Maintaining cabin pressure

Call me nitpicky or misguided but, as silly as this WhatsApp message sounds…


… how many people stop to check how cabin pressure really works? Clearly the message is wrong but where does our impression that “it’s wrong” come from? Does it come from the fact that

  1. it’s an unqualified WhatsApp forward?
  2. its author appears to be a ‘bhakt’?
  3. our experience travelling on flights seems so very at odds with what’s being described?, or
  4. one knowing how cabin pressure is maintained on aircrafts?

I think it’s a combination of the first three, and the third one more so, and I admit I’m quite cynical that it’s the fourth one. This isn’t an accusation that some people don’t know that air comes in from somewhere and leaves from elsewhere, and that there’s a pump that regulates this – but more a suspicion that, to the same people, the “somewhere” and the “elsewhere” might be irrelevant parts of the explanation.

Consider the following. There are two possible kinds of rebuttals to the WhatsApp message. First, the intuitive, and possibly experiential, kind that contrasts the great – and anecdotal – complexity of keeping an aircraft airborne and the almost abject mundanity of the drag caused by flatulent passengers. Second, the more technical kind that describes how an aircraft works with an engineer’s rectitude and, on occasion, authority. The first kind is a recourse to faith; the second is a recourse to knowledge. My contention is that the first kind (also as an encapsulation of the first three reasons from above) excuses us from dealing with the science and the engineering in a way that imposes some cognitive stress, which the second kind does.

When you receive such a WhatsApp message, the point is to disqualify it through the most effective means possible; from the outset, there is no diktat as to how it can be achieved (even if I’d disagree on principle), there is a preference for the faith-based mode of reasoning over the knowledge-based mode of reasoning. And over time, this preference builds tendency, which builds into bias through ignorance.

“What’s the point of learning the science?” Learning how cabin pressure is maintained (see below) isn’t going to help you in any way (unless your work has something to do with it) – but it is the lowest epistemological substrate upon which all faith-based reasoning on the topic can be founded. This isn’t to indemnify science against its many flaws as much as to suggest that without the foundation of knowledge-based reasoning, faith-based reasoning turns meaningless, and that the other way will never come to be. So why not begin with an awareness of the engineering behind maintaining cabin pressure so we remain forever sure about the provenance of our faith in it, at least?

Maintaining cabin pressure

The “somewhere” that the air comes into the cabin is first sucked in from the atmosphere by ‘taps’ installed on jet-engine compressors. They are then processed for humans by modifying their humidity and temperature using air-cycle machines, and fed into the fuselage. The “elsewhere” that air leaves the fuselage from is the set of outflow valves at the rear end of the aircraft. The air-cycle machines work constantly while the outflow valves regulate pressure within the cabin by opening and shutting accordingly.

When more air is let out than what’s coming in, the cabin will depressurise, and vice versa. This is because the ratio of cabin pressure to the aircraft’s ambient pressure is allowed to vary within only a certain range (and not too drastically when it does) as well as because the cabin itself will have been designed to be able to withstand a maximum pressure. The outflow valves are controlled by a computer.

Featured image credit: LittleVisuals/pixabay.