Flames during start on a turboprop.

jd117

jd117

New Member
Does anybody know how common or what the reason is that sometimes flames come out of the exhausts when a turboprop is starting up? I saw it on a King Air today on the number 2 and I've heard of some fairly large flames coming out of the back of MU-2s that people said are "normal."
 
During startup on turboprop engines, the fuel source is not introduced into the burner can until the N1 reaches 15% and stabilizes. However, during this time, the compressor and power turbine wheels are spinning up to reach a speed that will allow the fuel/air mixture that is being created to be exhausted and not toast the can itself when fuel is introduced. Sometimes when this is happening, the fuel/air mixture might not ignite right away and is vented out of the exhaust ports. As this is going on, the ignition might happen and follow the unburned fuel/air mixture out of the ports and into the open air resulting in a long flame. I used to see them all the time on BE1900s when I ramped for Lakes and then again when I started flying the Shorts. Normally, it's only a second before all the fuel is consumed but it can scare the poo out of passengers if they are looking out the windows when it happens.

Additionally, the igniters themselves are only necessary for the initial light-off. Turboprop engines are very similar to jet engines and diesels that they only rely on the associated heat of highly compressed air and the low flash point of Kerosene to produce ignition.
 
jml2992 said:
Turboprop engines are very similar to jet engines and diesels that they only rely on the associated heat of highly compressed air and the low flash point of Kerosene to produce ignition.
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Not even close. Turboprop engines ARE jet engines, and they don't rely on compression or flash point to set themselves on fire. Kerosene actually has a relatively high flash point, as does diesel fuel. Once lit, the flame is constant, there is no "bang, bang, bang" like reciprocating (diesel) engines have. The igniters are only needed during start because after it starts, the flame never goes out. Same reason you don't have to hold a match to the fire in your fireplace after you get it started.

As far as the flame that you sometimes see, it's due to excess fuel when the engine lights off. Often it follows an aborted start, or when the igniters are worn out and take a long time to light it off. It's more common with APUs also, as generally they don't keep turning after an aborted start to blow excess fuel out.
 
Only flames I've had during start are on the Alarus CH2000. Either google that thread or go on youtube and hear it. It's unreal.
 
I've shot four feet of flames (reportedly) out of the APU exhaust when starting it before, it's not usually something to be alarmed about. When the APU hangs during start you can sometimes see flames licking the exhaust as well. Exactly what you see depends on the airplane and APU configuration. In my airplane you don't usually see flame from the engines when they start (smoke is common) unless you introduced fuel too early or did something else weird (there are a couple ways to induce the fire shooting).
 
jml2992 said:
During startup on turboprop engines, the fuel source is not introduced into the burner can until the N1 reaches 15% and stabilizes.

Additionally, the igniters themselves are only necessary for the initial light-off. Turboprop engines are very similar to jet engines and diesels that they only rely on the associated heat of highly compressed air and the low flash point of Kerosene to produce ignition.
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Ehhh....Hmmmm, Ehhhh...ohhhhhh....Sighhhhhh....
 
Probably a king air with glow plugs instead of igniters. They make for a good way to clean the soot of the the nacelles! That, or somebody forgot to clear the engine and started it with fuel in the burner can(s).
 
Thanks for the replies. So I'm gathering it's normal. I'm kind of curious though, since I work line service. At what point you might want to signal the crew that something might be wrong with their plane?
 
BajtheJino said:
Ehhh....Hmmmm, Ehhhh...ohhhhhh....Sighhhhhh....
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Whoops. Sorry about that, kersosene is more difficult to burn...meaning that it has a higher flashpoint than avgas. Never could keep that straight. And the constant flow of fuel coming into the combustion chamber results in a continuous ingnition source...therefore eradicating the need for igniters past the initial start sequence. Company policy used to dictate that they came on below 3000' AGL and during heavy turb/precip to safeguard against flameout.
 
jd117 said:
Thanks for the replies. So I'm gathering it's normal. I'm kind of curious though, since I work line service. At what point you might want to signal the crew that something might be wrong with their plane?
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If you see flames coming from somewhere other than the exhaust, it's probably bad.
 
Ralgha has it right. Turbo props are turbine engines, hence jet engines-just with propellers. They share nothing in common with diesel engines. Ig-nit-ors start the fire, then it becomes self sustaining. It has nothing to do with hot air or compression. (Just to nitpick, not all engines start the same either, so forget about 15% and such;))
 
If the flames are more than transient on start-up, or if there is residual fire after shut-down those are times I would expect my line-man to give me a heads-up. Only turboprop I have logged much time in had the exhaust stacks pointed right back at my face though, so this was not ever an issue there.....
 
BajtheJino said:
Ralgha has it right. Turbo props are turbine engines, hence jet engines-just with propellers. They share nothing in common with diesel engines. Ig-nit-ors start the fire, then it becomes self sustaining. It has nothing to do with hot air or compression. (Just to nitpick, not all engines start the same either, so forget about 15% and such;))
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But the compression of the intake air is require in order to make the fuel combustible...stemming from jet fuel/kerosene/diesel type fuels needing to be heated to a point where it will reach its autoignition temperature. If it weren't for the compression of the centrifugal stages in front of the combustion chamber, the ignitors would only spark and never produce a flame for intial light-off. The fuel nozzles provide a constant flow of fuel onto the flame and keep producing the expanding gases that drive the power turbines. Sorry, all of my experiences with PT6A-45R or -65 series. I guess the stabilized statement was more important than the 15% part. Someone told me the ratio of the air passing through the engine to cool it versus the amount being used in actual combustion is about 9:1. Can someone out there confirm it?
 
jml2992 said:
But the compression of the intake air is require in order to make the fuel combustible...stemming from jet fuel/kerosene/diesel type fuels needing to be heated to a point where it will reach its autoignition temperature. If it weren't for the compression of the centrifugal stages in front of the combustion chamber, the ignitors would only spark and never produce a flame for intial light-off.
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That's a common point among all engines, it is still nothing like a diesel engine. Diesels ignite the mixture by compressing it until it spontaneously ignites, there is no other ignition source. Ever.

jml2992 said:
I guess the stabilized statement was more important than the 15% part.
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It still depends on the engine. In the PW118A you introduce fuel between 10 and 19% Nh, and it's not stabilized. If you didn't introduce fuel it would stabilize around 30%.
 
If you saw flames on a PT6A-67D during start, bro, you've got MASSIVE problems. MASSIVE.
 
jml2992 said:
But the compression of the intake air is require in order to make the fuel combustible...stemming from jet fuel/kerosene/diesel type fuels needing to be heated to a point where it will reach its autoignition temperature. If it weren't for the compression of the centrifugal stages in front of the combustion chamber, the ignitors would only spark and never produce a flame for intial light-off. The fuel nozzles provide a constant flow of fuel onto the flame and keep producing the expanding gases that drive the power turbines. Sorry, all of my experiences with PT6A-45R or -65 series. I guess the stabilized statement was more important than the 15% part. Someone told me the ratio of the air passing through the engine to cool it versus the amount being used in actual combustion is about 9:1. Can someone out there confirm it?
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I think we're splitting hairs here, or not. In your original post I thought you were saying that the compression of air itself and the heat associated with that is the reason that the flame in the burner can is sustained. I disagree with that statement.
 
I have flown various PT-6 and CT7-5A powered turboprops and can only remeber seeing a brief flame out the stacks during start a couple of times and both times were after an aborted start. The most common thing is a little smoke on start, especially on a cold soaked engine.
 
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