United B763 over Atlantic on Jan 19th 2018, 15 people felt unwell
Last Update: October 5, 2020 / 10:51:36 GMT/Zulu time
Passengers reported a bad odour making them feeling sick, other passengers reported lack of oxygen.
The airline however suspected an acute outbreak of flu or food poisoning reporting a check of the aircraft had not found anything out of the ordinary.
The Dutch Safety Board advised they have opened an investigation after a number of occupants of the aircraft felt unwell.
On Oct 5th 2020 the DSB reported in their quarterly bulletin, that the occurrence was not extensively investigated although rated a serious incident. The DSB summarized the sequence of events:
An hour into the flight, while flying straight and level at FL320 over Scotland, a growing number of the aeroplane’s occupants indicated they were not feeling well. Eventually all cabin crew and fifteen passengers (most of them from rows 27 through 30) were reporting similar symptoms. These included blurred vision, sensations of the aeroplane banking to the right and climbing, nausea and a general feeling of drowsiness.
While cabin pressure altitude was confirmed to be normal at around 8,000 feet, pulse oximeter readings with the cabin crew indicated low blood oxygen saturation levels. Administering oxygen and medication against motion sickness did not lead to structural improvements. Two hours into the flight, the captain instructed the cabin crew to stop their duties and decided to return to Schiphol for a precautionary landing.
The aeroplane made an overweight landing as a result of the smaller-than-planned amount of fuel used, and was directed to a remote parking position. There, one of the main wheel brakes overheated and the tyre thermal fuse plug blew, allowing the tyre to deflate to prevent a tire burst. The aeroplane did not sustain any further damage. No one got injured.
After the brakes had cooled, fourteen sick passengers were taken off the aeroplane for tests by airport medical services. These tests (anamnesis, blood pressure, oxygen saturation, ECG) did not yield any abnormal results. All passengers indicated that their symptoms disappeared as the doors were opened and fresh air entered the cabin. The cabin crew members were taken to a mobile medical facility set up near the aeroplane. Three of them were still showing some symptoms, most notably high blood pressure. One person showed high blood carbon monoxide levels, but the measuring equipment used was later suspected to be unreliable. All three were given extra oxygen after which all symptoms disappeared. No blood or urine samples were collected. The flight crew never suffered any symptoms. There was no need for any of the occupants to go to hospital.
The DSB described the steps taken by the investigation:
The investigation focused on probable causes of the cabin crew members’ and passengers’ symptoms, rather than on flight crew crisis management, the overweight landing, or the emergency services at the airport. Flight data recorder (FDR) data from the event flight, as well as the three previous (nominal) flights, were downloaded and analysed. The analysis did not indicate an increased likelihood of motion sickness on this flight, relative to the preceding nominal flights. Furthermore, Significant Weather Charts of 0600 and 1200 UTC on 19 January 2018 mentioned no significant weather on the route between Amsterdam and Ireland.
The aeroplane’s environmental control system (ECS) uses bleed air from the compressor of its Pratt & Whitney PW4000 turbofan engines for pressurized cabin air supply and controls cabin temperature and humidity. 50% of this conditioned cabin air is filtered and recirculated. Inspection of the aeroplane on the day after the event revealed a small puddle of synthetic motor oil at the 6 o’clock position in the fan cowling of the right-hand engine (#2). According to the aeroplane’s maintenance manual this is not unusual and may occur during wind milling or engine shutdown. Engine maintenance records demonstrated that both engines’ oil consumption had been within limits during the previous ten flights.
At the right-hand inner tube that connects the ECS bleed air inlet to the ozone converter outlet, a powdery black substance was found. This material contained small amounts of pentaerythritol triesters and tricresyl phosphate (TCP) isomers also found in the oil in the #2 engine cowling, very small amounts of some triglycerides used in lubricants, and paraffin. At the intake of the right-hand air recycling fan, an oily substance was found containing dioctyl sebacate (DOS), which is used in plastics, lubricants and flame retardant.
While the crew had not noticed any abnormal smells during the flight, it was decided to measure cabin air levels of oxygen, and of carbon monoxide and other intoxicating gasses or fumes which may originate from oil or de-icing fluid leaking into the ECS. Concentrations of toxic substances like volatile organic compounds (including organophosphate esters/TCP and aldehydes) were also measured. Nothing out of the ordinary was found during either high and low power engine test runs at Schiphol, or a flight test performed by the operator in the United States a week after the event. However, as all measurements were carried out after the cabin had been ventilated, cabin air contamination during the event flight could not be ruled out.
Based on the data available and previous occurrences on this type of aeroplane, the manufacturer concluded that the events were likely caused by degraded and uneven airflow and/or temperature control issues in the cabin. Local blockage of airflow in the conditioned air distribution system may be caused by ambient air contamination, internal lining from ground conditioned air hoses, wrappers of papers or debris from degraded recirculation air filters. The manufacturer therefore issued a service letter to operators, suggesting the implementation of a maintenance program to clean the air distribution system on a regular basis.
The Dutch Safety Board was able to exclude a number of other factors (including intoxication from food or beverages, infectious diseases, and leakage from oxygen bottles, pantry ovens or fire extinguishers in the cabin) that might contribute to the experienced symptoms. However, conclusive evidence regarding their probable cause was not found.
Aircraft Registration Data
This article is published under license from Avherald.com. © of text by Avherald.com.
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