October 11, 2017

Horse 2332 - Fly The Friendly Fart Filled Skies

A little over a week ago on Twitter, I made the assertion that one of the consequences of smoking being banned on aircraft was that the airlines could recycle the cabin air on a flight less frequently. I was chastised for suggesting such a thing and because text doesn't lend itself to nuance, I decided not to pursue this until I was in possession of a larger number of facts because specificity is the soul of narrative. Before I begin this story, I'm going to throw around some science.

Most passenger aircraft operate at many thousands of feet above sea level. I reckon that on a Sydney to Melbourne flight, that I've seen 39,000 feet listed on the in flight entertainment panel on the back of the seat in front; which I suspect means that the plane was following a semi ballistic trajectory. Officially Mount Everest used to be listed as 29,002 feet tall and what the exact number is doesn't matter, the point is that people suffer from altitude sickness up that high and aircraft operate another several thousand feet above that. Because of this, aircraft need to pressurise the passenger cabin or else have everyone black out.

The actual process of pressurising and air conditioning the passenger cabin is quite a complicated one but the short story is that air is drawn through the compressors in the engines, passes through the air-conditioning units in the aircraft and is piped into the cabin where all of the paying meatbags they call passengers are. The actual process is a bunch of electricity and goblins as far as I'm concerned, before lovely moist air comes out while you're blissfully unaware while you watch The Empire Strikes Back again. Here's where the story gets interesting.

I emailed someone at Airbus UK after Boeing didn't get back to me and according to their PR department, the air-conditioning system on an A320  is rated to able to completely replace the air in the cabin 45 times per hour; so that mean that there's a fair amount of over engineering built into the system. They also told me that in the event that the main engines expire, that the Auxiliary Power Unit makes enough electricity to drive all of the flight surfaces and the necessary systems required to make the plane controllable and keep the cabin pressurised. In my mind the APU looks like a cat's bum staring at you from the back of the plane.
Secondary to that, the usual operating settings for the aircraft in question are usually set to run a mix of 50:50 fresh versus recycled air. What this means is that assuming that the airlines run their aircraft according to the ideal specifications set down by the manufacturer, the air inside the cabin should be recycled and replaced at 30 times per hour.

This is where the story gets ridiculous. The US Department of Transport and the FAA first tried to impose smoking bans on aircraft in the 1980s but there was pushback from the airlines. The best available information that I have is that they wanted to keep the ability for passengers to smoke on board an aircraft due to passenger demand. Over time, bans slowly spread from flight of less than two hours, to less than six hours and finally only to all flights but only after a passenger died due to anaphylaxis caused by passive smoking. USDoT passed its set of no smoking rules in 2000 and the FAA followed suit.

There was an on effect though. Once it was realised that because people weren't smoking on board aircraft any more, the airlines soon realised that they didn't have to recycle and replace the cabin air as often. Since airlines are businesses which already run exceptionally tight margins, saving fuel and money because​ the cabin air didn't have to be recycled and replaced as often, is a very easy method of increasing profit margins with minimal effort. Because of this, that same person at Airbus UK told me that the airlines that they deal with, if the airlines want to be really tight with their money, will turn down the recycle and replace rates of cabin air to as little as 15 times an hour. Apparently that saves as much as 4% of fuel on a transatlantic flight.
It has been pointed out to me that the air on board an aircraft is cleaner than that in an average office building but again the person at Airbus UK put that down to a better HEPA filtration system on their aircraft and that the incidence of filters being changed in an average office air conditioning system are practically never. The actual air quality on board an aircraft, works out to be lower than what it used to be when smoking was still in place because the ratio of fresh air used to be higher and the rate of recycling and replacement was also higher.

I'm not saying that smoking on board an aircraft is either a good thing or even desirable. What I am saying is that airlines, who are after all it is said and done nothing more than businesses, saw that opportunity to turn down their air-conditioning systems in the pursuit of higher profits and did so.
Back in the days of smoking still being allowed on an aircraft, the airlines had a duty of care to replace the air as quickly as they could; whereas now when the air inside the cabin is being recycled less often, we're all living in more of our collective fart filled funk for longer.

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