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Continuous Climb Operations (CCO)

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Continuous Climb Opearions (CCO)


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Category: General General
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Introduction

Continuous Climb Operations (CCO) is an aircraft operating technique facilitated by the airspace and procedures design and assisted by appropriate ATC procedures, allowing the execution of a flight profile optimised to the performance of aircraft, leading to significant economy of fuel and environmental benefits in terms of noise and emissions reduction.

Description

The optimum vertical profile of a departing aircraft is a continuously climbing path with optimal fuel conserving climb rate. The fuel used in climbing to the most fuel efficient level can be a significant part the overall fuel used for the flight. CCO allows the aircraft to reach the initial cruise flight level at optimum air speed with optimal engine thrust settings, thus reducing total fuel burn and emissions for the whole flight. When CCOs are in effect, appropriate airspace design and ATC procedures should be used to avoid the necessity of resolving potential conflicts between the arriving and departing traffic flows through ATC level or speed constraints.

Benefits of CCO

Work by the European CCO / CDO Task Force[1] has demonstrated that the benefit pool from optimising the climb and descent phase (CCO / CDO) consists of fuel savings of up to 350,000 tonnes per year for the airlines (that is, over 1m tonnes of CO2) or ~150 million € in fuel costs[2]. While these numbers represent a theoretical maximum of potential benefits, it is important to note that the achievement of 100% CCO and CDO across the European network may not be possible for a number of reasons. It should be noted that the same study identified that the total fuel / emissions / cost savings from optimising CDO are in the range of 10 times those available from optimising CCO regardless of the high fuel burned in the climb phase. The same studies indicate noise impact on the ground may be reduced (in the descent phase) by around 1-5 dB per flight.

European CCO / CDO Action Plan

In 2020, a new Pan-European CCO / CDO Action Plan was adopted. The Action Plan includes new definitions for CCO and CDO developed by the European CCO / CDO Task Force and introduces the noise CDO (focusing on optimising that part of the arrival profile where noise is the primary environmental impact) and the fuel CDO. The Fuel CDO measures the environmental performance of the entire arrival phase from top of descent, in terms of fuel burn / CO2. [3]

The Action Plan also advocates a new harmonised metric for CCO / CDO performance measurement – “average time in level flight” - agreed by the Task Force taking into account current best practices from European stakeholders, and additional parameters for measurement, urges all stakeholders to collaborate on optimising vertical flight efficiency and recommends a set of Key Principles that should be communicated on CCO / CDO.

The European CCO / CDO Action Plan calls for a step change in the facilitation, promotion and implementation of optimised climb and descent profiles (CCO / CDO) so that the significant noise, fuel burn, emission and fuel cost savings generated by these techniques can be realised by Stakeholders. The Action Plan describes the latest technologies, procedures, harmonised performance measurement and stakeholder good practices for improved CCO / CDO facilitation, in order to enhance flight efficiency.

The Action Plan together with CCO / CDO performance dashboard and a set of supporting resources detailed on the web pages of the European CCO / CDO Task Force, constitute the CCO / CDO Tool Kit[4]. The objective of this Tool Kit is to provide information for stakeholders so that they can collaboratively implement more optimised climb and descent profiles and generate significant performance improvements for the climb and descent phases respectively.

SESAR: Optimised Climb Operations (OCO)

As the technological pillar of Europe’s ambitious Single European Sky (SES) initiative, SESAR is the mechanism that coordinates and concentrates all EU research and development (R&D) activities in ATM, pooling together a wealth of experts to develop the new generation of ATM.

For the descent phase, the SESAR concept includes a holistic view of the vertical flight efficiency: when ATC needs to impose constraints on a flight during the descent, it may be more fuel efficient for that flight to level-off after the constraint than to continue descending. The principles of Optimised Descent Operations (ODO) are explained in the Continuous Descent SKYBRARY entry.

Restrictions

In real life fully optimal CCOs to the top of climb may not be always possible, due to a number of reasons:

  • Limited airspace: Insufficient amount of vertical airspace to be reserved to protect the climb due to interactions with other traffic flows, particularly pronounced in busier airspaces.
  • Terrain and obstacles: risks to obstacle clearances associated with lower performing aircraft.
  • Environmental restrictions: noise abatement procedures might be in effect which may impose restrictions to the optimal departure climb.
  • ATC Procedures: procedures (such as radar handoff local procedures, or specific flight level allocation specified in letters of agreement with adjacent ATC units) and SID designs might impose restrictions to the continuous climb.
  • Weather avoidance: when weather avoidance is in effect the CCO procedures are normally cancelled due to the inability of departing aircraft to follow the published CCO-based departures.

