Why is it Always So Freaking Cold (or Hot) on my Plane?

Freezer or Sauna?

You get to the airport parking lot, run to catch your shuttle, make it through the TSA body cavity search, and then schlep your stuff a thousand yards to the gate. You board and heave your rollaboard into the overhead bin. After finally sitting down you notice two things: you are drenched in sweat and there is almost no air coming out of the vents. Or if there is air, it is warm.

Or perhaps it's July and you have a light shirt and slacks on for your trip but have brought no jacket. But shortly after takeoff you notice that it's cold in the cabin. I mean really cold. Your hands are blue and you are shivering.

So why the heck can the airlines never seem to get the temperature right? How difficult can it be?

As it turns out, getting it right is more difficult than you would think. This issue has about 85 moving parts involving both human and mechanical factors. I'd like to go over each aspect of what goes wrong, but first let me give you a quick description of the systems in place which provide heating and cooling aboard your aircraft.

Heating and Cooling At the Gate

In years gone by, heating and cooling at the gate were mostly provided by running a unit on board the aircraft known as the auxiliary power unit or APU. This is a small turbine engine usually mounted in the tail which provides both electrical and hydraulic power for use during preflight and also pressurized air to run the air conditioning or heating system. 

It generally worked well but consumed a lot of fuel and the technique was eventually replaced by the use of large heating and cooling units mounted directly on or near the jet bridge. Ground crews are required to attach a large air hose to the belly of the aircraft to allow this unit to heat or cool the interior of the plane. The systems are either programmed to provide a preset temperature or a temperature probe might be hung in the cabin to provide feedback to the system.

737 Pneumatic System

737 Air Distribution

Cooling and Heating While Under Way

After the airplane is away from the gate and under its own power, all heating and cooling is provided by onboard systems which are powered by compressed air from the engines. These onboard units are known as pneumatic air cycle machines or PACs (on Boeing aircraft) and not only provide heating and cooling but also pressurization to the cabin while at altitude.

Without going too far down the rabbit hole concerning Carnot cycles and thermodynamic flow equations, suffice it to say that the units take hot compressed air from the engines and make cold air out of it or use the hot air directly for heat. Yes, all the air that you're breathing on an airplane is brought in through the mouth of the engines. It is also why an engine malfunction can quickly fill the cabin with smoke, but that's a topic for another time.

After going through some plumbing and a water separator, the air is distributed to the cabin through ducting and the gasper outlets, which are those little twisty vents over your seat. The system temperature is controlled through the use of a thermostat which is usually located in the cockpit. It is supposed to be a "set and forget" type of arrangement which should always provide a comfortable temperature over a range of aircraft operating states such as taxi, climb, cruise, or descent.

At least that's how it is supposed to work. Let's now take a look at the many things that can go wrong to make you miserable.

Human Factor Errors

One of the basic problems concerning complex feedback systems is that the user...you freezing or sweating in your seat...is not the controller. A systems engineer might say the feedback loop of this control system is in an open state. My suggestion is that you attempt to close the loop by hitting your call button and complaining. Many times certain parts of the plane may be warmer or cooler than others. The galley where the flight attendants spend most of their time may be fine. Let them know that you are not fine.

Another issue could be that the user is feeling perfectly fine, but that person is not you, it is a flight attendant. They are the ones who call the cockpit to request a warmer or cooler temperature. On some airplanes, they can control the temperature directly. Remember, they are constantly on their feet and are likely to appreciate a cooler cabin than you sitting in your seat motionless. Again, if no one complains, they have no way of knowing.

The same dynamic is true for the pilots. If they don't hear any complaints from the back, they'll just assume everything is OK. And speaking of pilots, they are sitting up front in a glass house. It is the guy in the right seat who controls the temperature, so if he is on the sunny side of the airplane and is warm, he'll just dial it down.

Another thing I've noticed is that some folks just naturally run cold or hot. Heavier people seem to like it cooler than thin people. So if your first officer appears as an endomorph and is sitting on the sunny side of the plane, that may explain why you're freezing in your seat. Again, hit that call button and complain.

