I am curious about this PT6 auto ignition bomb theory I've been hearing discussed lately. The idea is something like this:
If you have the fire go out, the power drops and the auto feather dumps oil from the prop of the affected engine feathering the blades.
Once the power drops to whatever percentage the auto ignition kicks in and potentially relights the fire but the props are now feathered and torque rockets through the limits very rapidly. Potentially the relight is so violent with so little airflow into the engine that the re light can be explosive.
Has anybody ever actually heard of this happening? Is this really possible?
Auto Ignition Bomb?
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Axial Flow
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So I guess the scenario you are using is for a twin engine PT6 and that it would be in take off or climb with the auto feather system still on.
Now I am thinking of it do twin's have auto ignition ?
The PC12 has auto ignition and the NG has to drop below 500 ITT which lately at -15 happens even before I hit flight idle and an NG of 85-90. Personally I can't see what you are talking about because this low NG you are talking about and blowing the engine can't really happen if just the fire goes out.
The only other time auto comes on in the PC12 is on start above 10 ng.
Can someone give me any scenario that this could happen because from my experience it can't in the PC12 and the Caravan isn't auto and I don't know any other engine models.
Now I am thinking of it do twin's have auto ignition ?
The PC12 has auto ignition and the NG has to drop below 500 ITT which lately at -15 happens even before I hit flight idle and an NG of 85-90. Personally I can't see what you are talking about because this low NG you are talking about and blowing the engine can't really happen if just the fire goes out.
The only other time auto comes on in the PC12 is on start above 10 ng.
Can someone give me any scenario that this could happen because from my experience it can't in the PC12 and the Caravan isn't auto and I don't know any other engine models.
King Air's auto ignition are not based on Ng but rather a torque rating.
Pratt and Whitney and plane manufacturers do extensive testing for all sorts of cases. They would have probably thought of this. Plus if the flame goes out at a high power setting it's probably not going to start again, easily.
The only bomb case I know of is if you started the engine with ignition circuit breaker pulled. Hit the starter and introduce fuel. Let it go about 20-30 seconds. Then push the ignition circuit breaker in. It won't be long before the fireworks start.
Pratt and Whitney and plane manufacturers do extensive testing for all sorts of cases. They would have probably thought of this. Plus if the flame goes out at a high power setting it's probably not going to start again, easily.
The only bomb case I know of is if you started the engine with ignition circuit breaker pulled. Hit the starter and introduce fuel. Let it go about 20-30 seconds. Then push the ignition circuit breaker in. It won't be long before the fireworks start.
It's better to break ground and head into the wind than to break wind and head into the ground.
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twinpratts
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Last time I was at Flight Safety for the -350, they started talking about this stuff... I think it came from P&W. (Of course they always conclude everything with 'however, check your SOP's, and follow them accordingly'). 
I want to die like my grandfather did, peacefully in his sleep. Not screaming in terror like his passengers...
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rightseatwonder
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co-joe
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Yeah that's the scenario I've been told can lead to an explosion Twin Pratts. You have the power levers set at max/ takeoff power (100% torque) so the FCU is still scheduling a large amount of fuel and if the fire goes out you have a burner can basically full of fuel, then as torque drops to around 17% the auto ignition kicks in but the props are on their way to feathering.
Because of the low N1 and N2 not very much air gets to the burner can so if/when a relight happens, you have a very rich mixture of fuel that could potentially explode. Some of the pilots I talked to said they don't use auto ignition on takeoff for this reason. Basically they're saying that a flame out and feather is controllable, an explosion might not be.
This is so contrary to everything I've been told in the past so I wonder if it is conceivably possible. Does this make sense to anyone?
Because of the low N1 and N2 not very much air gets to the burner can so if/when a relight happens, you have a very rich mixture of fuel that could potentially explode. Some of the pilots I talked to said they don't use auto ignition on takeoff for this reason. Basically they're saying that a flame out and feather is controllable, an explosion might not be.
This is so contrary to everything I've been told in the past so I wonder if it is conceivably possible. Does this make sense to anyone?
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300_hour_wonder
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the pt6's i know and fly are set to auto feather at 350 ft lbs but auto ignition kicks in at 750 lbs lbs so auto ignition would kick in and re start before it feathers and furthmore the FCU is set up so that if there is a reduction of airflow due to a flame out lets say it will lower fuel flow to an appropriate amount for that airflow so you wouldn't wind up with an excessivly rich mixture so from what i can tell this is not very likey but who knows i sure don't know everything thats for sure
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Flying Nutcracker
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Just a thought... the flame goes out, the torque will drop pretty fast, while the N1 will still be spinning fairly fast. It takes time to spool down in the first place, secondly, with a forward motion comes airflow through intake that slows the spool down...
