How do jet engines actually start? crossposted
Posted by Colonelmann@reddit | aviation | View on Reddit | 133 comments
cross-posted
I was on an aircraft at Lexington that had been parked overnight. The pilots couldn't get the engines started because the APU wasn't running. A vehicle was brought over to assist engine start, but it failed. American Airlines borrowed a "jump start" vehicle from another airline.
After 20 minutes, each engine started, and eventually, we took off.
But then I wondered how aircraft engines are fired up. I also wondered if the plane has a set of keys the pilot receives before boarding the aircraft? Or does the pilot have a code to initiate the startup?
Wh hashtag: curiouspassenger
The sound of each engine starting up sounded brutal, lots of banging and rakcet until they finally got going. It did not sound good. Normal?
What did the jump-start vehicle actually do? apply electrical current or provide air pressure?
ArctycDev@reddit
The main question is answered so I'll just cover this bit
Nope, no keys, no code. The security at the airport and the knowledge of how to do it are the barriers.
Some small personal GA planes do take keys, but not airliners.
Foundrynut@reddit
A FedEx maintenance guy taught me how to fire up the APU on their 727s. This was so that I could fuel their aircraft. When he finished showing I asked is that the same process for the engines. Yes he replied. (Might have needed another switch flipped in the process. It’s been a while).
Fed Ex freight yard was at the end of the un way and a 727 is capable of power backing.
Wow. Nothing (except for morality or big balls) stood between me and a rogue flight of a 727.
ArctycDev@reddit
And possibly a tug, depending on where it was pointing lol.
I don't know the 727 startup process, but it really does essentially boil down to flipping switching or pressing buttons in the right order/timing
jlp_utah@reddit
As u/Foundrynut mentioned, the 727 is capable of power backing. Start the engines, engage thrust reversers, back plane up far enough to get out of wherever you're nosed into, disengage thrust reversers and away you go. No tug needed.
joesnopes@reddit
But apply brakes carefully when you want to stop going backwards.
Foundrynut@reddit
That would have been my undoing! I would have been ass planted in the freight yard, looking skyward and going nowhere. Thankfully my level of disgruntlement never got there.
aerocheck@reddit
Add enough power and it might push your nose back down :)
joesnopes@reddit
Probably. But will it take the dents out of the rear pressure hull? Will it restore the damaged outflow valves and drain masts?
ArctycDev@reddit
I suppose if you're stealing a plane, a powerback isn't off limits lol
blueflash775@reddit
Just remember mirror, mirror, mirror, look over shoulder. then you're ace.
homeinthesky@reddit
Also… don’t. Touch. The brakes. Repeat. Do not. Touch. The brakes.
MaverickTTT@reddit
Didn’t work out too well for that SkyWest pilot at SGU back in 2012.
Colonelmann@reddit (OP)
And I don't need a license?
Yussso@reddit
Let me check the law real quick.
Nope you don't need a license to steal a plane.
joesnopes@reddit
Good research! Google?
22Planeguy@reddit
The real security is the fact that there is essentially zero chance you get away with stealing an airplane. Once you steal one it's essentially a certainty that you'll go to prison, if you survive anyway.
eron6000ad@reddit
https://en.wikipedia.org/wiki/2003_Angola_Boeing_727_disappearance
Foundrynut@reddit
I had recognized that it would be my ultimate reconciliation of disgruntlement!
roguestephe1@reddit
Sanity is the biggest barrier. The massive balls are needed for the barrel roll…. RIP Sky King:
https://www.reddit.com/r/interesting/s/PjfJesmME4
I-r0ck@reddit
One thing to note is that unlike cars, GA planes share keys. The same model airplane might only have three different keys that it’s randomly assigned and they’re very easy to pick if someone wanted to.
dumptruckulent@reddit
I’ve always said the cobra is more important than the Huey and the evidence is that our canopy handles actually have locks. I’m not really sure who has the keys, but they must exist.
monroerl@reddit
The Huey has door locks (pilots doors) and locks for the cabin doors. It also requires a key (in and on) to start the engine. Keys have been lost numerous times. Crewmembers accidentally took home keys. Keys fell out of the logbook. All kinds of issues with needing a key to access and start a helicopter.
