Continuous Descent Approach
Continuous Descent Approach (CDA) or Optimized Profile Descent (OPD) is a method by which aircraft approach airports prior to landing. It is designed to reduce fuel consumption and noise compared to certain conventional approaches and involves maintaining a constant three degree descent angle during landing, until meeting the Instrument Landing System (ILS). Basically, instead of approaching an airport in a stair-step fashion, throttling down and requesting permission to descend to each new (lower) altitude, OPD allows for a smooth, constant-angle descent to landing.
Continuous Descent Approach starts ideally from Top of Descent, i.e. at cruise altitude, and allows the aircraft flying its individual optimal vertical profile down to runway threshold. Some airports apply constraints to this individual optimal profile such as imposing a constant descent angle (e.g. 3 degree) or starting the CDA only after having left the holding pattern (e.g. London, at about 7000 feet) due to Air Traffic Management constraints.
Studies of CDA
Continuous Descent Approaches (CDAs) have noise and emission benefits in low traffic environments based upon the study at the Louisville airport.
The noise and emission benefits derived for the nighttime arrivals at the Louisville airport are well documented scientifically, however, the implementation of full-time continuous descent approaches (CDAs) at Los Angeles World Airport (LAX) were done with altitudes that were exactly opposite as to what was done at Louisville.
What this means is that noise (especially for those from 15-30 miles away from the airport) and emissions/pollution have actually increased in communities surrounding LAX. .
Louisville Study: http://web.mit.edu/aeroastro/partner/reports/cda_rpt.pdf
The benefits of CDAs, as outlined in the nighttime Louisville study, concerning altitude profiles, are outlined on page 56 and state the following:
As shown, the two aircraft that performed the conventional approach descended sooner and thus flew lower over the community. The figure clearly shows the conventional flights performed step down descents.
Between 15 and 20 nm away from the runway, CDA flights were up to 1,500 ft higher. This is one of the reasons why CDA has a lower noise impact than conventional approaches.
However, when you look at the vertical profiles of current CDAs being flown into LAX, which were compiled by the Los Angeles World Airport staff, you can see on page 6 and 7 that the CDA altitudes at 20nm away are anywhere from 617 feet to 1101 feet lower than the conventional, pre-CDA altitudes. Because of this, LAX has more noise and emissions after the implementation of CDAs into LAX than before CDAs.
The reason that the vertical altitudes are so significantly lower at LAX using CDAs is due to the fact that CDAs, when utilized in high traffic environments, must be kept flying at low altitudes so that the planes do not catch up with each other.
In fact, Eurocontrol, which built upon the work started at the Louisville airport, said in a report presented at the 29th IEEE/AIAA Digital Avionics Systems Conference on October 3-7, 2010 that (first page):
Abstract - Continuous Descent Approaches (CDAs) can significantly reduce fuel burn and noise impact by keeping arriving aircraft at their cruise altitude for longer than during conventional approaches(to descend as late as possible)and then having them make a continuous descent to the runway at near idle thrust with no level flight segments. The CDA procedures are fixed routes that are vertically optimized.
Noise-efficient operations, such as Continuous Descent Approaches (CDAs), can significantly reduce the noise impact of landing aircraft by keeping aircraft at higher altitudes on approach to the airport and by reducing power during descent , .
Eurocontrol also said in the same report on page 9 under “Multiple Aircraft Scenario” that:
CDA operations are usually conducted at night and/or in low traffic conditions. One reason is that for CDA, the landing interval has to increase to guarantee sufficient spacing between aircraft on the final landing segment . The increased landing interval is necessary because of the large dispersion in aircraft approach speeds.
In addition to the supposed noise benefits of CDAs, based upon the nighttime Louisville study, the FAA asserted that emissions would be less utilizing CDAs, as the jets spent less time under 3000 feet in the Louisville study. Page 73 of the report states:
As was seen in Figure 6-2 and figure 10-1, conventional aircraft are typical vectored at significant distances at low altitude thus spending greater time below the mixing height of 3000 feet AGL. Further analysis confirmed that the greater time that the aircraft is spending below the mixing height is overwhelming the slight decrease in the CO emission rate that occurs at the higher throttle settings that are typical of the conventional approach.
However, because CDAs spend more time under 3000 feet at LAX, as noted on the 24R - Arrival Flight comparison done by the Los Angeles World Airport (page 7), and the CDA lower throttle gives off more CO emissions, than the CDA at LAX is clearly creating more emissions/pollution when compared to the conventional approach.
All of the benefits of CDAs are extrapolated based upon the results of the nighttime Louisville airport study. Unfortunately, because the data (altitude descents) utilized to formulate the conclusions are inversed when evaluating LAX, the reports cannot continue to support that the same noise and emission/pollution benefits apply. And, in fact, there is increased noise and emissions when their own theories, explained in the Louisville study, are used with LAX altitude data.
Due to the negative association with the term CDA, new procedures introduced at LAX and other US airport are instead called Optimized Profile Descent (OPD).
The Spanish Development Ministry has studied how to ensure aircraft land using the CDA method at all major airports. The Defense Ministry has also opened the military airways to commercial planes.
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