737 Pneumatics System

[@greatflyer_aviation]

Chris, these videos are a BLESSING to those of us not been flying for a while and wanting to brush up in sight of the summer season! Thank you so much!

[@ChrisBrady737]

My pleasure!

[@NicholasRiviera-Dr]

If you are watching this video from a general information point of view, then it’s interesting. But if your viewing this from a technical point of view, there are a few things I would like to point out, @1:38 that engine is a CFM56-3, seen on the B737-3/4/500 series aircraft. One way to tell is by identifying the generator cooling shroud (green cover below the starter). The Classic B737 has a Constant Speed Drive and a generator mounted to it. The B737NG has the CFM56-7 engine and has an Integrated Drive Generator fitted instead @2:36 where is the pneumatic panel? This is a video about pneumatics @3:44 called the bleed valve. It’s actually called the Pressure Regulating and Shut Off Valve (PRSOV). The engine bleed switch on the pneumatic panel is used by flight crew to open and close the PRSOV @4:15 technically that is where the air is “supplied to” for engine start. When you say that is where the air is taken from, it makes it sound like the engine can start itself. The air is supplied to that point for engine starting from another pneumatic source such as the APU, the other engine or from the ground pneumatic source @5:01 the overpressure switch. Apart from the fact it’s a switch and doesn’t give a pressure reading, no one can read that pressure from the flight deck. The switch is located within the Bleed Air Regulator and senses pressure from the interstage duct using a sense line as per the schematic. When that pressure is sensed at 220 psi the switch closes, this causes a relay to close in the Air Conditioning Accessory Unit which in turn causes a BLEED TRIP. The PRSOV then automatically closes. If you look at you duct pressure indicator, it only goes up to 100 psi and that pressure is sensed in the ducting that goes through the air conditioning pack bays @6:15 and @7:59 the preconditioned air is generally used to cool the cabin. The cabin heats up on the ground from cabin lights and passengers @6:20 the ground pneumatic air for starting the engines can also be used to drive the air conditioning packs while the aircraft is on the ground. But you are correct that this is a different connection to the preconditioned air connection @7:08 the turbofan operates only during ground air conditioning operation or in flight when the flaps are extended. The turbofan is part of the ram air system. The function is to draw more airflow across the air conditioning pack heat exchangers when ram air flow is low for better cooling efficiency. This air does not go through the packs @10:30 there is talk about the DUAL BLEED light. It’s important to note for anyone watching this video and viewing it from an maintenance troubleshooting perspective, the DUAL BLEED light illuminates for: Engine 1 bleed “switch” on and APU bleed “valve” open, OR Engine 2 bleed “switch” on, isolation ”valve” open and APU bleed “valve” open @15:19 the 220 psi over pressure switch is part of the Bleed Air Regulator, not the High Stage Regulator. There is a sense line from the engine interstage ducting that goes to the BAR @15:50 I don’t consider this good advice. Using the Wing Anti Ice will have zero effect on reducing the pressure within the interstage duct on an engine that has had a BLEED TRIP. With a BLEED TRIP, the PRSOV closes automatically. The WAI valve is down stream of the PRSOV. If you open the WAI valves, it will cause more problems. Agreed 100% it will reduce your duct pressure, but it’s because the remaining engine is now venting its pneumatic pressure overboard via the WAI valve. The single WAI switch in the cockpit opens both WAI valves, one for the left wing and one for the right wing, you will now be taking air away from the only remaining air conditioning pack and aircraft pressurisation will most probably be affected. Using Engine Anti Ice will reduce the pressure in the interstage duct because you are creating a “hole” in the system. But why are you trying to do that? If you experience a BLEED TRIP, follow the QRH and do the reset, if it won’t reset leave it alone. How do you know it’s not some other issue such as a wiring harness or a faulty over pressure switch. Write it up for the maintenance at the next port and let them worry about it

[@ChrisBrady737]

All points noted, thank you for taking the time to write this feedback.

[@NicholasRiviera-Dr]

@@ChrisBrady737no problem. I am not trying to be critical just accurate. It must have taken some time to put the presentation together. I think it’s great that you are giving the public an insight into these wonderfully complex and technically advanced machines. My sources are the Aircraft Maintenance Manual (AMM), System Description Section (SDS), System Schematic Manual (SSM), Dispatch Deviation Guide (DDG) and the Quick Reference Handbook (QRH)

[@simonryan5248]

With over 20,000 hours flying the 737. I like to keep my technical knowledge sharp and current. I find your videos excellent. Thank you for all your hard work and effort.

