Pressurization Systems

[rfresh]

Education Fun Time -

The pressurization systems on most commercial airliners are dependent upon many other aircraft systems besides just the pressurization components themselves. Without these 'other components', the pressurization system would not work.

1. What are the other aircraft systems that the pressurization system relies on to do its job?

2. Where does the air (for the pressurization system) come from in the first place?

3. Does the air conditioning system have anything to do with the pressurization system?

4. Do the jet engines have anything to do with the pressurization system?

5. What does 'cabin differential pressure' mean and why is it important for the pilots to know about it?

6. When automatic and semi-automatic control of the pressurization system is lost, most airliners can legally dispatch a line trip using the manual system. This means one of the pilots must manually control the pressurization outflow valve to control pressurization - however, many airlines will not dispatch this way - Why?

 

[Flying Illini]

With an FE rating and what would appear to be FE time in a B-727 and an L-1011 I would guess that you know all of this stuff...so what gives?

Just remember...Bleed air does it all.

 

[81Horse]

1. What are the other aircraft systems that the pressurization system relies on to do its job?

a) The Carbon-based Interactive Flightdeck Monitoring Units (CIFMU's).

2. Where does the air (for the pressurization system) come from in the first place?

a) "Air" derives from the Big Bang, which created the universe approximately 13.7 billion years ago; an alternate "theory" is that air was created about 5000 years ago, on a Monday, by an Intelligent Designer. <discuss>

3. Does the air conditioning system have anything to do with the pressurization system?

a) Air Conditioning System won't have a thing to do with Pressurization System since that big brouhaha at Hydraulic and Fuel Systems' party last Saturday.

4. Do the jet engines have anything to do with the pressurization system?

a) Yes, the Jet Engines still call Pressurization occasionally to play nine holes.

5. What does 'cabin differential pressure' mean and why is it important for the pilots to know about it?

a) Cabin differential pressure is a measurement of the hostility and aggression created when passengers on a given flight compare actual individual airfares paid; it's not important for the CIFMU's to know about it, as they have no control over route structures, pricing, marketing, strategic planning, or management compensation.

6. When automatic and semi-automatic control of the pressurization system is lost, most airliners can legally dispatch a line trip using the manual system. This means one of the pilots must manually control the pressurization outflow valve to control pressurization - however, many airlines will not dispatch this way - Why?

a) CIFMU control of the pressurization system can result in deployment of the Rubber Jungle, and is therefore not cost-effective.

 

[rfresh]

With an FE rating and what would appear to be FE time in a B-727 and an L-1011 I would guess that you know all of this stuff...so what gives?

Yes, I know how they all work. 20 years in the business, 17 as an instructor. I thought we could have some fun and at the same time educate some of the pilots who may have an interest in knowing but who have not had had an opportunity to learn about the 'big iron' systems yet. That's all.

 

[rfresh]

Let's answer question No. 2 first:

The source of the air into the cabin for pressurization purposes comes from the jet engines. The jet engines have a high pressure bleed air system that is piped into the airplane in various places. One place is into a set of metal ducts called the pneumatic system or bleed air system. The pipes are metal because of the air is under pressure and is very hot.

Depending on the type of engine, there is more than one stage of bleed air tapped into. While beyond the scope of this thread, jet engines have several stages of air compression as it moves air back into the turbine section. Bleed air is tapped into usually at 3 locations within each engine, although some may use only 2 locations: a 'low' pressure location on the engine case and at an intermediate pressure location and then finally a high pressure location.

So, your bleed air source comes from the jet engines themselves and - usually - from several compressor stages so that, depending upon the thrust setting at any given moment, a computer can mix and match what ever air pressure it needs for the bleed air system.

As a point of interest, while we are primarily talking about this air for the pressurization system, bleed air is also used for other things:

1. Engine anti-ice protection. Fly into some ice and you'll need to turn on engine anti-icing. This taps the hot bleed air from the engines too to keep the cowl warm so ice won't build up on it and get ingested into the engine which is very bad news if that happens!

