Engine fire during start...

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John777

Pre-takeoff checklist
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When the engine catch fire, we continue to crank it until the mixture and shutoff valve is cutoff..

Do you know why we keep cranking the engine?

Is it so that fuel in the plumbings going to engine gets ingested into engine cylinders?

Lastly, where and in which part of the engine is susceptible to catching fire?
 
It's possible to have an engine compartment fire. That is why no amount of fuel leakage is allowed in an engine compartment. You can have a carburetor fire, sometimes triggered by a backfire (continue to crank the engine for that one), exhaust fire (incomplete fuel burn in cylinder) and even an accessory gearbox fire, engine oil fire.
 
There is a big difference between an accessory section fire and an induction system fire, and is very important to be able to tell the difference.
Some bigger aircraft have fire warning elements in the nacelle that will light a warning light in the cockpit. our smaller aircraft we in most cases are not aware that we have a fire until some one out side the aircraft tells us.
Accessory section fires are usually caused by leaks ignited by starter sparks. or electrical over loaded wires.
induction fires are almost 100% caused by over priming and ignited by a back fire.
 
So in case of backfire, there are too much amount of fuel primed into the cylinder and as the mixture combusts, the fire spread out to the intake manifold?

John.
 
John777 said:
So in case of backfire, there are too much amount of fuel primed into the cylinder and as the mixture combusts, the fire spread out to the intake manifold?

John.
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On both major brands of aircraft engine we find in our aircraft are up draft induction. the carburetor is on the bottom of the engine, and connected to an airbox. when you prime a Lycoming the fuel enters into the induction system at the junction of the intake pipe and the cylinder head. IF you are not cranking sucking this fuel into the combustion chamber this fuel simply runs down hill until it exits the air-box and drips into the cowl.
When you prime a continental the fuel enters the induction in much the same way, but at a several different places depending upon model. but it will still run down and out if you are not cranking the engine.
once it is exposed to the open cowl is is simply ready to ignite by any source.
 
John777 said:
So in case of backfire, there are too much amount of fuel primed into the cylinder and as the mixture combusts, the fire spread out to the intake manifold?

John.
Click to expand...
Priming does not place fuel in the cylinder.
 
John777 said:
So in case of backfire, there are too much amount of fuel primed into the cylinder and as the mixture combusts, the fire spread out to the intake manifold?

John.
Click to expand...
Very seldom does the intake system contain enough O2 to create a big fire. but once the fuel is out in the cowl, it is a different story.
 
Tom-D said:
On both major brands of aircraft engine we find in our aircraft are up draft induction. the carburetor is on the bottom of the engine, and connected to an airbox. when you prime a Lycoming the fuel enters into the induction system at the junction of the intake pipe and the cylinder head. IF you are not cranking sucking this fuel into the combustion chamber this fuel simply runs down hill until it exits the air-box and drips into the cowl.
When you prime a continental the fuel enters the induction in much the same way, but at a several different places depending upon model. but it will still run down and out if you are not cranking the engine.
once it is exposed to the open cowl is is simply ready to ignite by any source.
Click to expand...
So it can be ignited by any source like sparks ?!

John.
 
So far, I understood priming as adding fuel into the cylinder; the combustion chamber...
Now I realize that the fuel would be staying between cylinder head and intake pipe or manifold...

What if the intake pipe is open and lets the fuel into the cylinder?
Isnt it going to let the fuel into the cylinder?


John.
 
The majority of engine fires occur at start-up. Most are the result of guys using the carburetor to "prime". If you take an average carburetor off an airplane and actuate the throttle you'll shoot a pressurized stream of raw fuel straight up from the carb throat. That's the aircraft equivalent of an accelerator pump. When guys pump the throttle to start an airplane they use that pump to shoot fuel into the induction. If the engine starts it creates a significant draft and all that fuel is drawn into the engine. If the engine doesn't start right up some of that raw fuel will fall back through the carb, into the airbox, and drip into the engine cowl. Not a good place for fuel and it can definitely catch fire if an ignition source is added. The most common ignition source is an engine backfire and those are usually caused by a lean condition, whether because of induction leak or poor fuel vaporization after pumping the throttle. If you have fuel drop out and a backfire and the fuel ignites your best action is to continue cranking and hope the engine starts and sucks the burning fuel into the engine. If it fails to start your best action is to grab a fire extinguisher. Most of us that live in cold climates have primer lines into all cylinders. I can pump my panel mounted primer as much as I want and fuel will not drain into the induction and into the airbox. Using the primer system is much, much safer than pumping the throttle. If you must use throttle to prime make sure you turn the key and start the induction draft before pumping the throttle. Never pump throttle on an engine that isn't spinning.

I had an induction leak lean cylinder, was frustrated that the engine wouldn't quite light up, and pumped the throttle to help it. The resulting induction fire was extinguished quickly. It took me about 5 seconds to unstrap, slide the seat, get out, grab the extinguisher, etc. The engine compartment damage was about a thousand dollars a second. Without the extinguisher the plane would have burned to the ground.
 
Had a carb fire on a C150 once, the starter was slow and battery low so all I could do is let it burn.

I got very lucky as the fire went out by itself. Removed the carburetor and found nothing wrong other than a fire singed air filter element. Replaced the starter and the battery and flew it several years after that without a problem.
 
