Airbus A321 Air Conditioning System - Looking to understand the basic system, redundancy, and what parts can be inoperative and still remain airworthy

[Slick]

Background (Why I ask this question)
Recently on a flight from DFW to DTW we were taxing to the active on an A321 when Pilot indicates we have an error message with the Air Conditioner and we would be returning to the the gate for maintenance check. A long trip from Gate D16 to E31. We were told all passengers would have to exit the aircraft during the maintenance check as one engine was required to be running during the check. We re-board and are taxing out to the active runway for the second time and Pilot indicates that the same same error message has returned. Return to gate E31. Since it was cooler in the cabin than in the passenger boarding bridge before; I was one of the last passengers to deplane and heard the Airline Mechanic tell the Pilots the aircraft was now "out of service". Later the Airline advises they have a replacement aircraft (slightly smaller A320) that will be searched and readied for flight.

Summary after about 3 hours we are now taxing out to the active on a A320 with a thunderstorm approaching DFW. Another 2 hours in the cabin watching mother nature provide a lightening display; we are now departing. The bonus of course was to conserve fuel the pilots had to shut down the engines on the taxiway to conserve fuel. Good choice in my thinking. I suspect they were using the APU as the cabin air temperatures seemed tolerable. Ground OAT before thunderstorm was 89F.

Summary of added time was around 3 hours for mechanical delays and 2 hours for thunderstorm delay. Pilots I am sure did what they could and sadly the mechanical delays pushed us into a weather required delay.

So I am looking to better understand the A321 Air Conditioning System, redundancy, and what if any part of the system can be inoperative and still allow the aircraft to remain airworthy. Additionally my information above is what passengers were told so the "air Conditioning error message" could have been much more than just the cooling portion of the HVAC/Pressurization system. Since the pilot had advised our cruising altitude would be FL39, I was more concerned with bleed air heating and pressurization functioning properly and safely.

Thanks I am trying to gain a better understanding of the system. By the way airline sent me an email the next day giving me a $30 voucher if used within a year.

Best regards
Slick

 

[kschachn]

One thing you could do is download the MMEL for the airframe and look in chapter 21 for the air conditioning system.

 

[Just a civilian pilot]

Background (Why I ask this question)
Recently on a flight from DFW to DTW we were taxing to the active on an A321 when Pilot indicates we have an error message with the Air Conditioner and we would be returning to the the gate for maintenance check. A long trip from Gate D16 to E31. We were told all passengers would have to exit the aircraft during the maintenance check as one engine was required to be running during the check. We re-board and are taxing out to the active runway for the second time and Pilot indicates that the same same error message has returned. Return to gate E31. Since it was cooler in the cabin than in the passenger boarding bridge before; I was one of the last passengers to deplane and heard the Airline Mechanic tell the Pilots the aircraft was now "out of service". Later the Airline advises they have a replacement aircraft (slightly smaller A320) that will be searched and readied for flight.

Summary after about 3 hours we are now taxing out to the active on a A320 with a thunderstorm approaching DFW. Another 2 hours in the cabin watching mother nature provide a lightening display; we are now departing. The bonus of course was to conserve fuel the pilots had to shut down the engines on the taxiway to conserve fuel. Good choice in my thinking. I suspect they were using the APU as the cabin air temperatures seemed tolerable. Ground OAT before thunderstorm was 89F.

Summary of added time was around 3 hours for mechanical delays and 2 hours for thunderstorm delay. Pilots I am sure did what they could and sadly the mechanical delays pushed us into a weather required delay.

So I am looking to better understand the A321 Air Conditioning System, redundancy, and what if any part of the system can be inoperative and still allow the aircraft to remain airworthy. Additionally my information above is what passengers were told so the "air Conditioning error message" could have been much more than just the cooling portion of the HVAC/Pressurization system. Since the pilot had advised our cruising altitude would be FL39, I was more concerned with bleed air heating and pressurization functioning properly and safely.

Thanks I am trying to gain a better understanding of the system. By the way airline sent me an email the next day giving me a $30 voucher if used within a year.

Best regards
Slick

While I can’t comment on your flight ( what fault the pilots got in the cockpit ) , all I can say is that there are two air conditioning packs ( AC, heating , pressurization ) and the MEL ( where we check to see if we can depart with something broken or “ unserviceable” ) says we can take off with only one working. There is usually a cruise altitude limit when flying with only one pack.

