45TH ANNUAL CONFERENCE, Kaohsiung, Taiwan, 27-31 March 2006
WP No. 85
Investigate Basic / Advanced Continuous Descent Approaches
Presented by TOC
1.1. The Technical and Operations Committee (TOC) has been asked to investigate Continuous Descent Approaches (CDAs). Such procedures are being introduced at various locations, and it is therefore useful for the Member Associations (MAs) to be made aware of such procedures.
2.1. Environmental issues have started to come to the fore in recent years. These issues include noise, emissions and visual pollution. Airports have come under increasing pressure to address these environmental issues. One specific area that airports are seeking to tackle is approach noise.
2.2. Approach noise is a specific measurement in the International Civil Aviation Organisation (ICAO) noise certification process whereby all aircraft types are assessed and certificated. The approach noise certification measurement point for large fixed wing aircraft is very close in to the airport (2 km from threshold), where aircraft configuration and flight conditions are very tightly defined. Studies have shown that further away from the airport there is significant variability in arrivals noise levels for a given aircraft type.
2.3. CDAs are being requested by airports in a bid to reduce both noise and emissions during the approach phase. The general principle of a CDA is that the higher the aircraft flies, the less intrusive it becomes. Aircraft operators are also eager to fly CDA procedures. A CDA tends to be flown at near flight idle. This results in a decreased fuel burn, and hence costs savings can be achieved by flying such procedures.
2.4. The London TMA has had CDA procedures in place for several years. The UK Aeronautical Information Publication (AIP) had the following definition of a CDA added in October 2001:
“A noise abatement technique for arriving aircraft in which the pilot, when given descent clearance below the Transition Altitude by ATC, will descend at the rate he judges will be best suited to the achievement of continuous descent, whilst meeting the ATC speed control requirements, the objective being to join the glide-path at the appropriate height for the distance without recourse to level flight.”
2.5. The UK’s “Arrival code of practise” definition of CDA is as follows: an arrival is classified as a CDA if it contains, at or below an altitude of 6000ft:
- no level flight; or
- one phase of level flight not longer than 2.5NM.
Note: ‘level flight’ is interpreted as any segment of flight having a height change of not more than 50ft over a track distance of 2NM or more.
2.6. CDAs offer environmental benefits in the form of reduced noise and a decrease in emissions. Studies have highlighted CDA as the leading technique for reducing arrivals noise. For example, a B747-400 on a CDA can be up to 5dBA quieter than when flying a 5NM level segment at a height of 3000ft above aerodrome level.
2.7. The theoretical “ideal” CDA profile is a descent at 3 degrees. A definition (covering both day and night) has been published that takes into account:
- ICAO PANS-OPS requirements: e.g. Vol II Pt III, 5.6 requires that, on the intermediate approach segment, “…a horizontal segment with a minimum length of 2.8km (1.5NM) should be provided prior to the final approach for Category C and Category D aircraft …”;
- the standard ICAO safety requirement of intercepting the final approach glideslope from below;
- at a given altitude, a level segment used to decelerate an aircraft, at or near thrust-idle, will tend to generate less noise than an extended level segment at or near a constant airspeed.
2.8. Continuous Descent Final Approaches (CDFA) is a subject that is being discussed in several different arenas at present. The concept of CDFA is similar to the CDAs discussed above. CDFA however is focused on the final approach, i.e. that part of the approach from the Final Approach Fix (FAF) to the Missed Approach Point (MAP). Final approaches that incorporate continuous descents are being promoted to encourage a move away from “Dive and Drive” techniques. Continuous descents in the final approach are therefore being encouraged on safety grounds.
2.9. There are a number of operational issues that affect air traffic controllers when providing CDAs. These operational issues include:
a) Minimum joining altitudes on to the ILS. Altitudes should continue to be specified such that aircraft do not descend below these altitudes unless they are established on the glidepath.
b) Ranges from touchdown. To assist pilots in the management of their descent, ranges from touchdown are to be passed as follows:
- When first issuing descent clearance from stack level (a best estimate only is required at this stage).
- As soon as possible after first contact with Final Director (the final approach controller).
- If the DME is unserviceable, ranges should be passed on the intercept heading to the ILS (these ranges should be as accurate as possible).