2. Wing anti-ice protection. LOTS of bleed air is needed to anti-ice the wings. As a matter of fact, when engine and wing anit-ice is being used, the thrust settings for the engines are affected quite a bit; meaning, for example, go around power is reduced if you fly an approach in ice and then have to execute a missed approached - your TOGA power setting will be less than if you were going around without engine and wing anti-ice being on.

3. Provide a source of power for air-driven hydraulic pumps such as on the L-1011 TriStar and many other jetliners.

4. And last but by no means least, the air conditioning system. On wide bodied jets there are 3 air conditioning 'packs'. All being feed air from the bleed air system to provide conditioned air into the passenger cabin for temperature comfort. Narrow bodied jets have 2 air conditioning packs; sometimes also called air cycle machines or ACM's. Marvelous little machines that are designed to do one thing and one thing only: make extremely cold air!

 

[Gulfstream 200]

so you asked a few questions then answered them for yourself.

I hope YOU were impressed!

 

[Clear-&aMillion]

While you could question his antics, many people don't know this stuff.

 

[mushroom]

I fly a wide-body jet and it has two packs, not three.

 

[FN FAL]

Intercourse

 

[rfresh]

Well, you try and some things work and some don't. No one posted any answers so either they don't care about the subect or they don't know. I thought I'd get the ball rolling and provided one answer. I got mildly flamed for it.

Obviously, the topic attracted more smart ass responses than serious answers, so that tells me no one is really interested in learning this stuff.

Point taken. I will now let this thread die. This will be the last time I try to share my knowledge with those who may have been interested in learning something new.

Have a nice flight.

 

[atrdriver]

I think that most passengers would be surprised that ALL the air that they breath during a flight first passes through the intake of one of the engines.

 

[Gulfstream 200]

Point taken. I will now let this thread die. This will be the last time I try to share my knowledge with those who may have been interested in learning something new.

great idea Ralph.

 

[luvz2fli]

great idea Ralph.

rfresh don't let the thread die. Don't even pay attention to that guy. You're right, not everybody flies that sort of stuff and I enjoyed reading what you had to say. Thanks for doing the thread.

 

[Gulfstream 200]

OK I see your point.

Maybe post these little quizzes under "training"?

 

[Gorilla]

Gulfstream, there's nothing wrong with this, IMO. Yeah maybe training would have been a good choice, but I think it's entirely appropriate and interesting.

 

[Lrjtcaptain]

1. What are the other aircraft systems that the pressurization system relies on to do its job? In a nutshell, the engine for 1, precooler/heat exchanger, Flow control units, bleed air valves, and outflow valves to name a few.

2. Where does the air (for the pressurization system) come from in the first place? How about from outside the airplane!

3. Does the air conditioning system have anything to do with the pressurization system? Yup, uses bleed air.

4. Do the jet engines have anything to do with the pressurization system? They suck in all the air. The bleed air comes from the compressor section of the engines.

5. What does 'cabin differential pressure' mean and why is it important for the pilots to know about it? Its the differential between the PSI inside the pressure vessle and outside. Pretty important to know when your can expect a CABIN ALT light. Also good to know incase your outflow valve fails its good to know when your airplane is going to pop. J/K

6. When automatic and semi-automatic control of the pressurization system is lost, most airliners can legally dispatch a line trip using the manual system. This means one of the pilots must manually control the pressurization outflow valve to control pressurization - however, many airlines will not dispatch this way - Why? Because the precious FE is no longer around.

 

[rfresh]

Yes, your absolutely right, I should have posted this thread under training.

 

[rfresh]

I fly a wide-body jet and it has two packs, not three.

Yes, you must be on a twin-engine wide body. That would make sense. I probably should have been more clear when I said that; I was referring to 3 and 4 engine wide bodied jets. They are the ones who have 3 packs.

