When starting a turbine engine, how do you know its started?

beasly

beasly

Well-Known Member
I listened to one start up yesterday and its always bugged me. With a piston engine you know it starts by the sound and noise. With a turbine, its just a whine that gets louder. How do you know when to take your hand off the starter?
 
In the Citations i fly these days, we have a battery master that is turned on, allowing power to be available to the start CBs. We push a button labled LH or RH Start, which initiates the start sequence. When the button is pushed it illuminates showing that the power relay is now closed allowing power to the starter. It also powers the electric fuel boost pump (gotta get that fuel moving to the engine) then arms the ignition switches (gotta make a spark somehow) while starting a starter/generator which gets the N2 section goining. And for some reason the cockpit flood lights also come on full bright. When we have reached a certain N2 % I bring the associated throttle out of cutoff and into flight idle, which then introduces fuel into the engine. With fuel, rotation and ignition, the engine lights off and the n1 begins to spool up. At around 48%n1 (on a few of my airplanes) the start sequence is terminated by a speed sensing switch and the starter generator goes from being a starter, to a generator as evident by the load visable on the associated amp guage. In addition, the fuel boost pump is powered off and the exciter box for the ignition is no longer on.

Someone once said it was like "releasing the dogs" when you pushed the start button and the speed sensing switch recalled them.

Short version - after we push the button it spools up and stabalizes at a certain N1 :)
 
MikeOH58 said:
You are kidding right?
Click to expand...
No need to be condescending. I can see why this is a bit of a mystery to some folks, especially if they've only flown piston. Turbines are a bit of a mystery to folks... it's just a cylinder of fire, noise, and magic. I'd venture to say that quite a few pilots that have never flown turbine (and even more of the general population) have very little idea of how they work, much less start or know how the start process works.
 
In the canuckjet you configure the bleeds, turn off the packs, turn on the ignition, hit start, watch N2 (core) spool up to 20%, make sure N1 (fan) begins turning, wait for ITT (inter turbine temperature) to be 120*c or below when you can introduce fuel. After that you watch everything increase while monitoring the temperature. at 55% N2, the starter should cut off automatically. The start is considered complete once everything is stable. The ITT increases somewhere to the 600* mark on most starts and then begins to fall. Once it falls to a more normal range you're pretty much done.
 
beasly said:
I listened to one start up yesterday and its always bugged me. With a piston engine you know it starts by the sound and noise. With a turbine, its just a whine that gets louder. How do you know when to take your hand off the starter?
Click to expand...

Primarily you are looking at the instruments. In the Van, you know because T5 decreases back down to a stable temp, and Ng stabilizes at 48%. There is a noticeable change in the sound as well.
 
Slight roll back and stabilized EGT start temps and stabilized N1/N2 numbers (around 28%/65% respectively). FADEC's make engine start pretty easy.:)
 
You can kind of feel it in my aircraft, and you can definitely hear it......there is a certain noise when it's cranking with the throttle in off/cutoff, another noise (very distinct) when you push the throttle to idle and add fuel, and then everything quiets down when it reaches idle. That and all the engine parameters stabilize and the associated gen comes on-line. Similar for the other turbine aircraft I have spent time flying.
 
MikeOH58 said:
You are kidding right?
Click to expand...

No, I honestly don't know. I was listening to a military trainer aircraft start its engine as I was passing by and I tried to hear when the engine was started. There was no definitive 'clang' like you get with a piston.
 
Part of the difference is just that a piston engine cranks over a couple times and then comes right to life (at least if you aren't hot wx starting an FI piper :) ). With a jet, you have to first get the compressor spooled up enough to provide the compression you need for combustion, then you have to introduce fuel and wait for it to light off, and then it has to spool up to ground idle. So the process is a little less binary than starting a piston engine. With that comes some of the start malfunctions common to all turbines.....hung start (engine spools up slowly without reaching ground idle), hot start (high EGT often the result of a prolonged hung start), and no start where you are dumping in fuel but it is not lighting off, so the rpms just stay at whatever the gpu/APU normally holds them at.
 
We have -12 Garretts... power lever on the gate, prop lever in low.. fuel on and hit the start button. Verifly SLR light off and ignition on at 10% RPM.. should light by 15%. EGT and RPM climb, the EGT peaks between 640 and 700 and drops to around 400 and the RPM is around 64%. Turn the start button off reset and turn the generator on. Oh ya.. also watch all the hanger and cargo doors close as you walk people to the airplane.
 
To start the 190 you turn a knob just as you would turn the key to start in your car. After you turn the knob the fadec works its PFM.

You monitor the start and know that the engine start is "stable" ie, done with its PFM by:

N1 ~21%
ITT ~480C
N2 ~63%
Fuel Flow ~450 lbs/hr
Oil Pressure greater than 25 PSI
 
In the King Air 90 we have the starter/ignition switch on until the N1 stabilizes at low idle, which is usually between 50%-54% N1
 
beasly said:
I listened to one start up yesterday and its always bugged me. With a piston engine you know it starts by the sound and noise. With a turbine, its just a whine that gets louder. How do you know when to take your hand off the starter?
Click to expand...
In the ERJ, it's when I hear "click. Click. CHUNK. CLICK. WHINE. CLICK" and the packs come back on and the engine parameters stabilize around 20ish percent N1, 400ish ITT and 60ish percent N2. The airplane (the FADEC) considers the engine to be running at 56.8% N2 and commands the air turbine starter valve to close, along with a few other actions including turning off the associated electric hydraulic pump with the switch in AUTO, and connecting both of that engine's generators to the associated DC bus (unless the GPU is connected and selected).
 
USMC-SSGT said:
To start the 190 you turn a knob just as you would turn the key to start in your car. After you turn the knob the fadec works its PFM.

You monitor the start and know that the engine start is "stable" ie, done with its PFM by:

N1 ~21%
ITT ~480C
N2 ~63%
Fuel Flow ~450 lbs/hr
Oil Pressure greater than 25 PSI
Click to expand...


LOL! I assumed that PFM meant 'Pure effF'ing Magic' and was gonna make a snide remark about it, thought better of it, decided to take the high road and looked up PFM on wikipedia. sure enough! :rolleyes:
 
Very interesting answers, all. Thank you!

My current working definition is that with a piston engine, the start-up sequence is just a forced cycle of the normal run cycle, while with turbines, you have more of an ignition sequence leading to the steady state burn. It makes a lot of sense to me now.

Thanks again.
 
It helps to think about what's going on inside the respective engines during start. Turbine engines handle the "suck - squeeze - bang - blow" sequence in a much simpler and more elegant fashion.

A piston engine is really pretty complex and has metal parts flinging every which way when it runs. A turbine engine just spins from one central axis.

So, the start sequence is just: spin the turbine to start moving air, start dumping fuel into the combustion chamber and ignite it with sparks. The flame will eventually sustain itself as long as it's fed fuel, and the compressor / turbine rotation stabilizes at a given RPM based on the amount of fuel that gets dumped in.
 
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