Operator Errors

Another class of error in temperature control might be classified as operator errors. For instance, on a coolish spring or fall day the ground crews may simply neglect to connect the air hose thinking that the temperature outside is cool so it must be OK inside the airplane. What they don't realize is that several hundred bodies in an aluminum tube will always result in a stuffy cabin even on the coldest of days. This problem is compounded when the pilots fail to look out the window to see a folded up or deflated air hose. The solution is to start the APU and get some air flowing.

Being a commuter, this is my personal pet peeve. We have a certain set of pilots who mean well but have their priorities askew. They are reluctant to start the APU because they've been told that it uses too much fuel, so in this situation they will call station operations on the radio to request that the ground air be hooked up. Station operations will then call the ramp agent who's probably loading bags and now has to stop what he's doing to hook up the air. All this might take five minutes. And surely you won't mind going into your meeting with sweat stains on your shirt.

My technique is to reach up and to start the APU, get some air to the customers, and to then perhaps chase down why the ground air isn't hooked up or working. Most jet bridges are owned by the airport authority which is usually a city-owned bureaucracy. If they are out of service for maintenance, making a call to get them fixed is literally the same as calling city hall to get a pothole fixed. Good luck with that.

I was even once parked at a gate without a working APU, so the only source of air was the ground unit. As I sat there in a full airplane on a summer day, a city crew pulled up, turned off the unit, and before I could shout at them, drained the coolant out of it to perform some maintenance. When I asked them if they noticed this big blue thing with wings and engines sitting there, the answer back was that they had their orders and didn't know nuthin about no airplanes. Luckily we were close to pushback, but this is part of the impenetrable stupidity that makes the job so enjoyable.

Lastly, sometimes the system is either overwhelmed such as waiting for takeoff on a 110 degree day in Phoenix with a full airplane, or it simply doesn't perform as expected. There's not much that can be done about the former, but if the system won't heat or cool properly, it needs to be written up and fixed. This can take some time.

In Conclusion

The heating and cooling systems on jet aircraft are charged with keeping you comfortable while the temperature outside the aircraft can range from over 100 degrees to 50 degrees below zero at altitude. They usually do a pretty good job but have their limits mainly due to human or mechanical error. The best thing you can do to ensure a comfortable ride is to speak up...and to bring a jacket.

Scarier Than Snakes: Sneezing on a Plane

In a new video and study conducted by the FAA Center of Excellence at Purdue University, particles from a sneezing passenger are modeled as they disperse in the cabin.

As we've mentioned before, the air coming from the airplane itself is most likely harmless. It's the next passenger over you need to worry about.


So cover your mouth already!

Is the Air on My Plane Safe?

Like all technical questions involving complex interactions between humans, other humans, and complicated mechanical systems, the answer is solidly "it depends".

For starters, it depends on the definition of the word "safe". Humans are famously bad judges of risk as I observe on a nearly daily basis. Statistically, perhaps only elevators are a less risky form of transportation than commercial aviation. And yet watching ostensibly normally functioning people come close to losing their minds as they board an airplane gives one pause when giving odds on the likelihood of the continued existence of the species. But never mind.

Let's start with the basics of how airplane pressurization and air conditioning work. Then we'll talk about the resulting air quality in the cabin and the risk of disease transmission which has been on everyone's mind.

It's Just Like a Balloon

Think of the aircraft as a balloon with two holes in it. Air is being blown into one hole to try to inflate the balloon but simultaneously leaking out of the other. If the amount of air entering equals the amount leaving, the balloon grows neither larger nor smaller. In a nutshell, that is how aircraft pressurization works.

The air we breathe on this planet is made up of many gases with the primary ones being nitrogen (78%), oxygen (21%) and argon (1%) along with many other gases making up the remaining bits. It is the partial pressure of oxygen which keeps you alive. As you climb higher into the atmosphere, that pressure drops. Most healthy humans can function normally up to an altitude of about 10,000 feet and above 14,000 feet cognitive function is impaired in most people.

