Carb Heat

[EXDAC]

Originally Posted by ahramin

No one is telling you that your technique is not acceptable for starting your aircraft in Arizona.

I think several have said my starting technique was incorrect and had a higher fire risk than following Piper's procedure. I have only defended use of throttle pump while cranking as an alternative to using the primer. It is not my only starting technique and the aircraft is not confined to Arizona.

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?

[ahramin]

Originally Posted by EXDAC

No one has yet supported the claim of increased fire risk.

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

1. Your technique can lead to a pool of fuel in the intake system
2. During a failed start attempt hot combustion gasses can end up in the intake system
3. When you combine fuel, air, and an ignition source, you get a fire

The primer system on our O-200 pumps the fuel into the air intake (the cylinders have no primer ports) so the risk is similar. The primer nozzle atomizes the fuel better though so less fire risk due to less pooling. Maybe next oil change I'll make a video of how much fuel ends up in the air box with each technique.

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]

Originally Posted by ahramin

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.

I am not stupid enough to pump the throttle when the engine is not turning but I think the amount of fuel would be very small. The volume delivered is limited by the bore and stroke of the accelerator pump which are both small compared to the primer.

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!

[ahramin]

Originally Posted by EXDAC

Please confine your justification of increased fire risk to throttle cycles while cranking!

No.

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

1. In moderate temperatures,
2. while cranking,
3. one to three applications of the accelerator pump,
4. while continuing to crank until the engine starts or the starter limit is reached,

will not have a higher risk of a carb fire than using the primer system, then yes I agree unreservedly. Thank you for sharing your technique.

[Pilot DAR]

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.

The volume delivered is limited by the bore and stroke of the accelerator pump which are both small compared to the primer.

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.

[Fl1ingfrog]

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.

No contributor gains ownership by their duration of contribution. everyone is free to add their knowledge at anytime and this is always valuable.

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.

I have explained that priming with the throttle, in the way that I described, was taught to me by highly qualified and experienced aircraft engineers. From the tips document provided by Lycoming yes they do recommend that using the primer is "preferred". However, they do not bar the use of the accelerator pump but only suggest that priming this way increases the risk of an airbox fire. Lycoming do not publish a formal warning notice on this although it is well known to them. When priming the fuel will always settle into liquid over time whether delivered as a mist or not. In the worst case scenario when the fuel has drained into the airbox, it will only ignite from a hot exhaust should the fuel leak out of the box onto it. It would be interesting to see what volume of fuel is required to be injected from the accelerator pump to then drain into the airbox and become a fire risk. Over priming is always problematic even using the primer pump. Excess fuel can be ejected into the exhaust and should the exhaust be hot there is a risk of fire. From my experience an exhaust fire has been the most common event. A carburettor fire being extremely rare and following gross mishandling.

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

Absolutely liquid doesn't burn, so over priming is the risk, less so the starting technique. My rule is: never prime a hot engine unless you have completed a fuel purging process first. To avoid an engine fire it is that simple.

[Pilot DAR]

When priming the fuel will always settle into liquid over time whether delivered as a mist or not.

I'm not a physicist, but isn't it the other way around? Gasoline would naturally be a vapour than a liquid in our normal atmosphere... Hand priming gets it to that state faster, will it will remain, until either burned or dispersed into the atmosphere...

[Pugilistic Animus]

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.

[Fl1ingfrog]

Gasoline would naturally be a vapour than a liquid in our normal atmosphere

I am very limited with my schoolboy physics but I'm sure somebody will comment who isn't. Water will evaporate also, but neither do in a sealed container. A stationary engine cylinder can be considered sealed can it not? Petrol can return to liquid and become heavy due to condensation albeit at a lower temperature than water. When a cylinder is not firing you can identify it from touch because it will be cold or cooler. Engineers always then go to the bottom plug first which will likely be wet if that is the problem plug.

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.

[Pilot DAR]

A stationary engine cylinder can be considered sealed can it not?

Happily, I know more about engines than physics! Yes, the cylinder can be considered a closed container - during the compression and power stokes only, as a valve would be open during the other two strokes. That said. gasoline exploding in a cylinder, is what is intended by design, though 20 some degrees before top dead center is the intended time for that.