[@ChrisBrady737]

You are very welcome, thank you.

[@rocotiger]

as a non-pilot viewer, I have to say your content is amazing. I don't know why but the pneumatics and packs systems on these commercial aircraft has always fascinated me. Really interesting stuff.

[@ChrisBrady737]

Great to hear that you find them interesting, thanks for watching

[@antoineyu2810]

Those videos are amazing and should be shown to all students currently in TR ! Thank you very much for sharing

[@ChrisBrady737]

Thanks Antoine, I appreciate your kind words.

[@MsYoungpilot]

The best explanation of the pneumatic system I’ve ever heard ! Perfect 👍🏻

[@ChrisBrady737]

You are very kind, thank you

[@gadgetpad2012]

Thanks Chris, the detail and the pictures are definitely a lot more easy to understand than looking at a schematic diagram. Having spent the time to get a technician/engineer to remove panels to see what we could only imagine before is really appreciated. Definitely going to subscribe to see what else you have in store for us.👍

[@ChrisBrady737]

Hi Allan, glad you found it useful. I will pass on your thanks to my engineering friends who made many of these photos possible.

[@allenwilliams7367]

I’m in class to be a A&P mechanic. These videos are helpful when studying. Thanks for making them.

[@ChrisBrady737]

Hi Allen, That is great to hear. Best of luck with your exams.

[@Rosseloh]

I'm just a flight simmer and aviation enthusiast, but thanks very much for this and your other/future videos. I always appreciate deep technical dives (your book is fantastic).

[@ChrisBrady737]

Thanks for your kind words. Whilst the videos are aimed at pilots I am pleased that they can also be enjoyed by a wider audience.

[@T0MaTOS]

THANKS CHRIS for sharing all of your knowledge on the 737!

[@ChrisBrady737]

My pleasure. Tell your colleagues

[@laurentkahn4745]

Amazing content through those videos. It really allows us to develop ourselves as professionals. Thank you for your dedication Chris.

[@ChrisBrady737]

You are very welcome, thanks for watching.

[@009raptor]

Just loving these deep technical dives Sir. Although I am already 33 and just a simulator flyer, I do aspire to fly in real life one day, and I wanna be ready in every way I can! Thank you so much. I would really appreciate a presentation with an overview discussion all the different systems (A/C, Fuel, Power APU etc.) focusing on a bird's eye view of how different systems interact with each other on a broader level, if possible Sir.

[@ChrisBrady737]

Thank you for your kind comments and your suggestion. I will keep it in mind for a future video.

[@catalincoco419]

Thank you cpt. Chris! Great presentation and looking forward on the next ones! Hope you will cover all chapters.

[@ChrisBrady737]

I have nearly finished all of the chapters, what other topics would you like to see covered?

[@danielfreifeldtaisen]

@@ChrisBrady737 In-dept coverage of the FMC and (LNAV/VNAV Automation), perhaps a video only about performance, GREAT videos as usual Chris

[@Ryan-ek7gh]

Very good videos, thank you for doing the Max differences aswell, I am on a Max differences type course this month so that was very helpful!

[@ChrisBrady737]

You are very welcome, enjoy the MAX

[@ElliotSmith1396]

Just discovered your channel and your site. I'm only a flight simmer but this is a fantastic resource to take my knowledge of the 737 to the next level. Thanks for what you do. Book Purchased!

[@ChrisBrady737]

Glad to help!

[@T0MaTOS]

EXCELLENT presentation Chris! Thanks again!

[@ChrisBrady737]

Thanks Jay, glad you enjoyed it.

[@767stick]

Thanks Chris, I liked and subscribed. Well presented information with some interesting tidbits as well. Look forward to the next one.

[@ChrisBrady737]

Thanks for the sub. More videos to come.

[@cptjack3dpilot242]

Great video! I'm doing my 737 type rating and found this very helpful 👌

[@ChrisBrady737]

Glad it was helpful, thanks for watching.

[@telescope64]

Thanks Chris,this is a brilliant explanation of the system and also flight considerations. Best wishes from Paul.

[@ChrisBrady737]

Glad you enjoyed it

[@yams900]

Very interesting and informative, just what I needed to know. Started to fly 737 in FS2020 :)

[@ChrisBrady737]

Great to hear that you are finding the videos useful.

[@astim8789]

You are doing great. Good job. Waiting for more ATA chapters.

[@ChrisBrady737]

Thank you. Yes lots more chapters to come. Elec next

[@iandudman9529]

Thanks Chris a fantastic presentation, don’t apologise for simple explanation the more experienced of our have forgotten some of it or have not flown enough recently 😂😂

[@ChrisBrady737]

👍

[@ethansaviation2672]

Great video chris😃👍, really helps me to understand

[@ChrisBrady737]

Glad it helped!

