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13 Upon landing, aircraft taxi to the gate where passengers disembark and cargo and baggage is off-loaded. While at the gate, a variety of activities occur in preparation for the aircraftâs next flight. During this time, aircraft require energy to power onboard systems. Aircraft are equipped with APUs. These engines not only provide the power necessary to start the main aircraft enginesâthe APUâs primary purposeâbut also continuously supply onboard power to support aircraft electronic systems (i.e., avionics) and heating and cooling needs. Like main aircraft engines, APUs are gas turbine engines and use jet fuel. However, once the aircraft arrives at the gate, on-ground power sources can provide an electric alternative to the onboard APU engine. This chapter provides an overview of aircraft power needs, power supply options, gate electrification equipment, and operations and maintenance considerations, as well as descriptions of the incentives driving the installation of gate electrification systems. 2.1 Aircraft Gate Power Needs Aircraft parking time at the gate varies for different aircraft sizes and types, among other factors. Smaller aircraft with quick turnaround times stage at the gate for as little as 20 minutes. Larger passenger and cargo aircraft can remain at the gate for several hours, with unexpected delays potentially extending the stay. While parked at the gate, these aircraft have two primary power needsâelectrical power and cabin-conditioning controlâwhich are briefly described below. 2.1.1 Electric Power The aircraft requires electric power for avionics and cabin operation. Aircraft undergo a variety of system checks while on the ground to verify functionality in advance of the next flight and to update system support for navigation. The flight plan is transmitted to the flight crew, reviewed, and loaded along with other information for the flight, such as onboard fuel and weather data. As part of the process of cleaning the cabin and readying it for the next flight, electricity is needed for lighting and powering onboard water systems and other appliances, including refrig- eration and kitchen equipment. The mechanics of the onboard water system is used as part of the cleaning process. 2.1.2 Cabin-Conditioning Systems Maintaining a comfortable cabin temperature is important for efficient preparation of the plane between flights for crew, service personnel, and passenger comfort during departure and arrival. C H A P T E R 2 Ground Power and Air Conditioning Systems
14 Optimizing the Use of Electric Preconditioned Air (PCA) and Ground Power Systems at Airports Cabin temperature demands are highly variable based on the ambient on-ground temperature. According to the VALE Program Technical Report (FAA 2010), aircraft cabins typically require heating when ambient temperatures are below 45Â°F and cooling when temperatures are greater than 50Â°F. When temperatures are between 45Â°F and 50Â°F, aircraft cabins will typically need ventilation (though not necessarily cooling or heating). Demand for cooling is critical in warm climates, particularly during the second half of the day as temperatures rise, as well as in summer months. In colder climates, the demand for heating takes precedence. In extreme cold, cabin heating may be necessary to prevent water onboard the aircraft from freezing. During shoulder seasons (i.e., fall and spring) or in locations where weather is variable and unpredictable through- out the day, the air-conditioning needs of aircraft cabins may vary and ultimately are subject to pilotsâ discretion. 2.2 Power and Cabin-Conditioning Equipment There are three main sources of power to meet aircraft power needs while on the ground: onboard engines (i.e., APUs), ground-based systems, and mobile units. This section describes the options for providing power to aircraft and provides a high-level description of ground power and PCA systems. 2.2.1 Onboard Engines Aircraft APUs are engines that provide power for aircraft functions beyond propulsion. The APU is typically located in the tail of the aircraft (Figure 1) and is powered by the same jet fuel as the main engines. Its primary purpose is to provide power to start the main engines. Once the main engines have started, the APU is typically shut off. Additionally, most aircraft are able to use the APU during normal flight conditions as backup power sources or in case of generator failure. The APU can also be used to provide a power supply to run the aircraft systems while on the ground between flights. In this case, the APU simply remains running while the aircraft is parked at the gate. Use of the APU to provide ground power is influenced by the expected time at the gate and availability of alternative power supplies. As with all engines, some regular use is important to keep the APU functioning. But long idle times can result in buildup and clogging. However, the APU has an expected runtime life conditioned on adherence to a prescribed maintenance schedule. When the APU is used to Figure 1. Aircraft APU.