Of course conditioned and trained people can go much higher with climbs up Everest at 35,000 feet being accomplished without oxygen. But for most people, going above 10,000 feet results in discomfort and light hypoxia. For this reason, aircraft cabins are always pressurized to an altitude below 10,000 feet.

Why not keep airplanes pressurized to sea level pressure? It's an engineering tradeoff. Most commercial aircraft are built to withstand a pressure differential of about 8 psi meaning the maximum difference between the air inside the plane and that outside is about 8 psi. This translates to a cabin altitude of about 8500 feet when the airplane is at it's maximum altitude of 40,000 feet. It could be built to withstand higher pressure (and some business jets are) but the cost and weight goes up greatly with thicker skin.

Now back to our balloon analogy. The air coming into the cabin is bled from the engines. Air enters the engine and travels through a series of compressor stages where most of it is used in combustion with the fuel. However some high pressure and temperature air is bled off and fed to a unit known as the air cycle machine. Using a linear flow model as opposed to a Carnot cycle used in your car's air conditioner, the air is then cooled and expanded before being fed into the cabin. Hot air from earlier stages in the process is mixed with cold air from the air cycle machine to attain a desired temperature.

On the back side of the plane is the second hole known as the outflow valve. This is where the pressure in the plane is controlled as there is a door which modulates open and closed to let air out at a rate to control the pressure inside the plane. For a given inflow rate, when the outflow valve closes, the pressure in the plane goes higher. A pressure relief valve keeps the "balloon" from bursting from over pressure in the case of a malfunction.

The inflow rate is modulated by automatic valves and is kept mostly constant while the outflow valve is controlled by a digital pressure controller. This provides for a gradual pressure climb and descent to match the pressure altitude of the landing airport which is set by the pilot. Were this not the case, the doors, which are essentially plugs, could not be opened at the destination if the aircraft was still pressurized. It's also why these doors can't be opened in flight. The air pressure keeps them closed with many hundreds of pounds of force.

Why Do My Sinuses Bother Me on an Airplane?

Well for one thing, the air in the stratosphere has almost zero moisture contained within it and as a result, the air coming into the cabin at altitude is also nearly zero percent humidity. This is why you will become quickly dehydrated on a long flight. Some business aircraft actually have humidifiers installed for comfort but these are not installed on commercial aircraft to my knowledge due to weight, cost, and corrosion considerations. Dry mucous membranes can also cause discomfort over long periods.

Pressure changes experienced on an aircraft are well known to cause discomfort. The reason for this is the construction of the human head. Aerospace travel was apparently not considered in its design. Put simply, air does not always flow freely into and out of the sinus cavities and ears of the mark-one mod-zero human. When the air pressure is changing outside of said human and congestion or some other problem prevents the air pressure from equalizing, pain often results.

This pain and discomfort most often manifests itself during descent as the pressure is increasing. During climb, air escapes more easily from sinuses but a "flapper" like effect makes increasing pressure more difficult to equalize resulting in sinus and ear blocks. They can be excruciatingly painful. Not flying with a cold is always a good idea but using a nasal inhalant such as Afrin can help greatly in the event of discomfort. It should not be used prophylactically though as there is a "bounce-back" effect and it can become habit-forming if used continuously.

Are the Pilots Turning Down the Oxygen?

Why yes. Yes they are up there wearing oxygen masks while turning the oxygen valve  to make passengers fall asleep and to also save money on oxygen tanks. And cackling wildly as they watch you turn slowly blue. Sometimes, though they need to take a break from that duty to monitor the chem-trail dispersal systems.

Seriously, though, per the laws of physics, the oxygen content of cabin air will decrease with cabin altitude. For customers who have hit it especially hard the night before or have health considerations, the resulting lack of oxygen may result in an extended nap or shortness of breath. My airline does not allow bottled oxygen to be brought on board by customers, but does allow battery powered oxygen concentrators. Checking ahead of time what the policies are is always helpful.

And no, there are no oxygen tanks in use during a normal flight. Your ability to remain conscious at altitude is a function of the cabin pressure. As long as it remains below about 10,000 feet, you'll stay awake and alive. There are oxygen tanks aboard but they are for crew use during emergencies. 

