Carb Heat
What starting technique works best depends on many variables and cannot possibly be covered in a single procedure that is followed by rote. While it may be possible to reliably start an engine by following a rote procedure I maintain that the same engine may be started more quickly if all the environmental variables are considered and the procedure is adjusted accordingly. Modifying the procedure depending on the conditions takes skill, experience, and perhaps some empathy with the engine. Such deviation from published procedure will be unacceptable to some.
No one has yet supported the claim of increased fire risk. How much fuel is delivered by two cycles of the throttle pump. Where does it go if the engine is being cranked? Where can any fire be sustained?
Last edited by EXDAC; 3rd Dec 2021 at 18:01.
Well the document you referenced mentions it, but not under the conditions later specified (warm OAT, max 3 pumps while cranking). The point is priming with the throttle has additional risks which should be considered, not dismissed.
EXDAC why don't you try a simple experiment: The next time you have your cowl off in cold weather, pump the throttle twice and then watch the fuel running out of the air box. I think you will then understand how much fuel is being delivered and where the fire will be.
Like I said there is no database of carb fire incidents that we can analyze to prove the increased risk. You just have to understand that
PilotDAR, do you have any pictures of the damage from your carb fire? Did you take the carb apart yourself or just send it out?
EXDAC why don't you try a simple experiment: The next time you have your cowl off in cold weather, pump the throttle twice and then watch the fuel running out of the air box. I think you will then understand how much fuel is being delivered and where the fire will be.
Like I said there is no database of carb fire incidents that we can analyze to prove the increased risk. You just have to understand that
- Your technique can lead to a pool of fuel in the intake system
- During a failed start attempt hot combustion gasses can end up in the intake system
- When you combine fuel, air, and an ignition source, you get a fire
PilotDAR, do you have any pictures of the damage from your carb fire? Did you take the carb apart yourself or just send it out?
Last edited by ahramin; 3rd Dec 2021 at 17:49.
The reason a couple of cycles of the throttle, while cranking, works better than the primer is that the fuel is ingested by the engine instead of pooling in the induction system. Please confine your justification of increased fire risk to throttle cycles while cranking!
EXDAC there is something you are obviously missing here. Fl1ingfrog stated "In fact, by utilizing the accelerator pump you can prime the engine for starting and this is often the preferred way." He's not the only one out there who thinks the accelerator pump should be used instead of the primer to prime the engine. I've run across dozens of pilots who use this technique because they heard from another pilot that it works better. When PilotDAR and I tried to point out the risks of this procedure, you jump in with "Lycoming says it's ok and it works very well. Don't knock the technique if you have not actually evaluated it." If the discussion is left there, someone could easily walk away without understanding that Lycoming specifies that this technique has risks and recommends using the primer system instead.
You don't get to jump into the middle of a conversation and dictate that all discussion now only applies to your personal technique which you later specify. If you're going to lay out your technique and explain what steps you take to prevent the increased fire risk, that's fine. If you're going to jump in and say there are no risks by quoting a document and leaving out the part that outlines the risk, you need to be corrected so others don't walk away with incorrect knowledge.
However if you're just desperate for me to acknowledge that
- In moderate temperatures,
- while cranking,
- one to three applications of the accelerator pump,
- while continuing to crank until the engine starts or the starter limit is reached,
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Sorry, no photos, I was already a bit sad about that. But it looked about the same as the photo above. The insulation on the oil breather continued to burn, and it was that which I could see burning, and extinguished with the fire extinguisher in through the oil door.
I took the carb apart myself, the engine shop then overhauled and certified it for me. The two piece venturi was blackened, and distorted enough that the engine would not have run with it that way. It was about ready to drop out of the three clips. Though not melted, it was evident that the inner venturi got hot enough to begin to melt, I could see it.
That is the reason for the AD to inspect the venturi's. A carb fire could have distorted the venturi a little, but the pilot got it running, and went flying unknowingly. The only way to know will be to look at it, and assure it's correctly in place. They have fallen out of position later, and they you're gliding.
