key: cord-0814443-k05b3qgl authors: de Wit, Mr Anthony John; Coates, Mr Ben; Cheesman, Mr Michael John; Hanlon, Mr Gregory Richard; House, Mr Thomas Giles; Fisk, Dr Benjamin title: Airflow Characteristics in Aeromedical Aircraft: Considerations During COVID-19 date: 2020-11-02 journal: Air Med J DOI: 10.1016/j.amj.2020.10.005 sha: dbb55d6c5a3f42178a7ca24cc90052d830b8b488 doc_id: 814443 cord_uid: k05b3qgl • Aeromedical transport of COVID-19 patients creates exposure risk to clinicians and non-medical aircrew; • Airflow characteristics in fixed-wing and rotary-wing aircraft vary depending on environmental control system settings; • Physical barriers were observed to limit airflow movement from the cabin to the cockpit in both aircraft types; • Patient positioning may decrease the risk of exposure to aircrew positioned in the cockpit; • Understanding cabin airflow movement can assist in flight planning. * Airflow characteristics in fixed-wing and rotary-wing aircraft vary depending on environmental control system settings * Physical barriers were observed to limit airflow movement from the cabin to the cockpit in both aircraft types * Patient positioning may decrease the risk of exposure to aircrew positioned in the cockpit * Understanding cabin airflow movement can assist in flight planning The coronavirus pandemic has had an unprecedented impact on service provision across many areas of health care. The learning curve related to the pre-hospital management of suspected and confirmed COVID-19 cases has been steep, with many aspects of clinical practice still being refined. The unique challenges of aeromedical transport (1, 2) have been magnified in the current climate (3, 4) . The aeromedical transport of COVID-19 patients presents risks to clinicians and aircrew due to the proximity to patients and exposure to aerosolised particles. Not only do aeromedical providers need to consider how to manage surge capacity (5) related to COVID-19, but they also need to determine how to safely transport patients in both pressurised and non-pressurised aircraft. Key strategies for the safe and effective transport of COVID-19 patients include selection of appropriate patients for transport (6) , minimising the utilisation of aerosol generating procedures (AGPs), and ensuring the correct use of personal protective equipment (PPE) (3, 7) . Despite some consensus on personal protective equipment (PPE) guidelines (8) (9) (10) , the utilisation of these guidelines is challenging for clinicians and aircrew in both rotary and fixed-wing aircraft. The COVID PPE and patient transfer guidelines utilised by Air Ambulance Victoria (AAV) are summarised in Tables 1 and 2 (11): ( The utilisation of PPE must provide protection against contact, droplet, and airborne transmission (12) . The correct level of PPE is determined by the risk and type of exposure, and donning and doffing procedures need to be followed (13) . Additional strategies to minimise the exposure of pilots and aircrew must be considered to ensure that patient transfers can be performed safely (3, 14, 15) . Patient isolation may be possible in different aircraft types (16) , however portable isolation units are expensive, may require re-configuration of existing aircraft layouts, and have limitations such as patient access and restraint (17) . Alternatively, the use of barriers such as screens or curtains may provide some level of protection for personnel positioned in the cockpit, and their effectiveness is reliant on airflow and the movement of airborne particles within the aircraft. Despite recommendations for patient positioning based on airflow (18) and previous reporting of airflow rates in fixed and rotary-wing aircraft (15) , testing designed to observe air movement in aeromedical aircraft had not been published at the time of writing. This report describes airflow testing that was undertaken on the Hawker Beechcraft B200C fixedwing aircraft and the Leonardo AW139 rotary-wing aircraft. The intent of the testing was to determine the safest positioning of clinicians and actual or suspected COVID-19 patients during flight. The testing also aimed to assess the risk of exposure to aircrew seated in the cockpit of each aircraft type. The methodology used to test and observe cabin airflow in the two aircraft types differed due to configuration and functionality. The methods for each aircraft will be outlined separately. Testing was completed in a stationary aircraft on the airfield apron at Essendon Airport, Melbourne, Victoria, Australia on the 23rd of April 2020. Testing of the Beechcraft was conducted over two sessions on the 23rd of April 2020 between 12:11 pm and 3:16 pm. The outside temperature was recorded as 20 degrees Celsius and the cabin temperature was recorded as 19 degrees Celsius. One person was seated in the cockpit and three people positioned within the cabin for each test. Each person observed and reported on the flow and movement of smoke in the aircraft during the phases of testing. The aeromedical fit-out of the subject aircraft consisted of two stretchers (a forward left hand and aft right-hand stretcher), three medical seats, and associated medical supply systems and components. The aircraft is fitted with an optional Keith Dual Zone Air-Conditioning System. This system distributes pressurised air from each engine to the fuselage through the wings via an air-toair heat exchanger. The air is then directed to a mixing plenum for either distribution to the lower heating outlets or through the evaporators, to the cockpit or the cabin. Table 3: ( The aircraft engines and environmental control system were started to provide bleed air and power to the environmental control system. The cockpit and cabin temperature were set to approximately 21 degrees Celsius, being the default position for aeromedical operations in this aircraft. Four states of pressurisation were simulated during testing: unpressurised, increasing