Ground support equipment (GSE), such as air conditioner (AC) carts or Preconditioned Air Units (PCA), are stationary equipment that deliver cold or hot air into a parked aircraft. AC carts are primarily used to maintain design thermal conditions throughout an aircraft’s parked phase, including temperature, humidity, and dew point. An AC cart keeps critical and sensitive electrical systems inside the plane from failing due to extreme heat or cold conditions when the engines are turned off. These units also offer enough air volume and temperature to improve boarding experiences by improving passenger comfort and cabin air quality at boarding gates or maintenance areas.
What are AC Carts necessary?
Although aircraft GPUs are connected to the aircraft to run the onboard AC systems, these AC systems do not operate on electricity and cannot be powered by electrical power from an AGPU. In addition, onboard air conditioning packs (or Air Cycle Machines) take bleed air from one of the jet engines or APUs (auxiliary power units) at high temperature and pressure to provide conditioned air onboard. As a result, when the plane is grounded, AC systems cannot obtain source air for cooling electric systems, necessitating a PCA or AC Cart unit for air conditioning.
PCA Units – Operational Principle and Types:
The chilled air is drawn through the PCA unit and converted to cool or hot air, and delivered through the aircraft preheating system. The preheating system directs cool outside pre-conditioned air into the cabin and airplane electrical systems to keep it at a comfortable temperature while the passengers are boarding or when the plane is parked or while the passengers are boarding.
PCA units can be categorized based on the power source and installation types:
- Classification based on Power source: Preconditioned Air (PCA) units are categorized into two types, Engine-driven Preconditioned Air Units and Electric Preconditioned Air Units.
- Engine Driven PCA units are driven by an IC (internal combustion) engine, most often a diesel engine, are referred to as ICPCAs. The engine powers a generator that powers up the HAVC refrigeration compressors in the PCA unit. These are conventional PCA units that have been used at airports for many years. Advanced IC units employ direct drive technology, which drives the refrigerant compressor and eliminates the electrical generator between it and the motor. These portable units are more prevalent in areas without access to an electricity source or battery packs.
- Electric Driven PCA unit is connected to the airport’s electricity supply and uses utility power in its operation. It receives electrical energy as input from the airport’s power plant. This source supplies all of the compressors and internal technology within the machine. The absence of emissions on the ramp allows these units to be more energy-efficient than engine-driven PCA devices. Furthermore, because they generate less operational noise, they are often preferred over engine-driven PCA devices.
- Classification based on Installation type: PCAs are available with Bridge mount, Apron mount (or Fixed on the ground), and Mobile PCA units (or AC Carts).
- Bridge mounted PCA unit: Bridge-mounted PCA units are positioned on the jet bridge, as the name implies. They’re suspended from one of the telescopic tunnels of the bridge using a unique mounting frame or supports. Instead of beneath it, some arrangements have a PCA unit mounted over the bridge. Electric PCA units are the most common type of bridge-mounted PCA equipment. Engine-driven PCA units aren’t placed under the bridge for fear of increasing vibrations, which might harm the jet bridge’s structural integrity.
- Fixed on-ground PCA unit: If a PCA unit is not installed under the bridge, it’ll be fixed to the ground. These PCA units are generally powered by an electrical source and are found on aircraft stands (ramp). The supply air enters the aircraft through a utility pit system or a duct (or pipe) installed beneath the jet bridge from the PCA unit.
- Mobile PCA units (or AC Carts): These are PCA units installed on a cart and can be towed to any position or mounted over a truck chassis to travel it to the desired aircraft location. These units have all of their PCA mechanism and parts put together on a platform positioned over the cart or truck chassis, with a tray to hold the PCA hose.
PCA units – Selection criteria:
While selecting a PCA unit, factors such as capacity requirements, energy efficiency, emissions and environment, space utilization, and operations efficiency should be considered.
- PCA Capacity Selection: The capacity of the PCA unit to be installed on an aircraft stand is determined by the maximum aircraft it can support and the climatic conditions. Higher capacity PCAs are required in regions where summer temperatures are extremely high, as opposed to coastal areas, where temperatures are more or less constant throughout the year.
A single PCA unit in the bay can service a wide range of narrow-body aircraft, from small Cessnas to large Boeings. The capacity for narrow-body aircraft is typically between 40 and 60 TR (Ton of Refrigeration), depending on the airport’s climate. The demand for air conditioning in wide-body aircraft is greater. To service these planes, multiple PCA units or a single large PCAs may be required. Wide-body aircraft, like the Airbus A380, has four PCA receptacles for ground connection to meet their air conditioning demands. On such aircraft stands, a single 90 TR capacity PCA or multiple 60 TR capacity PCA units may be installed.
