Preconditioned Air and Providing Comfortable Aircraft Cabin Conditions on the Ground
The importance of a PCA unit and proper ventilation
PCA (Preconditioned Air) systems are used to introduce fresh conditioned air into an aircraft cabin while the aircraft is parked at a gate or maintenance site. Air is filtered, heated, or cooled, and moisture is removed before delivery to the aircraft. Once the conditioned air enters the aircraft coupler at the end of the PCA hose, it is distributed through the aircraft cabin’s duct system and into the cabin.
The goal of this paper, among other things, is to explain the purpose of a PCA unit and some of the challenges faced by airports, maintenance contractors, and airlines regarding the ability to maintain comfortable aircraft cabin conditions. An additional motive of this paper is to provoke continuous dialogue for mindful design and usage of PCA equipment and systems. It is beneficial for end users, airport designers, and PCA equipment manufacturers to discuss things observed in real-world PCA applications. This can better help PCA professionals assist the airports and airlines to create a more comfortable cabin. Sometimes the adverse conditions realized in the field aren’t noticed during controlled laboratory tests in the factory. Possible end user issues could be overlooked by manufacturers. This paper will explain how PCA equipment operates and will also provide ideas for some areas of improvement.
Aircraft Cabin Ventilation
An aircraft in flight needs a fresh makeup air source, a comfortable cabin pressure, and the ability to purge high CO2 accumulations (human exhalation) from the cabin. Fresh oxygen containing makeup air is introduced into the cabin by an on-board environmental control system. This outside air is obtained from the aircraft’s jet engine compressor. Makeup air heating and cooling is necessary for passenger comfort and temperature control of sensitive electrical and electronic systems. The aircraft’s environmental control system does an extremely good job providing a satisfactory cabin temperature and humidity level in flight.
When an aircraft is on the ground and without the propulsion engine’s air source for cabin conditioning, an aircraft auxiliary power unit (APU) may be used to supply the conditioned air. The APU is a smaller onboard engine for powering aircraft systems when not using the main engines. Point-of-use air conditioning equipment at a gate, hangar, or remote parking area can also be used for cabin climate control needs. This equipment can be an air handling unit fed from a chilled water source or it can be a packaged DX (direct expansion refrigeration system) PCA unit. The packaged air conditioning unit contains a complete refrigeration system and air delivery source. This PCA equipment can be fixed or mobile. The electrically powered DX point-of-use packaged PCA unit is common in the U.S. and is the topic of this discussion. With a cabin door open during boarding, oxygen level and atmospheric pressure are not an issue; however, air comfort and quality concerns exist.
The point-of-use electrically powered PCA is a piece of equipment of great significance in an airport ramp environment. The electrically powered point-of-use PCA reduces noise and pollution levels on the ramp. It does so by eliminating APU noise and exhaust. It also eliminates the need for a diesel driven mobile cooling unit and subsequent noise and exhaust from that type of equipment. These reductions are important for obvious environmental and human health reasons. The elimination of APU operation while boarding and deplaning also offers significant cost savings to airlines from reduced jet fuel usage. With the APU being used less, ensuing maintenance costs are also reduced. Considering the benefits, the electric PCA is regularly used in place of APUs or diesel mobile air conditioning carts for aircraft cooling on the ground. A PCA unit is expected to provide the same comfort levels as the aircraft’s built-in source of cabin climate control; therefore, it must work with equal ability.
Differences Between PCA and Commercial Air Conditioners
Without understanding the complex physics involved, some may not appreciate why it’s so difficult to cool an aircraft parked at a gate or hangar, using ground source equipment. After all, isn’t it just an air conditioner with a hose? What’s the big deal? Understand that the design principles must be approached very differently with this type of unique air conditioning system. Below are some bullet point differences.
- Commercial Air Conditioning — Recirculates air from a space being conditioned. Reduces the air temperature by 15°–20°F in cooling mode, before discharging the air.
- PCA-Uses 100 percent outside air from the ramp area, filters and conditions it and delivers it to the aircraft. Sometimes required to reduce outdoor air temperatures by 60°–70° (or more) before discharging it.
