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Continuous Descent

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Continuous Descent Operation (CDO)
Continuous Descent Arrival (CDA)


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Category: General General
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Description

Continuous Descent Operation (CDO) or Continuous Descent Arrival (CDA) is an aircraft operating technique in which an arriving aircraft descends from an optimal position with minimum thrust and avoids inefficient segments of level flight to the extent permitted by the safe operation of the aircraft and compliance with published procedures and ATC instructions. (EUROCONTROL).

The objective of a CDO is to reduce the environmental impact of the arrival phase of flying by both maintaining a fuel optimal profile (thereby minimising gaseous emissions) and keeping engine and aircraft noise to a minimum, prior to intercepting the approach glide path at an appropriate altitude for the distance to touchdown.

By keeping the aircraft as high as possible for as long as possible, this ensures that the aircraft spends the least amount of time at non-optimal lower intermediate cruising levels. It should be noted that keeping the aircraft as high as possible for as long as possible can be more effective at reducing noise impact on the ground than Low-Power/Low Drag (LP/LD [1])) techniques alone even though they are complementary techniques: CDO will reduce the noise at intermediate distances from touchdown (8 to 25 NM) and LP/LD is applied on final approach.

Approach Profiles: CDO versus non-CDO

In a conventional non-CDO approach, the aircraft descends stepwise, in accordance with ATC procedures and airspace constraints that have been defined to maintain sector throughput and transfer of control agreements. This essentially means that arriving aircraft may be subject to flying a stepwise trajectory between the constraints with portions of level flight in-between. By performing a CDO the aircraft remains higher for longer and operates ideally at idle engine thrust. Both of these elements induce a reduction in fuel use, emissions and noise along the descent profile prior to the point at which the aircraft is established on the final approach path.

The ideal CDO starts at the top of descent and ends when the aircraft starts the final approach and follows the glide path to the runway. Typically CDOs are not possible all the time, not for all arriving flights and not always for the whole descent profile. But at more and more airports measures are taken to use CDO to the extent possible and to gradually increase the percentage of CDO-flights. It should be noted that CDO procedures may be defined from top of descent for use in core European airspace during busy traffic periods.

Comparison of CDA and non-CDA-profile

Benefits of CDO

Work by the European CCO / CDO Task Force [2] 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[3]. 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 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 harmonised definitions, metrics and parameters to measure CCO and CDO operations in Europe, developed by the European CCO / CDO Task Force. The Action Plan 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.[4]

The Action Plan also advocates a new harmonised metric for CCO / CDO performance measurement – “average time in level flight” - agreed by the Task Force. This metric takes 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.[5] 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.

ATC Facilitation

In a perfect world without constraints, all flights would fly an optimal profile, a CCO or CDO but this should not be achieved at the expense of safety. Likewise, there are many interdependencies between other performance areas that ANSPs have to take into consideration e.g. capacity, cost efficiency, etc.

Nonetheless, there are measures that air traffic controllers can take to facilitate CDO without losing the flexibility to fully optimise their traffic sequencing and arrival flows.

Where possible, controllers should aim to:

  • Allow an aircraft to descend from their optimal Top of Descent (ToD) point,
  • In cases where the flight will not be able to start descent at their optimal ToD point (and time permits), provide the flight crew with information about when they can expect the descent clearance,
  • Provide information on the type of arrival procedure to expect as soon as possible and preferably before ToD, as this information influences the descent profile of the aircraft,
  • Provide more direct routings but if possible, information on expected tactical interventions or shortcuts in the descent phase to be delivered prior to ToD,
  • Sequence traffic while the aircraft are still either in the cruise, or in the early part of their descent. This would minimise the sequencing required at lower altitudes and reduce the aircraft fuel burn and noise footprint,
  • Whenever the traffic situation allows, aircraft should be permitted to follow a closed STAR, as this adds predictability to the operation and enables the flight crew to descend as close as possible to the optimum profile,
  • If an arriving and departing aircraft are in conflict – if possible, let the arriving aircraft use its preferred descent profile, instead of instructing the departing aircraft to change track or level-off.

SESAR: Optimised Descent Operations (ODO)

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. This section outlines the SESAR concept for the optimisation of the descent.

For FMS-equipped aircraft, the most efficient descent profile is a continuous unconstrained descent starting at the FMS-calculated Top-of-Descent (ToD), where the engine is set to idle, and forward thrust comes solely from the aircraft’s potential energy (height) being traded for kinetic energy (speed). However, ATC often needs to impose vertical or speed constraints along the descent in order to ensure that separation between aircraft is always above the separation minima. Both tactical and published constraints may limit the freedom for aircraft to fly their optimum FMS-calculated flight profile.

A particular scenario where a potential for optimisation has been identified by SESAR research is the case where aircraft are required to descend early. Early descent is often required in order to meet a published constraint, but it can also result from a tactical ATC clearance. The SESAR demonstration project Optimised Descent Profiles (ODP) found that Letters-of-Agreement (LoA) requiring aircraft to descend between 30NM and 80NM before their ToD were not uncommon in the core area of central Europe.

