How A Jet Engine Starts

your pins are checked onboard override projections so the receiving checklist is complete sir all right - before start push all right I bet every pilots wanted to do this pick up sorry engine number two I'm gonna go to run here gonna go start for two seconds in a startup time there we go we got into percentage all right hey even if you don't want to fly a jet for a career it would be nice to start a jet at least once and maybe push the throttle up jet engines are usually a lot easier to start than a prop and the process is pretty simple it comes down to lots of air under pressure some fuel and boom you're let the tough part is getting enough compressed air to understand how I term an engine starts you should understand how it works they run off a suck squeeze bang blow principle take this generic turbofan engine like the one that powers the ERJ it sucks air through the intake and then it sends the air through a fan compressing it in the ERJ the fan is a single stage one row of blades then most of the air goes around the engine this is called bypass air we use this for most of the engine's thrust and for cooling the rest of the air goes through the hot section the part of the engine that burns air traveling through the hot section gets compressed even further by passing through 14 rows or stages of compressor blades at the end of the compressor section the highly compressed air which by now is extremely hot from all of that compression passes into a diffuser where it slows down and stabilizes then it moves into the combustion chamber where it mixes with fuel and that's where it burns normally the fuel air mixture and a jet engine burns constantly unlike in a reciprocating engine some people call this the bang section but I like burn better because it makes it easier to understand that the fuel air mixture it's always burning it's not a series of individual explosions when the fuel burns the air heats up but this time it can expand and it's forced out through the turbine blades in the ERJ there's five rows called stages the first two are called high pressure turbines in the last three are called the low pressure turbines the passing air spins a turbines that drives a shaft connected to the fan of the compressor blades spinning them and then after passing through the turbines the hot air leaves the engine generating a little more thrust so air passing through the combustion chamber needs to be highly compressed to handle all of the fuel that mixes in if you can't get enough compressed air you can't burn enough fuel and the turbine is not going to light so to start a turbofan engine you need to spin the compressor fast enough to start pushing compressed air into the diffuser and then into the combustion chamber simple right not so jet engines have spark plug like igniters that initially light the fuel air mixture and they keep it lit in turbulent or wet conditions during a start sequence the engine core needs to spin at 14% of its maximum speed before the igniters begin to light the engines core speed is called n2 and it's expressed as a percentage of maximum RPM on the ERJ 100% of n2 is roughly 16,000 rpm so the engine needs to reach 2200 rpm before the igniters start firing so you're not going to hand prop that and then the core needs to spend up to 28.5% of n2 over 4,500 rpm before the engine can introduce the fuel and light the mixture and when it adds fuel it does so at about 200 pounds per hour that's roughly half a gallon each minute so you need a lot of compressed air coming into the combustion chamber to handle all of that fuel smaller turbines like many turbo props they're light enough to start using an electric battery and motor but even a regional jets engines are too large to start using that electric motor so instead we take pressurized air and run it through an air turbine starter that's another small turbine and it's attached to the side of the engine the air spins the air turbine starter which is connected to the engine shaft through a clutch so spinning the air turbine starter in turn spins the engines compressor and you rely on that system until the engine reaches 28.5% of n2 and it lights then the engine powers itself so where does the compressed air come from for most civilian turbines it comes from one of three sources the most common is the AP or the auxiliary power unit it's a small turbine engine usually located that provides compressed air and electricity it powers the aircraft's electrical systems on the ground it supplements compressed air and electricity while flying and it acts as an all-around backup for electrical and pneumatic needs and it can send compressed air to both of the engines air turbine starters it's small enough to start with a battery so when you normally start the jet you first start the APU simple enough what happens if it's broken now this happens a little bit more often than you'd actually think in that case you'll need an external source of compressed air and the most common is the hovercard it's an air compressor that hooks have to the side of an aircraft and it provides compressed air for engine starting it's officially called an air start unit but no one really calls it that so this is an external start pretty good start number two all right pickup sorry engine number two I'm gonna go to run here I'm gonna go start for two seconds and a start time there we go we got into percentage all right ignition starts at about 17 percent roughly should get up to about twenty-eight percent and we'll see a fuel flow and a light off you'll flow light off and rn1 which is the big blade up in the front starting to spin it about seven and a half percent now now you hear a bunch of clicking at about fifty six point four percent and two and that will be all of our buses popping out of the generators that looks good good and now that the engine 2 is up we need to start engine 1 but we don't need the Kart anymore we can actually use engine 2 for that turbine engines bleed off bypass air for aircraft systems like the heated di systems in the cabin pressurization system and we can route compressed air from engine 2 to the air turbine starter on engine 1 we simply open the bleeds on engine to open the cross feed to engine 1 o Allah ng ones ready to start like this ok now to start engine number 1 for the bleed air from engine number 2 yep we will run up number 2 up to roughly 83% and - that provides enough pressure the 14 stage bleed valve does push through our system we have the cross beads open so the piping between the number 2 and number 1 is completely open so when that pressure comes through it's going to go through the system and actually gonna spin our number 1 engine up to 56 point 4 will become self-sustaining alright number 2 is coming up again here we go alright go ahead alright 80% engine number 1 moving the start/stop selector to the run position momentarily start for 2 seconds this is where to start our time and we're starting to see the n2 rotation on engine number 1 egytian a is on across the evil trooper there's light on feel oh there goes n1 so I'd 1 start to spin up just nice we have a time limit of 1 1 minute and that's a limit on the air turbine start of the 80s it gets incredibly hot that air that we're taking off the 14 stage believe it is really hot and so it's a structural limitation on the air turbine starter side of the edge that's fine that is associated with a number one running so we get a maid we have a we get a master warning if we haven't made any main service door or main cabin door opened with number one running that's because that's where passengers if they're going to disembark the air point they'll go off the main cabin door and obviously with number one running that is a definitely a hazard at the 121 apartment absolutely so it could get a good stabilized engine will bring the power back down so now we have two perfectly good idling engines once you get the engines started you need to check a couple of indications quickly unless you want to burn up the engines core but we'll show that in our next video you

Channel: Boldmethod

Views: 4,671,630

Keywords: aviation, flying, airline pilot, airline jet, embraer regional jet, boldmethod, ifr, engine, jet engine, turbofan

Id: GzhdxSsoT0g

Length: 9min 4sec (544 seconds)

Published: Fri Nov 11 2016