The Secret Boeing 737 Jet engine

hi everybody very welcome to mentor yet another video podcast as always i hope you're doing absolutely fantastic today we are going to be talking about the secret third jet engine aboard the 737 800 the apu what does it do why do we have it and what happens if we don't have it stay tuned [Music] three one zero one six this video is brought to you in cooperation with brilliant now maybe you are like me and you find yourself together with your kids with a little bit too much extra time and you want to start to improve things like for example science maths or physics in that case i highly recommend you to check out brilliant use this link here below because they have more than 60 different courses which are really really well built in order to help you do exactly that improve and do it in a way that is actually fun to do i've done it together with my oldest son lucas and he gets so happy when he managed to solve some of their puzzles for example so if you want to try it out use this link here below you'll get 20 off the annual fee and the link is brilliant.org mentor pilot check it out all right my friend so this is a video that i've had coming for quite a long time it's about the auxiliary power unit the apu which is actually a jet engine that we have in the back of the aircraft in order to give us electrical power and pneumatic pressurized air for a few different reasons all right i bet you think you know all that you need to know about the apu but in fact there are some quite interesting things that i'm sure that you don't already know so first of all why have they chosen to have a jet engine to produce electricity and pressurized air couldn't they just have a normal compressor the fact is that a jet engine is uniquely positioned to do exactly those two things very very well and they can you can do it at almost any different altitude with almost any different temperature or pressure there are very very few moving parts in a jet engine which makes it very reliable and the fact is that since we already have jet fuel available well then you know whatever drives the engines might as well drive this generator so a jet engine basically now the apu is situated in the very back of the aircraft it's likely that you have seen it if you have boarded the aircraft from the back steps for example you will see the exhaust coming out of the very back of the aircraft and you will also obviously hear it right it takes the air in from the right side so behind the back right emergency exit you'll see there's something that looks kind of like a little wing sticking out that's a vortex generator that forces the air into the air inlet when when we are using the apu in flight and from there the air goes in through just like a normal jet engine the compressor stages we take the bleed there for the automatic power from the compressive stages then it goes into combustion chamber and after that you have the turbine stages after the turbine stages it goes through a muffler because it makes a lot of noise and a lot of noise is not a good thing on an airport and there's actually quite a lot of rules and regulations surrounding the use of the apu and then it's being tossed out but those of you who have checked the 737 800 would have noticed that there's not only one exhaust there's actually two holes in the back of the of the aircraft those holes are both to do with the apu so the bigger hole that's the exhaust exhaust muffler and you know where the exhaust gases comes out at the hole above that is actually where the cooling air for the apu is taken from so the cooling air is being sucked in there it's then being routed around the hot part of the apu and then it's being exhausted out together with the rest of the exhaust through the the bigger hole the fuel is taken from the number one tank that's the tank on the left hand side and it can either be pumped up to the apu using the normal fuel pumps or it can be suction fed to the apu in any case here is one thing that we need to think about in the cockpit and that is the fact that since the epu is taking its fuel only from the left hand side if we have a let's say we have a long slot for example and we need to run the apu for air conditioning well in that case it will start depleting the fuel on the left hand side but not on the right hand side so if you're not careful you might develop an imbalance which is where they're more fuel in the right tank than the left tank so sometimes we actually have to open the the crossfit valve and turn off the fuel pumps on the left hand side in order to force fuel from the right side through the crossfield valve and then up into the apu so it's one of those things that you have to be a little bit careful with but in all fairness the apu doesn't use that much fuel it's only about 40 to 50 kilos per hour so in order to get about a 200 kilo imbalance you would have to wait about four hours but i should be saying that if you have to use a lot of air conditioning it does use a bit more fuel than stated so it's still worth to kind of keep track of okay so what does the apu do then well i've already mentioned that it is an electrical generator and that it produces bleed air so that gives you a bit of a clue to why we have it um on the ground we tend to use it primarily to give us electrical power until we can get ground power so for example as we're taxing in we tend to start up the apu um and then when we get on the stand we can immediately put the apu on the electrical