In an early descent, after the constraint has been met (e.g. a waypoint has been crossed at or below a certain flight level), the majority of current FMSs compute a (geometric) descent path that continues to bring the aircraft even further below its ideal profile, typically at a reduced rate of descent (e.g. 1.000 ft/min) until the original unconstrained (ideal) descent profile is intercepted, as shown in the figure below.

FMS early descent path.jpg

SESAR research has shown that it is often more efficient for the aircraft to level off and calculate a new top-of-descent (which is referred to as re-cruising, or re-calculating the ToD), from where an idle descent can be started, as shown in the figure below. During the level-off (the re-cruise phase) more thrust is needed than if the aircraft had continued in a continuous thrust descent (like most aircraft do today), but this extra fuel for the level-off is compensated by the longer idle thrust descent leg. In this case, for a flight which must respect an ‘at or below’ constraint, a continuous descent profile is not the most efficient.

Recruise after early descent.jpg

The ICAO CDO Manual (Doc 9931) provides guidance for airspace and procedure design that allow a continuous descent profile using published constraints. The objective is that as many aircraft as possible fly their unconstrained FMS profile; the design is based on assumptions of what the most common unconstrained descent rates are. The manual also highlights that “at or below” or “at or above” constraints are preferrable to “at” constraints, because they increase the flexibility for aircraft to choose their own descent profile.

The SESAR re-cruise concept builds on this guidance and proposes a way to realise maximum benefits from the flexibility for aircraft to choose their own descent profile afforded by “at or below” constraints, by levelling off when this is more efficient than continuing the descent. SESAR proposes that instead of aiming at achieving a CDO, we aim at achieving an ODO (Optimised Descent Operations), based on the research results showing that a more continuous profile may not always be the most efficient when the aircraft needs to comply with an ATC constraint along the descent. SESAR projects have not found a straightforward geometric criterion (like the length-of-level-segments) that correlates with the fuel efficiency of the descent; the metrics for fuel efficiency in the SESAR Performance framework are Kg of fuel burn, and SESAR projects assess this metric using advanced models developed within SESAR, e.g. the IMPACT tool.

For the flight crew to be able to optimise their descent taking into account the ATC constraints, these constraints should be known to them as far in advance as possible.

FAA NextGen Optimised Descent Profile (OPD)

As a component of the NextGen initiative, the FAA has authorized development of arrival procedures with vertical profiles optimised to facilitate a continuous descent from the top of descent to touchdown. OPD flight procedures use the capabilities of the aircraft Flight Management System (FMS) to fly a continuous, descending path without level segments, based on the actual performance of the aircraft under current flight conditions.

Two vertically-optimized arrival procedures were designed and successfully instituted at Los Angeles International Airport (Los Angeles) as part of the Southern California Redesign. The routes into KLAX allow aircraft to glide down to the runway, using minimal power, starting approximately 70 nautical miles east of the airport.

Since 2004, OPD procedures have been evaluated extensively at Louisville-Standiford International Airport, are being tested at Hartsfield-Jackson Atlanta International Airport (Atlanta/Hartsfield-Jackson International), and were demonstrated at Miami International Airport (Miami) in May 2008.

Identified Safety Issues

Typically in a continuous descent approach, an aircraft begins its final descent from a distance of about 12 nautical miles and an altitude of 4000 feet. It then maintains a steady 3° angle of descent during its approach. For many airports, the opportunity to implement a CDA is limited because of the volume of air traffic on approach and in the vicinity of the airport especially during busy daytime periods. When approaching traffic is heavy, a pilot may need to adjust throttles, flap settings, and extend landing gear to maintain safe and consistent spacing with other aircraft in the terminal airspace. Extending flaps, and landing gear increases drag, which requires the application of additional thrust to keep the aircraft flying at the same speed.

EUROCONTROL recommends that before CDA trials or operations commence, the proposed implementation shall be the subject of a local safety assessment, in accordance with the safety management principles of EASA regulations. A number of safety issues were identified as part of the safety assessment of this guidance that should be considered as part of the local safety assessment:

  • The interaction between CDA and non-CDA traffic.
  • Cockpit workload, in particular where radar vectoring and profile management can impact onto a phase of flight that is already subjected to increased workload.
  • Accurate DTG information is required for the aircrew to ensure that they are not “rushed” and that the aircraft is able to achieve a stabilised configuration on final approach.
  • Account should be taken of variability in descent paths and speed management depending on aircraft weight, the type of FMS, wind component, and pilot training.

Related Articles

Further Reading

  • ICAO Doc 9931 Continuous Descent Operations (CDO) Manual

EUROCONTROL

SESAR

FAA

Notes

  1. ^ LP/LD is a noise abatement technique for arriving aircraft in which the pilot delays the extension of wing flaps and undercarriage until the final stages of the approach, subject to compliance with ATC speed control requirements and the safe operation of the aircraft.
  2. ^ Continuous Climb and Descent Operations, EUROCONTROL
  3. ^ Based on IATA fuel price and USD/EUR exchange rate June 2018
  4. ^ Note: SESAR analysis indicates that noise is the primary environmental impact below FL75 for arrivals
  5. ^ Available here