As the world’s airlines gird their loins in preparation to get flying again it may be worth considering the measures being proposed by the carriers to mitigate possible viral cross-infection between passengers. At one point during the lockdown the suggestion of blocking middle seats was mooted. easyJet at first suggested they might adopt such a measure, but then seemed to talk themselves out of it. Over at Ryanair the ever vocal Michael O’Leary opined from the outset that if his airline was forced to adopt such a plan, he would simply leave his aeroplanes on the ground! One can perhaps understand his position as the whole business model of the Low Cost Carriers (LCCs) is based upon effective yield management – ‘full aeroplanes’ to you and me.
Once on board it seems likely that the new requirements will be limited to having to ask permission to visit the smallest room for a ‘de-fuel’, wearing masks throughout, possibly no in-flight sales (although these too form a big part of the LCC’s business model so there might be some opposition to this suggestion) and deep cleaning the aeroplane every night. Note: every night, not every turnaround. These machines fly many sectors every day (high utilisation is another part of the LCC business model), with each sector seeing a new backside belonging to another potentially pathogen-shedding passenger, occupying what will eventually become, on perhaps the day’s ninth or tenth sector, your seat!
But are the risks any greater than travelling on say a full train, or bus? Well firstly you are probably going to be on an airliner rather longer, so if there is anything nasty floating around you’re likely to cop a higher dose of it. But what about the air conditioning? Many people believe that airliner cabin air is recycled and mixed with fresh air throughout the flight in order to save fuel, and that pilots are instructed by their employers to use as much recycled air as possible, as a percentage of the total. Is this true?
Well, like with a lot of layman’s knowledge where specialist issues are involved, some of it is, whilst the rest is something of an urban myth!
An unpressurised commercial aircraft is limited to an altitude of 10,000 feet. Above this the air is considered sufficiently thin to cause some people to suffer hypoxia. On pressurised airliners, high pressure air is pumped into the cabin from the compressor stage of the engines, effectively replicating an altitude of 8,000 feet (6,000 ft on the latest models). In an ideal world it would be closer to sea level but a higher pressure differential (the pressure inside v the pressure outside) puts great strain upon the airframe.
Compressed air is very hot, far too hot to introduce into the cabin in its raw state, so it goes via a pair of Pneumatic Air Cycle Kits (known as ‘PACKs’) which control the ambient temperature required in various areas of the cabin, flight deck, and underfloor holds (remember, live animals travel in the hold!). It also provides cool air for the ‘gasper’ vents (the little doll’s eye vents above the passengers’ heads). It must be stressed that the compressor stage of a jet engine is before the combustion stage, so no combustion gases can enter the cabin.
The used air is routed to the lower part of the pressure hull, where some of it is dumped overboard via the outflow valves, controlled from the flight deck to achieve the desired pressure differential, whilst roughly half is routed back to the PACKs to be mixed with new, fresh air arriving from the compressors. However, it always travels via HEPA (High Efficiency PArticle) filters, which, according to airline trade organistion IATA, “…are effective at capturing greater than 99% of airborne microbes”. However, like everything Covid, we do not yet have the full picture. HEPA filters are generally held to be effective at arresting virions (virus particles) down to between 0.1 and 0.3 microns in size. According to non-profit research organization ‘eLife’, SARS.CoV.2 virions are believed to be around 0.1 microns, so if this is true then we can see that they are at the very lower end of the HEPA’s efficacy. Unfortunately the social enterprise organisation ‘Smart Air’ claims that some SARS.CoV.2 virions can actually be as small as 0.06 microns, which is probably not what you want to hear!
So why do airlines use recycled air on board their aircraft? Is it to save fuel, as many believe? The simple answer is they do it because they have no choice. That’s how airliners roll off the production line. On Boeing aircraft, as used by Ryanair, the crew have no control whatsoever of the ratio of fresh to recycled air. It is a factory pre-set. They have control over the flow of air and its temperature, but not the mix.
Airbus models (a la easyJet) give the pilots very limited control over how much recycled air is used (just three settings, normal, high, or low), and this is based solely upon how many passengers are being carried, and subject to standard operating procedures set down by the manufacturer.
The reason that recycled air is used at all is to do with humidity. The fresh air coming out of the compressors at altitude is as dry as a stale lasagna – the thin high altitude air has a very low relative humidity. Dehydration is a real problem. If you are near the front of an airliner cabin, you will see a constant stream of soft drinks being taken up to the flight deck. That is why.
One of the unpleasant effects of dehydration is to dry the body’s mucus barriers, which play an important part in filtering out and killing infections. They are nowhere near as efficient if they dry out, so theoretically at least, there is a slightly higher risk of cross infection from someone in close proximity who is shedding than, for example, there is on a London bus.
So the factors when tossing up the respective risks of staycation, jetting off to the Mediterranean, or simply hunkering down at home appear to be:
- Yes, any form of public transport carries a slightly higher risk than personal transport.
- Some of the air we breathe on an airliner has already been used by other passengers, but has passed though HEPA filters which there is no doubt filter out most pathogens but
- depending upon which research you choose to follow, some small SARS.CoV.2 virions may evade the HEPAs. Finally, high altitude air is extremely dry, marginally increasing your risk of infection further. So if you do decide to roll the dice and jet off, whilst on board, drink LOTS of fluids!
But bear in mind, in terms of dehydration, your nerve calming gin and tonic or double scotch will have the precisely opposite effect to that desired!
So the bottom line is, depending on the size of the Covid-19 virions, that the air conditioning is probably fairly low risk; the greater risk is quite simply the proximity of your nearest neighbour, and what previous passengers may have left behind. In other words, much as it would be on any form of public transport where social distancing is not possible.
Capt J W H Quayle 30-06-20
Photo: John Quayle, 737-400 G-BOPK London-Las Palmas 1980.
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