The CH-47 did not need a key to start the APU nor the engines but cabin door needed a key to open. There were ways to lower the cargo ramp without a key and gain access to the inside of the helicopter. Good luck starting the aircraft if you intend to steal it. It takes two pilots and at least one Crewmember about 20 minutes to start that aircraft properly.
We could do a combat start if prearranged but even that took around 8 minutes to get the engines running to 100% and all systems online.
dumptruckulent@reddit
Not anymore it doesn’t
monroerl@reddit
Yeah, I'd heard from them that it didn't do that at all unless the other thing was.
rocket_randall@reddit
I'm told that the Huey's anti-theft system is mast bumping.
Itaintall@reddit
and getting the generators paralleled.
dumptruckulent@reddit
Not on the yankee
serrated_edge321@reddit
Most military aircraft also have keys.
They found out the hard way that it was necessary. 🤭
IChurnToBurn@reddit
As we’ve seen in the past though, security and knowledge aren’t always enough to stop someone from stealing an airliner.
StandByTheJAMs@reddit
Absolutely true. OTOH, losing a set of keys could ground an airliner so there's quite a trade-off. 😄
pope1701@reddit
Imagine insuring a 200 million $ set of keys
ArctycDev@reddit
Right. RIP Sky King.
Express-Way9295@reddit
Is that Rick Russell?
ArctycDev@reddit
I only know him as Sky King, but apparently yes.
RhoadsGoneWylde@reddit
When I was in the Air Force we would always make the new guys go get the keys to the jet from the tool room as a joke. Good times.
Clockdistrict@reddit
Hopefully you specified you needed the left hand keys!
rmhallus@reddit
When I flew Eastern Airlines 727s, both pilots and flight attendants were issues a cockpit door key. That key would open any 727 cockpit door. The key was from Boeing. It would also work on any other airline’s 727 cockpit door if they had the lock installed.
Colonelmann@reddit (OP)
And TSA can use it on luggage
pylotsven@reddit
I have a Boeing key from my 757/767 days…it also work on my airbus
ArctycDev@reddit
Yeah I guess cockpit doors lock, but don't the just get left open?
agadir80@reddit
Is the cockpit door not locked when there's no pilot?
RadosAvocados@reddit
I've worked with a few carriers and only one had keys for the cockpit door that locked when the aircraft was unattended. It was one company key that each crew member had a copy of. All of the other carriers left the doors open.
ArctycDev@reddit
I can't speak for everywhere, but I am almost certain that yes, on the ground, the cockpit door is left open.
tantricbean@reddit
Commercial buses, at least the ones I drove a decade ago, were the same way. Right sequence of turning things on, then push a button or twist a knob.
iowanawoi@reddit
Gilligs are just push button start
Lampwick@reddit
Likewise with pretty much any military vehicle. Can't have missions compromised because a lieutenant dropped the keys to the commander's tank into the mud.
Colonelmann@reddit (OP)
Thank you!
a_scientific_force@reddit
I'll add, some smaller jet engines (think tiny business jets, or airliners with tiny business jet engines like the BAe 146) have electric starters. The most common turboprop engine (the PT6) also has an electric starter. But pretty much anything you'd fly on would start with bleed air from either an APU, an external huffer cart, or a cross-bleed from another running engine's bleed air.
Zealousideal_Put_501@reddit
I thought only the two inboard engines on the 146 had electric starters, and the outboard were air? It’s been like 20 years since I’ve worked on those…
Ancient_Mai@reddit
The Chinook uses hydraulic fluid to start the APU/Engines.
Colonelmann@reddit (OP)
Thank you
DueOwl1867@reddit
Everyone laughs but all if the us army aircraft have keys. It is funny. The airforce huey used to have keys. I dont know if they still do.
e28Sean@reddit
To answer your other question, airliners do not have ignition keys, nor is a "code" required. You just need to know the startup procedure.
Colonelmann@reddit (OP)
Do ramp agents start up aircraft in advance of pilots? Or pilots only?