Despite the aforementioned restrictions, the implementation of CCO can provide significant benefits even over shorter sections of the climb.

Departure profiles: CCO versus CDO

In TMAs and near to airports, there are inevitable conflicts between arrival and departure flows that the controller must manage. When these routes cross, the controller conventionally has two ways of resolving traffic conflicts at these points:

  • aircraft are vectored to avoid the conflict. In this case, the vector will mean a deviation from the planned aircraft route and therefore increasing/decreasing the distance flown (with a consequent increase/decrease in fuel burnt and CO2 produced); or
  • aircraft are kept vertically separated until their paths have crossed and they are clear of each other. Using vertical separation at the crossing points will often mean that at least one of the aircraft (and sometimes both) will have to maintain level flight for a period of time. As a result, one or both of the aircraft will not be able to maintain a continual climb or descent.

Depending on the length of the level-off or the off-track distance flown, recent studies show that a small level segment may be preferable to increasing the distance flown. This is more important for climbing aircraft than descending aircraft, as the fuel flow per extra nautical mile is higher, thus making the lateral efficiency a priority over vertical expedition. For the same reason, short cuts should be given to climbing aircraft when there is a choice between a climbing and a descending aircraft. If there is a need to vector one of the aircraft, from the fuel burn perspective and where practicable, consideration should be given to vectoring the descending aircraft, as it has a smaller fuel flow.

When a climb / descent conflict is solved by levelling-off one of the aircraft, priority should be considered for the descending aircraft which is carrying out a continuous descent. Descent is a much more complex flight phase than the climb out. A descent has to end at an exact 4D-point (FAF/FAP), within a specified performance window (stabilized) and often has to be managed taking into account the ad-hoc restrictions that are imposed (e.g. height or speed) as a result of other aircraft that are also descending to land at the same, or nearby, aerodromes. Whereas, a climb-out can end at almost an infinite number of points.

Also, studies show that it is not the fuel flow – heavier aircraft (departures) burn more fuel than lighter aircraft (arrivals) - that is important when making a decision on which flight to optimise and which flight to penalise. In fact, it is actually the inverse Specific Range differential[5](the fuel burned per nautical mile) between optimum and sub-optimum flight levels for heavier and lighter aircraft, that is the most important factor to take into consideration.

Therefore, prioritising a descending aircraft over a climbing aircraft may actually save overall fuel as the higher penalty (in inverse Specific Range differential) for a level segment is avoided.

When considering whether to prioritise CCO or CDO or bigger / smaller aircraft, other points to highlight include:

  • When comparing different aircraft types, the bigger aircraft should usually be the least penalised especially in the case of a lighter departure and a heavier arrival;
  • The above point should bear in mind that all airspace users have the equal right of access to the ATM resources needed to meet their specific operational requirements; and,
  • It should be noted that, in general, the fuel savings from prioritising a heavier arrival (without a level segment) over a lighter departure (with a level segment) are higher than prioritising the same heavier aircraft on departure (without the same level segment) over the same lighter aircraft (with the level segment) on arrival.

When looking at whether to give a small track extension to a departing aircraft to enable a CCO, as opposed to a level segment without track extension, from a purely fuel burn perspective, it should be understood that: “If you add a track extension, the additional distance flown will be always be flown at the cruising level fuel flow rate”.

This is because regardless of the track extension, the time spent ‘climbing’ will be approximately the same in both scenarios regardless of how long the track extension is or the duration of any intermediate level segments. Therefore, any track extension will result in additional fuel burn at the cruising level fuel flow rate.

Calculations may help to demonstrate this point. The table below shows fuel penalties in kg/nm for level segments at different flight levels for a medium-sized jet aircraft, commonly flown in Europe, at nominal weights.

Medium jet uel pentalty for level segments at FL.jpg

The figure 5.93 in the table refers to the fuel burn penalty in kg/nm for a 1nm level-off at FL330, i.e. the fuel flow rate in cruise. Therefore, if a departing aircraft has a track extension of 1nm in the climb (to facilitate CCO), this will result in an additional fuel flow of 5.93kg.