[@jhk580lpa]

great video! likes from s.korea. looking forward to learn more👍

[@ChrisBrady737]

You are very welcome!

[@matthpilot4361]

Precious informations, thank you very much!

[@ChrisBrady737]

My pleasure, thanks for watching

[@neekoweeclearwater2305]

great video. Thanks!

[@ChrisBrady737]

You are welcome

[@alimohammedhaider6611]

Chris, I can't thank you enough for what you've put out. Just a question about where you spoke about trim air only being present in the long body variations of the 737s. Does this only refer to the cockpit overhead switch for trim air, as trimming conditioned air following the turbine in the air cycle machine was performed in the classics too? Many thanks

[@ChrisBrady737]

Trim air was only used on the -4/8/900 and Max. The other series did not have it.

[@alimohammedhaider6611]

@@ChrisBrady737 Thanks for clarifying Chris. My confusion was due to the positioning mix valves present on the 300/500s which would proportion hot air with cold air from the conditioning packs, although those weren’t the same as the trim air modulating valves present on the long bodies.

[@mohamedzaytoun4966]

Thanks, very useful information

[@ChrisBrady737]

My pleasure

[@gwrallen]

If the old style bleeds use more bleed air than necessary, what happens to the excess bleed air? Is it shunted overboard somewhere? Awesome videos!

[@theHDRflightdeck]

Hi Chris. If there's a check valve from the L Pack to the mix manifold how are we getting recirc fan air, or any air for that matter, in the flightdeck with the packs turned off? Also, would it be possible to run the packs from an air start unit on a hot day prior to eng start?

[@ChrisBrady737]

Just with fans, it is not from the packs if the pack switches are off. No you cant run the packs from an ASU, the mix manifold is in a different place, hence the different connection point for ground conditioned air. I cover this either in this or another video.

[@theHDRflightdeck]

@@ChrisBrady737 Thanks for the reply! Just to be clear, I'm talking about an air start unit that provides compressed air for starting the engine and it's fed to the right side of the bleed system.

[@ChrisBrady737]

@@theHDRflightdeck Yes, you can not use ASU air for the packs.

[@NicholasRiviera-Dr]

Yes you can use an airstart unit to drive the packs

[@NicholasRiviera-Dr]

@@ChrisBrady737strange answer. Yes, you can use the air start unit to drive the air conditioning packs as long as the temperature and pressure limits of the delivered air are observed

[@NestOneofThree]

Will you be adding differences for a 737-800 BCF to any of your videos?

[@ChrisBrady737]

Hi Vivian, Yes I have a cargo conversions video in the pipeline. It should be out in a few months.

[@Debus005]

Hello Captain, i am a little bit confused about the anti ice bleed, why would the cowl heating air be taken before the pre cooler( so its hotter) and the LED taken after it ( isn't it better to be the other way around since it has to travel a long distance in the ducts untill it reaches its destination?)

[@envy4839]

I have heard that a small amount of air is supplied to the flight deck from the right pack, is that true? And if you had a Bleed Trip Off warning that couldn't seem to be fixed, would you continue to fly?

[@ChrisBrady737]

A small amount could come from the right pack but as you can see from the schematics here and in your FCOMs, the bulk is from the left pack. Re continuing with a bleed trip off, that comes down to your company policy/training and the Captains judgement. You should consider what would happen if the other bleed subsequently failed.

[@envy4839]

@@ChrisBrady737 Perfect, thank you!

[@buhlenkomonye451]

I heard you speaking about a book may I ask how can I get ahold of it because I’m quite new in aviation and I feel it may be helpful

[@ChrisBrady737]

My book, which accompanies this series of videos is available from Apple Books at this link: www.b737.org.uk/ebook.htm

[@buhlenkomonye451]

May you please make it to be available in Apple books in South Africa

[@ChrisBrady737]

Sadly the list of countries where Apple sells the book is set by Apple and RSA is not on that list, I dont know why. Sorry.

[@sharoz6300]

Hello capt. I am little confused if Bleed Trip off light illuminates and it is due to overpressure situation. Should i turn on engine anti ice to reduce the duct pressure or wing anti ice? (Qrh says first step turn wing anti ice Off) . Please throw a little light on it.

[@ChrisBrady737]

Always follow the QRH procedure. WAI has a far larger affect on duct pressure than EAI.