My thinking would be a "tiny" bomb perhaps, but as there still should be a fair amount of airflow the vapors should purge fairly quickly... me thinks it would not be a big problem.
Also remember that the FCU schedules fuel based on several inputs. P3 and N1 rpm being 2 of them. If you get a relight the FCU will then schedule fuel based on the "new" condition and the acceleration should be controlled.
Ofcourse... this is just a speculation on my part!
My thinking would be a "tiny" bomb perhaps, but as there still should be a fair amount of airflow the vapors should purge fairly quickly... me thinks it would not be a big problem.
Also remember that the FCU schedules fuel based on several inputs. P3 and N1 rpm being 2 of them. If you get a relight the FCU will then schedule fuel based on the "new" condition and the acceleration should be controlled.
Ofcourse... this is just a speculation on my part!
Only 2 ways to put the fire out - disrupt the airflow or stop the fuel (or a mechanical catastrophe!).
If its the former, re-establish the airflow (maybe recover from the stall!) and turn on the ignition (or auto-ignition). Ice might come loose and mess up the airflow, but you didn't forget the vanes, right? Seems to me that we had auto-ignition with the boots/vanes out?
If its the latter, reestablish the fuel flow and turn on the ignition. If there has been mechanical damage, all bets are off.
The rest doesn't make sense to me - if the engine is still turning there would be a large airflow through the engine, blowing unburnt fuel, if there is any, straight out the exhaust or out the burner drains. If the fuel ignited, it might light up the clouds a bit, but explosion - I don't think so. Remember that gas turbine engines use about 80%+ of the air that is passed through them strictly for cooling so the sheet metal they are made of doesn't melt, so there is plenty of hot air to flush out the fuel (remember dry motoring on the ground?). If it was not turning, neither would the fuel unit be, and it would stop introducing fuel - might be some left over though...
If the auto-ignition failed to relight the engine, it would just stop and feather and you would continue on one less. There would likely have to be a mechanical fault, though.
Rats! Now I'm going to have to dig out my old Dash-7 manuals to see if I'm spouting claptrap. What did I have for dinner....?
Edited to count the engines.
If its the former, re-establish the airflow (maybe recover from the stall!) and turn on the ignition (or auto-ignition). Ice might come loose and mess up the airflow, but you didn't forget the vanes, right? Seems to me that we had auto-ignition with the boots/vanes out?
If its the latter, reestablish the fuel flow and turn on the ignition. If there has been mechanical damage, all bets are off.
The rest doesn't make sense to me - if the engine is still turning there would be a large airflow through the engine, blowing unburnt fuel, if there is any, straight out the exhaust or out the burner drains. If the fuel ignited, it might light up the clouds a bit, but explosion - I don't think so. Remember that gas turbine engines use about 80%+ of the air that is passed through them strictly for cooling so the sheet metal they are made of doesn't melt, so there is plenty of hot air to flush out the fuel (remember dry motoring on the ground?). If it was not turning, neither would the fuel unit be, and it would stop introducing fuel - might be some left over though...
If the auto-ignition failed to relight the engine, it would just stop and feather and you would continue on one less. There would likely have to be a mechanical fault, though.
Rats! Now I'm going to have to dig out my old Dash-7 manuals to see if I'm spouting claptrap. What did I have for dinner....?
Edited to count the engines.
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Switchfoot
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Re: Auto Ignition Bomb?
An instructor we worked with just recently explained this theory to us and set it up for us to fly in the sim (BE-350) and let me tell you it was one hell of an abrupt and violent ride.co-joe wrote:Once the power drops to whatever percentage the auto ignition kicks in and potentially relights the fire but the props are now feathered and torque rockets through the limits very rapidly. Potentially the relight is so violent with so little airflow into the engine that the re light can be explosive.
Has anybody ever actually heard of this happening? Is this really possible?
Beech has modified the checklists to prevent pilots from having something catostrophic like this from ever occurring.
Let's just say you wouldn't want to be on board when the damn things kick in at night in IMC over unfriendly terrain!
Switchfoot
I call horseexcrement. try starting your pt6 with throttles full forward. until you hit above 60% or so the fcu will not schedule fuel thanks to surge protection from the P3/Py line. Be fast with the throttles though it will accelerate really fast from there to full power.