Ground Power and Air Conditioning Systems 15 regularly supply ground power to the aircraft, run time increases and maintenance schedules must be accelerated, often leading to an increase in overall maintenance costs. In addition, as the run time of the unit increases, it shortens the product life and requires accelerated component replacement and associated costs. Any time an aircraft is out of service due to unexpected APU maintenance needs, it may cause ripple effects across an airlineâs operations because of lack of availability of an aircraft for scheduled flight operations. APUs are not designed or economical for use as routine sources of ground power. However, when gate electrification systems are unavailable (e.g., due to equipment misuse or damage), pilots must use aircraft APUs to provide ground power while at the gate, increasing the wear on the engines. APUs are also utilized when ambient temperatures exceed the operating limits of the PCA system or when the PCA system is not sized appropriately for the aircraft model (i.e., the system is not powerful enough to heat or cool the aircraft models in use; or, in some cases, the system is too powerful). In these instances, APUs provide air supply, as well as energize the air-conditioning system on the aircraft to heat or cool the supply air to maintain a comfortable cabin temperature. 2.2.2 Ground-Based Electric Power and PCA The primary alternative to using the APU at the gate is ground-based electric power, which can be provided through a ground power unit (GPU) frequency converter with power supplied by the airport terminal. Alternatively, power can also be provided by a mobile ground support equipment unit, an option described in more detail in Section 2.2.3. The power must be con- verted to a voltage and frequency that is compatible with the aircraft systems. This is achieved through the GPU frequency converter. Cabin conditioning can also be powered on the ground instead of by engine. However, an air- conditioning unitâreferred to as a PCA unitâis necessary to provide efficient heating and cooling to the aircraft. Like the GPU, the PCA unit may be physically connected to the terminal via the gate or as mobile ground support equipment. Figure 2 shows a schematic of an aircraft parked at the Power Cable PCA Hose Bridge-Mounted PCA Unit Bridge-Mounted Power Converter Unit Main Engine Main Engine Auxiliary Power Unit Figure 2. Typical location of aircraft main engines, APU, and gate electrification equipment.
16 Optimizing the Use of Electric Preconditioned Air (PCA) and Ground Power Systems at Airports gate, with a jet bridgeâmounted PCA and ground power unit, along with a PCA hose and power cable connecting the aircraft. Figure 3 depicts the GPU frequency converter and the PCA unit positioned on the underside of the jet bridge, a common design for gate electrification equipment. Even at airports where gate electrification equipment and services are available, the APU remains operational for a short time during the transition at the gate. It is estimated that APUs are used for approximately 7 minutes per aircraft turn (2 minutes when aircraft first arrive at a parking position and for approximately 5 minutes prior to pushback [Environmental Science Associates et al. 2012]). FAA also assumes thatâas stated in the VALE Program Technical ReportâAPUs must be used for a minimum of 7 minutes per turn (time at gate during passenger loading and unloading operations), even if gate electrification equipment is available (FAA 2010). 18.104.22.168 GPU Frequency Converter As previously described, the GPU can be any equipment on the ground that supplies power to the aircraft. The GPU frequency converter specifically transforms the terminalâs power delivered from the electrical grid and consumed in the airport terminal to 400 Hz, a frequency that can be utilized by most aircraft. Smaller aircraft, such as regional jets, often are designed to use 28 volts direct current (VDC) power, and GPUs may be capable of serving both types of aircraft. The major- ity of airports interviewed and surveyed for this project provide electric 400 Hz power, with far fewer gates equipped with electric 28 VDC power. Therefore, discussions of electric ground power equipment and utilization in this report generally refer to 400 Hz, unless stated otherwise. When the unit is located at the gate, it is referred to as point of use. The unit may be located on the underside of the jet bridgeâoften close to where it connects to the aircraftâor on the ground if there is insufficient space to accommodate the unit on the bridge. It may also be part of a centralized system with primary components located in the terminal building with a smaller gate box unit with controls located on the jet bridge. Some airports have GPUs located in under- ground pits to minimize equipment on the ramp. The GPU frequency converter is connected to the aircraft with a cable (Figure 4), which is manually attached by the ground crew. 22.214.171.124 Preconditioned Air The PCA unit provides cooling, heating, and ventilation to the aircraft. Conditioned air is supplied through a large air-conditioning unit attached to the underside of the jet bridge Preconditioned Air Unit GPU Frequency Converter Figure 3. Gate electrification equipment attached to a jet bridge (Source: Steve Bivens, Cavotec).