That said, if a pack fails after pushback, taxing to the runway for take off, we have to return to the gate and have maintenance do their check before they will authorize departing with only one pack. The plane may require extra fuel if the MEL limits the cruise altitude and it’s a long flight ( 3 types of MEL deferrals on the Airbus ….some don’t have an altitude restriction, some do ) because fuel burn is greater the lower we fly.

Was it a A320 or A321?

Great APU and very reliable air conditioning system on the A320 ( why I don’t bring a suitcase ).

Yes, if it’s very hot, and humid, the APU has a harder time keeping the cabin as cool as when its shutdown and both engines are running.

Hope that answers your question.

 

[Cujet]

Here is a Gulfstream cockpit layout. This is a basic twin engine setup.

Since the crew did not clearly tell you what part failed, it could be a cooling turbine, a valve, the electronic control of a valve, or something else like a leak etc.

This setup allows the use of both "packs" (the 35ºF fan symbol) with any single air input. Depending on valve configuration.

In normal operation the systems are isolated. We would not dispatch with just one pack operative. But no problem if we are down to one in flight.

 

[Slick]

One thing you could do is download the MMEL for the airframe and look in chapter 21 for the air conditioning system.

kschachn

Thanks will give that a try

 

[Slick]

While I can’t comment on your flight ( what fault the pilots got in the cockpit ) , all I can say is that there are two air conditioning packs ( AC, heating , pressurization ) and the MEL ( where we check to see if we can depart with something broken or “ unserviceable” ) says we can take off with only one working. There is usually a cruise altitude limit when flying with only one pack.

That said, if a pack fails after pushback, taxing to the runway for take off, we have to return to the gate and have maintenance do their check before they will authorize departing with only one pack. The plane may require extra fuel if the MEL limits the cruise altitude and it’s a long flight ( 3 types of MEL deferrals on the Airbus ….some don’t have an altitude restriction, some do ) because fuel burn is greater the lower we fly.

Was it a A320 or A321?

Great APU and very reliable air conditioning system on the A320 ( why I don’t bring a suitcase ).

Yes, if it’s very hot, and humid, the APU has a harder time keeping the cabin as cool as when its shutdown and both engines are running.

Hope that answers your question.

Just a civilian pilot

It was a A321.

Appreciate your information on the AC, heating, pressurization system, and the MEL. Your detail was sufficient for me to understand the options and choices taken. Being an engineer and a pilot flying under part 91 general aviation, it is interesting for me to better understand part 121 operations when flying as a passenger with the scheduled airline carriers. I expected the A321 would have two systems. The Pilot had stated during taxi to the active that our cruising altitude would be FL39 which I believe is just under the service ceiling (39,200 to 39,800 feet?) for the A321.

I am sure the Dallas Based Crew was not happy with what they had to go through that evening. They were expecting to be into DTW by 19:46 EDT and it ended up being 00:37 the next morning. At least we made it to our destination after the Airline brought a replacement A320 to the gate.


Thanks for your excellent description so I better understand

Slick

 

[Slick]

Here is a Gulfstream cockpit layout. This is a basic twin engine setup.

Since the crew did not clearly tell you what part failed, it could be a cooling turbine, a valve, the electronic control of a valve, or something else like a leak etc.

This setup allows the use of both "packs" (the 35ºF fan symbol) with any single air input. Depending on valve configuration.

In normal operation the systems are isolated. We would not dispatch with just one pack operative. But no problem if we are down to one in flight.

Cujet

Appreciate the Gulfstream technical details with cockpit view for the pressurization system.

You indeed get to be involved with what I consider the highest end of the corporate aviation aircraft. Plus you get to pilot your own C177RG. I have not flown passenger in a Gulfstream (someday hopefully) but did get some time years ago in a Citation 10.

Best regards

Slick

 

[Cujet]

but did get some time years ago in a Citation 10.

We had a Citation 10 for a bit. Liked it. As I am sure you know, that little beast is stinkin' fast with an 0.93-0.935 MMO, wonderful for up and down the East coast. What's interesting though is that modern Gulfstream jets are even faster, block to block. Especially so on longer trips where the Citation might use a more economical cruise speed to make the destination. The climb speed of a G600, for example, is 0.087 and cruise is nearly always 0.91. A trip from NY to CA could put the Gulfstream ahead by half an hour or more. We learned this the old fashioned way. By real world experience.


More off topic thoughts. I've been working on a small air cycle machine for light aircraft. It is far more feasible than people realize. Not efficient in any way, but very practical and light.

