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rjsampson
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CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Mon Oct 02, 2017 8:08 pm

My understanding of the the 737 engine startup checklist involves pushing APU air to the starter, monitoring N2, and then manually introducing fuel with a lever a dedicated lever, under the thrust lever, to achieve light-off.

I've heard that the MAX will eliminate this relic of a step from the Jurassic period, as they have so done on pretty much every subsequent product they have been delivering to customers, since the 737-100.

I don't know what the minimum N2 needs to be to push forward the fuel levers (and too lazy to research it): But what could be the consequences of introducing fuel with igniters, before the appropriate minimum N2, as this is process is not automated, and susceptible to human factor error?
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Mon Oct 02, 2017 8:38 pm

I don't know about the 737, but on other engines you'll generally get a hot start if you introduce fuel too early. I think on the early turbojets I flew we needed 15% (maybe 20) before we could introduce fuel or open the condition lever.
 
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Mon Oct 02, 2017 9:59 pm

The limitation states that fuel can't be introduced until 25% N2.
 
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Mon Oct 02, 2017 10:01 pm

On the max there is still a fuel cutoff switch that has to be moved to introduce fuel, it isn't a lever on the max or NGs manufactured for about the last year or so.
 
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Mon Oct 02, 2017 10:47 pm

rjsampson wrote:
My understanding of the the 737 engine startup checklist involves pushing APU air to the starter, monitoring N2, and then manually introducing fuel with a lever a dedicated lever, under the thrust lever, to achieve light-off.

I've heard that the MAX will eliminate this relic of a step from the Jurassic period, as they have so done on pretty much every subsequent product they have been delivering to customers, since the 737-100.

I don't know what the minimum N2 needs to be to push forward the fuel levers (and too lazy to research it): But what could be the consequences of introducing fuel with igniters, before the appropriate minimum N2, as this is process is not automated, and susceptible to human factor error?



Not true. Redbellyguppy is correct, the new start levers, or "fuel control switches", have been introduced on the -NG since Nov 2015. These still require the same manual input as the -NG/-CL. There is different logic on the MAX for bowed rotor motoring, but still requires the flight crews to move the levers to "idle". https://goo.gl/images/WTJyYk

You can get a successful light-off in certain conditions with minimum of 20% N2. Under normal conditions its 25% N2, however it may not always be possible to achieve that. If you try below 20% N2, you may get a hot start, which the EEC can terminate the start sequence.
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Whiteguy
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Mon Oct 02, 2017 11:47 pm

25% N2 or when it stops increasing.....it has happened before where the lever or switch has been flipped below 20% and it's started with no issues.
 
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Tue Oct 03, 2017 12:56 am

Thank you so much for your answers. I had no idea that the 2015+ production 73-NGs (if I understand correctly) removed the fuel levers.

So 25% N2 is pretty much ideal to light off a CFM on a 737 (kinda what I assumed) -- or otherwise when N2 stops increasing from the APU pressure.

I'll be honest: I don't know the implications of a "hot start." Could such an event potentially have an adverse effect on the turbines? (eg, "roasting" them?) Could this be something that would require termination of the start sequence, and perhaps subsequent inspection of the hot section, and a required change of equipment?
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
Max Q
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Tue Oct 03, 2017 2:14 am

The answer is yes to all the above
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Rexus
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Tue Oct 03, 2017 2:58 am

Hi,

rjsampson wrote:
I'll be honest: I don't know the implications of a "hot start." Could such an event potentially have an adverse effect on the turbines? (eg, "roasting" them?) Could this be something that would require termination of the start sequence, and perhaps subsequent inspection of the hot section, and a required change of equipment?


here is an extract from a pretty good website on the B737:

http://www.b737.org.uk/cfm56_soi.htm wrote:
Unsatisfactory Starts

If a unsatisfactory start should occur, it will most likely be accompanied by one of the following conditions :

Caution : THE EXHAUST GAS TEMPERATURE (EGT) HAS A DIRECT EFFECT ON THE SERVICE LIFE OF ENGINE HOT SECTION COMPONENTS. EXCESSIVE AND REPEATED HIGH EGT ENGINE OPERATION RESULTS IN PREMATURE ENGINE DETERIORATION.
A. Hot Starts

A potential hot start is indicated by an abnormally rapid EGT rise after light-off. By monitoring fuel flow and EGT, a hot start can be anticipated before the 725 C limit is exceeded.