Ground Power and Air Conditioning Systems 17 (bridge mounted), on the ground adjacent to the bridge (ground mounted), or from a central location inside the terminal, as illustrated in the schematic in Figure 5. Point of use PCA units may provide cooling and ventilation exclusively or have the ability to provide heating if a heat pump is included. Central systems provide heated or chilled liquid to each gate through a series of pipes. Each gate unit contains a heat exchanger across which air is moved before being pumped into the aircraft. The liquid is heated or chilled from a central chiller or boiler unit at the airport and may be part of the airportâs overall HVAC system. With central PCA systems, an air-handling unit is typically still necessary at the gate to blow the conditioned air into the aircraft. The ground crew delivers the heated and cooled air through a flexible hose that is unrolled and connected manually to the underside of the plane. 2.2.3 Mobile Ground Power and Mobile PCA Units Ground support equipment refers to a variety of vehicles (such as fueling and baggage trans- port) used on the airside apron to support aircraft when parked at the gate. Ground support equipment may also include mobile 400 Hz or 28 VDC GPUs or mobile PCA units to serve multiple gates as an alternative to APUs when electric ground power and PCA are not available. Figure 4. Aircraft connection to ground power (black cable) with frequency converter (red arrow), and PCA hose (yellow ribbed hose). Figure 5. Typical layout of gate electrification system and centralized cooling system (Fleuti and Ruf 2013).
18 Optimizing the Use of Electric Preconditioned Air (PCA) and Ground Power Systems at Airports The GPU and PCA mobile units are typically powered by fossil fuels (diesel or gasoline). Some units are powered from electricity supplied either from an onboard battery or by plugging into an outlet at the gate. In 2018, a mobile battery-powered GPU entered the market (Figure 6), though mobile PCA units require too much power to run on batteries at this time. Fossil fuelâpowered ground support equipment have been utilized most commonly where APUs are discouragedâor when airlines prefer to use ground support equipment instead of APUs for economic reasonsâand in cases in which the airport is not equipped with other means of providing electric ground power or conditioned air. When aircraft are parked at gates with unavailable gate electrification equipment, or equip- ment that is not sized or configured appropriately for the aircraft (e.g., a regional jet requiring 28 VDC power parked at a gate that only has 400 Hz available), mobile units may be used. In addition, remote gates and hardstandsâgates not attached to the terminal building by a jet bridgeâthat are not in close proximity to the terminal and associated power infrastructure are often supported by mobile units. Whether in use at a gate or a hardstand, the mobile unit is attached to a tug and driven to the aircraft location for service. The tug also requires power, which may be supplied by an onboard battery or a traditional combustion engine. A schematic of a mobile PCA is provided in Figure 7. Figure 6. Mobile battery-powered GPU, ITW GSE Model 7400 (Source: ITW GSE). Figure 7. Schematic of tug-towed mobile PCA unit.
Ground Power and Air Conditioning Systems 19 A considerable number of ground support equipment vehicles are often operating simul- taneously on the ramp to serve aircraft needs at the gate, particularly at commercial service airports. In addition to mobile GPUs and PCA units, there may also be baggage tugs, catering vehicles, fueling trucks, lavatory servicing equipment, deicing equipment, and water cabinets in service on the ramp. Therefore, ramp traffic congestion and vehicle storage associated with mobile units should also be considered when selecting the type of gate electrification equip- ment to employ. 2.3 Operations and Maintenance Gate electrification equipment is typically owned and operated by the airline or the airport, as discussed in Section 2.5. Most of the airports interviewed for this project owned all or the majority of gate electrification equipment at the airport. Maintenance is performed by a number of different parties, including airport staff, contracted third parties, and equipment manufacturers. There are several operations and maintenance variables that may impact the use of gate electrification equipment, including the competency of ground crew opera- tors, the functionality of monitoring equipment, and equipment maintenance scheduling considerations. 2.3.1 Ground Crew Operators The gate electrification equipment requires action by the ground crew to physically connect the gate equipment to the aircraft (Figure 8). This is conducted by extending the GPU power cable and the PCA hose from its position at the gate and connecting it to the underside of the aircraft. The ground crew must be trained in the proper use of the equipment or it may be damaged and become inoperable. Ground crew must also be instructed by the managersâeither at the airport or from the airlineâabout the necessity of using the gate electrification equipment. If aircraft are not connected to the gate electrification equipment, or not connected quickly enough, the pilot may instead continue to run the aircraft APU for ground power. If the ground crew is unable to use the gate electrification equipment because of damage, crew members must have access to direct communication with airline and airport operations staff to alert them to take action to repair the equipment. Figure 8. Ground crew connecting jet bridge at San Diego International Airport (Source: San Diego International Airport).
