Carb Heat

[Pilot DAR]

Elsewhere, I posed some questions about carb heat, to offer a new poster some research points. I'm not sure if the poster has taken up the challenge, but it's been a few days, so I owe some thoughts to the points I raised. Everyone is eagerly encouraged to offer their thoughts/experiences/exceptions to these points:

Some GA planes have a carb heat some don't - which are which and why?

Airplanes with fuel injected engines do not have venturi carburettors, so are naturally immune to carb ice formation.

What is the relationship to carb heat and alternate air?

Alternate induction air is a design requirement for certified planes. The carb heat doubles as the alternate air control. Alternate air is not filtered (by design requirement), and usually drawn from within the cowling.

Do planes which do not have carb heat still have alternate air?

Yes, it's still a design requirement.

Do planes have an alternate air control if not a carb heat control?

Some planes have an alternate air control, others have a spring loaded door, which pops open if the manifold pressure demands airflow and the main induction is blocked

What information is necessary for you to apply partial carb heat?

If you have a carb air temperature indicator, you can use it to apply partial carh heat, to just melt ice. Otherwise it is recommended to apply full carb heat, if you need to use it.

If you apply carb heat for a rough running engine, and the engine then runs worse, what should you do? Why? For how long?

When you apply carb heat, you're melting any ice which may have accumulated in the carburettor. As it melts, the water will enter the engine, and the engine may run worse. Keep the carb heat applied at least until the engine continues to run smoothly, though you may want to leave it applied for the rest of the flight in those conditions.

If you apply carb heat, and it's not enough, what two engine controls can you adjust to get a little more carb heat? Adjust how?

Leaning the mixture may increase the exhaust gas temperature - so hotter carb heat air, and closing the throttle a little will reduce the amount to [moisture containing] air entering the carb, so reduce the demand for heating.

When might you need to urgently apply carb heat, but have not encountered carb icing conditions?

If something else blocks your main induction air filter

Why should you never use carb heat while taxiing?

Carb heat is unfiltered air, so any dust, grit, etc. will be ingested by the engine.

If your plane is equipped with an indicator associated with carb heat, what is that indicator?

A carb air temperature indicator will show you the temperature at the venturi.

What does the yellow arc on that indicator mean? Where will the yellow arc be on the scale? Will there be any green or red lines on that indicator?

The yellow arc on the carb air temperature indicator is the icing range, so avoid continued operation of the engine in that range. Applying carb heat will heat out of the range (in most cases).

Will a piston engined airplane which does not have a carb heat control always have an electric fuel pump? Why?

If a piston engine does not have a carburettor, it is fuel injected. Fuel injection systems always require pumped fuel pressure.

Could an airplane which does have an electric fuel pump also have a carb heat control?

Yes, you can have fuel pumped carburettor equipped planes, so understand your systems!

Where does the heat resulting from the application of carb heat come from?

The heated air comes from a muff around the exhaust system/muffler

If you have had an engine failure, will applying carb heat help you get it started again? Why?

If your engine has stopped as a result of carb ice, you will no longer have exhaust heat to heat carb heat air, so getting it running by normal means is unlikely - start looking for a forced landing place!

What would be a practical warm outside air temperature where you would be unconcerned about carb icing?

I've seen charts up into the 20+ C temperature range, it's not just a cold day thing...

What other atmospheric factors will affect carb icing?

The moisture content of the air

Can you get carb icing on the ground?

Yes, so if in doubt, a brief application of carb heat prior to takeoff, and then assure expected full power is available before committing to take off.

If you're flying in very well below freezing air, is carb heat a risk?

Much less so, as moisture in very cold air will pass through the carburettor venturi frozen, so not accumulate there.

[Russell Gulch]

The smaller-horsepowered Continental carburetted engines (A-40 to O-300) will suffer carb ice more often than similarly-powered Lycoming engines due to the differences in their construction, and the attachment of the carb to to the engine. So don't put the carb heat to cold at 300 ft AGL on final approach in a C150 :ouch:

I don't know enough about Franklin engines of similar vintage to be able to make a comment.

Russ

[mikehallam]

Ha Ha.... Not quite 100% there DAR.
c.f. Para. #7.
Some aeroplanes do not have alternate air.
e.g. UK and USA at least, Rans S6 series.
Most in the UK are classed as 'microlights' so perhaps you can wriggle out there, but the lusty S6-116 carburetted model definitely "an Aeroplane" !

[Pilot DAR]

Some aeroplanes do not have alternate air.
e.g. UK and USA at least, Rans S6 series.

Interesting, are they certified? Alternate air is a certification requirement. And a fine idea. I did hit very narrow band of freezing rain in the 150 once, and both the windshield and air filter iced over instantly, followed by the engine stopping completely. I pulled carb heat for the alternate air, and got the power right back, as I turned 180, out of what I had thought was just rain!

[Maoraigh1]

"So don't put the carb heat to cold at 300 ft AGL on final approach in a C150."
I was persuaded to do that on our Jodel. Landed at Ashaig, Skye. Opened throttle to turn and backtrack. Engine spluttered. Attempting a go-around would have been an accident.Went back to carb heat until landed.

[ahramin]

Good thread.