ShaemusOdonnelly@reddit
It depends. Airliners are started by the pilots from cold & dark (if they're the first flight of that day). I fly cargo in the military and normal procedure for us is that the loadmasters or maintenance will hand over the aircraft to us with most of the systems running (except for the engines) and we hand it back to them at the destination/homebase in that state, and they shut it down. We basically only check that all of the systems are in the correct configuration, enter the flight plan, brief the departure and then we'll start the engines and depart. At the destination airport we shut down the engines, return the systems to a pre-start configuration and then hand them over.
Colonelmann@reddit (OP)
It seems engine startup is a rather involved process. Not like turning the key in a car and taking off
ShaemusOdonnelly@reddit
The actual engine start is just as easy as the engine start in a car. Turn on the ignition, flip the master lever and the engine will start automatically. Checking wether all the systems are working correctly and are configured as required, as well as programming the flight management system before we can start the engines is the actually involved process.
Colonelmann@reddit (OP)
Thx
Shadowrend01@reddit
Pilots only
The only time non pilots start the engines is when maintenance crews are doing engine checks post repairs
Colonelmann@reddit (OP)
I guess it was maintenence I saw starting up an aircraft. I saw the High-Viz vests in the cockpit.
soarer205@reddit
Pilots will wear high viz jackets when doing their pre flight walk around. It's possible they had just not taken their vest off yet or one planned to go back outside prior to leaving
Colonelmann@reddit (OP)
Makes sense
Ruepic@reddit
We’ve had maintenance guys taxi the plane off the gate to be parked, so not always for doing checks.
anonymous4071@reddit
my company has maintenance power the aircraft in the morning before the flight flight of they day. this is done so they can perform checks and make sure everything is operating normally to avoid any last minute maintenance delays.
In this case, they bring the aircraft up on the APU which provides electrical power and bleed air for air conditioning. they don’t start the engines. If the APU isn’t working, they’ll likely power it using a ground power connection.
murlyy@reddit
Air or electric starters start spinning the N2 (High Pressure Spool) until a certain percentage, it varies, maybe 10-25% N2. Then fuel is introduced into the combustion chamber and ignition. Once N2 reaches around 50% the starter is disengaged and then the engine accelerates on its own until idle. 55-65% N2.
Again all the exact parameters will vary depending on the engine but that’s the gist of it. Ignition will cut off with the starter then it just keeps itself running until fuel is cut off.
Colonelmann@reddit (OP)
Interesting
prof-bunnies@reddit
N2 for twin spool engines, used on most large high bypass ( in the US)... For single spool there is only N1 ( but not used on large engines since the 40-50's... Then you always have our Britt friends that do the triple spool so N3 for the large RR ( and I think maybe some eastern block folks too). For Bus Jets:(and turbo prop) it use to be single spool but most use twin spool now for higher power to weight.
dat_boi_whit_da_stik@reddit
T56/501 still single spool (except for a couple of the 501K industrial versions) and in widespread use with new ones still being built
Colonelmann@reddit (OP)
The gritty details! Thanks
SCGSKnucklez@reddit
This guy engines.
murlyy@reddit
Haha, been an AME for 12 years now. I just love the theory behind turbines. Suck, Squeeze, Bang, Blow.
AlternativeSweet9307@reddit
Once started it's: suck squeeze burn blow No bang after starting
SCGSKnucklez@reddit
If it burns you need to see a doctor.
Tangram11@reddit
That is quite funny because it is also the same process for a typical 4 stroke car engine. 😁
ASD_user1@reddit
Yep, the main difference between 2/4-stroke engines and turbine engines is working in a step-by-step cycle vs a continuous flow through all the steps.
subWoofer_0870@reddit
I always describe it as, “piston engines step through the cycle in the same place at consecutive times, while jet engines step through the cycle at the same time in separate places”.
glbtroters@reddit
We used to say suck, squeeze, burn, phooey to distinguish Brayton from Otto.
Technically no bang...