20 Optimizing the Use of Electric Preconditioned Air (PCA) and Ground Power Systems at Airports 2.3.2 Monitoring Equipment Functionality Gate electrification equipment owners and maintainersâwhether they are employed by an airline or the airportâmust be able to determine if the equipment is functioning properly and being put into service. Airports monitor the functionality of their equipment in varied ways, often including some combination of the following actions: â¢ Conducting regular manual equipment checks; â¢ Relying on operators to report malfunctions; and â¢ Utilizing centralized monitoring systems, such as building automation systems or central HVAC controls. The monitoring capabilities in place at each airport depend on the equipment installed, equipment ownership structure, maintenance responsibility, and metering availability (includ- ing gate electricity meters and gate electrification equipment use meters). Many newer gate electrification systems include options for remote monitoring through a connection that transmits activity information to a centralized control center, where the infor- mation may be viewed from a central location or from an Internet-enabled deviceâsuch as a computer or smartphoneâas illustrated in Figure 9. The information available may vary, depending on the interests and access level of the user. However, a monitoring system generally provides real-time information about the operational status of the gate equipment, including equipment that is in active use, faulty, in maintenance, or remains idle. A depiction of elec- trification equipment operational status by gate for a single concourse is shown in Figure 10. Figure 11 depicts a screenshot of SeattleâTacoma International Airportâs software interface for monitoring the PCA status of individual gates. This airport has a central chiller and boiler plant that provides heating, cooling, and ventilation to the PCA system, which is incorporated into the airportâs HVAC system. The PCA monitoring software has been integrated into SeattleâTacoma International Airportâs building automation system. Aircraft Passenger boarding bridge PCA hose Power cable Bridge-mounted PCA unit Bridge-mounted power converter unit Remote monitoring through Internet- enabled computers or wireless devices Figure 9. Diagram of gate electrification equipment at a jet bridge (passenger boarding bridge) being accessed remotely.
Ground Power and Air Conditioning Systems 21 Figure 10. Software display for gate equipment tracking and monitoring (GSE = ground support equipment) (Source: Steve Bivens, Cavotec). Figure 11. PCA unit monitoring software interface for one gate at SeattleâTacoma International Airport (Source: SeattleâTacoma International Airport).
22 Optimizing the Use of Electric Preconditioned Air (PCA) and Ground Power Systems at Airports If a remote monitoring system is not a component of the gate electrification systemâs con- figuration, usage may be monitored by installing a meter at the point where the GPU and PCA units are connected to the terminal. Such a meter may also be tied into a centralized monitoring systemâsuch as an airportâs existing building management system or building automation systemâto allow for remote monitoring using existing systems. If monitoring information is not tied into a centralized system, confirming equipment functionality may require staff to physically inspect the equipment, which can be time consuming and inefficient. Absent the ability to electronically monitor use and functionality of gate electrification equipment (or to provide supplementary information), owners and maintainers may choose to undertake formal or informal surveys to observe how the gate electrification equipment is being employed. These visual surveys are useful to confirm if the PCA and/or ground power equipment is physically connected to the aircraft but cannot necessarily confirm if the equip- ment is actually in use. While each of these monitoring options can be used to assess the opera- tions of the gate electrification equipment, none of these systems is useful in confirming whether the APU has been turned off. APU use is monitored by some airlines to understand fuel consumption. However, that information is not regularly provided to airports. 2.3.3 Equipment Maintenance Like other aircraft service equipment, the gate electrification components should be inspected and maintained according to a regular preventative maintenance schedule to avoid operational failure. In addition, quick response time concerning corrective maintenance is necessary to max- imize equipment availability in the event of an unexpected breakdown. All require sufficient budget, staff, and inventory on hand to ensure minimal repair time when problems do occur. As discussed in Chapter 3, equipment availability is heavily impacted by improper use and storage of gate electrification equipment components and maintenance practices, which can speed or hinder the return to service of the equipment. For example, making spare parts available and even storing backup units that can be deployed quickly as a replacement will reduce equipment downtime and loss of service. 2.4 Ownership Models for Gate Electrification Gate electrification systems may be owned, operated, and maintained by individual airlines or by the airport, or by any combination thereof (i.e., equipment may be owned by the airport but maintained by the airlines). 2.4.1 Owned by Airline Terminal and gate use and ownership vary from airport to airport and often change over time. At some airports, airlines own (or used to own) their gates or have exclusive use or preferential use leases (i.e., they do not share gates with other airlines). In other cases, multiple airlines may share the sameâor commonâgates. In some airports where airlines own or have exclusive use leases of gates, airlines have acquired and operate their gate electrification equipment to meet their respective operations and maintenance needs. The deployment and utilization of gate elec- trification equipment supports airline economic interests by reducing the use of the APU. This, in turn, increases its useful operational life and decreases costs associated with powering the aircraft at the gate using jet fuel. In the airline-owned scenario, the airline workersâor their contractorsâoperate and main- tain the gate electrification equipment. In some cases, the airlines also maintain their own