Any twin engine airliner is going to have only 2 air conditioning packs because one engine cannot feed more than one pack.

 

[PC12Cowboy]

Information is a very powerful thing and some people enjoy learning new things.....and some people already know it all...I would prefer to be aligned with R A Bob Hoover who was asked on his 80 th birthday when he stopped learning about flying and bob replied " when I stop flying or Die"

 

[rfresh]

1. What are the other aircraft systems that the pressurization system relies on to do its job? In a nutshell, the engine for 1, precooler/heat exchanger, Flow control units, bleed air valves, and outflow valves to name a few.


Great answer and more detailed than I was thinking. I was thinking about the larger systems that are indirectly associated with the pressurization system that most people don't realize:

(1) Engines. The air comes into the cabin from the engines as others have already pointed out. Once sucked into the engine, part of the air is taken away from the engine compressors via it's bleed air system; bleed air that is needed by other systems on the aircraft as I previously mentioned in answering question 2 the other day (recall we're talking about the engine and wing anti-icing systems, etc. in addition to the air conditioning systems)

(2) Air Conditioning systems. The following a bit long regarding the air conditioning system but it does relate to the pressurization system, so please bear with me. For those pilots who know about the air conditioning sytem and it's related ACM, you may want to skip this part. After the technical explanition I'll tie it all together for you.

The air coming into the cabin is directed through the air conditioning system; sometimes called the A/C Packs. There are usually 2 or 3 packs. Without going into a lot of detail, the core of an a/c pack is a device called an Air Cycle Machine or ACM. An ACM is pretty much like the turbo charger on your car; that is, it has a turbine wheel and a compressor wheel. Even on a large jumbo jet, this is a fairly small device. Hot, relatively high pressure bleed air enters the pack through an On/Off valve called a Pack Valve. This valve does not modulate; it's either open or closed. It is operated by the pack switch in the cockpit. Switch ON, the pack valve opens and the pack is working. Switch OFF and the pack valve closes and the pack stops working. It's very binary for you computer geeks.

Once the pack valve is open, the air first passes through a primary heat exchanger. The purpose of this primary heat exchanger is to remove some of the heat from the air but not reduce the pressure too much. The heat exchanger is like an air-to-air radiator vs an air-to-water radiator like we're used to seeing on our cars. Anyway, after passing through the primary heat exchanger the air finally enters the pack itself and specifically, it enters the compressor section. There the air gets compressed to a higher pressure and along with this change also an increase in temperature. There is a key temperature senor down stream of the compressor section called the Pack Temperature Senor, which feeds the Pack Temperature Gage in the cockpit. More on this important gage in a minute.

There are some additional temperature sensors in this duct (down stream of the compressor), including one that will shut down the pack (close the pack valve) if the temperature in this section overheats but we will not go into that much detail. The air, leaving the compressor, now passes through a secondary heat exchanger where much of the heat is removed but most of the pressure is still retained. The air then flows into the turbine section (of the ACM) where the air is now *expanded* across the turbine wheel and this is where the magic happens: As the air flow *expands* across the turbine, there is a very large pressure drop and along with that comes a very large temperature drop! The air temperature is now down to about 32 to 34 degrees F (approximately).

ACM's are wonderful little machines that are great for one thing and one thing only: making very cold air. I'll ignore the other parts of the air conditioning system that ensure the air stays above freezing as it is not uncommon for the air to come out of the turbine section at or slightly below freezing. Maintaining the air temperature above freezing is desirable as mositure (always present) in the air would freeze and the air flow making it's way into the cabin and cockpit, would turn into snow flakes or hail. If you have ever been in a jetliner and had snow flakes or hail come out (I have) of the over head vents, that indicates some malfunction of the sub-system which ensures enough hot air is bypassed around the compressor inlet of the ACM and added to the outlet of the turbine section to maintain the target 35 F temperature of the air flow.