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....

[Fl1ingfrog]

But gasoline does not pool in the cylinder,

Leave it long enough it will.

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 trenches

It 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.

[Pilot DAR]

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....

[Pugilistic Animus]

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!

[EXDAC]

Originally Posted by Pilot DAR

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.

A cold Lycoming O-360 will not start unless the mixture is richer than delivered by normal cranking air flow through the carburetor. The fact that a cold Lycoming O-360 will start easily with a couple of cycles of the accelerator pump, while cranking, shows that the fuel delivered by the accelerator pump did in fact richen the mixture that was drawn into the engine. If it richened the mixture drawn into the engine then the cranking airflow must have been sufficient to overcome gravity.

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.

[Pilot DAR]

Can we please stop assuming that fuel delivered by the accelerator pump while cranking will pool in the air box.

I choose not to, I've read the POH's.

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. ....

For both the Continental and Lycoming powered 172's, and the 182, the POH says:

IMPORTANT
Pumping the throttle may cause raw fuel to accumulate in the intake duct, creating a fire hazard in the event of a backfire....

The Beechcraft Musketeer POH is more brief:

Do not pump throttle to start

When I started my 150, which I had preheated for the cold day, I gave it the normal two shots of prime, and started it. It was not idling smoothly, which is usually an indication that not all of the cylinders are running yet, but all four usually catch after a few seconds. As the POH says, as it stumbled, I added a few more primer shots to keep it idling. It stopped, so one more shot of prime, and start again. It ran, then stopped. I saw the smoke coming up through the windshield defogger, knew I had a carb fire (and my wife was phoning me to tell me she could see it). So I pulled the mixture, opened the throttle, and cranked until the battery was discharged. I hopped out, and could still see fire in through the oil door, so I hit it with the fire extinguisher. It was the oil breather insulation burning, not terribly serious. But, the airbox, and carburettor were fire damaged, and the cowling discoloured.

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....

[EXDAC]

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.

[Pilot DAR]

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.

I do agree, and I am in the habit of checking the full travel of the throttle as a part of control checks before starting the engine (lesson learned from a bad experience!)

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!

[Jhieminga]

Can we please stop assuming that fuel delivered by the accelerator pump while cranking will pool in the air box.

Originally Posted by Pilot DAR

I choose not to, I've read the POH's.

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. If we spotted the behaviour in time we could intervene, park the aircraft in a corner where the fuel could happily drain away and cancel that student's flight. But my photo above was not the only occasion where we had a carb fire.
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.

[Fl1ingfrog]

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.

Light aircraft are subject to many variables which are very difficult and sometimes impossible to pin down but they dominate. The fueling systems are very basic and crude. The pilot therefore must use some initiative in managing the fuel for starting. It is possible to operate large aircraft exactly in accordance with its flip charts and the maintenance regime is very detailed. Fuel flows for instance can be measured precisely. However an over rich turbine engine can cause havoc just the same as our basic pistons; some years ago a heavy jet was cleared to start. My C150 was parked at the approved Avgas fueling point and being fueled from a Bowser. The distance from the MD80 was of over 80 metres.. The MD80 had failed to start and air traffic reported to the pilots that they had observed a lot of black smoke. The captain in spite of this continued to a second attempt. Apparently the flame from the engine was spectacular, my C150 was launched into the air, flipped 180 degrees and its port wing struck the Bowser on landing inverted. Fortunately the fueler had finished and replaced the fuel cap and he was able to jump clear. There was no fire, thank god. Two very shaken refuelers and my C150 written off.

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.

It is not the method that is always the hazard rather it is simple common sense. If an engine does not start as it should then clear the engine of the excess fuel that WILL be present. It is unlikely that you will be certain of why it did not start.- and only then start again. Have in your mind that the failure to start may well be an over priming rather than too little. By clearing the engine you will not be at risk.

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?

[Pilot DAR]

From the facts that you reported shouldn't you have cleared the engine of fuel and then attempted a second start?

Yup!

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.