An Update on Aircraft Oil Bearing Chamber Sealing - European Sealing Association

Dr Susan Michaelis PhD, MSc, ATPL, University of Stirling

The issue of jet engine oils and other fluids contaminating pressurised aircraft breathing air (bleed air) supplies continues to be widely discussed and reviewed within and external to the aviation industry. The main concern continues to be related to small amounts of oil leakage from the engines and Auxiliary Power Units (APU) into the cabin environment.

A couple of recently completed and on-going initiatives include air monitoring and oil pyrolysis studies undertaken by the European Aviation Safety Agency (EASA), with a further two-million-euro study looking into acute high dose oil leakage events led by EASA and the European Commission. The European Chemicals Agency (ECHA) is undertaking a review of Tricresyl phosphate (TCP), the organophosphate used in most engine oils and the International Civil Aviation Organization (ICAO) has published fumes guidance-training material for aircrew, maintenance personnel and management. Of major concern is the degree of impairment incurring in flight when people are exposed to these fluids, which are reported as hazardous.

Recent studies by Susan and her colleagues report varying degrees of impairment occurring in 30% of reported fume events, increasing to 93% in 15 selected events of which 87% were confirmed leaking oil. [1] Aircrew are suggested to be more susceptible, as a pattern of chronic background exposure is reported with acute events on top.

There continue to be 2 opposing views related to how oil can leak into the aircraft bleed air supply. Many in the aviation wider industry suggest oil leakage occurs only under seal failure conditions or operational factors such as seal wear or oil overfilling. There is growing awareness (outside of sealing experts) that the design using pressurised air allows low level oil leakage past the seals with some entering the air supply for the cabin.

Dr Susan Michaelis, who spoke at the 2016 ESA annual meeting, has now been awarded a MSc for her extensive work in researching the issue of oil leakage past seals in aircraft gas turbine engines. [2]

The research sought to assess whether there is a gap between aircraft certification requirements for the clean air in crew and passenger compartments of transport aircraft using the bleed air system and the theoretical and practical implementation of the requirements. The 3 main areas undertaken were a review of the regulations and standards related to the ventilation air, a review of the documented understanding of oil sealing and interviews with 12 experienced aerospace and sealing engineers and the 2 main aviation regulators (FAA, EASA) on their professional views on the oil sealing system and the aircraft/engine certification process respectively.

Key findings include:

• Regulations and standards and acceptable means of compliance related to cabin air quality exist. Low-level oil leakage over the bearing seals into the bleed air is an expected normal condition at various phases of flight. The required bleed air quality is not being met, as the standards and compliance material are not specific enough to ensure suitable bleed air quality, or application. The focus is placed almost entirely on the prevention of incapacitation, while ignoring impairment, with the clean air requirements open to interpretation.
• Design: Although many suggest the certification requirements for clean air supplies are being met, careful review and research shows this not to be the case. Oil leakage past the bearing seals associated with impaired or degraded performance occurs more frequently than the ‘major’ (< 10^-5/efh or /flight hour) remote or improbable regulatory and compliance criteria allow. Oil leakage associated with impairment is probable (>10^-5/efh or /flight hour) or above and is an ‘unsafe condition.’
• Compliance: The lack of detection systems to identify the air quality in flight causes ongoing compliance problems. Additionally, the ventilation requirements are not specific enough to ensure occupants will remain free of adverse effects.
• Preventative control measures: Low-level and transient oil emissions are not adequately taken into account when considering acceptable leakage levels. The designs are based on steady state conditions, there are no filtration or detection systems to identify and prevent exposure with rigorous controls lacking.
• Expertise and communication: Oil contamination of the air supply is a highly specialist area, with inadequate communication between all relevant parties to ensure compliance and airworthiness.

Other recent initiatives:

In September 2017, a two-day International Aircraft Cabin Air Conference (endorsed by EAS) was held at Imperial College London. Topics covered included: health and toxicity; engineering; compliance; history; flight safety, solutions, amongst others. John Morton, of the ESA co-presented the mechanisms of oil leakage into the cabin air supply, along with Susan and has since co-authored a new paper on this topic with various experts covering areas of oil leakage; toxicology & health, engineering and lubricants. The conference was extremely well received, with a key positive outcome being that EasyJet plans to trial the in-development bleed air filtration system and sensor technology later this year. PDFs and video footage of the entire conference can be found on the conference website. [3] However many still suggest oil leakage is not intentional and occurs only under failure conditions.

In mid-2017 Susan, now a visiting researcher at the University of Stirling, was awarded the Cranfield University Course Director’s award for best MSc student for her MSc work including her thesis work on the oil seals. In early March 2018, Susan is a keynote speaker at the 24th International Conference on Fluid Sealing in Manchester.

Further information can be found on Susan’s cabin air quality page of her website.

A further international conference on the cabin air topic will take place at Imperial College London in September 2019.

References

1. Michaelis S, Burdon J, Howard C. Aerotoxic Syndrome: a New Occupational Disease? Public Heal Panor 2017; 3: 198–211. http://www.euro.who.int/en/publications/public-health-panorama.
2. Michaelis S. Implementation Of The Requirements For The Provision Of Clean Air In Crew And Passenger Compartments Using The Aircraft Bleed Air System. (MSc thesis) Cranfield University, 2016. http://www.susanmichaelis.com/caq.html.
3. IACAC. International Aircraft Cabin Air Conference. In: International Aircraft Cabin Air Conference, Imperial College London, 19-20 September 2017. https://www.aircraftcabinair.com/films (2017).