Hot Starts may be caused by :
(a) Inadequate starter air pressure, resulting in N2 too low to provide sufficient compressor air flow.
(b) Faulty starter valve action, preventing proper operation of starter, with same result as item (a)
(c) Premature starter deactivation
(d) Incomplete purging of fuel in the combustion chamber from the previous start attempt.
(e) Foreign object damage (FOD) preventing sufficient engine acceleration and airflow.
(f) Faulty pressurizing valve (hung open) resulting in fuel, under low pressure, puddling in the combustion chamber before light-off.
(g) Faulty main engine control (MEC) resulting in incorrect start fuel scheduling.
(h) Incorrect scheduling of variable stator vanes (VSV)



Also, here is a video on how a hot start looks like on the central upper display unit: https://www.youtube.com/watch?v=im_WKy0bdOk
 
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rjsampson
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 3:06 am

So for you 73 drivers: Do you push the fuel levers up based on N2 RPM or EGT Temp?
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 5:35 am

rjsampson wrote:
So for you 73 drivers: Do you push the fuel levers up based on N2 RPM or EGT Temp?

Not a real 737 driver but I have studied the FCOM: It is always based off of N2.
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rjsampson
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 6:21 am

speedbird52 wrote:
rjsampson wrote:
So for you 73 drivers: Do you push the fuel levers up based on N2 RPM or EGT Temp?

Not a real 737 driver but I have studied the FCOM: It is always based off of N2.


Haha what was I thinking... of course you gotta light off on N2 (I mean, how can there be EGT without combustion?)

Max Q: I am very curious what the repercussions of a Hot Start could be. From, say:- "no biggie, let's shut her down, let her cool, and crank it again,". ... to "oh boy: We gotta call dispatch and get another aircraft to transport our passengers"

Thoughts?
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
Max Q
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 6:44 am

If you’ve exceeded the start limit the engine Is going to have to be inspected

Depending on how far above the limit you’ve gone maintenance will decide whether it will have to be borescoped


If you’re lucky the engine will not have permanent damage, if not it may literally be trashed, damaged beyond repair and we’re talking millions of dollars to replace it


I always believe in max motoring (within starter limits) to get the coolest start possible


I figure if I take care of the engines I stand a better chance of them not failing when I need them most!
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 8:12 am

You use the N2 rpm because that is what the starter is turning. The N1 is much slower to spool up. The start is one of the most critical phases as an engine can be damaged beyond repair if the temps are exceeded. If you put fuel in too early you might also get a hung start but as long as the temp limits aren't exceeded you can try a a second start. There is also the starter duty cycle limit so you wouldn't want to motor the engine too long before putting in the fuel.
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fr8mech
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 12:03 pm

rjsampson wrote:
I am very curious what the repercussions of a Hot Start could be.


While, I don't normally use absolutes when dealing with aviation, I feel comfortable in writing this: a hot start ALWAYS results in some kind of inspection from maintenance. The inspection may be simply interrogative in nature (e.g. accessing EICAS to determine temperature and time) or a hot section borescope inspection followed by an engine change.

Now, it is my opinion, as an engine run & taxi instructor, that a hot start can usually be avoided by an attentive engine operator, whether that operator is flight crew or maintenance crew. A hot start is usually proceeded by a...wait for it....impending hot start. A rapidly increasing EGT without a corresponding increase in N2. A start should be terminated before the EGT reaches the red line. Sometimes, with a tired engine, you get close, but you should be able to avoid the hot start.

As for fuel introduction...we've always trained to introduce fuel at max motor (the highest N2 the starter can provide) or the manual minimum, whichever is greater.
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 2:32 pm

rjsampson wrote:
speedbird52 wrote:
rjsampson wrote:
So for you 73 drivers: Do you push the fuel levers up based on N2 RPM or EGT Temp?

Not a real 737 driver but I have studied the FCOM: It is always based off of N2.


