Understanding Continuous Descent Operations
As every pilot strives for the perfect, most direct route, they know the close scrutiny and often unrealized potential for more efficient operations. Operators and regulatory authorities are similarly pushing for increasingly efficient flight operations, but balancing noise, weather, traffic and other important constraints. Climb and departure procedures aim to minimize an aircraft's noise footprint while complying with local airspace requirements, ultimately striving to enable the aircraft to reach its most efficient operating altitude as safely and expeditiously as possible.
Enroute, the focus shifts to meteorological conditions, flying the most direct route and seeking out tailwinds. Other operational factors come into play, such as airspace restrictions and ETOPS limitations that limit overwater flight, if applicable. Depending on the aircraft type, weight, and atmospheric conditions, an aircraft may need to utilize step climbs – periodic ascents to higher altitudes that are performed as fuel is burned off and the aircraft weight allows.
Finally, an aircraft must descend from its cruising altitude to the destination airport. While straightforward in theory, operational realities such as routing and sequencing require aircraft to periodically interrupt their descents by leveling off for various reasons. This results in additional fuel burn and generates a corresponding increase in carbon emissions.
To minimize these interruptions, continuous descent operations (CDOs) are being implemented at various locations worldwide. The International Civil Aviation Organization (ICAO) has adopted this generic term to include operations variously known as continuous descent arrivals, continuous descent approaches, optimized profile descents, and tailored arrivals, among others.
What Are Continuous Descent Operations?
Source: FAA
ICAO defines a continuous descent operation as "an operation, enabled by airspace design, procedure design, and ATC facilitation, in which an arriving aircraft descends continuously, to the greatest possible extent, by employing minimum engine thrust, ideally in a low drag configuration, prior to the final approach fix/final approach point."
In simple terms, a CDO is a descent in which interruptions to the descent are minimized to the greatest extent possible. Ideally, the engine thrust is reduced to flight idle at the top of descent and remains there until the aircraft is reconfigured shortly before landing.
Always secondary to safety, CDOs are voluntary in nature. As air navigation service providers, aircraft operators, airports, and regulatory authorities continue to coordinate their efforts, CDOs are likely to become more commonplace and could become the default method of descent/arrival as the practice evolves.
How Can CDOs Minimize Aviation’s Carbon Footprint?
As a CDO minimizes or eliminates the need for flight crews to periodically increase engine thrust and level off, the most significant benefit of CDOs is the reduction in fuel consumption and the corresponding reduction in carbon emissions.
According to NATS, the primary air navigation service provider in the United Kingdom, a single CDO from 20,000 feet can save up to three tons of CO2. On a broader scale, achieving a 5% increase in CDOs across the UK alone will save around 10,000 tons of CO2.
What Other Benefits Result From Utilizing CDOs?
CDOs provide benefits above and beyond the reductions in fuel consumption and carbon emissions. Chief among them is the reduction in the aircraft's noise footprint as it descends and overflies populated areas.
CDOs are helpful in minimizing the noise at urban airports
Compared with traditional step-down descents and approaches, CDOs reduce the noise footprint in multiple ways. First, because the descent profile is predicated upon an idle or near-idle thrust setting, the engines themselves generate less noise than if they were producing higher levels of thrust during level flight. Second, CDOs keep the aircraft higher for longer, inherently minimizing the time spent flying at lower altitudes where jet engines are least efficient.
Third, while many traditional operations see airspeeds up to 250 knots…the maximum allowable speed below 10,000 feet…CDOs are typically flown at speeds of approximately 180-220 knots. Because aerodynamic noise created from turbulent air flowing around the airframe, wing, and landing gear comprises a significant portion of an aircraft's total noise signature, this speed reduction alone will result in a correspondingly notable reduction in the noise footprint. The general lack of speed brake usage during CDOs also contributes to a lower aerodynamic noise footprint.
Beyond noise benefits, crew safety is said to be improved during CDOs, as well. Similarities between vertical guidance and precision approach operations help standardize flight crew procedures, and the safety benefits inherent in stabilized approach criteria also apply to descent planning. Any standardization among flight procedures tends to increase the flight crew's situational awareness while reducing their workload and mitigating task saturation.
Finally, CDOs provide a more pleasant ride for the passengers. Because the aircraft's pitch remains relatively constant throughout the descent, passengers susceptible to motion sickness are not subject to a regularly changing deck angle. Cabin noise is also kept to a minimum throughout the descent, compared with traditional step-down approaches in which the thrust level regularly alternates between flight idle and cruise settings.
What Obstacles Hinder The Widespread Use of CDOs?
One of the most significant challenges to implementing widespread CDO usage is the variability in speed and descent profiles from aircraft to aircraft. Because an individual aircraft's optimum speed and rate of descent will vary based on aircraft weight and top of descent (TOD) location, there can be significant variation among arriving aircraft. This makes precise sequencing difficult and hinders the ability of air traffic control (ATC) to space arriving aircraft tightly during high-density operations.
Additionally, specific performance data from each aircraft's onboard flight management system (FMS) isn't shared with air traffic controllers (ATC). This may require controllers to increase separation intervals among arriving traffic. In Japan, CDO arrivals are only conducted during low-density operations.
Further obstacles emerge when flight crews must alter routing for traffic separation and weather avoidance. Compared to terrain clearance, for example, these factors are more fluid and are ever-changing from one day to the next. Similarly, airspace in the form of military operation areas (MOAs) and temporary flight restrictions (TFRs) could make implementing CDOs more difficult.
In addition to operational challenges, the widespread adoption of CDO usage also requires cooperation among operators, airports, and regulatory authorities. The resulting logistical challenge can be a significant one. A number of airlines, for example, incorporate CDO modules in their initial pilot training. Should this training become mandated, it would present a not-insignificant challenge for airlines experiencing bottlenecks in their initial and recurrent pilot training pipelines.
Summary
The benefits to CDOs are significant. For each group of CDOs used at an airport, the FAA estimates that operators save an average of 2 million gallons of fuel and eliminate 40 million pounds of emissions annually. And with the increased efficiency comes a reduced noise footprint, a more comfortable passenger experience, and potential increases in safety.
One of the most intriguing aspects of CDOs is that their widespread adoption is primarily driven by software, communications, and coordination. Unlike other avenues to increased efficiency, like technologically advanced airframes or powerplants, the path to widespread CDO usage is primarily software-driven and contingent upon cooperation among the involved entities.
As roadblocks to progress go, this seems to be one of the more easily surmountable ones and thus, the future of CDO usage appears to be bright.