Engines are air pumps. Continuing to turn the engine pumps the fire into the engine and keeps the fire from spreading. Most fires start at the carburetor. That's where the fuel air mixture is.
 
coloradobluesky said:
Engines are air pumps. Continuing to turn the engine pumps the fire into the engine and keeps the fire from spreading. Most fires start at the carburetor. That's where the fuel air mixture is.
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What if the fire is caused by the main battery cable to the starter?
 
bnt83 said:
Had a carb fire on a C150 once, the starter was slow and battery low so all I could do is let it burn.

I got very lucky as the fire went out by itself. Removed the carburetor and found nothing wrong other than a fire singed air filter element. Replaced the starter and the battery and flew it several years after that without a problem.
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Very typical of the Cessna 150. it was over primed or there would have been no fuel to burn.
 
Stewartb said:
The majority of engine fires occur at start-up. Most are the result of guys using the carburetor to "prime". .
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That is not true. 100 % of induction fires are caused by adding fuel when the engine is not cranking. When the fuel is added to the induction system when there is no airflow to carry the fuel into the combustion chamber the fuel will simply run down the induction pipes to the carb, drop thru the throttle plate and into the airbox and out into the cowl.

It does not matter how the fuel got into the induction system.
 
John777 said:
So far, I understood priming as adding fuel into the cylinder; the combustion chamber...
Now I realize that the fuel would be staying between cylinder head and intake pipe or manifold...
What if the intake pipe is open and lets the fuel into the cylinder?
Isnt it going to let the fuel into the cylinder?
John.
Click to expand...
When there is no airflow to carry the fuel into the combustion chamber it will not flow there by its self, no matter if the valve is open or not. gravity will control the flow.
 
coloradobluesky said:
Engines are air pumps. Continuing to turn the engine pumps the fire into the engine and keeps the fire from spreading. Most fires start at the carburetor. That's where the fuel air mixture is.
Click to expand...
Nope there is no ignition in the carb. and gravity will not allow the fuel to stay in the carb venturi. the only fire you can see is in the open.
 
Remember ignition in the combustion chamber does not occur until 10 degrees after the intake valve closes. So fires in the intake system can't be caused by the spark in the cylinder.
Induction fire are very rare, the fires we see are fuel in the cowl and ignition occurred from out side sources. so continuing to crank is useless. your first and only responsibility is to your pax, get them out of the aircraft as fast as possible because any aircraft that is on fire belongs to the insurance company.
 
How induction fires occur:
The engine was not retarded when a cylinder fired, it then kicked back, as it rotated backward it dumped the burning fuel air mixture out the intake valve which will open 180 degrees after the engine started to rotate backwards. NOW you have a burning mass in the induction system. and most likely the cowl too.
 
Let's stop and think,,, if the engine did in fact "After fire" in the exhaust the flame won't ignite anything until it exists the exhaust pipe. When there is fuel out there, who screwed up?
Did you allow the fuel drained from the gascolator to drop to the ground? (bad on you)
Did you over prime? if there is gas out there, how much is still in the cowl?
 
Stewartb said:
Never pump throttle on an engine that isn't spinning.
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Overstated. There are some airplanes where pumping the throttle is SOP. A common example is Cessna 172RG. It's safe because it's not an updraft carb; you can't put the airbox below the engine because the nosegear retraction/extension and door mechanisms are there. The engine start checklist and expanded procedure explicitly say to pump the throttle.

The correct statement is not to make up procedures. If the POH doesn't say to pump the throttle, don't pump the throttle.
 
Some aircraft have no primer at all. A Cherokee I look after has just the carb. It has a good starter and battery. Get it spinning over and pump the throttle twice. Works every time.
 
Yep, read the manual for your plane. I have no primer, and The Book says to pump the throttle twice before turning the key. The Cherokee above is the opposite--turn the key then pump the throttle.
 
Stewartb said:
The majority of engine fires occur at start-up. Most are the result of guys using the carburetor to "prime". If you take an average carburetor off an airplane and actuate the throttle you'll shoot a pressurized stream of raw fuel straight up from the carb throat. That's the aircraft equivalent of an accelerator pump. When guys pump the throttle to start an airplane they use that pump to shoot fuel into the induction. If the engine starts it creates a significant draft and all that fuel is drawn into the engine. If the engine doesn't start right up some of that raw fuel will fall back through the carb, into the airbox, and drip into the engine cowl. Not a good place for fuel and it can definitely catch fire if an ignition source is added. The most common ignition source is an engine backfire and those are usually caused by a lean condition, whether because of induction leak or poor fuel vaporization after pumping the throttle. If you have fuel drop out and a backfire and the fuel ignites your best action is to continue cranking and hope the engine starts and sucks the burning fuel into the engine. If it fails to start your best action is to grab a fire extinguisher. Most of us that live in cold climates have primer lines into all cylinders. I can pump my panel mounted primer as much as I want and fuel will not drain into the induction and into the airbox. Using the primer system is much, much safer than pumping the throttle. If you must use throttle to prime make sure you turn the key and start the induction draft before pumping the throttle. Never pump throttle on an engine that isn't spinning.

I had an induction leak lean cylinder, was frustrated that the engine wouldn't quite light up, and pumped the throttle to help it. The resulting induction fire was extinguished quickly. It took me about 5 seconds to unstrap, slide the seat, get out, grab the extinguisher, etc. The engine compartment damage was about a thousand dollars a second. Without the extinguisher the plane would have burned to the ground.
Click to expand...

This may have saved me from starting a fire someday. I was throttle pumper. No more. Thanks @Stewartb and thanks for asking @John777
 
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