(I mean, how can there be EGT without combustion?)


Quick technicality. You will see a EGT rise with the N2 shaft spinning because adiabatic compression of a gas also inherently has a rise in temperature. That being said, the rise in EGT would be proportional to N2 speed (higher N2, more compression, more temperature rise in EGT), so the driving factor is still N2 speed.
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FlyHossD
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 3:20 pm

akiss20 wrote:
rjsampson wrote:
speedbird52 wrote:
Not a real 737 driver but I have studied the FCOM: It is always based off of N2.


(I mean, how can there be EGT without combustion?)


Quick technicality. You will see a EGT rise with the N2 shaft spinning because adiabatic compression of a gas also inherently has a rise in temperature. That being said, the rise in EGT would be proportional to N2 speed (higher N2, more compression, more temperature rise in EGT), so the driving factor is still N2 speed.


That's technically true (the EGT rise from compression alone), but it's very small and nowhere near the EGT rise experienced from combustion.
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yeelep
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 5:22 pm

The start lever on the Jurassic and Classic were operating cables to the engine, as well as switches. On the NG the cables were eliminated and the start lever is connected to six switches in a brake assy. that simulates the cable operation of the earlier planes start levers. The start switch on the latest NG's and MAX has eliminated the brake assy. and just has six sets of contacts within the switch.

Starting:
Jurasic: 15% N2 min, 20% recommended.
Classic 20% N2 min. 25% or max. motoring.
NG 20% N2 min. 25% or max. motoring.
MAX ? I don't know.

Quick technicality. You will see a EGT rise with the N2 shaft spinning because adiabatic compression of a gas also inherently has a rise in temperature. That being said, the rise in EGT would be proportional to N2 speed (higher N2, more compression, more temperature rise in EGT), so the driving factor is still N2 speed.

That is only true for an engine that is cold. During normal ops (turns) the core will still be above ambient temp. and therefore the EGT will decrease during motoring before light off.
 
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 6:39 pm

akiss20 wrote:
You will see a EGT rise with the N2 shaft spinning because adiabatic compression of a gas also inherently has a rise in temperature.


Not to get into a pissing match, but by the time the air has reached the EGT probes, it has already expanded through the diffuser, combuster and across the the HPT...and, depending on engine, the LPT.

I have seen EGT driven lower during the start process, especially when the engine is still warm or in high ambient temps, but I can't really recall seeing it go up. Doesn't mean it hasn't happened.
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 7:33 pm

fr8mech wrote:
akiss20 wrote:
You will see a EGT rise with the N2 shaft spinning because adiabatic compression of a gas also inherently has a rise in temperature.


Not to get into a pissing match, but by the time the air has reached the EGT probes, it has already expanded through the diffuser, combuster and across the the HPT...and, depending on engine, the LPT.

I have seen EGT driven lower during the start process, especially when the engine is still warm or in high ambient temps, but I can't really recall seeing it go up. Doesn't mean it hasn't happened.


Also not trying to get into a pissing match, but I want to clarify some things. Diffusion or expansion of a gas through non-rotating components such as the combustor (the combustor would actually diffuse the flow, not expand it) do not change the stagnation temperature, only the static temperature. Unlike pressure, it is near impossible to measure static temperature (it can only be computed through the Mach number and stagnation temperature) so the EGT is measuring stagnation temperature. The stagnation temperature will drop through the HPT and LPT, but it will still be greater than the ambient temperature due to non-ideal compression and expansion.

That being said, the case of a lower EGT being seen during a start for a hot engine makes sense. The hot engine will have heated the quiescent air around the EGT probe. Drawing in fresh, colder ambient air will register as an EGT drop.
Last edited by akiss20 on Wed Oct 04, 2017 7:35 pm, edited 1 time in total.
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 7:35 pm

FlyHossD wrote:
akiss20 wrote:
rjsampson wrote:

(I mean, how can there be EGT without combustion?)


Quick technicality. You will see a EGT rise with the N2 shaft spinning because adiabatic compression of a gas also inherently has a rise in temperature. That being said, the rise in EGT would be proportional to N2 speed (higher N2, more compression, more temperature rise in EGT), so the driving factor is still N2 speed.