cabin pressure, steady cabin pressure and decreasing cabin pressure. Manual override was used to achieve and maintain a maximum pressure differential of approximately 0.7 pounds per square inch (psi). Two overhead cockpit cooling outlets, and the cabin overhead outlets were adjusted during testing to determine their effects on airflow. Tested configurations consisted of the following: Leonardo AW139: Ground testing was carried out on the AW139 to simulate airflow and small-particle aerosol The most dramatic change to the cabin's mass airflow behaviour was observed when the cockpit curtain was closed ( Figure 2 ). With the curtain closed smoke was only observed to enter the cockpit in very small quantities, predominantly in the gap between the curtain and headliner. No appreciable difference in the mass airflow behaviour was observed between unpressurised and simulated cruise conditions. Smoke that passed through the air flow from the overhead outlets was observed to only be disturbed locally. No change was seen in the mass airflow behaviour. When the aircraft engines and environmental system were shutdown smoke was observed to take longer to dissipate than in any other tests. The orientation of the smoke generation from various backrest heights and positions resulted in initial local variations to the smoke propagations, before conforming to the cabin's mass airflow. Smoke generated from halfway between the forward and aft positions was observed to split into forward and aft main volumes across the fluid boundary, then continue to behave as smoke generated from either the forward or aft positions respectively. Smoke generated from various locations within the cabin, including a forward position on the longitudinal stretcher and from the lateral stretcher, was observed to expand to fill the cabin evenly before dissipating. Smoke was observed to remain within the cabin over 3.5 minutes with the cabin ventilation off, and cockpit curtain raised. As with the Beechcraft B200C a dramatic change to the cabin's mass airflow behaviour was observed when the cockpit curtain was opened. With the curtain open, smoke would equalise between the cockpit and cabin, as shown in Figure 3 below. The overhead cabin outlets were observed to move smoke from the upper cabin downwards and accelerate the spreading smoke throughout the cabin. Smoke generated from the forward position, was observed to spread to the aft section and filled the cabin volume quicker than when smoke was generated in the aft position. COVID-19 can be spread via direct droplet and airborne transmission (12) . It has been reported that droplet spread of the disease can occur when fluid particles greater than 5 microns directly contact a person, but that microscopic aerosol particles can also be inhaled when droplets < 5 microns remain airborne for longer durations (19) . COVID-19 may be detected in aerosols for up to 3 hours (10) and a high percentage of aerosols have been reported to be deposited on surfaces close to the expiratory source in aircraft cabins (20) . These points reinforce the risks to aircrew (21, 22) working in the confines of aeromedical aircraft. There are limitations of the testing which must be considered. All testing was conducted while the aircraft were stationary on the ground, and the nature of airflow whilst at altitude would need to be studied further to definitively report on dynamics during flight. The use of smoke as a medium for testing, the environmental control systems on both aircraft types, and varying aircraft configurations warrant discussion. The characteristics of the smoke generated during testing are important, as a direct comparison with the characteristics of COVID-19 movement and transmission is difficult (23) . The use and type of smoke as a medium for testing has limitations (24-26), but has been used to simulate airflow around oxygen masks (27) , air escape in hospital isolation rooms (28) , and to estimate the pattern of movement of aerosolised particles (26). Computational fluid dynamics (CFD) modelling has been used to simulate aircraft cabin airflow (29) but such a method was beyond the scope of this testing. Smoke was selected for this testing for several reasons. A primary aim of the testing was to assess the movement of small particle aerosols entering the cockpit and exposing flight crew who are unable to wear full PPE. Using smoke to demonstrate small particle movement was deemed more appropriate for this reason. Also, smoke is highly visible, easily generated and able to visually demonstrate air movement throughout the cabin and cockpit. The smoke medium used for testing had a relative density of 1.050, being slightly denser than the surrounding air and is designed for longevity of visibility. This density resulted in the smoke gradually sinking to the cabin floor before dissipating. The outside temperature was relatively constant during testing and greater variations would be expected during flight in both aircraft types. There may be variation in the extent of heating and cooling regulated by the automated systems during flight. During flight, cabin temperatures are more stable and the effect of the air circulation will be greatest because the cabin environmental control systems will be operational. Testing was completed within closed cabin environments, such that no wind would affect results, and external air-conditioning would have a negligible effect. In addition, a maximum pressure differential of 0.7 pounds per square inch (psi) was achieved during Beechcraft testing compared to a pressure differential of 6.5 +/-0.1 psi which can be encountered during flight. This pressure differential was deemed to be sufficient to measure the effects on the airflow characteristics during pressurisation and de-pressurisation cycles. Kingair 300 aircraft models. The main difference between models is the method of temperature control. The more recent systems