Ground Power and Air Conditioning Systems 23 jet bridges and ancillary equipment. Interactions between the ground personnel, pilots, and onboard crews remain within the same company, which may improve consistency of commu- nication and use of the gate electrification system, since company policies and procedures apply to all parties. 2.4.2 Owned by Airport It has become more common for airports to invest in gate electrification equipment to main- tain greater control over facilities, to meet airline needs, and to manage environmental impacts better. In some cases, airports have reassumed ownership of gates and equipment that were previously owned by airlines. Access to funding available from FAA and other government entities has also encouraged airport investment in the equipment (e.g., through VALE grants or state grant programs). In the airport-owned scenario, airport workers or their contractors maintain the gate electri- fication equipment (although airline ground crew often predominately operate the equipment). Interactions occur between the ground personnel, the pilot and onboard crew, and airport staff. Airlines may have their own policies and procedures with regard to APU use, which can be facilitated or hindered based on whether the airport has the appropriate gate electrification equipment. In some cases, U.S. airports have policies encouraging airline use of gate electrifica- tion equipment instead of APU use. But none of the U.S. airports interviewed and surveyed for this research project have policies mandating the use of gate electrification systems. 2.5 Motivations for Installing Gate Electrification Systems Whether the airline or the airport owns and/or operates the gate electrification systems, there are a number of reasons why these systems are more frequently being installed to provide ground power and cabin conditioning at airports. This information can be useful in understanding potential solutions for optimizing their use. 2.5.1 Preserve APU Critical Functionality Use of the gate electrification equipment while the aircraft is on the ground decreases use of the APU. This switch allows the APU to be preserved for its primary purpose, which is to be available to start the main aircraft engines prior to flight orâin the event of an interruptionâ during flight. As a result, airlines are placing greater emphasis on reserving the APU for these safety reasons. 2.5.2 Reduce Aircraft Operational Costs Complementing the preservation of the APU is the additional economic benefit to the airline from using the gate equipment instead of the APU. The economic benefit can come in two forms. First, when the APU is not used as often, maintenance and replacement costs are pushed further out into the future, which decreases associated costs on an annual basis. Second, operating the APU burns jet fuel, which, in most cases, is comparatively expensive. While airports use a variety of systems to charge airlines for electricity use, the cost of electricity is cheaper than jet fuel. Therefore, costs to the airline for using the gate equipment are often lower. Optimizing the use of gate electrification systems, then, will accelerate the return on investment for the costs to install the equipment. Labor costs for the ground crew are comparable for APU use and gate electrification equipment use.
24 Optimizing the Use of Electric Preconditioned Air (PCA) and Ground Power Systems at Airports 2.5.3 Local Air Quality and Noise Improvements As discussed in Chapter 1, running the APU emits pollutants from the APU exhaust at the back of the aircraft, while increasing associated noise on the ramp. These emissions and related noise can impact ramp workers and communities adjacent to the airport. For these reasons, many airportsâmost outside of the United Statesâhave enacted policies restricting the use of APUs. The Boeing Company manages a list of international airports with airport noise and emission regulations (The Boeing Company 2018). Converting to gate electrification equipment decreases localized air pollution from running the APU on site at the airport, though emissions are still associated with producing electricity off site and pulling it from the electrical grid. The gate electrification equipment does emit noise, as well. However, the noise is at lower levels than an APU (Tam et al. 2005, Kwan and Yang 1992). Reducing local emissions is particularly important in urban areas that have been designated as nonattainment zones (i.e., those areas not meeting air quality standards) under the federal air regulations. While direct impacts on neighboring communities remain a strong motivation for airport improvement projects, airport authorities are also developing broader environmental and sus- tainability goals consistent with the public policies of local, regional, and state governments. Sustainability goals have been implemented at airports that seek to be leaders in the industry, as well. The conversion of ground power to electric sources may assist airports in contributing to many of these environmental initiatives, not only improving local air quality but also decreasing the airportâs greenhouse gas emissions when comparing APU emissions to those associated with the gate equipmentâs electricity demand.