Commercial Air Conditioning
Regarding commercial design of air conditioning equipment, a general requirement is to deliver 350-400 CFM (cubic feet per minute) of air, per nominal “ton” of refrigeration (“Ton” definition - Removal of 12,000 BTU’s per hour per 24h hours, from 32° water to create 1 ton of 32° ice). The above air volume per refrigeration capacity is assuming something referred to as “standard air.” Standard air is a theoretical term assuming air at “normal” atmosphere, with a consistent room temperature and consistent comfortable indoor humidity level. A constant standard air value is inserted into design calculations for air conditioning equipment design/sizing, duct design, blower sizing, and supply air outlet layouts, to provide adequate air delivery and cooling for a conditioned commercial space. A standard air constant number can be used in the calculation because commercial systems bring the air from the conditioned space, back to the air conditioning equipment via a return air duct. The return air maintains the consistent condition of the space being heated or cooled. It’s not a difficult task to condition air that’s already near its delivery set point. See Typical HVAC Example Below:
Aircraft Preconditioned Air
PCA units deliver a heated or cooled 100 percent outside air source to the aircraft cabin. In cooling mode, the temperature reduction can be 60°-70° or even more. Heating mode temperature rises of 100° or more, over ambient temperatures, are not uncommon. Heating is accomplished by electric heaters in the unit. These temperature alterations alone are difficult, without factoring the complexities of conditioning outside air. Outside air does not have the consistent air property values of indoor air; therefore, commercial air conditioning design mathematics aren’t used in PCA design. The “12,000 BTU’s/hr./24 hr.” formula assumes the theoretical process of freezing water at a steady 32° temperature. We don’t necessarily ever achieve a steady temperature or conditions with outside air. Outdoor air comes with ever-changing psychrometric properties (psychrometrics is a field of engineering pertaining to the physical and thermodynamic properties of air). Atmospheric pressures, moisture content per air volume, wet bulb and dry bulb temperature, relative humidity, etc. are related to one another and affect the conditional variables of the air to be conditioned. Commercial indoor design calculations, using “standard air,” need not consider this the way a PCA design does.
Imagine taking a 5 gallon bucket and scooping a bucket of air in Chicago at 12:00 noon on August 1st. Then, do the same thing at the same time in Houston. Those two same sized buckets would theoretically hold 5 gallons of air with very different properties when plotted on a psychrometric chart. Air density, grains of moisture per volume, etc. would all be different in these equal sized buckets. The psychrometric state of air is important to understand when tasked with a specific air volume and temperature delivery goal. In this case, comfort cooling of an aircraft cabin using an air source with ever changing conditions. Outside air properties are subject to change, with the always changing atmospheric conditions. See PCA Graphic Example Below:
The term tonnage is often used to distinguish PCA models/sizes from one another because the non-engineer customer can often only visualize size differences by the tonnage number. Customers also incorrectly use the tonnage number to compare one OEM’s price against another’s. For example, a 25-ton refrigeration compressor would not achieve 25 tons of heat removal during most outdoor air cooling conditions and thus the tonnage number nomenclature is a misnomer for PCA size description. It is better to let the designers design for specific aircraft air volume and temperature delivery specifications and forget the tonnage number altogether.
Commercial vs. PCA Air Flow
Commercial Air Conditioning — If we calculate airflow for a standard 60-ton commercial roof mounted air conditioner at 400 CFM (cubic feet per minute) per ton, the total airflow required is 24,000 CFM.
60 X 400 = 24,000. There’s a bit more to it; however, this is abridged to get the point across. 24,000 CFM at a standard design friction and velocity) may require engineers to specify a duct of around 60 inches diameter or its rectangular equivalent. This dimension is approximated for example purposes.
PCA — Note that aircraft manufacturers already selected the duct sizes in their aircraft as well as the external connection point. They also specify rated flow and static pressure requirements for their respective models. The PCA is designed to deliver air at a specified air volume and static pressure since the distribution duct size was already determined. A PCA discharges conditioned air through a 14” diameter hose with an 8” diameter aircraft coupler at the end. This is quite a bit smaller than the commercial system ducting size in the example of the previous paragraph. Remember, outside air density, humidity, temperature, etc. all play a role in controlling the delivery of the rated volume and condition of the discharge air to the cabin. A PCA’s control system is required to command control logic alterations, in real time, to deliver steady discharge conditions in dynamically evolving environments.