- At any time if a previous estimate has become invalid, e.g. following a change in landing sequence, or if a controller considers that a range check would assist a pilot with descent management.
2.10. A potential concern is that in the not-too-distant future controllers in certain States may become liable for non-compliance with CDAs, based on environmental performance indicators. It may be possible that policy will need to be developed to safeguard controllers from being penalised for non compliance with CDAs.
2.11. The concept of CDAs is not new. CDAs have been discussed for many years. One of the earliest articles on the concept of CDAs was an article entitled “How to Reduce Noise and Save Fuel” in the March 1974 edition of the Journal of the Guild of Air Pilots and Air Navigators (GAPAN).
2.12. An article also appeared in Flight International on 25th September 1975 describing ‘Lufthansa Managed Drag Approach’ procedures. The concept is similar to that described in the GAPAN Journal article.
2.13. Low power/low drag (LP/LD)
This has been defined in the UK AIP as “A noise abatement technique for arriving aircraft in which the pilot delays the extension of the 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.”
2.14. This broadly means the aircraft being in as ‘‘clean’’ a configuration as possible, for as long as possible. During the intermediate approach, including the closing heading, and on final approach, thrust reductions should be achieved where possible by maintaining a ‘clean’ aircraft configuration and by landing with reduced flap.
2.15. The following diagram depicting the noise benefit realised from a CDA:
2.16. Basic CDAs
2.16.1. The most widely used type of CDA is what is known as a ‘Basic CDA’. These are CDAs resulting from controllers radar vectoring aircraft, and providing descent instructions in a timely manner so as to ensure that the flight crew can arrange for a continuous descent. Basic CDAs can also be known as ‘tactical CDAs’. They have been in use in the London TMA for several years at the major airports, including Heathrow, Gatwick and Stansted.
2.16.2. A code for arriving aircraft has been published. This code has assisted in raising awareness for controllers, flight crew, airport operators and regulators on minimizing noise from aircraft landing at airports. Noise from Arriving Aircraft; An Industry Code of Practice was published in February 2002. An arrival is classified by the code as a CDA if it contains, below an altitude of 6000ft, no level flight, or one phase of level flight not longer than 2.5NM. The key objective of CDA is to ensure aircraft remain as high as possible for as long as possible.
2.16.3. One of the techniques that is encouraged is the provision of ‘Distance to Go’ information by the controllers. A CDA becomes more achievable when the crew are given an indication of the track miles to run to landing by ATC. The standard rule of thumb for track miles is to multiply the height (in thousands of feet) by three, e.g. 3000 feet requires 9 miles of track distance. This information is used by the flight crew to plan for their CDA, and to check conformance against it. This is a very basic calculation, but the crews also have to consider the changing effect of the wind, aircraft performance and other factors. If the information received is inaccurate and the aircraft is taken further than the stated track miles then the aircraft will become lower and will have a level flight segment. Conversely if the actual track is shorter than the estimated distance then the aircraft will be high on the approach leading possibly to an unstable or rushed approach. Pilot workload is slightly increased with a CDA, but the airlines routinely using CDA procedures indicate that it is not unmanageable with the proper training and the use of the automated aircraft systems.
2.16.4. As the provision of CDAs becomes more prevalent at various locations, some controllers have expressed a concern about workload. Mixed experiences have been encountered at ATS units where CDA procedures are use. For instance, the increase in workload is seen as being minimal by the London Approach controllers who are experienced at providing CDAs. However, other MAs have indicated that they suspend CDA procedures during busy periods due to workload issues.
2.16.5. Concerns have also been expressed that the provision of a CDA will be detrimental to capacity. Basic CDAs have been provided at London Heathrow for several years on a 24 Hour basis. Even during peak traffic times, when the arrival rate is about 42- 44 aircraft per hour, controllers endeavour to give instructions that will enable a CDA to be flown. The adherence to CDAs, even during busy times, is good, with approximately 80-85% of aircraft flying a CDA.
2.17. Advanced CDAs
2.17.1. In addition to the Basic CDAs, in the last few years a new type of CDA has been introduced. This is known as an ‘Advanced CDA’. This is where the vertical profile is embedded in a published arrival procedure, such as an RNAV Initial Approach Procedure (RIAP).