[@NicholasRiviera-Dr]

Obviously the advice of following the QRH is correct because it is written by the manufacturer and they should know what they are talking about. With regards to your question about Engine Anti Ice (EAI) and Wing Anti Ice (WAI) I offer the following based on the B737NG: When an engine BLEED TRIP occurs, the Pressure Regulating and Shut Off Valve (PRSOV) closes automatically. This means that no air is being supplied by that engine to that side of the pneumatic system. BUT, the 5th stage check valve and high stage valve (9th stage) are unaffected by a BLEED TRIP and one of those will still be open supplying pressure to the ducting on the engine side of the closed PRSOV. Note: this cannot be seen on the duct pressure indicator because those transducers are located in the air condition pack bays. The air supply for the EAI comes from the interstage manifold ducting which is on the engine side of the PRSOV. So no matter if the PRSOV is open or closed, you will always have air available for EAI. In theory (against the QRH) if you turn on the EAI, yes this will reduce the pressure in the engine pneumatic manifold because essentially you are creating a “hole” for the air to escape. The engine will continue to supply air pressure while it rotates, but with the EAI valve open will the “hole” be big enough to remove the pressure on the 220psi overpressure switch (sensing the pressure in the interstage manifold duct) located in the Bleed Air Regulator which is causing the overpressure condition in the first place? Who knows as there are many factors to consider. I suspect that the QRH says to turn OFF the WAI off first because of what happens later in the checklist. If the BLEED TRIP remains illuminated, the affected side PACK switch is now turned OFF. If the ISOLATION VALVE is in the AUTO position, that valve will now open so that the other engine bleed can be used as the source to drive both air conditioning packs for pressurisation and ventilation purposes. So now you have a single engine supplying the bleed air for both air conditioning packs. If the WAI was left on, a massive hole would be in the system that is trying to keep the aircraft pressurised.

[@MAhmedShah89]

Thanks for sharing.

[@ChrisBrady737]

Thanks for watching!

[@spiritzm]

Thank you, sir!

[@ChrisBrady737]

My pleasure, thanks for watching

[@aviationds5624]

Hi Chris, These videos are helping me through my engineering degree. You are a life saver. I have a question about the Ground conditioned air and the external air used for engine starting. Do they both come from the Ground cart?

[@ChrisBrady737]

No, they come from different sources because the external air for starting has to be delivered at high pressure.

[@NicholasRiviera-Dr]

No they come from a different source. Conditioned air can be supplied from either a mobile cart or an aerobridge mounted unit. A hose from the cart/unit connects to the aircraft ground conditioned air connection which is a standard 8 inch bayonet connector (with check valve). This air feeds directly into the airconditioning mix manifold. When the air is supplied the air pressure should not exceed 15 inches of water (.54 psi) gauge pressure and the air temperature should not exceed 71 degrees C (160 degrees F). Contrary to what is said in this video @6:17 ground conditioned air is used to COOL the cabin while on the ground. Heat is generated in the cabin by lighting and passengers External air for engine starting is supplied from a different ground source. Usually this is a compressor mounted on the back of a truck. The hose connects to the aircraft via a ground pneumatic service connector which has a standard 3 inch quick disconnect nipple (with check valve). This air is supplied to the aircraft pneumatic manifold, right hand side on the B737-3/4/5/NG. The external pneumatic source must have a maximum supply pressure of 60psi and temperature of 450F (232C). Having said that, this pneumatic source from the external compressor can also be used to drive the air conditioning system.

[@zaqzaq7426]

Thank you Sir!

[@ChrisBrady737]

You are welcome, thanks for watching.

[@mikoajkula4093]

I'm just a fan of aviation and a pilot in the simulator, I'm just wondering why do we turn off the isolation valve when starting the engines? greetings.

[@ChrisBrady737]

We don't. We just switch off the packs for engine start to enable maximum bleed air available for engine start. The isolation valve must be open to enable both engines to receive air from the APU.

[@spiritzm]

+1

[@sergiolopez1118]

Very noise

[@ChrisBrady737]

?

[@Joeys6thchin]

This makes me wonder what would happen in the event of a pre-cooling failure?

[@NicholasRiviera-Dr]

Are you referring to the engine precooler? If it has a failure such as a split or crack in the welding for example, then hotter air would be allowed to enter the pneumatic system ducting. Upstream of the precooler there is an over temperature switch set to 490F (254C). If the air gets to that temperature, the system shuts off the air supply to the pneumatic system for that engine. It’s called a BLEED TRIP. The valve known as the Pressure Regulating and Shut Off Valve closes automatically and stops the supply of air from the engine