Check again.... I think you'll find that they have very similar volumes. The accelerator pump (of an MA-3 carb) is in the range of 5/8" diameter, and strokes about an inch. A Piper hand primer has a plunger diameter if less than half an inch, and about the same travel if I recall (it's been a few years since I flew a Piper. More importantly, the more slow movement of the hand primer is because it is making much greater pressure, and atomizing the fuel to a much more fine mist in the air. (so flow is much less). This mist will quickly evaporate into vapour (which is what burns - liquid does not burn well). The fuel vapour will hang in the induction air passages for some time. Compared to the stream of liquid gasoline (about like a water pistol). If the accelerator pump squirt is drawn up the induction by good airflow (running engine), it gets to the cylinders as intended. However, if the airflow is poor, and gravity good, the liquid fuel will just drop down through the carb, into the airbox, and pool there. Once the accelerator pump fuel has dropped into the airbox, it is of no use as priming fuel, and the start may fail. More priming will be necessary, which will probably get the engine running, but there's still the the pooled fuel. Only a while to evaporate it, or a running engine to draw it through will get rid of it. That'd be why getting the engine up to full cranking speed before introducing the full prime make some sense (and works well for my O-360). No matter what anyone says, a pool of fuel, where there is a possible source of ignition, is not as safe as no pool of fuel there.
In aviation (like many other things) . There are "other ways" to do things, some of which work - but the fact that they work does not mean they are the better way to do it! Noting Lycoming's own remark about the primer being the "best" method to start their engines.
I took the carb apart myself, the engine shop then overhauled and certified it for me. The two piece venturi was blackened, and distorted enough that the engine would not have run with it that way. It was about ready to drop out of the three clips. Though not melted, it was evident that the inner venturi got hot enough to begin to melt, I could see it.
That is the reason for the AD to inspect the venturi's. A carb fire could have distorted the venturi a little, but the pilot got it running, and went flying unknowingly. The only way to know will be to look at it, and assure it's correctly in place. They have fallen out of position later, and they you're gliding.
The volume delivered is limited by the bore and stroke of the accelerator pump which are both small compared to the primer.
In aviation (like many other things) . There are "other ways" to do things, some of which work - but the fact that they work does not mean they are the better way to do it! Noting Lycoming's own remark about the primer being the "best" method to start their engines.
You don't get to jump into the middle of a conversation and dictate that all discussion now only applies to your personal technique...................... If you're going to jump in and say there are no risks by quoting a document and leaving out the part that outlines the risk, you need to be corrected so others don't walk away with incorrect knowledge.
I've run across dozens of pilots who use this technique because they heard from another pilot that it works better. When PilotDAR and I tried to point out the risks of this procedure, you jump in with "Lycoming says it's ok and it works very well. Don't knock the technique if you have not actually evaluated it." If the discussion is left there, someone could easily walk away without understanding that Lycoming specifies that this technique has risks and recommends using the primer system instead.
the more slow movement of the hand primer is because it is making much greater pressure, and atomizing the fuel to a much more fine mist in the air. (so flow is much less). This mist will quickly evaporate into vapour (which is what burns - liquid does not burn
Last edited by Fl1ingfrog; 4th Dec 2021 at 13:08.
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When priming the fuel will always settle into liquid over time whether delivered as a mist or not.
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Regarding the primer, I treat it like a syringe because that's what it is, basically, just slowly pull the primer. If done too fast you would not be giving the cylinders enough gasoline and then you would prime and prime again until you finally get an engine fire.
Gasoline would naturally be a vapour than a liquid in our normal atmosphere
I'm very open to knowing more on the physics that takes place in the cylinder prior to ignition. If I'm completely wrong in my current understanding then I will stand corrected.
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A stationary engine cylinder can be considered sealed can it not?