Ancillary factors must also be considered as propelling the specified volume of air through such a small duct is already a great challenge. Though some call it a hose, it can also be called the duct that delivers the air to the aircraft. The terms hose and duct may each be used.
To explain hose management, we can review some commercial air conditioning rules. Due to air’s affinity to take the path of least resistance, a straight, short air path offers the most optimal results.
- 40 ft. of duct is going to have better flow than 100 ft.
- A duct that is straight impedes flow less than one with a 45-degree bend.
- A 45 degree bend impedes flow less than a 90 degree bend.
- A 90 degree bend impedes flow less than a kink in the duct.
ASHRAE Manual D (Duct Calculation and Design) declares “a 90-degree elbow can have an equal resistance to air flow, as 30 feet of the same diameter duct.” Commercial duct systems are thoughtfully designed to minimize drastic transitions, directional changes and lengthy distances due to the impedance of air flow that they cause. The same can be done with PCA duct.
Thermodynamics dictate that a mass with more heat will give up heat to a mass of lesser heat value. The greater the heat imbalance, and the greater the difference in mass, the greater the rate of exchange will be. This is why PCAs discharge much colder air than commercial systems. The limited duct size in an aircraft requires very cold air delivery to make up for the limited available duct volume. Very cold air at a trickle, due to a poor duct configuration, will not cool properly. A high volume of air that’s not cool enough will also not cool properly. An impeded airflow; due to twists, turns, kinks, and extreme lengths, can absorb heat from the ramp beneath it or environment around it. This is because the “slowed air” will spend more time in contact with the exterior sources of heat. The air may then enter an aircraft at a much higher temperature than when it was discharged from the PCA in cooling mode. This creates an appearance of a PCA not working correctly. The conditioned air delivery package should be considered as an entire system from the PCA to the aircraft cabin.
Airlines sometimes have very diverse aircraft parking layouts at their gates. Every aircraft type has a different PCA hose connection location, in relation to the mounted PCA. This requires differing lengths of hose to get to the connection point. Simply supplying the longest hose needed isn’t a good option if it’s kinked most of the time.
• A hose retrieval system is a great way to solve hose length and hose stowage issues. This system is designed to only dispense the exact amount of hose for the aircraft type currently in the gate. The remainder of the unneeded hose is contained within the unit.
Right sizing hoses at the gates, per aircraft type, is another method of hose management. If most of a gate’s parked aircraft require 40 feet of hose and two flights a day require 100 feet, all flights are subject to poor airflow for the sake of the two that need the 100 feet length. A basket which contains 40 feet of hose, along with staged extension reels or carts for additional lengths, would be a better solution.
Hose Reels/Extension Carts
• Another solution is a dedicated aircraft parking layout. If aircraft of similar sizes, with similar connection points were parked in the same gates, hoses could be installed based on common configurations.
It may not be practical to undertake one of these suggested remedies exclusively; however, a combination of them might be a possibility along with some dialogue promoting additional ideas. The important thing to take away is that the best refrigeration machine with the lowest discharge temperature possible can’t cool an aircraft without the accompanying specified airflow. The lack of air flow can also harm the packaged PCA’s refrigeration components.
We have discussed how design airflow and refrigeration capacity require an intricate balance in relation to one another. We also discussed how the physics of air is related to what levels of comfort can be achieved and how we may get to that comfort level. A well-designed control scheme and some standard functions are necessary for proper control of a refrigeration machine’s components.
- Aircraft Selection Switching — An electronic controller with an HMI (Human Machine Interface) programmed for every type of aircraft in the fleet would be the first choice of this author. All aircraft have unique air volume requirements as specified by their respective designers. A second choice would be to have at least “Regional Jet,” “Narrow Body,” “Wide Body,” and “Jumbo Jet” selections, on the control, based on relative size of the parked aircraft. There is an argument by some airports and airlines for simple ramp equipment operations. As diverse as the aircraft model’s climate control requirements are, a simple “ON/OFF” control is not a worthy solution. Improper aircraft and mode selections are often mistaken for malfunctioning equipment.