 

[Astro14]

Here is an overview of a typical airliner air conditioning system:

There are two air-conditioning compressors known as “packs”. They consist of an expansion turbine (that looks like a turbocharger in design, but much bigger) where pressurized air (from engines or APU compressor) is first compressed, then run through a heat exchanger while hot, then through the expansion side of the turbine and cooled.

The heat exchangers are cooled in flight by ambient air taken in through a low drag duct nearby. On the ground, they are cooled by fans that draw air through that duct over the heat exchanger.

It is a basic air conditioning cycle, compress the gas, and it gets hot, cool it while it is hot, then expand it, and it cools even further.

The outlet temperature of the packs is generally determined by the lowest temperature desired in the cabin. In the air conditioning manifold that feeds the cabin, there is a mix of both cool pack outlet air, and “trim“ air, that is, air that is still hot, coming directly from the bleed air source. The manifold mixes the air at each output point, to give the desired temperature.

The system typically feeds avionics cooling as well, but the temperature there is not regulated with trim air, the way that each cabin, and cockpit, zone is regulated to achieve a specific temperature.

There are pressure and temperature sensors in each phase, and they are looking for over pressure, under pressure, over temperature, or signs of a leak. Any one of those sensors can cause an air conditioning pack fault.

That fault may be mild, or it may be severe, it may be able to be reset, or it may require turning off the pack. It may require putting the pack in a manual, fixed temperature, mode.

Every airplane manufacturer has fault, or failures, that are subject to the “MEL“ or minimum equipment list. On an Airbus, you can fly with one pack inoperative, as long as you’re not going over water, or ETOPS. I had a pack over temperature, on a 767 going to London Heathrow, a couple months ago. It happened right as we were ready to push back from the gate.

That was not something we could defer under the MEL for a transatlantic flight. We allowed our mechanics to work on it for a couple hours, and they decided that it was the pack itself that was at fault, not a sensor, and not a computer. They took the aircraft out of service. We left for London, three hours late, in a different aircraft.

There is an altitude limit on the airbus, as was already mentioned, if you have a pack inoperative. It’s fairly low, like FL310, and it has to do with the amount of flow from one pack, and the balance of that flow, leaving the airplane through the outflow valve. If you have less flow coming in, it is harder to pressurize the airplane, even with the minimum flow going out.

From your description, it’s impossible to tell what was actually wrong with the aircraft. It would be a disservice to the passengers for the pilot to precisely describe the problem because it is a degree of technical understanding that no one else in the aircraft, not even the flight attendants, actually possesses. If I stated that I had a pack fault due to an outlet overtemp, as I did on that London flight, not one person of the 240 on the plane that night would have understood, so, like your crew, I simply stated that we had a problem with one of the air conditioning units, and that we needed both of them to be working correctly in order to fly 4000 miles over the Atlantic to London.

There may be a very good reason why you don’t want to MEL a system problem, even though it is legal to MEL, and instead would take the aircraft out of service.

It may be that the MEL is fine for the flight that you’re operating, but not for the subsequent flight that the aircraft is scheduled to operate, so the company wants to dispatch an aircraft that is legal for the next leg that aircraft was intended to fly.

It may be that the altitude restriction for a pack inop simply imposed too many operational limitations, like the ability to avoid icing, or severe weather, or to carry sufficient fuel for the length of the flight.

Here is an example, let’s say the weather is great in Newark, and in Chicago, but on the subsequent leg, the aircraft was supposed to go into Denver, and there is icing in the descent. I need both packs operating for that leg, because of operational considerations. So, while it might’ve been great to execute that first leg, then the airplane gets stuck in Chicago, because it’s not suitable for the next leg.

Airlines look at that stuff, and they think about that stuff, when making a decision on MEL.

 

[Cujet]

I am one of a few anywhere who has experienced multiple rapid cabin pressure loss situations at FL450. It is ugly, uncomfortable and for some, makes any form of human functionality impossible for quite some time, as O2 deprivation can affect people differently. One crew member did not recover fully until after the flight was over. Despite getting the mask on him right away and forcing the O2.

In our case the outflow valve was motoring full open. So the airflow into the cabin was normal. Manual control got the cabin back fairly quickly.

The loss of one pack should never be a disaster in flight. However, the loss of the other one (let's say they are exactly the same age and number of hours) in flight might require more extreme measures to maintain cabin pressure. Sure, it could be that expansion turbine cooling is not required at altitude, and the hot air is cooled enough to provide cabin pressure. Or not. Depending on what went wrong and why.

We've seen this, and the result was duct overheat. Which has its own set of smelly problems.