Also for OP in case its not clear enough. The air starter looks like a turbo charger. It is mounted to the gearbox. Air spins the the starter which, through the gears, spins the N2.
greatlakesailors@reddit
That's it for most planes. There are a handful of exceptions. To start an SR-71 you bolt two Buick Wildcat 400hp V-8s to a combining gearbox on a cart, then stick a driveshaft from that gearbox up into a socket in the bottom of the engine. Start both Buick engines, rev them both up to full throttle, they start the jet engine core spinning, then at 3200 rpm you start the fuel injectors and squirt a bit of triethylborane into the engine.
The suit worn by the guy who fills the triethylborane squirter tank is really quite something.
rayrevolveray@reddit
Never heard of an electric start cart, but will look them up.
We call the air start carts "huffers." And they are loud and are basically a small turbine engine in a cart.
lujimerton@reddit
Those blades need to be spinning fast enough so you dont have something called a "hot start" or "hung start". It's where there isn't enough airflow to create a sustainable start, and the engine sort of mushes or burns it self up.
it's anywhere from 10-20% of the RPMs its capable of before you can start pouring sparks and fuel to it.
They start by whatever can spin that. It could be electrical. Usually it's high pressure air, which can be supplied by a smaller engine called an APU, a ground cart that supplies high pressure air,. Sometimes its hydrazine in the military which is super toxic but works really well, sometimes its air off the other engine called a cross bleed start, it can be and sometimes its wind milling it at 23,000 in the case of a flame out.
whatever it is, you need those compressors spinning to give the engine the airflow it needs to light, and not melt itself in the process.
Conceptually think of critical mass. you need a certain amount of momentum however you get it to get a sustainable reaction. It's totally different, but its a metaphor. APU's are kind of like a pilot chute that pulls out the main chute. Small enough to start with electricity, and powerful enough to blast engines with enough high pressure air to spin them.
Colonelmann@reddit (OP)
Really interesting! Another commentor sent a YT video link. I watched it and they alluded to this concept but didn't go in to detail like you did here. Thank you
nukii@reddit
If you ask the engineers that model the things, they think it just begins life at idle and everything before that is someone else’s problem. Not bitter.
Dapper-Bar1746@reddit
The APU provides compressed air to the ATC. Air Turbine Starter ... turn-n-burn. Usually a ground cart provides the air ,,,
Condurum@reddit
Basic physics of it, simplified a bit.
You need very high compression for the turbine to work, and thus drive the compressor. Basically, all of this needs to spin really quickly before it’s possible to attempt to light it.
This heavy shaft, with many compressor blades, takes A LOT of power to get going, and usually external compressed air is provided to a geared mechanism to start the engine from either: The APU (small auxiliary engine in the tail), or a start cart which is just a huge diesel compressor motor, or from another source (maybe another aircraft). The 2nd engine of the aircraft usually gets its compressed air from the first engine.
To add: There are smaller aircraft engines that can be started with electric motors, and some military aircraft can be started in emergencies with small charges, almost like a shotgun shell.
Intigracy@reddit
Suck Squeeze Bang Blow
Colonelmann@reddit (OP)
Well put. Thank you
Condurum@reddit
If you want to get into jet engines, Agentjayz is amazing, and have answers for every question you can imagine.
https://youtu.be/g4pqEzfKXcA?si=1A_ASlG5osiJ5Wiy
Colonelmann@reddit (OP)
I'm watching it now! Thank you
CelendilAU@reddit
This video series is a fantastic series of videos that covers the entire start process - focusing on the engines of a 777: https://youtu.be/AdCcbBhondA?si=eYKjb_YXvfeJ8qt6
Colonelmann@reddit (OP)
I'll watch now. Good timing. Thx
Several_Hospital_129@reddit
Ah, the memories. I used to work for United Express out at DEN. We would arrive in the morning on the employee bus 🚌, with the planes towering over us. The planes were empty, waiting for their crew and passengers. I always wondered how you started them up.
Colonelmann@reddit (OP)
Remote starter fob from the pilots break room
SuperFrog4@reddit
Pretty simple actually.
The engine has a front fan (compressor section), an ignition (combustion section), and an exhaust area. Additionally, there is an auxiliary power unit or APU (small engine) as well.
Engines work by blow air into via the can and compressing that air so it heats up. Once it is warm enough it enters the combustion section and ignites with the fuel. The resulting hot gas exits through the exhaust area that has fans on it as well. That exhaust spins the fans which are connected to the front fans that compress. Additionally that hot exhaust exists as thrust to push the engine forward.