But gasoline does not pool in the cylinder, it either burns there, or is pumped out (where it can pool in the muffler, and explode there - caution about those live mag checks!). The danger is the unburned fuel dripping down through the carburettor, into the airbox, which is never a closed container. If gasoline is a puddle, it will slowly evaporate off the top, or burn as a pool of fuel. A pool of gasoline in the airbox will not be drawn up into the inductions system, which is why, in the case of a carb fire, you must crank the starter (mixture idle cut off) until you're battery is flat (the cost of the starter motor is secondary at that point). You have to pull all of the fire through the induction system, which means keeping cranking until the fuel is exhausted in the airbox. Yes, by design, airboxes are required to drain, but from my experience, that's never perfect.
In the case of the (C150) carb fire I had last year, (which looked the same as the photo), and from my wife's report, there had been enough gasoline in the snow under the cowl, that it too was burning - there was a tiny bit of charring on the nose tire, which I replaced. In simple terms, pumping the throttle allows you to pump out a lot more gasoline than pumping the primer, I think it's designed that way on purpose....
But gasoline does not pool in the cylinder,
Weight of Gasoline Vapors
Gasoline quickly evaporates when exposed to the atmosphere; the vapors are not lighter than air. This characteristic is not uncommon, according to the Canadian Centre for Occupational Health and Safety, which notes, “The vapors from nearly all flammable and combustible liquids are heavier than air.”Dangers of Gasoline Vapors
Because gasoline vapors are heavier than air, they sink through the normal atmosphere. Flammable, explosive amounts of vapors can therefore collect around floors or in basement structures, pits, sewers, sumps and trenchesIt may seem that I'm arguing against using the primer which I am not. But machines are not all the same they have their own idiosyncrasies. Whenever flying someone else's aeroplane I always ask the owner how I should start it and do as they say - they are always right. They know their aeroplane.
Doesn't aviation teach us; never be rigid on your views and dig a hole.
Last edited by Fl1ingfrog; 4th Dec 2021 at 22:25.
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Well, yes, gasoline vapour will settle in the lowest available area, but will remain a vapour. This is common with many "fuel" type vapours and gasses - but, I'm not a physicist. The amount of gasoline vapour which could "pool" or otherwise accumulate in the cylinder, would probably be just right for the intended combustion, when ignited. Now, if it's a radial engine, Hydraulic lock becomes a concern, but that's not quite this topic....
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Quite simply gasoline vapors are heavier than air, we all know that.
I once had a fire in organic chemistry lab where someone put a hollow glass tube instead of a solid glass tube and the vapors got into the heating element and ignited...We were able to extinguish the fire and we got good yields that day...
In case anyone wonders why I was in organic chemistry, that's because Chem was major number 1 and mechanical engineering is 2 major 3 Biology and biochem
I've learned a lot here about Carburetors!
I once had a fire in organic chemistry lab where someone put a hollow glass tube instead of a solid glass tube and the vapors got into the heating element and ignited...We were able to extinguish the fire and we got good yields that day...
In case anyone wonders why I was in organic chemistry, that's because Chem was major number 1 and mechanical engineering is 2 major 3 Biology and biochem

I've learned a lot here about Carburetors!
Last edited by Pugilistic Animus; 5th Dec 2021 at 01:02.
If the accelerator pump squirt is drawn up the induction by good airflow (running engine), it gets to the cylinders as intended. However, if the airflow is poor, and gravity good, the liquid fuel will just drop down through the carb, into the airbox, and pool there. Once the accelerator pump fuel has dropped into the airbox, it is of no use as priming fuel, and the start may fail.
Can we please stop assuming that fuel delivered by the accelerator pump while cranking will pool in the air box.
I have flown about 40 different powered aircraft types and not set any of them on fire. Always open to learning from those who have though. What exactly were the steps you followed to set fire to your C-150? Hopefully we can learn from that unfortunate experience.
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Can we please stop assuming that fuel delivered by the accelerator pump while cranking will pool in the air box.
The POH for the 150 says:
FIRES.
Engine Fire During Start On The Ground.
Improper starting procedures such as pumping the throttle during a difficult cold weather start can cause a backfire which could ignite fuel that has accumulated in the intake duct. ....
Engine Fire During Start On The Ground.