There aren't even any oxygen tanks hooked to the emergency oxygen masks that will drop if the cabin pressure rises above 14,000 feet. Those are actually connected to "oxygen generators" which is a fancy way of saying a chemical stew that "burns" when activated but gives off oxygen as a byproduct. A number of these oxygen generators which were improperly packed caught fire in the cargo hold and brought down a Valuejet MD-80 about 15 years ago. As installed in the aircraft overhead passenger service unit, the heat generated during use is harmless.

Should the "rubber jungle" ever fall down in your face, you will have about 12 minutes of oxygen mixed with ambient air to keep you awake while hopefully your pilot gets the airplane pressurization under control or gets below 10,000 feet. But remember that since the mask mixes oxygen with ambient air, it won't keep you from breathing smoke should a fire ever break out. 

Another favorite conspiracy theory is that pilots recirculate used air to be breathed again and again...to save money. Well, this one is true. A certain amount of the air on most commercial aircraft is in fact recycled. A "recirc" fan recirculates a certain amount of air through a filter like the HVAC system in your house, car, and office. This results in higher humidity and less fuel consumption. And while 100% of the air from your home air conditioning system is recycled, the number is closer to 50% on an aircraft depending on the model.

But How Safe is it Really?

Again, it depends, but air quality on a modern commercial aircraft is probably about as safe as it can reasonably be. According to Boeing, passengers receive between 14 and 20 cubic feet of air per person per minute and this air is essentially particle free. Boeing claims that the HEPA filters used can remove particles as small as .3 microns which includes most microorganisms.

So while the air coming from the airplane is probably safe, it's that guy sitting next to you hacking up a lung you're probably worried about. And probably with good cause. It's not the airplane itself that is the problem, it's the proximity to sick people in the cabin with you.

Put several hundred people in any enclosed space be it airplane, elevator, or subway, and someone with an infectious airborne disease will probably transmit something to someone. But of all those transportation methods, the airplane probably has the freshest air source. Directing one of the overhead gasper air vents towards your face may have some effect in keeping ambient air particles out of your lungs but how much is unknown. And as I mentioned earlier, the air is probably fresher than your home or office. 

While the practice is more often seen in Asian countries than the US, wearing a mask can certainly cut down the risk of acquiring an airborne infection. But other than wearing a mask, there's probably little that you can do to reduce your risk of airborne infection other than staying off the plane altogether.

Is There Anything Else I Should Worry About?

The other and probably equally dangerous threat on an aircraft is the tray table. And also the armrest, and the lavatory. And probably your seat too.

All these things are public spaces and most likely contaminated with germs, fecal matter, and other gross stuff. Just consider the interior of the airplane as one giant Petri dish. It probably isn't a bad idea to carry a supply of anti-bacterial wipes and to wipe down your immediate area when you sit down. Use the anti-bacterial soap in the washroom and learn to unlock and open the door with your elbows.

I personally don't eat or drink on an aircraft either. The flight attendants are handling cash, credit cards, trash and empty cups all day from hundreds of people. Sure they wash their hands and wear gloves at times but is that diet Coke worth a few weeks of flu, or worse? And keep your hands away from your face.

Yes, but aren't the airplanes cleaned every night? Sure they are. We park and a crack crew of uniformed professionals are waiting to scrub down every bit of the aircraft to CDC approved standards using industry standard anti-microbial cleaning agents. Not.

No, the "cleaning crew" consists of minimum wage immigrant labor armed with a vacuum cleaner and a few rags. The plane is also tidied up between flights but not "cleaned".

So What Now?

Well I guess in the words of Dirty Harry, "Do ya feel lucky?" If you've got to go fly, then go fly. I'll be there because I have to be. 

Your odds of catching Ebola or Enterovirus are probably infinitesimal but increasing, due to the madness of our open borders policies inviting the third world and their diseases to our shores.

You are probably more likely to get something more common like the flu on an airplane but a few common sense steps as mentioned above should help. And voting. That will help as well.