So how do you start the engine. You use the APU to provide the combustion section with compressed hot air to start the ignition process which then spins the exhaust fans that spin the front fans which then compresses air to keep the engine going.
So the APU is a little engine that blows air into the main engines into the ignition sectio
Colonelmann@reddit (OP)
This makes sense now
xyeahtony@reddit
In a very simplified way: jet engines turn on by having the turbine spin fast enough to ignite. This happens by using compressed air which is generated by the APU. Now some fancier engines in modern planes (like the 787) have electric start instead of "bleed air"
Colonelmann@reddit (OP)
👌
LethalDan@reddit
Not too much to add to the on ground engine starting stuff. One cool thing about jet engines is you dont need that starter to go fly. The odds of the engine failing are so insignificant. The jet is certified to fly safely on the remaining engine. Lastly, if the engine somehow gets shut off when you are altitude, you can start descending to get your airspeed up, and you can restart the engine just with the air rushing in
Colonelmann@reddit (OP)
Like rolling an old standard transmission car and popping the clutch.
Far-Plastic-4171@reddit
Pair of Buicks start the SR-71
Colonelmann@reddit (OP)
Good grief!
MoiraRose2021@reddit
Suck squeeze bang blow.
Colonelmann@reddit (OP)
It was the bang part that's a bit startling
davidb4968@reddit
Yardi breeze training
anonymous4071@reddit
Completely normal for a huffer cart. They’re loud and obnoxious. They’re doing the same function as the APU, generating compressed air to spin the starter. Ground crews start them before we’re ready to start the engines to make sure it’s working properly and supplying enough pressure. They make a terrible racket until we begin the start and the start valve opens allowing the air to move, then get quite loud again when the start valve closes.
Automatater@reddit
About like a aircraft or car piston engine. If you get it rotating and it has air, fuel and ignition, it'll just start going by itself.
Ancient_Narwhal_9524@reddit
On larger aircraft they mostly use an air starter. It’s just a motor powered by compressed air. Usually this is provided by the APU, which can start off the batteries or an external electric power cart.
If the APU doesn’t work they need an air cart that is pretty much an APU on a cart. Some are also a very large air bottle and a compressor. It has a hose they attach to a port on the aircraft. Once an engine is started they can do a cross bleed start from the operating engine once away from the gate. You usually have to throttle up a bit to get enough bleed pressure to start the other engine.
RumSwizzle508@reddit
The usaf used to be able to start all the b-52 engines using explosives, to quick scramble jets on nuclear deterrent.
RAAFStupot@reddit
So did some engines on 707s
rockfordstone@reddit
Cartridge starts were not uncommon on cold war jets. Quite a few used to use them. Quite fun to watch
railker@reddit
And some pistons, too.
Ancient_Narwhal_9524@reddit
Later they put 2 APU on the KC-135 so they could start 2 engines at once. The APU start switches are right by the crew door, so they will be running by the time the crew hops in their seats.
BoringBob84@reddit
True. The 787 is the exception. It has electric engine start.
discombobulated38x@reddit
And also uniquely, the Trent 1000 on the 787 is the only Trent to start from the IP spool not the HP spool.
BoringBob84@reddit
Interesting. You obviously know more about jet turbine engines than I. I know that there are fires and fans in there. 😉
Colonelmann@reddit (OP)
It sounded like this is what was going on. The booster cart stayed in place. Thank you
xXCrazyDaneXx@reddit
https://youtu.be/bYjQ9fzinT8
grasib@reddit
That's quite a cool video, linking the sounds to what is actually happening.
Question tho. I see the white smoke, which you see in the video around 5:05 quite often. I assumed it is from the fuel injection and light up. In the video, however, this only seems to happen at 5:52.
What exactly is the white smoke caused by?
ASD_user1@reddit
The video labels that as when the fan connected to the N1 shaft, and the way the white dissipates makes it look like that may be moisture in the engine or ambient air being pressurized and blown out while the engine is still cold. The rapid pressure changes before introducing heat might be the cause (watch a video of a fighter jet maneuvering aggressively at low altitude, you will see the same color of white off the wing tips).
mjdau@reddit
New Pope just dropped.
