To pressurize the cabin and meet the regulatory requirement of 0.25Kg/min (0.55lb/min) per person, approx. 50% of the air in the cabin is recirculated and mixed with fresh air supplied from the engines (bleed air). In a typical commercial cabin air recirculation system, the air supplied into the cabin consists of approximately:
- 50% outside air from either the engine's compressor stage (engine 'bleed air') or the Auxiliary Power Unit (APU) mixed with approximately
- 50% of filtered, recirculated air.
(Note however, that some aircraft designs are now moving to 'bleed air free' ECS but these aircraft still use HEPA and HEPA/odor filters in the system).
HEPA (High Efficiency Particulate Air) filters were introduced on board aircraft to ensure the required level of cabin air quality and provide excellent standards of protection against dust, fibers, allergens and microbes (‘microbes’ includes viruses and bacteria). More recently, Pall Aerospace also introduced the A-CAF (Advanced Cabin Air Filter), a combined HEPA/Carbon filter that removes odors and volatile organic compounds (VOCs) that may be present from engine oil/oil degradation, ingestion of de-icing fluids or hydraulic fluids etc.
These filters only treat the recirculated air so some particulate contamination, odor causing compounds and volatile and semi-volatile organic compounds (VOCs and SVOCs) may enter from the outside air system. Air purification systems that treat both the outside air ('bleed air') and the recirculated air are in development.
Figure 1: schematic representation of the air delivery system in an aircraft
WHERE ARE THE CABIN AIR FILTERS LOCATED?
Cabin air filters can normally be found just before the location where the air to be recirculated and the air from the outside mix, which is typically either underneath or over the main passenger cabin.
HOW ARE THEY MADE?
- What materials are the filters made of?
Materials vary for different filter configurations, but usually include glass fiber filter medium, activated carbon, epoxy resin sealant, aluminum hardware, and plastic hardware.
- How long does it take to make one filter?
Although the timescale will vary from application to application and project to project, as a guide, manufacturing a batch of filters can take up to 8 weeks.
- What is the manufacturing process?
There are a number of processes that include pleating of the glass fiber HEPA filter media, forming the hardware from sheet aluminum, assembly of the odor and HEPA stages, assembly and sealing into the hardware and efficiency testing every batch.
The process to make HEPA media is a high-tech version of paper making, where fibers are suspended in a liquid which is poured over a screen so that the liquid can drain away. Quality control checks that the distribution of pore sizes within the fibrous structure are within defined limits. The efficiency of the medium, particularly with smaller particles, is related to the thickness of the fibrous medium, the density of fibers, and the thickness of the individual fibers. Optimizing these three parameters ensures that the diffusional interception mechanism (the one that removes the smallest particles, such as viruses) plays a large role in particle removal.
HOW DO CABIN AIR FILTERS WORK IN REMOVING VIRUSES AND BACTERIA?
Modern Cabin Air Filters remove particles throughout the spectrum of sizes which are found in the aircraft cabins. There are three primary methods by which a filter removes particulate, and all three mechanisms are in operation in fine air filters. The three mechanisms are “Direct Interception,” “Inertial Impaction,” and “Diffusional Interception”:
1. Direct Interception
Filters have matrices of defined pores. If the particles are larger than the pore sizes, then they are unable to penetrate into the medium and are said to be removed from the air stream by Direct Interception.
2. Inertial Impaction
Filters remove particles smaller than the pore size by inertial impaction. Particles have a higher density than the air, will deviate from the streamlines of the airflow and impact on the surfaces of the pores formed by the fibers of the filter medium, where they adhere and are captured. This mechanism removes particles in the 0.3 microns to 10-micron range approximately, depending on the filtration rating of the filter medium.
3. Diffusional Interception
For very small particles, Brownian motion results in rapid random movements about a nominal trajectory, which causes the particles to come into contact with, and to be collected on, the fibers and pore walls of the filter medium. This mechanism is effective for very small particles below about 0.2 microns, and in fact the smaller the particle, the greater is the Brownian motion, and the higher is the capture efficiency.
WHAT IS THE HEPA FILTRATION EFFICIENCY FOR PALL FILTERS?
Tests by an independent laboratory show that Pall HEPA Cabin Air Filters have a microbial removal efficiency of greater than 99.999% with bacteria and viruses which is also sometimes referred to as equal to an H13 level classification.
Note that HEPA filters are usually rated as either 99.99% by sodium flame test efficiency test or 99.97% by the D.O.P. (Di-Octyl Phthalate) efficiency test. According to ASHRAE Standard 161-2007 “HEPA filters….. shall meet or exceed the requirements of Institute of Environmental Science and Technology IEST-RP-CC007.2 Filter Type "A," or MERV 17 or H13 according to EN 1822-1, and shall provide a minimum of 99.97% collection efficiency for 0.3 micron particles.”
The standard tests challenge the filters with what is termed the “maximum penetrating particle size.” This means that particles smaller and larger than the tested size will be removed with a higher efficiency. This is seen in the graph below, which shows a significantly higher removal efficiency for particles smaller or larger than the 0.1 to 0.3 micron range.
 Centre for Applied Microbiology and Research (CAMR), now known as Public Health England.
WHY DO YOU NEED CABIN AIR FILTERS?
Due to the large number of passengers in the aircraft cabin, there may be high concentrations of dust, fibers, bacteria, and other microorganisms. All of these are nuisances and potential health hazards to crews and passengers. In order to enhance passenger and crew health and comfort, HEPA filters must be fine enough to control these contaminants.
Odor generating contaminants as well as bacteria, microbes, and ultrafine particles (UFPs) that emanate from inside the cabin are removed by the Pall Cabin Air Filters before the recirculated air is mixed with the fresh air supply and re-introduced into the cabin. This takes place approximately every 2 minutes.
WHAT IS THE RECOMMENDED FILTER ELEMENT CHANGE-OUT INTERVAL?
Recommended filter element change out intervals are typically specified by the system integrator or aircraft manufacturer. The time interval varies between aircraft types and between aircraft systems. Pall also works with airlines and aircraft manufacturers to determine the optimum service interval for specific operating environments.
WHAT IS THE RECOMMENDED FILTER ELEMENT CHANGE-OUT INTERVAL IN THE CASE OF A PANDEMIC?
We recommend following the aircraft/system integrator guidance and any local airworthiness/regulatory guidance.
IS THERE A DANGER TO MAINTENANCE PERSONNEL IN REMOVING USED CABIN AIR FILTERS?
There is no more risk involved in replacing a used HEPA or A-CAF filter than carrying out general maintenance on any in-service aircraft part. Maintenance staff who may be concerned about the general accumulation of dust, debris on cabin air filters should wear the same personal protective equipment as for other similar aircraft maintenance tasks. Airline safety and health personnel should be contacted to advise if specific PPE is required.
IS THERE A DANGER TO MAINTENANCE PERSONNEL IN REMOVING THE USED FILTERS DURING the SARS-cov-2 OUTBREAK?
We recommend following the official WHO and local government guidelines.
NOTE: The above applies to all Pall Aerospace cabin air filters, OEM or IPC listed filters and any Pall PMA approved parts.
 World Health Organization, Naming the coronavirus disease (COVID-19) and the virus that causes it (accessed March 2020).
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