buses so that becomes the electrical power source and then we can shut the engines down without any delay then as soon as we get the ground power connected if we don't need to use it as an air conditioning source then we turn the apu off right and the reason we do that is like i mentioned before the there's quite a lot of exhausts coming out of the apu and on a major airport if you have hundreds of aircraft standing with the apu running constantly it means that there's going to be quite bad air quality as well as noise so generally speaking we normally have about five minutes after we pull on to stand and about 10 minutes before we're about to push back that we're allowed to use the apu if we don't have any external air conditioning so here in spain for example right now in july the temperatures can rise up to 35 40 degrees celsius easily you need air conditioning inside this metal too but otherwise it will become unbearable well then we can ask from for permission to use the apu while we're standing on the ground in order to cool the air down right most major airports here in spain have external air conditioning you probably see in this big kind of plastic tube that goes out and is connected to the lower part of the aircraft that's the air conditioning but if that's not available you need to be able to use the apu now to start the apu it's really really simple you have the apu control panel on the overhead panel and you just take the apu switch you bring it down to start and then it's spring loaded to the on position and you leave it on that will start the startup sequence and the apu actually monitors itself for most malfunctions so if it has a hot start or a hung start or an rpm over speed or low oil pressure anything like that it feels that it's going to shut the whole starter sequence down itself and it will just illuminate a fault light for us right so there's nothing we need to do to monitor it if it starts well then once it has heated itself up enough then we will see that by the apu becoming available as a power source we can use it immediately then but in order to use it as a bleed air source we need to wait 60 seconds so one minute after the little blue light comes up in the electrical panel then we can use it as a asymptomatic source for air conditioning as well when we push back that's what we had tried to do a couple of minutes before we are ready we start the apu we do the before start checklist whilst the apu is available well then we can finish everything else off and we can push back and we can just start our engines the normal way which i've done videos about if the apu is not available then it becomes a little bit more tricky because without an apu we are going to need not only ground power but also an external start source so we need to get a special big machine that can produce a lot of bleed there connected and it connects on the right hand side of the aircraft which means that we can only start engine number one normally we start two and one in this case we're going to start at number one on stand once that's done we can use that engine as an electrical source and a submatic source so once we start at number one we can disconnect the the um the power sorry the starter unit from the right hand side we can push the aircraft back set the parking brake disconnect all of the ground equipment and then we can do something called a cross bleed start which is where the engine number one is being throttled up in order to get enough kinematic power to start the remaining engine number two so if you've ever been on an aircraft which have done this it's pushed back you've heard how the engine is kind of rubbing up normally to about 35 40 then the remaining engine has started that's why right that's because the apu hasn't been working so what about when you use the apu and the air done generally speaking under normal conditions we almost never start the apu in the air all right the apu is only used on the ground for the reasons that i've already said however if we have for example a really short runway but then we might want to use as much trust as possible from the engines and remember if you take blee there from the engines that's actually taking away a little bit of that trust so we can do something called a bleed off takeoff that means that we stop taking bleed air from the main engines all of that trust is being focused in order to give us as much power as possible during the takeoff but instead we take the air for the pressurization of the aircraft from the apu okay in that case we're using a special checklist because it's really important otherwise you might forget to switch the the engines back on as pneumatic sources and the apu can only be used as a sole pneumatic source up to 17 000 feet okay there are basically three different limitations the apu can be used as both an electrical source and the pneumatic source up to ten thousand feet it can be used as a pneumatic source only up to seventeen thousand feet and it can be used as an electrical source all the way up to the maximum certified ceiling of 41 000 feet and the reason for that the reason that you can use it as an electrical source that high up is because if you would have an engine failure if the engine would fail for whatever reason well then we need to have a backup electrical source that's the apu so we can start the apu all the way up to maximum certified ceiling what i should say though is that even though it can run all the way up to 41 000 feet you might