- Cabin sensor — This is a means of sensing cabin temperature to modulate the PCA discharge temperature accordingly. Generally, there are not complaints of an aircraft being too cold in summer, so cooling discharge at setpoint temperatures are usually ok. Heating is typically where discharge temperature regulation is necessary to not overheat the cabin. It’s common to have a temperature sensing probe connected to the PCA which is brought into the cabin during PCA use. The PCA control system will adjust discharge temperature based on current cabin temperature and anticipated ability of achieving a setpoint at the cabin’s current rate of change in temperature. This prevents over or undershooting the setpoint.
- “Heat- Vent - Cool” manual override — “Auto mode only” sometimes promotes nuisance calls. At times the machine won’t allow cooling if the aircraft is warm and outdoor conditions dictate no cooling. Auto mode switchover ranges can end up getting reprogrammed by people who may not understand the functions. Suitable operations can suffer. A dedicated “heat/ vent / cool” override function (in addition to “Auto”) is preferred. Refrigeration compressors shouldn’t operate in dangerously low outside temperatures; however, a low ambient limit switch can keep this from happening. Vent mode on a cool day would cool the cabin adequately in this case.
- No Bridge Pre-Conditioning Mode – Boarding Bridge pre- conditioning is when a PCA is used to quickly heat or cool a bridge. A dedicated hose and control functions are built into the PCA as an option for this purpose. Most climates would require the size of boarding bridge air conditioning units to be 5-7 ½ nominal tons. A PCA will have a much greater minimum compressor capacity among respective unit sizes, brands, and configurations. PCA compressors tend to be too large for bridge cooling purposes. It is not possible for many large capacity PCA units to “turn down” or trim capacity for such low refrigeration capacity and airflow needs. This places stress on the equipment’s components and compressor failure can result over time. Bridge pre-conditioning also won’t offer continuous bridge climate control. A boarding bridge and an aircraft can’t be simultaneously heated or cooled with bridge pre-conditioning. A roof top HVAC unit or ductless split system and a thermostat will do a better job of keeping a constant comfortable temperature in the boarding bridge, thus eliminating a need for bridge pre-conditioning.
Consider the discussions of the complexity of the relationship between refrigeration capacity and airflow. We are told that it’s easier for the ramp personnel to push a start button only and not have too many setting choices. Often the correct setting is not chosen with the multi-switch control station, so customers sometimes don’t want aircraft selection modes. There is no conceivable way to achieve the correct air flow and accompanying necessary refrigeration capacity using only an “ON/OFF” type of controller. Over or under feeding of aircraft and PCA compressor damage can occur without synchronizing airflows and refrigeration capacities. With modern building automation, it’s conceivable for an agent to set up a fixed PCA per aircraft type parked at the gate. This could be accomplished from a host computer in a command center. The ramp worker could then press a start button on a pre-programmed machine. Even more advantageous, would be to have the aircraft in the gate automatically broadcast information directly to the PCA programmable logic control system for an internally automated calibration. That type of connectivity is being discussed among stakeholders.
Left: Example of an On/Off controller
Right: Example of a controller with some of the necessary selectable control options with a display screen
Air Discharge Temperatures
Air discharge temperature plays a major role in heating or cooling parked aircraft. Regarding PCA using electric heat, heating operation is generally an easy task. Of course, discharging the coldest air possible will cool an aircraft rapidly; however, we are still at the mercy of physics and atmospheric conditions. Wet air over a sub-freezing surface makes ice. Without getting into all the supportive science, this is the simple explanation. By attempting delivery of air below the freezing temperature of water, moisture contained in the air freezes on the evaporator/evaporators of the PCA. This ice can block any additional air flow. The ice also has an insulating effect on the evaporator surface and refrigerant tubing making it a less efficient heat sink. Low or no continued heat removal can result from the frost or ice buildup. To simply try to deliver the coldest air possible needs careful consideration.