2.17.2. The vertical profile is specified by means of flight level or altitude constraints at some of the waypoints that make up the procedure. The waypoints specify the lateral profile, whilst the constraints published at the waypoints specify the vertical profile, and the speed profile, if applicable.
2.17.3. Advanced CDA trials have been conducted in a number of locations. There is some concern that the use of Advanced CDAs may not be suited to areas of high density operations. Indications are that the level of capacity may be adversely affected by Advanced CDAs. One of the reasons why this occurs is the current inability of different flight management systems to adhere to speed constraints in a consistent manner. This results in potential catch-up scenarios, preventing aircraft from being spaced at the minimum longitudinal separation.”
2.17.4. The concept of Advanced CDAs involves aircraft flying on specified vertical profiles, normally with an associated lateral profile in the guise of an RNAV procedure. At present controllers are responsible for monitoring the conformance of aircraft to these profiles. However, information received about these trials indicates that there is a fundamental shift in the controller task from an active, hands-on way of working, i.e. providing radar vectors, to a passive, hands-off way of working, i.e. monitoring aircraft flying the profiles. Humans in general are poor at undertaking monitoring tasks. Controller tools are therefore needed to automate the task of monitoring aircraft flying Advanced CDAs. Such tools will also assist in optimising spacing, thereby reducing the risk to capacity.
2.18. Tailored Arrivals
2.18.1. Tailored Arrivals consist of a customised RNAV arrival created to provide the optimal descent trajectory for each arriving aircraft based on the particular aircraft type. In this context, optimisation is based on aircraft performance, but is also modified by the needs of the air traffic management environment.
2.18.2. Tailored Arrivals have been performed as part of a trial in Melbourne between Airbus, Boeing, and Airservices Australia.
2.19. Continuous Descent Final Approaches
2.19.1. CDFA is a subject that is being discussed in several different arenas at present. The concept of CDFA is similar to the CDAs discussed above.
2.19.2. CDFA is focused exclusively on the Final Approach, i.e. that part of the approach from the FAF to the MAP. Final approaches that incorporate continuous descents are being promoted to encourage a move away from “Dive and Drive” techniques. Continuous descents in the final approach are therefore being encouraged on safety grounds.
2.20. Controller training needs to be provided to ensure that controllers participating are aware of the requirements of the procedures, and how they can best provide a CDA to participating aircraft. Controller training can take the form of briefings, simulator sessions and on-the-job practical experience whilst under supervision. Similarly, pilots need to be aware of the requirements of the CDA procedures, and should check the relevant AIPs for appropriate information.
2.21. There is a significant amount of activity taking place at present regarding CDAs. Some of this activity is detailed below:
- Pan-European Guidance on ‘Basic’ Continuous Descent Approaches. This is part of the Eurocontrol SOPHOS program;
- Providing advice and guidance to Cambridge University on the Silent Aircraft Initiative;
- Providing input to a US-funded, NASA/Boeing-run CDA arrivals study at Louisville, Kentucky (KSDF) airport;
- P-RNAV trials at Heathrow and Gatwick Airports;
- Input into CDAs at ECAC Aerodromes — Concept Paper derived from a presentation given to Eurocontrol on the impact of the code in February 2003.
3.1. A recent review has found that CDA remains the most effective single way of managing arrival noise.
3.2. Workload associated with the provision of CDAs does not increase significantly for either the controllers or the pilots.
3.3. Capacity is not adversely affected by the provision of Basic CDAs, i.e. radar vectored ones. However, Advanced CDAs are currently only utilised during times of low traffic density as the RNAV procedures are not suited at present for use in high capacity situations.
3.4. Concerns have been raised by controllers regarding their responsibility for non- compliance with CDAs. Consideration needs to be given in the future as to controller liability for non-compliance.
3.5. The widespread introduction of Advanced CDAs, especially at high density airports at busy times, is likely to require the provision of appropriate controller support tools.
3.6. IFATCA should closely monitor the developments of CDAs as the introduction of these procedures in various guises becomes more common place.
It is recommended that:
4.1. This paper is accepted as information material.
Last Update: September 29, 2020