 | Scott Tatge
is a Learjet Instructor for FlightSafety International here in Wichita. He teaches the ground school and simulator sessions for corporate and charter pilots new to the airplane, and those coming for recurrent/update training once or twice a year. Prior to this position, Scott was a Production Test Pilot for Learjet, testing new airplanes off the assembly line and preparing them for certification. Scott has worked as a Flight Instructor, and taught in the Aerospace department at Metro State College in Denver, CO. Scott has worked for the airlines both on the ground and in the air as a pilot. He has a Bachelors degree in Aerospace, and a Masters degree in Education. You may contact Scott at sion for Scott, and he is honored to write for this periodical. You may contact Scott at (316) 943-3394, or by e-mail at scott.tatge@flightsafety.com |
Health & Medicine
2002-09-01 11:00:00
Can re-circulated air in a plane make you sick?
: Why is it that airplanes use re-circulated air instead of bringing in fresh air from outside? Doesn't the engine exhaust and everyone's exhaled air make a lot of people sick?
Scott Tatge
Question: Why is it that airplanes use re-circulated air instead of bringing in fresh air from outside? Doesn't the engine exhaust and everyone's exhaled air make a lot of people sick?Answer: This question comes from a news story that was aired a couple of years ago. The story went that people were getting sick from air that was re-circulated inside the airplane cabin. This news story was not correct. That is not how air is used in pressurized airplanes. Outside air is used through the engines, sent into the cabin, then out what we call an outflow valve. So the air is continually refreshed from outside then sent back outside. That is the short answer. The second part of your question is about the engine exhaust. That is a common misunderstanding about how the airflow comes into the cabin. In order to understand how and why air comes from the engines, you need to understand the basics of how an aircraft is pressurized, and why. First let's talk about how our bodies work. We breathe air into our lungs. This air has oxygen in it that our body requires to live. Down here at the lower altitudes, there is enough pressure all around us that it is easy to breathe it in. As we go higher, the pressure all around our body reduces, thus we cannot breathe as much air into our lungs and we do not get enough oxygen into our bodies. Try running a mile at sea level, then going up to Denver at 5,650' above sea level and running that same mile. You will notice that you are breathing much harder in order to stay conscious. That's because there is so much less pressure at that altitude. The amount of oxygen is the same throughout our atmosphere; it is just that there is not enough pressure to breath it in. Now let's take our bodies to 45,000'. The time of useful consciousness at this altitude is about 5 seconds simply because there is so little pressure way up there. Now most airliners fly at only 35,000', so your time of useful consciousness is increased to about 10-12 seconds. So, the environment that airplanes fly in is not hospitable for human, or any mammal's survival. Most airplanes fly around with a cabin altitude of about 2000' to 4000' even though the airplane is flying at 35,000'. That is why your ears "pop" while climbing and descending. The airplane cabin altitude is changing in order to maintain a proper pressure. This is the secret to our survival in this hostile environment. As you recall, there is plenty of pressure at these lower altitudes to inhale the required oxygen into our lungs. So the trick is to get enough compressed air into the cabin of the airplane, then regulate the air inside the cabin to a proper pressure. This is where the engines come in. Jet engines operate on similar principles as your car engine, except at much higher levels of pressure torque and thrust. First the outside air is brought into the engine at the front. Then it is highly compressed, so much so that it becomes extremely hot (in the neighborhood of 400-800 degrees). Then fuel and ignition is added creating an "explosion". This "exploding" air expands rapidly and is sent out the back as thrust/exhaust. This thrust pushes the airplane forward, and drives the front part of the engine. Notice that the outside air is compressed prior to making exhaust. It is at this point where the air is compressed that we tap some of it off and send it into the cabin. That is why there is no exhaust in the cabin. Remember, the air still has the same amount of oxygen in it at these high levels of compression. This air is also very hot. So, the air first goes through an air conditioning unit that cools it down to whatever level is requested by the pilots, or flight attendants to warm you up or cool you down. Now, we are forcing all this air into the cabin. The cabin is very tightly sealed (notice how big those entry doors to the airplanes are) so that it can hold the pressure required to inhale sufficient oxygen. Since we are forcing all this compressed air into our tightly sealed cabin, we must let it out or the airplane will explode, like blowing up a balloon too much. In order to control how much air is let out, we have a pressurization system. In the older airplanes, the pilot's had to set the cruising altitude into the system. This system regulates what we call an "outflow valve" that lets the air out of the cabin and maintains the proper pressure inside the cabin. So, as you can see, there is always a constant supply of fresh air from outside the airplane brought in through the engines, then allowed to escape back outside through the outflow valve. The air coming out of the vents above your passenger seat is actually coming from the engines, cooled, then sent into the cabin. That is why the airplane does not cool down until the engines are started. While the airplane sits at the gate and before engines are started, the air comes from a ground air conditioner that is hooked to the airplane.