A recommendation from some in the industry is to discharge air below 32° in certain conditions and with certain aircraft models. Based on this author’s experience, sub-freezing discharge is a challenging task for many OEMs of PCA machines. Even when delivering air just above the freezing temperature of water, the surface of the evaporator would be below the freezing point. If one attempts to deliver air below 32°, one can quickly freeze the makeup air’s contained moisture on the evaporator and end up needing some form of a defrost mode. During the defrost mode of some manufacturers, all or some refrigeration circuits are turned off temporarily. Air delivery temperature can increase to the point where warm air is sensed entering the aircraft cabin and or flight deck. The gaspers that disperse the conditioned air are located just feet from a passenger’s face. Passengers and crews will quickly notice warm air blowing on them regardless of overall cabin temperature. Sub-freezing discharge can also be damaging to a compressor as extremely low heat loads can promote liquid refrigerant ingestion, which can be catastrophic to a compressor. Controls and components for compressor protection are a must here.
Sub-freezing air delivery units are being developed by most of the PCA manufacturers because airlines, airports, and aircraft manufacturers are demanding them. Careful defrost and low load control schemes need to be designed so that a constant cool air discharge can be maintained for prolonged periods. Only very limited and slight deviations from discharge setpoints are being tolerated today. Flight crews will demand a PCA be removed and they’ll use an APU after the slightest sensation of discharge air which does not feel cool.
Cabin discharge air temperature somewhat above the freezing point of water, for a sustainable length of time, is the best option in this author’s experienced opinion. The continuous delivery of air slightly above sub-freezing offers better cabin comfort than the “freeze and off” cycle. The goal is to deliver as low a temperature of air as can be obtained within air’s physical limitations and sustain the cold air delivery for hours. Defrost and low load condition control, which creates drastically warmer discharge temperatures, result in flight crews terminating use of a PCA. Even a momentary deviation from a discharge air setpoint is noticed. A continuous cold air delivery of 34°- 40°, without hysteric overshooting and undershooting of the discharge air setpoint, should be the goal. Advanced controls and components along with continuous research/development testing are necessary for this type of prolonged low temperature discharge to be successful. Frost mitigation will be necessary but should not drastically warm the discharge air; nor, should there be a long duration before recovery. The same rules should pertain to low load condition corrections. Quick recovery is a must. Operations are being monitored with building automation these days.
Low ambient conditions are another area where some OEMs of PCA equipment struggle. It’s very easy to make a unit that can cool during 100° temperatures in the desert. Lots of cooling capacity (e.g., more or bigger compressors) may be able to handle a consistently high load. What about a jumbo jet rated machine that was built for a hot Phoenix July afternoon, what if it’s late fall and only in the low 80’s? One compressor may not be enough, and two compressors may be too much. We often see a PCA that struggles to cool an aircraft and delivers too high of a discharge temperature when its 80° outside; but, can deliver 38° when it’s very hot. This is a design flaw regarding the ability to control capacity. It’s important to maintain constant discharge temperatures in many ambient environments.
There are refrigeration system components and engineering techniques to overcome low ambient condition obstacles, improve capacity turndown, and provide a steady cool discharge temperature. However, it does take a commitment to research and testing. To list the types of technology which could be used would take a great deal of basic refrigeration explanation and wasn’t the goal of this paper. Just realize that better capacity control can be accomplished with the right components and programmable logic in a refrigeration machine.
Many of the “cheaper” means of capacity control being used today are not working well. Superior designs in control logic and sophisticated technological attributes can make a unit cost a bit more than a competitor. Many customers don’t understand the price differences. The higher price may be worth it if the result is a consistently comfortable cabin and less jet fuel usage.
Maintenance and Service
Regular maintenance of this equipment is necessary to sustain optimal performance. Cleaning intake filters, condensers, and evaporator coils is an easy way to keep the units running well, if they are performing well otherwise. Clean condenser coils are necessary to maintain compressor discharge temperatures and head pressures at a level where they can be most beneficial to the machine’s operation and function. High compressor discharge temperatures and pressures can lead to compressor failures. Clean inlet air filters and evaporator coils will help maintain optimal discharge air temperatures, indoor air quality and protect the compressor/compressors from catastrophic liquid refrigerant ingestion which can result from low or lack of air flow.
