The main objective of the research project FACTS is to determine how the cabin air quality of an aircraft is affected by engine oil entering the bleed air system in a so-called fume event. The project is centered around an experimental campaign, involving in-flight tests and tests on the ground, where the chemical composition and possible toxicity of air samples will be analyzed after being deliberately contaminated with engine oil. This direct approach differs from previous measurement campaigns, in which a large number of commercial flights were sampled in the hope of catching random fume events.
The project is divided into four work packages. The first work package is a background literature search to identify existing knowledge in the area of cabin air quality. The second work package involves the experimental test campaign, in which data will be collected about the chemical and toxicological aspects of contaminants resulting from engine oil as well as hydraulic oil or de-icing fluid entering the bleed air system. In the third work package a risk assessment methodology will be developed, building on the results from the experimental campaign and the literature. The fourth work package focuses on countermeasures which can mitigate possible contamination of cabin air. The work packages are being explained in more detail below.
The objective of this work package is to conduct an inventory of former studies related to aviation air quality measurement, turbine engine oil composition, or other possible effects on passengers and crew from contaminants. This will determine the loopholes, prevent repetition, and identify knowledge gaps not yet assessed in the earlier studies. The results of this research will provide a knowledge baseline for the other work packages.
Since there is no clear definition of a fume event in terms of type of contamination, concentration or chemical characterization, the experimental campaign is divided into three test programs to obtain the best possible data under different conditions:
A limited number of in-flight tests with specially equipped test aircraft, without passengers, will be performed in which oil-related fume events will be provoked and cabin air will be sampled for chemical analysis. In-flight tests are preferred because of the real-life flight environment, ambient conditions and system mechanics. However, flight tests also have some limitations: engine failure conditions in-flight cannot be provoked easily; the cabin air may contain compounds from other sources than engine oil (e.g., flame retardants from carpets or aircraft seats); and for safety reasons it may be difficult to provoke extreme, or “failure events”, which are believed to produce a relatively high amount of contaminations. For these reasons, the flight tests will be complemented with ground tests at an engine test bench.
In this test program, ground-tests at an engine test bench will be performed in which known amounts of engine oil are being inserted into an operational engine. Similar to the in-flight tests, the outflow of the engine will be sampled, and used for chemical analysis of possible contaminants. In addition, the sampled gases will be used for toxicological screening using various non-animal bioassays representing lung and brain tissue. Ground-based tests are the “next-best-thing” regarding real engine conditions, with the advantage that the amount of oil, or other contaminants, can be controlled. The limitation, however, is that the results may be specific for a certain engine type in terms of temperatures and pressures at certain engine settings. For that reason, the engine tests will be complemented with tests in the laboratory.
The laboratory test program within FACTS will use an experimental set-up where contaminants can be inserted into heated and compressed air, representative of the engine conditions at the compressor stage where bleed air is deducted from. This set-up, designated the “Bleed Air Conditions and Contamination Simulator” (BACCS) enables the investigation of a wide range of temperatures and pressures up to 600°C and 8 bar, respectively, independent of aircraft or engine type or flight phase. Another advantage of this laboratory set-up is the exact control of the amount of contaminant injected into the system. This provides the most direct and scientifically sound way to measure the by-products resulting from the break-down of engine oil. Similar to the engine tests, the gases at the outflow of the BACCS will be sampled and used for chemical and neurotoxicity testing. The results of the three test programs will be compared, where similar results will reinforce the underlying chemical process, and potential differences can be used to explain the “unknowns”.
Previous studies focused on volatile organic compounds (VOCs) in the cabin during regular flights, where passengers comprise an additional source. In additional air- or wipe-samples specific compounds, such as organo-phosphates (mostly TCP), were analyzed. Also, oil vapors were chemically characterized at elevated temperatures under oxygenized and oxygen-depleted laboratory conditions. In the FACTS project, more extensive analysis of contaminant constituents and their reaction- and decomposition products will be performed based on international standards (e.g., the ISO 16000-series for air pollution monitoring). As explained above, this will be done at temperatures and pressures representing the engine conditions where the bleed air is drawn for cabin air supply, and without the possible confounding from passengers.
Besides being analyzed chemically, the fumes produced in the ground-based test program will be screened for their possible toxicity. The objective is to allow predictions of actual hazards from fumes generated during in-flight fume events, with inhalation as most important route of contamination, and the nervous system as most important biological target system. As a first step, various mixtures of selected fumes will be screened for possible neuro-toxicity, resulting in a ranking relative to a positive control compound. For this purpose, the FACTS consortium has access to state-of-the-art bioassay technologies, such as the Microelectrode Array (MEA). This comprises in vitro cultures of brain cells to determine the impact of contaminated air on neuronal activity. Another in vitro method uses cultures of zebrafish larvae, which is an accepted bioassay to determine the neurotoxicity of contaminants by changes in the larvae’s’ mobility. In a second step, the most toxic fume will be tested in a tandem set-up, consisting of an Air Liquid Interface (ALI) and the MEA. The ALI contains in vitro lung cell cultures, representing the lung-blood barrier which does not let through all contaminants. The contaminants which do pass the lung cells are being collected, and subsequently exposed to the MEA to obtain insight in their neurotoxicity. Finally, the most potent fume will be used in a limited in-vivo mice behavior test, to assess the impact of direct inhalation exposure.
An important element of the bioassay experiments is the screening for potential biomarkers in relation to the produced fumes. The research will focus on biomarker formation, stability and half-life under controlled exposure conditions. The results of this work will be useful to identify possible biomarkers which can be used in future studies involving human volunteers, or in the examination of passengers or flight personnel after they have encountered a fume event.
The FACTS team explicitly decided not to include human exposure experiments because the physiological effects of contaminated air are still unknown. Furthermore, there still is a lack of knowledge on useful biomarkers to examine the degree of exposure. However, the FACTS project will produce results which can be used to carefully define future human exposure studies. First, the risk assessment methodology in Work Package 3 will provide guidelines on assessing the risks of bleed air contamination. Second, the animal study in Work Package 2 will include the investigation of useful biomarkers.
In work package 3, a stepwise approach will be taken to develop a risk assessment methodology that is applicable for cabin air quality. This methodology will serve future assessment of health risks (both acute as chronic exposures) of cabin air quality, providing best-practice support for stakeholders within the aviation community. The results of the other work packages will contribute to the development. Essential aspects of this strategy include risk assessment of chemical mixtures, grouping of chemicals, and the applicability of existing reference values. Several risk assessment approaches will be explored that especially focus on the risk assessment of chemical mixtures. Two different types of risks need to be addressed: one for the acute, incidental exposure to fume events and one for the more long-term exposure to cabin air during normal flight operations.
Work package 4 will provide an overview of technical measures that potentially can be taken to reduce, or eliminate the intrusion of engine oil into the aircraft. The measures will be characterized by their point of application, i.e., 1) source elimination, 2) source control, 3) propagation restriction, 4) propagation indication, and 5) remedy controls. For each of the categories the potential measures will be assessed on several aspects, including certification, Technology Readiness Level and aircraft integration. The overview will provide a ranking of the measures towards those with the highest mitigation potential and the highest potential for integration in the retrofit and forward fit market. This will serve as guidance and advice on the focus of subsequent research projects.