That's technically true (the EGT rise from compression alone), but it's very small and nowhere near the EGT rise experienced from combustion.


Never said it was large compared to combustion, only that compression from spinning the N2/N1 shafts without combustion will result in an EGT rise.
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litz
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 8:48 pm

Not being a jet engine mechanic or engineer ... is the idea that too little airflow (due to lower rpms) doesn't allow the hot air/gasses from combustion to get out the back end fast enough?
 
Redbellyguppy
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 04, 2017 10:54 pm

So let's say I land in a station that has the ability to do a super fast turn, like Burbank. On pushback and the beginning of engine start those egt readings will be in the mid 200s when the engine starts turning. On the 300 I had a limitation that said 200C or less before introduction of fuel but neither the NG nor the max have such a limitation. That said I usually let it spin away until the egt falls below 200 anyways before I introduce fuel and in the example above it's not much past when I hit 25%N2 on an average day... maybe 28%n2 and to get to 200c and introduce fuel with average temps and average density altitude. Typically though on an average 30-35 minute turn the temp is already a little below 200 when I start the engine start process and by the time I get to 25%n2 the egt is ariund 130-150 or so. You can blow heat out right quick when you start spinning the thing.
 
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Thu Oct 05, 2017 5:13 pm

Balerit wrote:
You use the N2 rpm because that is what the starter is turning. The N1 is much slower to spool up.


It certainly would be pretty pointless to have the starter turn N1 :)

Hey, this may be hearsay -- but someone once said on the forum they started a saw turbofan (military I believe) start during a tailwind, and the engine lit off with the fan (N1) still spinning in the opposite direction! Any validity to this being possible?
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Thu Oct 05, 2017 7:52 pm

rjsampson wrote:
Balerit wrote:
You use the N2 rpm because that is what the starter is turning. The N1 is much slower to spool up.


It certainly would be pretty pointless to have the starter turn N1 :)

Hey, this may be hearsay -- but someone once said on the forum they started a saw turbofan (military I believe) start during a tailwind, and the engine lit off with the fan (N1) still spinning in the opposite direction! Any validity to this being possible?


I've done a few push-backs on B747's where the fan was turning in the wrong direction but once the N2 spools up, it stops and turns the correct way.
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Redbellyguppy
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Thu Oct 05, 2017 11:09 pm

A stiff tailwind and the n1 can indicate backwards rotation. I've seen what amounted to -2% (it's not indicated as such). By the time that n2 gets to about 15% or so the n1 is stopped and will shortly start turning the proper direction.
 
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Fri Oct 06, 2017 1:06 am

Redbellyguppy wrote:
A stiff tailwind and the n1 can indicate backwards rotation. I've seen what amounted to -2% (it's not indicated as such). By the time that n2 gets to about 15% or so the n1 is stopped and will shortly start turning the proper direction.


Yes, however you increase your risk for hot starts as you're fighting reverse flow. This is one of the easiest ways to get a hot start, is having a stiff tailwind.
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barney captain
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Fri Oct 06, 2017 2:36 am

I know a guy who raised the start lever at 10% N2 on an NG. You know what happened? Nothing. Nothing until appx 22% N2 - then it lit off on it's own. Just like the electronic fuel control dictates. I'm not recommending it, just pointing out that in a normally operating CFM-56, the start lever only "permits" the fuel to flow - it doesn't directly command it (during the start sequence).
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Sun Oct 08, 2017 1:58 pm

Balerit wrote:
I've done a few push-backs on B747's where the fan was turning in the wrong direction but once the N2 spools up, it stops and turns the correct way.


Very common on the legacy Jumbos. In fact, I recall that part of our start procedure was for the ground guy to confirm N1 rotation since the N1 indication system was just about useless below idle. It would even indicate an N1 speed even if the fan was turning the wrong way.
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Redbellyguppy
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Re: CFM Startup on the 737: What happens if fuel is introduced too early while spooling?

Wed Oct 11, 2017 5:58 am

So I had a chance to start a jet with "room temperature" engines today as it had been sitting out on the hard stand all day. So I spun up the engines and they indicated 27c when I started turning them. At 30%n2 they indicated 29c.

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