 

[Slick]

We had a Citation 10 for a bit. Liked it. As I am sure you know, that little beast is stinkin' fast with an 0.93-0.935 MMO, wonderful for up and down the East coast. What's interesting though is that modern Gulfstream jets are even faster, block to block. Especially so on longer trips where the Citation might use a more economical cruise speed to make the destination. The climb speed of a G600, for example, is 0.087 and cruise is nearly always 0.91. A trip from NY to CA could put the Gulfstream ahead by half an hour or more. We learned this the old fashioned way. By real world experience.


More off topic thoughts. I've been working on a small air cycle machine for light aircraft. It is far more feasible than people realize. Not efficient in any way, but very practical and light.

Cujet

Yes the Citation was very fast for shorter trips. Longer trips like you note surely favor the G6000. Not the status of a G6000 and we were the "little bird at DTW for sure". We used the on a M-F up and back trip from DTW to BUF, so depending on vectoring and which runways were in use, around 220 NM each segment. Corporate passengers were ready on each end so a quick up an back.

Appreciate the great pictures of the G6000 multiple displays with enhanced vision and synthetic vision(?). I especially like the HUD. Does the pilot (assuming the HUD visor is only for the left seat pilot position) use it often or is just too much input. The G6000 panel is sure a different world than the analog "steam gauges" in my C182.

Interested in learning more about the small air cycle machine for small aircraft. If you have time and ever want to share and it is not priority.

Best regards

Slick

 

[Astro14]

Here is a pack from a 757. We simplify the entire machinery set by calling it a “pack” but in reality, there are a lot of parts, a lot of valves, complex air routing, and a lot of sensors.

 

[Slick]

Here is an overview of a typical airliner air conditioning system:

There are two air-conditioning compressors known as “packs”. They consist of an expansion turbine (that looks like a turbocharger in design, but much bigger) where pressurized air (from engines or APU compressor) is first compressed, then run through a heat exchanger while hot, then through the expansion side of the turbine and cooled.

The heat exchangers are cooled in flight by ambient air taken in through a low drag duct nearby. On the ground, they are cooled by fans that draw air through that duct over the heat exchanger.

It is a basic air conditioning cycle, compress the gas, and it gets hot, cool it while it is hot, then expand it, and it cools even further.

The outlet temperature of the packs is generally determined by the lowest temperature desired in the cabin. In the air conditioning manifold that feeds the cabin, there is a mix of both cool pack outlet air, and “trim“ air, that is, air that is still hot, coming directly from the bleed air source. The manifold mixes the air at each output point, to give the desired temperature.

The system typically feeds avionics cooling as well, but the temperature there is not regulated with trim air, the way that each cabin, and cockpit, zone is regulated to achieve a specific temperature.

There are pressure and temperature sensors in each phase, and they are looking for over pressure, under pressure, over temperature, or signs of a leak. Any one of those sensors can cause an air conditioning pack fault.

That fault may be mild, or it may be severe, it may be able to be reset, or it may require turning off the pack. It may require putting the pack in a manual, fixed temperature, mode.

Every airplane manufacturer has fault, or failures, that are subject to the “MEL“ or minimum equipment list. On an Airbus, you can fly with one pack inoperative, as long as you’re not going over water, or ETOPS. I had a pack over temperature, on a 767 going to London Heathrow, a couple months ago. It happened right as we were ready to push back from the gate.

That was not something we could defer under the MEL for a transatlantic flight. We allowed our mechanics to work on it for a couple hours, and they decided that it was the pack itself that was at fault, not a sensor, and not a computer. They took the aircraft out of service. We left for London, three hours late, in a different aircraft.

There is an altitude limit on the airbus, as was already mentioned, if you have a pack inoperative. It’s fairly low, like FL310, and it has to do with the amount of flow from one pack, and the balance of that flow, leaving the airplane through the outflow valve. If you have less flow coming in, it is harder to pressurize the airplane, even with the minimum flow going out.

From your description, it’s impossible to tell what was actually wrong with the aircraft. It would be a disservice to the passengers for the pilot to precisely describe the problem because it is a degree of technical understanding that no one else in the aircraft, not even the flight attendants, actually possesses. If I stated that I had a pack fault due to an outlet overtemp, as I did on that London flight, not one person of the 240 on the plane that night would have understood, so, like your crew, I simply stated that we had a problem with one of the air conditioning units, and that we needed both of them to be working correctly in order to fly 4000 miles over the Atlantic to London.

There may be a very good reason why you don’t want to MEL a system problem, even though it is legal to MEL, and instead would take the aircraft out of service.