PCA equipment is on the advanced side of the complicated technology spectrum, as far as refrigeration machines are concerned. The PCA requires a service technician with journeyman level refrigeration and HVAC understanding, to effectively diagnose and repair. Simple preventive maintenance techniques; however, can be easily taught.
Aircraft Interior Control Concerns
Generally, ground equipment technicians are unaware of how the parked aircraft is to be adjusted for proper ground operations. They would likely not know if poor PCA function is due to aircraft system control issues (valving, recirculation fans, etc.). The anticipation is that pilots will have adjusted the aircraft onboard ventilation systems accordingly for ground operations. All gaspers above the seats should be open while boarding and deplaning; however, there’s been a debate around where this responsibility lies. Window shades closed while boarding and deplaning can mitigate some solar infiltration and can assist air conditioning process.
A PCA is an HVAC machine, although it greatly differs from a commercial HVAC unit. PCA discharge air is delivered at a much lower sensible temperature due to the laws of thermodynamics and limited mass airflow ability (small duct size) of an aircraft ventilation system. A PCA needs to have state-of-the-art components and controls to be able to match capacity needs for any ambient air temperature and psychrometric state. PCAs need to be able to automatically adjust to climatic conditions nearly instantaneously. An integrated control system with carefully thought-out instructions and algorithms can command the needed machine actions for the situations encountered. The goal of a PCA should be to discharge a steady output of 34°-40° for hours in a very dynamic environment without drastic deviations from a programed setpoint. The right refrigeration system components and a well- designed operating scheme are critical for this to take place.
The right duct strategy will help a unit deliver the correct air flow. Proper lengths and limiting kinks and turns will help to get the proper amount of cool air into the cabin quickly and efficiently. There are products and techniques available to facilitate this; however, many don’t realize poor air flow is contributing to their ineffective operations.
Many units in the field today can’t maintain a low discharge temperature due to misuse, electrical/mechanical problems, or poor design. Sometimes misdiagnosis and improper repairs cause more harm than good. These are very complicated refrigeration machines and it takes a technician with a solid refrigeration and HVAC background to properly diagnose issues and understand if poor performance is due to misuse, design flaw, or electromechanical issues. It’s important to know who will be needed for service assistance (maintenance staff, OEM, HVAC contractor, etc.). Extensive training is necessary to really understand this technology well.
PCA units need to be properly adjusted for the aircraft being served. Improper aircraft type control selection, or the wrong mode being selected will not do the job of conditioning the cabin correctly. The correct airflow and matching refrigeration capacity, for each aircraft type, is essential for cabin climate comfort. Sometimes the equipment is looked at as malfunctioning due to the improper operational selections. Manufacturers are asked to simplify controllers by just supplying a means of turning the unit on. The only way an “on button only” controller can satisfy every operational need of every aircraft in every location is for an automated calibration of the PCA to be performed beforehand. Theoretically, programmable logic can custom configure the PCA when the aircraft arrives at a gate or hangar by having the aircraft electronically “tell” the machine what class of aircraft it is. PCA manufacturers and aircraft manufacturers are working toward making this a reality one day.
Many customers are requesting manufacturers build subfreezing units, which are difficult to produce with a sustainable constant discharge temperature. A carefully thought-out control instruction and technologically advanced system components will need to be the response to this challenge. If manufacturers can only discharge sub-freezing air for 15 minutes and then must thaw out the evaporator for three minutes or more while blowing ambient temperature air into the aircraft, the PCA will be ineffective and not used. At least that is the experience of this author.
Technology comes with a cost and competing manufacturers try to balance delivery of an adequate PCA unit while competing with one another on price. Often the customer doesn’t understand differences in price. However, sometimes quality comes at a higher price. Value is the better word to use here as a cheaper price may be wasted money if equipment isn’t used due to its ineffectiveness. It is important to realize that the ground source PCA is meant to substitute for the aircraft’s on-board climate control system and as an alternate to running an APU. The benefit is lower fuel costs and a cleaner environment. This means it needs to work equally as well if it is to be used.