The other boxes in orange indicate the additional fuel flow of a level segment at lower levels compared to the fuel flow at FL330 e.g. at FL190 the fuel flow is 8.23kg/nm, a difference from 5.93kg of 2.3kg.

Therefore, if an aircraft is levelled off for 1nm at 5000ft instead of flying a CCO, the additional fuel burn for that 1nm level-off will be the difference in fuel flow between 9.57kg/nm and 5.93kg/nm i.e. only 3.64kg. Therefore, a 1nm level-off in the climb with no CCO and no track extension, may actually save more fuel than a perfect CCO and a 1nm track extension. This is the same regardless of aircraft type.

Using this data, it can be seen that the fuel burn for a 1nm track extension (5.93kg) to enable a perfect CCO is ~equivalent to a 12nm level-off at FL260 (0.49*12 = 5.88kg fuel) whilst a 1nm track extension (5.93kg) is ~equivalent to a 1.5nm level-off at 5000ft (3.64*1.5= 5.46kg fuel).

The situation may however be further complicated. Tactical interventions are not however, simple fuel calculations, as ATCO track extensions are more likely to be subtle 5 or 10° turns.

Track mile extensions for approximate 10, 15 and 20° tactical vectors

In such circumstances, the diagram above shows that a 15° tactical vector for 20nm would result in a track extension of 1.8nm. In the previous example this would be an additional 11 kg fuel to enable a CCO. This would result in more fuel burn than a 2nm level-off at 5000ft or a 5nm level-off at FL120.

CCO Design Considerations

Ideally a CCO should be organised as a part of a Standard Instrument Departure (SID) so that both flight crews and controllers have a fixed procedure to refer to in advance. After departure a path to the destination or airspace exit point that supports the most optimised vertical profile is desirable. This should also provide for the shortest track distance to be flown. Unrestricted climb to the cruise flight level with no speed restrictions is also desirable. However, specific speed restrictions (to maintain separation between succeeding aircraft or to enable a smaller turning radius) may be required to allow CCO in high traffic density areas or in areas with airspace and terrain constraints. Speed restrictions reduce the flexibility of the CCO but can aid in enabling a CCO-based procedure where it might not otherwise be possible.

There is a difference in design philosophy between CCO and continuous descent operations (CDO). In surveillance environments the CCO design should take into account that tactical changes to the flight path, initiated by ATC, may be desirable. In general CDO aircraft should be left on the designed route and not given a vector “shortcut” because a CDO is already descending at flight idle and thus descending at the steeper angle a shortcut requires may lead to an unstable approach. In contrast, ATC tactical shortcutting of a CCO departure to take advantage of observed aircraft climb performance is desirable because it saves both flight mileage and time. The potential for tactical shortcutting should be considered in any CCO design, as well as the fact that other flow restrictions potentially restrict the opportunity of ATC to provide tactical shortcuts.

According to ICAO Doc 9993 Continuous Climb Operations (CCO) Manual (unedited advance version) in the most optimum situation, a departure route should be designed in such a way that there is no restriction that prevents an aircraft continuing its optimum flight profile. Both, the arrival (STAR) and the departure (SID) should be de-conflicted laterally or vertically. This optimum situation may not be reachable and therefore a balance must be found between the arrival and the departure routes. The spread of performance between aircraft in the climb is much greater than in the descent and a SID catering for all aircraft may present a height window prohibitively large for an unconstrained SID to be developed. One solution may be to develop different SIDs for different performance classes of aircraft. Compromises such as short intermediate level-offs for some aircraft, climb profiles at less than optimal rates, and route path changes, may also be needed. ICAO also specifies that the overall efficiency achieved for all aircraft operating within the system must always be considered.

Related Articles

Further Reading

ICAO

EUROCONTROL

Notes

  1. ^ Continuous Climb and Descent Operations EUROCONTROL
  2. ^ Based on IATA fuel price and USD/EUR exchange rate June 2018
  3. ^ Note: SESAR analysis indicates that noise is the primary environmental impact below FL105 for departures.
  4. ^ Available at: https://www.eurocontrol.int/concept/continuous-climb-and-descent-operations
  5. ^ Specific Range is the amount of distance flown by a unit of fuel burn. The inverse of this value (kg/nm) is used to check how much fuel is burned per extra nautical mile or level segment distance. Specific Range is used so that the variable of speed does not complicate the scenario.