It may be that the MEL is fine for the flight that you’re operating, but not for the subsequent flight that the aircraft is scheduled to operate, so the company wants to dispatch an aircraft that is legal for the next leg that aircraft was intended to fly.

It may be that the altitude restriction for a pack inop simply imposed too many operational limitations, like the ability to avoid icing, or severe weather, or to carry sufficient fuel for the length of the flight.

Here is an example, let’s say the weather is great in Newark, and in Chicago, but on the subsequent leg, the aircraft was supposed to go into Denver, and there is icing in the descent. I need both packs operating for that leg, because of operational considerations. So, while it might’ve been great to execute that first leg, then the airplane gets stuck in Chicago, because it’s not suitable for the next leg.

Airlines look at that stuff, and they think about that stuff, when making a decision on MEL.

Astro

Thanks the additional information, system descriptions, and example of real world reasoning for the decision that have to be made. You always (and the other ATP Pilots flying scheduled airliners on this site) offer great understanding for us that do not operate in that scope of aviation except as passengers. You are correct that in your "conditioning" announcements to passengers you must also consider their knowledge and ability to digest the technical details. Most don't care to know. I suspect I was the only one in the cabin who would have valued more information. The rest of the passengers were interested in when the free snacks and drinks wold be given out. I myself learn from experience and travel with my emergency snacks/water and ear plugs for the restless kids on the plane. I am just glad I had a window seat to watch the interesting 2 hour thunderstorm ground delay we entered after the mechanical delays.

It is interesting with the major scheduled airlines in the day of cost savings they can fairly quickly find a replacement airliner for a "ready to depart" airliner that is taken out of service.

I am thankful we have this great group on this aviation forum that we can read and ask questions to gain understanding and knowledge.

Appreciate you being a staff member/moderator as well.

Best regards

Slick

 

[Slick]

I am one of a few anywhere who has experienced multiple rapid cabin pressure loss situations at FL450. It is ugly, uncomfortable and for some, makes any form of human functionality impossible for quite some time, as O2 deprivation can affect people differently. One crew member did not recover fully until after the flight was over. Despite getting the mask on him right away and forcing the O2.

In our case the outflow valve was motoring full open. So the airflow into the cabin was normal. Manual control got the cabin back fairly quickly.

The loss of one pack should never be a disaster in flight. However, the loss of the other one (let's say they are exactly the same age and number of hours) in flight might require more extreme measures to maintain cabin pressure. Sure, it could be that expansion turbine cooling is not required at altitude, and the hot air is cooled enough to provide cabin pressure. Or not. Depending on what went wrong and why.

We've seen this, and the result was duct overheat. Which has its own set of smelly problems.

Cujet

I am glad that I have never experienced rapid cabin pressure loss. Not a pleasant experience and your description confirms that for sure. Interesting that O2 deprivation affects each of us differently. The one crew member in your experience is a good good example.

I have been to 14,000 MSL in unpressurized cabins with out O2 for less than 30 minutes to see the effects. I did not notice any effects but one rear seater did in our C182. (It was my lovely wife so I did not suggest differently. I have learned)

Your example is one of many justifications for the crew to know the aircraft systems from memorization, have excellent training, and experience to be able to react immediately when an emergency arises.

The G6000 is one very high flying aircraft. I think its service ceiling is 51,000 feet.

Thanks

Slick

 

[Slick]

Here is a pack from a 757. We simplify the entire machinery set by calling it a “pack” but in reality, there are a lot of parts, a lot of valves, complex air routing, and a lot of sensors.

View attachment 272036

View attachment 272037

Astro14

Thanks for the very detailed pictures of the B757 packs. I will never get any closer than those pictures. As you indicate much content from your earlier post. I can now visualize the system. With all the valves and sensors it is interesting that after many years of flying with airlines on US Domestic/Europe/Asia routes I have only had one known delay from the air conditioning system packs.

It is truly amazing that the scheduled airline system with all of the aircraft complexity/redundancy/operating extremes is as reliable and safe as it is.

I believe the airliners are designed for a 30 year useful life. Not sure when the major airlines start replacing the older fleets. Likely more replacement time is based on fuel efficiency, overall costs, and route profiles.

I could only wish modern automotive vehicles would have a 30 year service life with proper maintenance. Different cost and maintenance between automotive and scheduled airliners for sure. I have had (too bad I did not keep them) a 1969 Z28 and a 1971 Monte Carlo into the early 2000s and they were still operating fine when I sold them. Not sure my 2022/2023/2024 vehicles will have any hope of being in service after 30 years.

Thanks again for the great pictures

Slick