(photo: DFS Deutsche Flugsicherung)

Thu 11 July 2019 – A Continuous Descent Operation (or CDO) is an aircraft operating technique where an arriving aircraft descends with minimum thrust, ideally from top of descent, and avoids level flight to the extent permitted by the safe operation of the aircraft. The descent is performed in compliance with published procedures and ATC instructions. A CDO operation – enabled by airspace and procedure design, and facilitated by ATC – can provide substantial noise, fuel and CO2 savings, reports EUROCONTROL’s David Brain.

A CDO is a natural technique that can be flown by pilots in any aircraft: from an eight-engine behemoth B52 bomber or a single-engine piston aircraft. It is often referred to as being taught as ‘flying lesson number 2’ since the standard descent technique is to pull back the throttle and use the aerodynamics of the wings to glide down the descent path until you need to configure the aircraft for the final approach. ‘Flying lesson number 1’, of course, is teaching the pilot how to take off. Thus, the flying of efficient vertical flight profiles should be ingrained in a pilot’s mentality from day one. Despite being a natural flying technique, it is not always possible to fly a CDO or its departure equivalent, the CCO (Continuous Climb Operation), due to considerable traffic growth and airspace complexity, particularly in Europe.

Previous studies under the banner of SESAR (the Single European Sky ATM Research Programme) – together with individual studies focused on specific airports, city pairs, or airlines – have attempted to quantify what level of environmental savings in terms of noise and emissions are available by flying CCO and CDO compared to non-CCO or non-CDO profiles. On average, these studies concluded the fuel savings from a CDO flight (compared to a non-CDO flight) range anywhere from 50 to 150kg as there are many factors that can have an impact on the level of benefits. Several studies have also demonstrated that CDO can reduce noise by 1-5dB, compared to a non-CDO operation. However, as each study had a separate and local focus it was not previously possible to identify how representative each set of results was, or to extrapolate potential performance benefits to the network level.

In 2016, ICAO concluded a global environmental analysis of the estimated fuel and emissions savings of the global Aviation System Block Upgrades (ASBU) framework. That study estimated CCO and CDO were two of the four modules that could provide up to 85% of the expected fuel savings from global ASBU implementation between 2013 and 2019.

In 2015, a European CCO/CDO Task Force was set up, under the aegis of EUROCONTROL, with the objective of improving European CCO and CDO performance. A review of existing CDO performance measurement practices revealed measurements were often based upon local definitions and measurement criteria that differed from airport to airport. When CDO performance measurements had previously been shared on an international level, different definitions or performance measurement criteria often led to confusion and misunderstanding, generating a picture of European performance that was at best a patchwork approach and at worst providing an inaccurate picture of pan-European CDO performance.

To improve this, the Task Force delivered a set of stakeholder-agreed definitions, metrics and parameters for harmonised pan-European CCO/CDO. These included:

• A harmonised definition of both a noise and a fuel CDO;
• A harmonised definition of both a noise and a fuel CCO; and
• A harmonised set of metrics and parameters for CCO/CDO measurement relating to average time in level flight.

The definitions highlighted the potential for CCO and CDO to provide both noise and CO2 reduction benefits. The fuel CDO measures the vertical flight efficiency of the whole descent profile from top of descent (fuel CCO measures CCO from ground to top of climb). In contrast, the noise CDO/CCO measures the vertical flight efficiency of lower levels (from FL75 until FL105) and evaluates that part of the descent/climb profile where SESAR considers the primary environmental impact to be noise.

Historically, metrics that have been commonly used to measure CCO/CDO have relied on a percentage achievement level based on a binary indicator of whether there are any periods of level flight or not in a vertical profile. The Task Force considered that such a binary indicator did not reveal the full picture of CCO/CDO performance levels. For example, one unavoidable 30 seconds of level flight for separation or safety purposes should not be considered to have the same performance impact as 10 minutes of level flight to comply with inefficient transfer of control procedures between ATC sectors. The Task Force therefore advocates an indicator based upon time in level flight (seconds) allowing a better identification of the areas of inefficiency.

All agreed definitions and the parameters to measure CCO and CDO can be found on YouTube.

The outcomes of the Task Force have not been designed to interfere with existing national regulatory requirements on CCO/CDO reporting. However, stakeholders are strongly encouraged to follow the harmonised definitions and parameters of the Task Force when measuring and sharing results on CCO/CDO performance at the international or pan-European level in order to allow a harmonised comparison of performance.

Once the Task Force outcomes had been agreed, the first ECAC-wide CCO/CDO study was undertaken in 2018, using the harmonised definitions and parameters for measurement, to identify the current state of implementation and the extent of the benefit pool available from CCO/CDO performance improvements.

The results of the study revealed the following key findings:

• There are significant environmental savings that optimised CCO/CDO can provide;
• The amount of time flown level (a proxy for inefficiency) and consequently the amount of fuel savings available from optimising the descent phase (CDO) is significantly larger (about 10x) than the time flown level in the climb phase (CCO), therefore the greatest fuel saving benefits should be enabled by optimising CDO; and,
• The pool of potential performance improvements is much larger for the fuel CDO (CDO from top of descent) compared to low level (noise) CDO with the majority of airports having only minor performance benefits available.
• Based on these results, the Task Force concluded that the focus of improving the performance of the vertical flight efficiency of the climb/descent profiles should be to optimise CDO from top of descent or higher levels wherever possible.

