Archive for differential pressure

Ask an Airline Pilot: Why Do I Feel So Worn Out After a Flight?

Posted in air travel, airline pilot blog, airline pilot podcast with tags , , , , , , , , , , on September 19, 2012 by Chris Manno

This is a question that I get asked by the most astute travelers: why do I feel physically worn out after a routine flight?

That’s a very good question, born of savvy observation among those passengers who fly often. And they’re right: flying has a direct physical impact on your body for very clear reasons, which I’ll explain. And–good news–there are some things a passenger can do to minimize many of these effects.

The culprit with the most insidious effect is one of the most obvious factors in your flight–and also the most underestimated, and that is, ALTITUDE. I’m only talking about the aircraft’s altitude indirectly, because what really affects you as a passenger is the cabin altitude.

The controlling factor in the cabin altitude is the the hull design of the aircraft. Because the pressure at altitude is a fraction of that at sea level, the aircraft’s pressure hull must have the strength to hold survivable (for humans) pressure in an extremely low pressure environment at altitude. For the sake of structural integrity, the difference between inside and outside pressure has to be minimized and the way to do that is to allow the cabin altitude to climb with the aircraft altitude, gradually, to a higher altitude to minimize the differential pressure.

The picture above is of a pressurization gage that shows aircraft altitude, cabin altitude–and the difference between the two. The aircraft’s pressurization system gradually raises the cabin pressure with the climb to altitude to keep the differential within the aircraft hull’s design limits. The result? At 41,000 feet, the aircraft cabin is over 8,000 feet.

That’s like being transported to Vail, Colorado (elevation, 8,000′) in a matter of twenty or thirty minutes. Suddenly, your body must make do with a significantly reduced partial pressure of oxygen, and this after being depressurized like a shaken-up soda can: every bit of moisture in your body–and gasses as well–are affected by the rapid (compared to a gradual drive from sea level to Vail) change of pressure.

This affects your body like the bulging of a balloon animal: the pressure reduced around you makes everything swell. Maybe not as dramatically as the balloon animal, but with definite and perceptible effect. And that cycle is repeated on the way down, in the opposite manner: your body must accommodate the repressurization.

Some people are more susceptible to the effects of altitude than others: I’m one who doesn’t feel well doing prolonged physical activity at altitude above 5,000 feet. I don’t like skiing or the outdoors type stuff in the mountains because of the flu-like effects of physical exertion at higher altitudes. Common side effects, or what’s called “altitude sickness,” include flu-like symptoms such as headache, body and joint pain, fatigue and a feeling of being out of breath.

If you’re the type who is susceptible to “altitude sickness” in any degree, you shouldn’t underestimate the effects of cruising for several hours at a high cabin altitude.

Next on the list of stress factors has to be humidity–or more importantly, the lack of humidity at altitude. The cabin air comes from outside at altitude where the humidity is around 1% to 2%. Fresh air is drawn in, heated, then mixed with cooler air to provide a comfortable cabin temperature. Mythbusters: I’ve heard the urban legends about “restricted air flow” to save money; recirculated rather than fresh air in airline cabins. That’s all bunk, at least in Boeing and Airbus aircraft. The cabin pressure is maintained through constant circulation and in the Boeing, large fans draw the air throughout the cabin and cargo compartments, then through the electrical compartments for cooling, then to an outflow valve for metered release with respect to a stable cabin altitude.

The pressurization and air conditioning systems work great, supplying fresh, conditioned air but . . . the humidity is very, very low because the source air at altitude is exactly that way. So, if you as a passenger aren’t drinking water constantly–you’re going to feel the effects of dehydration quickly at altitude.

The last of the major but often underestimated stress factors is also obvious but insidious: noise and vibration. Consider the basic fact that each engine on a 737 weighs over a ton and each has a core that spins at over 30,000 RPM at idle power. At cruise power, the vibration is subtle but distinct, transmitted through the airframe to everything and everyone on board.

That has a physical side effect added onto the motion effects of flight: pitch and roll changes in combinations you don’t experience on Earth. Like trains, ships and cars on a rough pavement, vibration is transmitted to your body and has a wearing, fatiguing effect.

Noise? Absolutely: there is a layered level of noise from engines, airstream outside and air flow inside that is both fatiguing and cumulatively, wearing.

What can you do about these three factors that can literally wear you out as you travel by air? Let’s start with the last factor: noise. This one has a really easy solution:

Earplugs. The foam kind, available in any hardware store. We use them in the cockpit all the time, because wind noise in the pointy end is much louder than in the back, depending on altitude and speed. They’re disposable, they’re cheap–and they work. You can still hear normal conversation just fine and the important thing is that they will reduce the fatigue level you will feel after a long flight. Try them on your next flight.

Low humidity? This is tied to the altitude effects as well, and this is a simple solution as well: hydration. Certainly on board you should be drinking plenty of water (yes, you’ll have to bring water with you–it’s available in airports everywhere; if you’re really cheap, just bring a bottle and refill it at a water fountain in the airport) but also, you need to think ahead.

Like the preparation for a marathon, you need to stay well-hydrated the day and night before the event.

If you step on board marginally hydrated or actually slightly dehydrated, the 2% humidity at cruise altitude will outpace any attempt you make to catch up, much less keep up. Coffee? Alcohol? Diuretic soft drinks? See photo above.

Finally, the high altitude effects of cruise flight. The good news is, going forward, that the next generation aircraft like the 787 “Dreamliner” are capable of maintaining significantly lower cabin altitudes during cruise. This is very important on long flights and the fact that Boeing designers recognized this passenger stress factor and have designed a way to minimize the effects underscores my original point about the stress.

For now, though, the best you can do is to deal with the other two key stress factors–hydration and noise–and simply try to be in decent physical shape: those who are suffer less with the stress of altitude exposure.

In the final analysis, you can’t completely avoid the physical stress of air travel without avoiding air travel itself. But, if you are a savvy traveler, you can at least recognize the effects and take simple steps to minimize the effects of air travel on your body and ultimately, your trip.

Bon voyage.

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