Colonelmann@reddit (OP)
I'll watch it rn. Thank you
MaleficentCoconut594@reddit
Jet engines need 3 things to start: fuel, sparks, and air flow. Not just air, air FLOW. A jet engine is essentially a vacuum cleaner. Unlike a cylinder engine from a car that has cycles (strokes), a jet engine is an instant stream of, as we colloquially call it, “suck-squeeze-bang-blow”. Air is sucked it by the N1 (big fan you see from the outside), most of that air actually just flows around the engine core and is expelled out the back (hence the name “high-bypass turbofan”). The air that does go through the core is then compressed (squeezed) by the N2 stage before moving into the combustion chamber where fuel and sparks ignite it and it’s blown out the tailpipe. In the way, that exhaust spins another blade that is connected to the same single shaft as the N2 and N1, faster that spins the faster the rest spin etc etc which is why it takes a minute to fully spool up an engine. Once an engine is started, as long as there is fuel it’s self sustaining
Now, in order to get that airflow to spin that shaft, you need either the APU or an external air cart, both of which just force flow air through to get the shaft spinning and start compressing something in the N2 for combustion. This is why when they start engines you may notice the cabin air flow stops or is vastly decreased, that’s because all of that air flow is being redirected to the engine they’re starting
paulomario77@reddit
Do jet engines have critical speeds that must be avoided by means of an acceleration curve? I'm familiar with that on centrifugal compressors.
railker@reddit
Not sure about jet engines, listening to the LEAP-1B start it certainly sounds like there's a number of 'stages' it gets to before advancing to the next one through the start, takes forever.
On the Dash 8 (and I imagine ATR and other turboprops), there is exactly that. When feathered the propellers run at about 220-290 RPM. Once you unfeather and get into the flight idle-power regimes, the range is from about 785-1050 RPM. There's specific warning to avoid propeller operation between 290-785, I forget specifically why other than 'bad things happen'. And I can't remember if that's something you can even manually achieve.
legrenabeach@reddit
There are videos like this one showing how a cold start works from the cockpit POV.
I don't know the technical specifics though.
External-Creme-6226@reddit
That takes you from cold dark to apu running and powering both the a/c and electric. Does not include an engine start
Colonelmann@reddit (OP)
I'll watch it!
capn_davey@reddit
Regardless of how the spinny things start spinning, in a modern jet with FADEC I can almost guarantee you there’s some button labeled “engine start” somewhere. Because you can melt some really expensive bits if you mess up starting a jet and the goal is to make the minimally trained monkeys up front become even more minimally trained.
zylpher@reddit
IIRC the start up procedure for A+ Hornets correctly.
Battery on, APU start and wait for it to come online. Eng crank left. Wait for it to to come up to speed. Eng crank right. It's been since 2007. I'd need a checklist to get further and make sure il right.
All done of the left side panel with toggle switches.
rygelicus@reddit
Spin it to get airflow going through the engine, this sets up the different pressure zones within the engine.
Turn on igniters to spark the fuel that is about to arrive.
Turn on the fuel.
Because that pressure differential is already established by spinning the turbine, even if small, the fire will take the easiest escape route as its expands, which means going out the exhaust. This causes the turbine to spin faster, greater pressure differentials, more fuel, more fire, more noise, time to go.
Colonelmann@reddit (OP)
Making sense now
Comfortable-Dish1236@reddit
There are no keys and no codes.
The APU supplies bleed air to power the start valve that spools up the compressor fast enough to allow fuel and ignition for engine start. If the APU is inop (or the APU may be operable for electrical generation but the APU bleed air is inop), it will require either an air start cart, a portable GPU (ground power unit) or both. Once power is supplied and air is available, all that’s required is activating the required switches for engine start. Both engines can be started at the gate, or one can be started, and on taxi out, the operating engine can supply bleed air to start the other engine (crossbleed start).
jsmeeker@reddit
Huffer cart start. Cool
Colonelmann@reddit (OP)
My knowledge is expanded.