# Analytical Model for Enhancing the Adoptability of Continuous Descent Approach at Airports
## 1 Introduction  
The air transportation and aviation industry face several challenges due to projected increases in demand for air travel and freight accompanying limited airspace congestion and airport capacity. The International Air Transport Association expects 7.2 billion passengers to travel in 2035, almost doubling the 3.8 billion air travelers in 2016, with the U.S. as the second-fastest-growing market, after China, with 484 million additional passengers per year forecasted for a total of 1.1 billion passengers [1]. With increased pressure on the infrastructure of terminals, runways, airspace around airports, and air traffic control operations, the industry is struggling to cope with this demand, yet it has to limit the harm that aircrafts cause to the environment through carbon emissions and noise levels. With regard to aircraft emissions, the U.S. Environmental Protection Agency finalized its determination that greenhouse gas emissions from certain types of aircraft engines,primarily engines used on large commercial jets, contribute to the pollution that causes climate change and endangers Americans' health and the environment [2]. Other countries are taking strict measures to limit emissions from aviation operations at airports by setting penalties for emissions levels above a specified limit. Under the European Union Emission and Trading System, all airlines operating in Europe, European and non-European alike, are required to monitor, report, and verify their emissions and to surrender allowances against those emissions that exceed certain levels from their flights per year [3]. Aircraft noise, on the other hand, is the biggest concern for airport officials at 29 of the 50 busiest U.S. airports [4]. Airports' support personnel who work in proximity to aircrafts idling on the ground or taking off and landing may suffer hearing loss. Residents of communities surrounding airports suffer sleep disorders and interference with speech, both of which may lead to reduced productivity in learning and work. Furthermore, recent studies have linked noise to non-auditory health effects, such as hypertension, heart disease, and stroke [5]. These issues represent critical challenges to air transportation and aviation industry sustainability, development, and prosperity. The Continuous Descent Approach (CDA), also known as Optimized Profile Descent (OPD), is an advanced flight technique for aircrafts to descend continuously from cruising altitude to the Final Approach Fix (FAF) or touchdown without level-offs and with an idle, or near idle, thrust setting. Descending using the CDA procedure, an aircraft can stay as high as possible for a longer time than with a conventional descent, thereby expanding the vertical distance between the aircraft's sources of noise and the ground and thus, significantly reducing the noise levels for populated areas near airports. Furthermore, by descending with an idle, or near idle, engine setting, fuel burn is decreased, resulting in the reduction of fuel consumption and harmful emissions to the environment. A study that conducted flight trials of CDA at Kentucky's Louisville International Airport using an aircraft fleet of the United Parcel Service (UPS), an express package delivery company, quantified the benefits of CDA in terms of fuel savings by 400 lb. to 500 lb. per flight and noise level by 3.9 A-weighted decibels (dBA) [6]. Another study conducted at San Francisco International Airport estimated a reduction of CO2 emissions of between 700 lb. and 10,000 lb. per flight with CDA flights [7]. To cut down on aircraft emissions, airplane manufacturer Airbus recently has been working on unique ways used by birds and emerging concepts like tandem flying that could reduce fuel burn by up to 10% [8]. When compared with the widely used Step-down Descent Approach (SDA), in which the arrival aircraft descends in a step-like fashion, CDA reduces flight time by around two minutes [9].