incorporate a computer to control the cockpit and cabin separately via sensors in the ceiling and ducts, servo valves in the mixing plenums, and by directly controlling bleed air pass valves in the wings and the vapour cycle compressor/condenser blower. The older system manages temperature with thermistors, bridge balance circuits, valve position switches and a temperature selector rheostat. Both models generate similar mass airflow in the cabin and cockpit. On all models, pressurised warm air is distributed at floor level outlets, and cool air is distributed below the glare shield in the cockpit and at ceiling level in the cabin. On all models, air exits the cabin via the same outflow locations at various phases of flight and ground testing undertaken in this study aimed to replicate these phases of flight. In rotary-wing aircraft, or any unpressurised aircraft, the ambient air temperature and cabin pressure at normal cruise altitudes are typically less than at surface level, and these conditions were not able to be replicated during the ground testing of the AW139. Cabin temperatures will be subject to variation (cold winter vs hot summer) but this variation will be greatest when on ground during patient loading and unloading. During flight, cabin temperatures in rotary-wing aircraft are more stable and this is where the effect of the air circulation will be greatest because the cabin environmental control systems will be operational. Droplet movement, as well as virus survival in aerosols (30) , may be affected by changes in pressure, altitude, temperature and humidity. Testing did not replicate the lower outside air pressure and temperatures from increased altitude where these variations may influence airflow due to saturation and convection. Typically in Victoria, Australia the cruising altitudes of rotary-wing aircraft would not be expected to have a significant impact upon the test results. During testing of the AW139 the aircraft engines were not operated, and heating system and airconditioning functionality was not included. The effect on cabin airflow due to heated air being supplied to the cabin and cockpit floor level outlets was not tested, nor the effect of supplying the cockpit outlets and cabin overhead outlets with cooled air. AW139 testing was conducted using fanforced air at different settings in the cockpit and cabin outlets. The AW139 ventilation, heating and air conditioning systems can be operated with either ram-air, or fan-forced air. Stationary testing excluded the use of ram-air, but ram-air characteristics would be similar to fan-forced characteristics in flight. Differences in fan-forced airflow when the aircraft is stationary or in flight would not be significant unless a cockpit window or cabin door is open. For normal inter-hospital transport operations, the cockpit window and cabin door would be closed. The testing conducted on the AW139 was intended to reflect air circulation during a typical flight environment and the results obtained are reflective of this. Based on the results of this testing ventilation system settings in the AW139 can be used to generate air flow from the cockpit into the cabin to reduce cabin air entering the cockpit. Positive cockpit pressure scan be generated using one of the following ventilation system configurations: (a) Cockpit ventilation low and cabin ventilation off. (b) Cockpit ventilation high and cabin ventilation low. The air-conditioning re-circulation setting should be avoided as smoke was observed to linger for extended periods in the cabin when this setting was used. The results may assist clinicians with the positioning of patients in flight to minimise the risk of COVID-19 exposure. The safe and wise approach to the aeromedical transfer of confirmed or suspected COVID-19 patients in our current climate is to strictly adhere to accepted safety guidelines and infection control procedures (7, 13, 31) . The safe management and transport of COVID-19 patients requires the utilisation of appropriate PPE in combination with practical distancing measures applicable to various transport platforms. This report describes airflow testing procedures undertaken in fixed-wing and rotary-wing aircraft designed to assess the efficacy of physical barriers and patient positioning during flight. Observations from the airflow testing undertaken reinforce that physical barriers between the cockpit and cabin of both the Beechcraft B200C and Leonardo AW139 provide a degree of protection for non-clinical aircrew. Diligent use of these measures in addition to the stringent and disciplined use of PPE provide a degree of safety for staff engaged in the aeromedical transport of COVID-19 patients. The results of this airflow testing provide a baseline for further investigation into practical measures which can be adopted to enhance the safety of aircrew against infective aerosolised particles. Tables: to assist the landing crew and coordinate all other personnel, ensuring appropriate PPE is worn  A trolley with PPE and cleaning equipment should be positioned in the respective hangar  The aircraft is to be towed into the hangar nose first, except for HEMS -which is positioned tail first  The road ambulance is to be positioned outside the hangar ensuring that beacons are on  The Flight Coordination Centre (FCC) is to announce via PA that we are currently unloading a patient in the hangar  Unload patient from aircraft using current procedures  Load road Ambulance as per normal procedures  FCC to announce via PA that patient now departed from hangar  Aircraft to be cleaned as per respective procedures  PPE to be doffed within the hangar and placed into an infectious waste bag and then placed into infectious waste bin  Hand hygiene to be observed during the doffing procedure and before entry into main building  FCC to be advised ASAP that the aircraft is again operational. 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