Improper starting procedures such as pumping the throttle during a difficult cold weather start can cause a backfire which could ignite fuel that has accumulated in the intake duct. ....
IMPORTANT
Pumping the throttle may cause raw fuel to accumulate in the intake duct, creating a fire hazard in the event of a backfire....
Pumping the throttle may cause raw fuel to accumulate in the intake duct, creating a fire hazard in the event of a backfire....
Do not pump throttle to start
Prior to that, I had twice been a passenger for a carb fire, once in a C180 amphibian, which is a bit more difficult to exit and extinguish in a hurry. My 150 is the only plane I have set fire to of the 283 different aircraft I have flown since 1975, and when I did it, I had over 3000 hours in 34 years of owning it - I did what the POH said to do - and still got it wring that day.
It's my goal posting here to provide my experience and whatever wisdom I have to give, to those who wish it - advice as to have the best success at safe flying. I will not be advocating procedures which contradict what a POH says. That's just me....
I suppose all those quoted cautions would depend on the meaning of "pumping". I don't think advancing the throttle from idle to about 1/3 open would be "pumping". I don't even think that advancing the throttle from idle to full open would be pumping. If it's not pumping to do it once then does it become pumping if it is done twice?
Maybe I won't have the O-360 much longer and this will all be moot. I have more fun with my much newer IO-360 which has no primer or throttle pump. I also get to write my own AFM so the start procedure can be whatever I'd like it to be. That engine starts in about 2 seconds hot or cold too.
Maybe I won't have the O-360 much longer and this will all be moot. I have more fun with my much newer IO-360 which has no primer or throttle pump. I also get to write my own AFM so the start procedure can be whatever I'd like it to be. That engine starts in about 2 seconds hot or cold too.
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I don't think advancing the throttle from idle to about 1/3 open would be "pumping". I don't even think that advancing the throttle from idle to full open would be pumping.
It is true that for a non certified airplane, the procedures may be defined by, well, really, anyone! I speak only in the certified airplane world, as that is where I fly and work. There is certainly a grey zone in between what a flight manual/POH says is a procedure, through what it is silent on, through to prohibited. When I write flight manual supplements for approval with an STC'd mod to an aircraft, I will be explicit as to what is approved, and what is prohibited, with a purposefully small if any grey zone.
Procedures in flight manuals have evolved, I expect out of lawyer's reactions to bad experiences in their client's planes. When I compare the Owner's Manual for an older, very simple trainer, the Piper J-3, at 52 pages, (about a third of which is how to maintain it, rather than fly it) to Airplane Flight Manual for a much more recent simple trainer, the Diamond DA-20 at 376 pages, I can see that manufacturers generally feel the need to write more down about operating their airplanes!
Can we please stop assuming that fuel delivered by the accelerator pump while cranking will pool in the air box.
Please note that I am talking about the 150 here. This will of course be different on other aircraft and we have to be careful in this discussion as we are generalising in some posts and discussing specific types in others.
Last edited by Jhieminga; 6th Dec 2021 at 08:50. Reason: Added note about types.
There are "other ways" to do things, some of which work - but the fact that they work does not mean they are the better way to do it! Noting Lycoming's own remark about the primer being the "best" method to start their engines.
Even without the POHs, I have seen 150s where the fuel was literally dripping from the air box. Students who couldn't get the engine started on a cold day were prone to forgetting the briefing and just priming a bit more... and a bit more.... etc.
Pilot DAR, you reported over priming with the installed primer when you had your engine fire! From the facts that you reported shouldn't you have cleared the engine of fuel and then attempted a second start?
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From the facts that you reported shouldn't you have cleared the engine of fuel and then attempted a second start?
I hadn't realized, though should have, that I had failed to clear fuel, and created a hazard for myself. My point is that you can put a lot more fuel into the airbox faster with several throttle pumps than several primer strokes. If I bungled it using the primer, I would have really got it wrong pumping the throttle for the same intended outcome.
And yes, you can really get a turbine start wrong. I have seen it from right seat! A few turbines I have flown have very strict procedures about clearing fuel after a failed start.