- Energy efficiency and environmental considerations: Electric PCA units are becoming increasingly popular as a result of growing environmental restrictions in the airline sector. Furthermore, from an energy efficiency standpoint, electric PCA units are more efficient from an energy efficiency standpoint. Production of large amounts of energy is always less expensive than generating local energy onsite for equipment.
- Operation Flexibility in GSE Operations: At each airport, there are a certain number of contact stands at each airport, where aircraft stand and jet bridges are installed to link the ramp with the airport building. The selection of bridge-mounted or mobile PCA units may be determined based on redundancy and operational flexibility.
Contact aircraft stands with mounted electric PCA units are the preferred choice for contact airplane stands since these devices stay busy all day, handling numerous flights assignments to the aircraft stand. Mobile PCA units do not require a power supply and can be towed anywhere on any airline stand. This mobility allows airport management to deal with equipment problems more flexibly.
- Maintenance ease: The most common airport management worries include breakdown and troubleshooting procedures. To ensure that all airport facilities are available at all times, airport infrastructure and PCAs should be chosen for their simplicity of operation. Mobile PCA devices are once again the most advantageous since they do not suffer from flight operations or site limitations in maintenance.
Fixed on-ground PCA units are preferred second as they can be serviced without interrupting the flight schedule. They are generally placed in one corner of the aircraft stand where maintenance staff and specialists may disassemble and maintain it while a mobile PCA unit provides flights. From a maintenance standpoint, Bridge mounted PCA units are the least desired since servicing and maintenance teams cannot access different parts of the PCA units without using a ladder or an elevated platform. When an aircraft is parked on the bay and ground support activities are taking place, no maintenance work can be done on bridge-mounted PCA.
Operational Guidelines & Preventative Maintenance of Water carts:
FAA Considers PCA units and AC Carts as necessary ground support equipment, and proper maintenance and inspection are vital. OEMs make the following recommendations for operating the AC carts:
- LOTO (Lock out tag out): The Occupational Safety and Health Administration (OSHA) recommends controlling hazardous energy sources as part of the process. Following the OSHA guidance on managing unsafe energy sources during PM, the disconnected source should be locked and tagged first. Later, the main power will need to be “on” for operational testing purposes.
- Filter Maintenance: In order to effectively heat or cool air, the equipment’s design engineers established a mass airflow rate that must be followed. Dirty filters may impact how much air reaches the cooling and heating systems, which can ultimately lead to thermal stress or reduced cabin comfort. Filters replacement will be performed at the intervals recommended by the manufacturer. Filter types, sizes, and efficiencies differ from brand to brand.
- Cleaning the evaporator coils: The easiest method to keep an evaporator clean is never to let it get dirty and clog. There should not be a lot of concern about the evaporator, and many PCA manufacturers do not make it easy to access the evaporators. If troubleshooting reveals that the evaporate is unclean, consult OEM.
- PM of Heating Elements: Electric elements can be checked with an ammeter and circuit current values. Heating discharge temperature is verified using a thermometer. Check the manufacturer’s recommendations if the heating element needs to be cleaned. Additional inspections may be required in areas that are hot throughout the year.
- Condensation Pumps: Some PCAs use a pump to remove plenum condensation. The water is sometimes pumped directly to the ground beneath the PCA, and other times it must be pumped to a sanitary drain. Make sure the pump is in good working order. Check with the PCA’s manufacturer for recommended condensation pump “lay down” instructions during the winter.
- PM of Condenser coil: A condenser is an essential component of the refrigeration system and will not function effectively (or at all) if it is extremely dirty. Even a thin layer of oily residue may prevent heat transfer and endanger effective cooling operations. To clean the appropriate condenser coil cleaner, contact the PCA manufacturer.
- Hoses and Connectors: External and internal tears in hoses, undesirable connection cuffs (Velcro/zippers), exposed reinforcement rods, and other issues should all be visually detected. Each hole or tear indicates a loss of ventilation. Over time, hoses that have not been maintained can become contaminated with organic materials because of the normal condensation process. Check the aircraft connector for secure operation and general condition and cable seal’s integrity. Make sure the PCA outlet collar hose clamps are secure. Examine all of your hoses, baskets, and reels at 6-month intervals.
The operation and service manuals provided by the equipment manufacturers must be thoroughly studied by ground support personnel. Every PCA unit used must have a maintenance record, which must be filled out at each service interval showing all work performed on the system.
Aircraft operators should assimilate that AC carts are critical equipment and that failing to maintain proper hygiene and preventive maintenance standards will harm the passengers or aircraft components. It is essential to deliver clean, carefully regulated, conditioned air every time a water cart is connected to the aircraft.
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