Now, once this air leaves the ACM it enters into the air conditioning distribution ducts and that in turn finally delivers the air to the cockpit and cabin.

Putting It All Together (the A/C System)

Now, I know this is a long explanation about the air conditioning system but laying it's ground work is important in regards to the pressurization system. Once the air is now inside the cabin, pressurization is controlled by controlling how much of it we allow to escape through the ourflow valve(s). Remember, the engines are pumping a lot of air into the cabin so a computer will control how much the outflow valves move and modulate to release enough air from the cabin to maintain the target cabin altitude/pressure differential. The cabin altitude is usually set by the pilots or done automatically by the onboard computers based on the cruising altitude.

So, now, where does all of this air conditioning info come into play? Well, if you turn off a pack, the pack valve closes and no air can move through the ACM/air conditioning system and no air flows through the distribution ducting and into the cabin. Turn off all the packs at cruise and guess what? You'll loose your pressurization. Oh, not by alot but the older the plane the more leaks it will have. When I flew FE on the TriStar for Lockheed one of the production flying checks we had to do at FL350 was to turn off all 3 packs and make sure the cabin didn't climb more than 1,000 feet per minute. If it did then we had too many air leaks and they had to be found and sealed. And no, you don't do this in the airline flying world but I wanted to make the point that the air conditioning system is an important part of the pressurization system in that unless you have at least one pack on, you won't be able to pressurize the cabin.

Now, lets go back to the ACM's for a moment. Knowing that the packs only put out 35 F air, how does the air conditioning system provide for warm air when the plane is cruising at FL350 where it's -50F outside? On the ACM there is a bypass duct that takes the hot air coming from the engines via the bleed air system and through a computer controlled by the pilots in the cockpit as they set the cockpit and cabin desired temperatures, modulates this bypass valve so that when hot air is needed in the distribution ducts, it is added directly and mixed with the cold air for the desired temperature.

Now, picture the plane sitting on the ramp at the gate loading passengers in Las Vegas in July when it's 110F outside!! As the pilot, you've got one or probably two packs on and the temperature set to 65F or so. You're calling for cold air into the cockpit and cabin. The air coming into the packs, almost all of it is going through the compressor section because we want most of it to expand across that turbine section which produces the most (in volume) cold air. Very little air is being bypassed around the ACM in this condition. Now, remember our little friend the Pack Temperature Gage in the cockpit? What does that little jewel tell us? Since most of the air is being directed through the ACM machine in this situation, the pack temperature gage will read high; will read on the high end of the gage's temperature range. In other words, the Packs (and hence ACM's) are working their asses off and you *will* see high pack temperatures under this condition. (But not, normally, high enough to trip the over heat sensor and shut down the pack). High pack temperatures mean your ACM is working hard and therefore your asking for a lot of cold air!

Now you take off and start to climb out into cooler and cooler air. As this happens, the demand for cold air is reduced more and more and more hot air ends up being bypassed *around* the ACM now. Pack temperatures will also come down to verify this. By the time you reach cruise, the ACM's are receiving the minimum amount of air through them to keep them turnng. They are really just 'idling' in cruise as most of the bleed air is bypassing the ACM. Look up while in cruise and observe the pack temp gages: it will indicate a low temp on its range.

So, that's it for the air conditioning system, the Air Cycle Machines and how they relate to the pressurization system. Now if you find yourself in a simulator for an airline or other operator getting initial training or recurrent training and the instructor gives you a Pack over heat problem, as you work through the checklist, hopefully you'll understand what is happening to the pack as you try to bring it back online and why you want to bring it back online!

(3) Outflow Valves. Controlling the outflow valves means controlling how much air we allow to 'leak' out. There is usually multiple outflow valves and one pressure relief valve. If the modulating outflow valves were to ever slam close and stay closed (malfunction), pressure would build up inside the cabin so to prevent structural damage, a maximum pressure relief valve would begin to open to relieve excess pressures.