The study estimated the benefit pool in ECAC from optimising CCO/CDO could provide fuel savings of up to 350,000 tonnes per year for the airlines, equivalent to approximately €150 million ($165m) in fuel savings. CO2 savings would be greater than 1 million tonnes. The study also revealed that across Europe, the potential fuel saving benefits from CDO are in the region of ten times greater than those from CCO. Therefore, whilst at certain airports the potential individual benefits from optimising CCO may still be greater than those from CDO, at the European level the primary focus of vertical flight efficiency improvements for the climb and descent profiles should concentrate on optimising the arrival profile, and from top of descent wherever possible.

Challenges to implementation

Why can’t CCO and CDO be flown at all airports all of the time? Improving CCO and CDO performance is not a simple task. The first priority in any ATC system is safety and by its simple definition, separating aircraft for safety reasons either on a one-by-one basis or by de-conflicting multiple traffic flows, sometimes requires aircraft to be deviated horizontally or vertically away from their optimum, or most efficient, trajectory. Add in strong yearly traffic growth, congested airports, complex airspace and other factors such as ATC strikes, restricted airspace, delay minimisation and flow management, mean that an optimised vertical (and horizontal) profile is not an easy task to facilitate and fly.

The CCO/CDO Task Force recognises the barriers that exist to improving CCO/CDO performance in Europe and it is developing a tool kit for CDO performance improvement with the objective to create a step-change in deployment of CCO and CDO over the coming years. The cornerstone of the tool kit is the development of two key deliverables: the updated Joint Industry CDO Action Plan and the new CCO/CDO State of Play Report.

The CDO Action Plan introduces and promotes a set of actions that support a step-change in CDO performance improvement across Europe. These actions range from airspace and procedure design guidance that enable more optimised CCO/CDO, to focused ATCO and pilot training material related to CCO/CDO, together with the introduction of CCO/CDO performance tables for all airports in Europe and all airlines flying in Europe (subject to a minimum number of traffic movements per airport together with availability of the relevant data).

The State of Play Report will support the Action Plan and introduces the factors that may affect optimised CCO/CDO performance. The report aims to increase awareness about CCO/CDO and to support every stakeholder in better understanding the complexity of optimising CCO/CDO. It proposes potential best practice solutions and mitigations to overcome the barriers to CCO/CDO optimisation in European airspace.

The third part of the tool kit will be a set of additional resources that enable stakeholders to implement and optimise CCO/CDO procedures such as:

• Guidance material for implementation, for example ICAO documentation on CCO/CDO and impact assessment guidelines for ATM changes;
• Best practices on CCO/CDO implementation, case studies on implementation and industry developments on CCO/CDO;
• The promotion of operational fora where CCO/CDO implementation can be discussed;
• Support in stakeholder consultation;
• Advice on aircraft energy and flight performance management;
• Support in airspace and procedure design; and
• Support in training initiatives on CCO/CDO.

European CCO/CDO Tool Kit

One of the key outputs of the Task Force work will be the introduction of European CCO and CDO performance tables. History has shown that performance comparisons breed performance improvements. Thus the objective of publishing these tables will be to ‘report and support’ – to report CCO and CDO performances and to support stakeholders in identifying the causes of any inefficiencies and collaborating on performance improvements.

The basic data provision of the performance tables will be monthly CDO figures from top of descent (CCO figures to top of climb) for the airport/airline selected. Enhanced functionality will include the possibility to slice and aggregate, for example by aircraft type, country, alliance, type of flight, CCO/CDO, level band and time of analysis with vibrant charting possibilities. The graph below shows just one example of what could be provided with basic functionality.

Another key output of the Task Force is a proposed update to air traffic controllers’ (ATCOs) training material on CCO/CDO and the creation of an ATCO refresher training module on aircraft energy management. The training material update will introduce more mandatory content on CCO/CDO in ATCO training and introduce simulator training sessions on CCO/CDO earlier in the training schedule – so the ATCOs develop a ‘CDO mentality’ similar to that taught by best practice airlines for their pilots.

The refresher training will support ATCOs in understanding how their instructions to pilots in the descent phase influence the actions that pilots need to take in order to balance compliance with ATC instructions, required inputs to the Flight Management System (FMS) to manage the energy of the aircraft and the maintenance of an optimum descent profile. Experienced pilots will run through scenarios explaining how the provision by ATCOs of certain information such as ‘distance to go’ can help them optimise their descent profile by improving the predictability of the approach.

The objective of the Task Force is to have both the Performance Tables and the Refresher training material ready for the planned launch of the Action Plan and State of Play Report, provisionally expected late 2019.

For more information on the different work threads of the European CCO/CDO Task Force click here.

David Brain previously worked as an Area Air Traffic Controller in the UK and has over 20 years’ extensive experience in ATC, ATM and Project Management. He currently leads EUROCONTROL’s environmental efforts on reducing aviation’s operational impacts and co-chairs the European CCO/CDO Taskforce as well as leading several other European operational projects. David is a member of the ICAO-CAEP Airport and Operations Working Group where he has been responsible for estimating the global environmental benefits of the ICAO ASBU framework and for leading the first ever global flight efficiency analysis using a harmonised surveillance data source.

Copyright © 2007-2021 Greenair Communications