is equipped, introducing these approaches will introduce a mixed mode operation as the identification and assignment of these green approaches are determined and managed by the individual air traffic controller (tactical safety). Conversely, not introducing these new approaches (intrinsically safe) will not deliver the environmental benefits.

2.3.15  To keep the ATM system flexible, tactical safety and mixed mode operations will remain. For example, an ATM system with only one single wake turbulence category will not meet the required capacity and efficiency at many airfields. The introduction of a new mixed mode should be considered as part of the complete ATM system. It should consider the benefits of it, as well as the complexity for ATC operations.

2.3.16  The first part of the IFATCA Policy states:

This Policy statement appears to be hard to achieve, as it is almost impossible to conduct a ‘system wide’ safety analysis. Additionally, the mixed mode environment within the ATM system and any new introduction of a mixed mode operation must be proven to be safe according to all existing safety standards. While normal safety processes may have been followed, a mixed mode environment that relies primarily on tactical safety, must have consideration given to the workload of the individual air traffic controller. This Policy is therefore recommended for deletion.

2.3.17 The second part of the IFATCA Policy states:

This Policy statement only mentions capacity levels, which need to be revised to maintain the required level of safety. TOC believes it is very important to stress the need for intrinsically safe solutions. Whenever this is not possible, and a mixed mode operation is introduced, assessment should take place that the change in the ATM system does not increase controller workload to an unacceptable level. This Policy is therefore recommended for deletion.

3.1  Mixed mode operations are defined as ATM Operations that require different procedures due to variances in airspace users characteristics and/or ATM design within the same area of controller responsibility.

3.2  The move towards ATM in which more variances in airspace users characteristics and/or ATM design are the rule rather than exception, will put more emphasis on the need to find intrinsically safe solutions.

3.3  Also in current ATM system design, efforts should be undertaken to reduce unnecessary complexity for the individual air traffic controller by finding intrinsically safe solutions.

3.4  In case the ATM system relies on tactically safe solutions, assessment must be made to avoid an increase in controller workload to an unacceptable level.

It is recommended that;

4.1  Mixed Mode Operations are defined as:

Mixed mode operations are defined as ATM Operations that require different procedures due to variances in airspace users characteristics and/or ATM design within the same area of controller responsibility.

And that this definition is included on page 3 2 3 14 of the IFATCA Technical and Professional Manual.

4.2  IFATCA Policy is:

Efforts should be undertaken to reduce existing Mixed Mode Operations by creating intrinsically safe solutions.

And is included on page 3 2 3 14 of the IFATCA Technical and Professional Manual.

4.3  IFATCA Policy is:

Introductions of new Mixed Mode Operations should be avoided by creating intrinsically safe solutions.

And is included on page 3 2 3 14 of the IFATCA Technical and Professional Manual.

4.4  IFATCA Policy is:

When safety of a Mixed Mode Operation cannot be completely managed at an intrinsic level, assessment must take place that the change in the ATM system does not increase controller workload to an unacceptable level.

And is included on page 3 2 3 14 of the IFATCA Technical and Professional Manual.

4.5  IFATCA Policy on page 3 2 3 14 of the IFATCA Technical and Professional Manual:

In addition to the individual safety case, a system safety analysis should be conducted at the introduction of each and every technology which results in mixed mode operations.

The role of the controller as the mitigation must be considered in the context of the level of integration and, for safety assurance, capacity levels must be revised to maintain the required safety level.

is deleted.

IFATCA Statement on the Future of Global ATM.

ICAO Doc 9854 Global ATM Operational Concept.

IFATCA WP 169 ‘Investigate the professional aspects of the difference between intrinsic and tactical safety in the aerodrome domain’ (Arusha 2008).

IFATCA WP 96 ‘Mixed mode operations’ (Buenos Aires 2003).