not get it started up there so if you had an engine failure on your maximum certified ceiling then you need to do a drift down because you don't have enough trust to keep you up there i've done a video on that as well which you can check out and then you can try to start it this part checklist but if it doesn't start if you get a full like switch it back off again you know wait until you're down maybe to 25 20 000 feet or so and then you can start it again it's more likely that it's going to succeed on that second time so what can go wrong with the apu then well the fact is that most things that can go wrong with the apu is actually monitored by the apu itself so there's no need for the pilot to sit and constantly watch over the engine parameters if it feels that it has low oil pressure for example it will shut itself down and give us a fault light uh if it feels like it has any kind of other problem it will also shut itself down normally okay but as always when you're dealing with an engine of any sort um there's a possibility that it might catch fire okay and a fire in an aircraft is one of the worst things that can ever happen so there is a fire detection and a fire extinguishing system that is connected to the apu it consists of a single uh fire loop that's basically a temperature meter that's looping around the apu and if that one feels that the temperature exceeds a predetermined limit it will give us a fire warning the fire warning in the cockpit sounds exactly like you get for an engine fire you get the fire handle illuminated the apu fire handle which is in between the engine fire handles that illuminates you get the fire bell ringing that will definitely get your your pulse up that's for sure and then you get the two master fire alarms getting on when that happens the apu shuts itself down right without you doing anything the apu will already automatically shut itself down but we have memory items connected with an apu fire and the memory items are that we have to pull the engine sorry the apu fire warning switch then we have to rotate it when you pull it that will shut down the fuel to the apu it will shut the bleed air from the apu the electrical circuits will also open up it will close the the inlet door and a few other things and when you rotate it that's when you're actually releasing the uh the fire retardant into the apu and there's only one fire bottle for the apu for the engines there are two fire bottles actually they're only two available so you can either use one or two in one engine but if you use both of them one engine there will be no one left for the remaining engine uh but but that's what you do anyway and the last part of the memory item once you have pulled and turn the the fire handle is to switch the apu off and that is really a redundant thing because like i said it's already been turned off but you still have to do it as part of the memory items okay then once you've done that you hopefully the fire has gone out in any case you're going to try to go down the land as quickly as possible because whenever you've had a fire indication on board it is better to be safe than sorry get the aircraft down on the ground and land as soon as possible okay other things to know about the apu um well the shutdown sequence for example um normally when we shut it off once again it take care of itself so we just turn the apu switch to off well then that will initiate the shutdown cycle which begins with 60 seconds of cooldown and once that cooldown is over well then it's going to take care of shutting itself down completely by itself there's nothing that you need to worry about it will stop producing electrical power immediately when you switch it off so you better make sure that you have connected it to a ground power source before you switch it off all right well that's basically all i had about the apu guys now i am guessing that you still have more questions and if you do or you want me to explain other technical systems or other things that are connected to aviation well then let me know here in the comments below make sure that you tell me exactly what you think about the video if there's anything that i should be improving anything else i should be doing leave a like and make sure that you have subscribed to the channel as well so that you know when i do for example spontaneous live streams or when i have guests coming on or when i do a video about something that's happened but if you want to discuss with other aviation enthusiasts or other commercial pilots out there and you want to talk about technical issues for example or whatever it might be in aviation well then get the free mentor aviation app okay i have the links to it down here in the description of the video it's completely free to download it you can talk to me or anyone else or if you just want to have it to get the latest 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Channel: Mentour Pilot

Views: 346,497

Keywords: boeing 737, mentour pilot, how to become a pilot, pilot life, fear of flight, boeing 737 max, airbus 320, pilot training, cockpit view, APU, auxiliary power unit, auxiliary power unit aircraft, Fear of flying, nervous flyer tips, Nervous flyer help, Boeing 747, Boeing 787, Boeing 777x, Airbus A380, Airbus A350, tecnical aviation videos, aviation, flying, pilot secrets, cockpit secrets, boeing 737ng training, boeing 737ng cockpit, jet engine, how do aircraft work

Id: EkFE-ZiDJqA

Length: 18min 37sec (1117 seconds)

Published: Fri Jul 24 2020