Recently I was flight testing a Murphy aircraft with a Van's style air box / filter. The air filter is in a plenum, when carb heat is applied the intake to the plenum is blocked and the heated air intake opens. This means that if the filter ices up, there is no alternate air intake, the only option is to melt the ice. Checking the certification requirements that might be applicable to a similarly equipped certified aircraft it requires the carb heat to be able of generating a 100°F rise. I've never seen a system that can deliver this much heat but we have a carb t° probe on the way and we'll see what it can do.

If you are flying an amateur built aircraft, best check how the carb heat / alternate air works.

[TheOddOne]

Applying carb heat will heat out of the range (in most cases).

Flying our PA28-180 today, fitted with a carb temp gauge, showed -10deg C, near the lower end of the yellow arc. Applying carb heat only raised the temp to the top end of the arc, not out of it. Still, got about a 50RPM drop in revs, followed by the classic return to the original RPM 'no rise, no ice'.
You might have mentioned NOT using carb heat in very cold conditions, when doing so can put the inlet temp INTO the carb ice range, from a nice too-cold position.
We regularly get carb ice in the PA28 when starting from cold over grass, good demo to the student of what it looks like. Personally, I've never seen significant carb ice in a PA28-180, over decades. Now, in my C150, wow, a different story!
I think there are very few days in the UK when the conditions are outside the critical part of the graph.

Next thread - 'Use of Pitot Heat'.

TOO

[Fl1ingfrog]

It's worth mentioning: that the venturi can reduce the temperature by up to 25 degrees centigrade, also that carb. heat enriches the mixture and therefore leaning is important when carb. ht. is to be applied for long periods.

Opened throttle to turn and backtrack. Engine spluttered.

The argument as to whether the carb heat should be maintained until touch down or not is an old chestnut. As many failures can be caused by the carb. ht. being left on too long and causing a rich cut instead. This can be particularly so during touch and go and a go around, and especially on hot days. Remember that when the throttle is increased an accelerator pump gives an extra squirt to minimize take up lag. I think that 300ft is too soon to go to cold air but below 200 ft is a good compromise. I suspect that 300 ft is rollover to do with multi-engine 'committal' heights than carb icing considerations.

[ahramin]

As many failures can be caused by the carb. ht. being left on too long and causing a rich cut instead.
We can put that one to bed. Carburetor heat induced rich cuts are an OPT (Old Pilot's Tale). Doesn't happen.

[ahramin]

near the lower end of the yellow arc. Applying carb heat only raised the temp to the top end of the arc, not out of it.

Does your gauge have actual temperature markings? I'd be very interested to know where the yellow arc for this aircraft lies.

[Jhieminga]

One tip that I was given long ago was to leave the carb heat on for a minute or two (or even longer) during the runup so that the carburettor body would be warmer. The idea was that no ice would then be able to form during the subsequent minutes of the take off. I can't claim the idea or confirm that it works... any thoughts on this?

[B2N2]

One tip that I was given long ago was to leave the carb heat on for a minute or two (or even longer) during the runup so that the carburettor body would be warmer. The idea was that no ice would then be able to form during the subsequent minutes of the take off. I can't claim the idea or confirm that it works... any thoughts on this?

Carburators are (were) made of pot metal so very conducive to temp changes.
Start with a hot carburetor body and within seconds after applying power you’d have a cold carb again. I would put that one to bed.

Carb heat associated rich cut? Definitely.
I’ve had two engine stoppages as a result of a too agressive application of power with carb heat on.
After that I started teaching the 3-second rule, 1-2-3 count from idle to full power or back to idle.
Carb heat OFF 1-2-3
Carb heat ON 1-2-3

[Fl1ingfrog]

We can put that one to bed. Carburetor heat induced rich cuts are an OPT (Old Pilot's Tale). Doesn't happen.

Unfortunately it isn't and it does. Complacency though, "doesn't happen" should be guarded against, it leads to too many avoidable and tragic accidents: "there old pilots and bold pilots but there are no old bold pilots springs to mind". The Lycoming 0-235 engine installed in the C152 is very prone to running very rich being just one example. Rich cuts as with carburetor icing leaves little or no trace and therefore makes it very difficult to establish the cause after the event.

[Jhieminga]

Carburators are (were) made of pot metal so very conducive to temp changes.
Start with a hot carburetor body and within seconds after applying power you’d have a cold carb again. I would put that one to bed.
My back-of-the-envelope calculations told me so too, but it's one of those things that you do as told by the instructor/examiner at the time but you don't really start thinking about it until later. As mentioned before, the temp increase from carb heat isn't all that much, so it is not as if you are heating up that carb with a blowtorch. Once you switch the carb heat off and go to full power, you've got the original -20C air going through it again and at a higher mass flow. As you say, it's not going to stay warm for long. If someone has a carb temp indicator and wants to try this on a cold day, I would be interested in the numbers.

[Fl1ingfrog]

The carburetor heat systems are very varied in design and effect. The techniques applied are also very varied. I agree with the point made by B2N2 that the benefit from carb ht once set to cold is short lived.

The purpose of the power checks and vital actions at the hold is to ensure that the aircraft is fit for flight. Any alterations once the checks are complete can introduce a problem, so should be done with caution. I've lost count on the occasions that an additional application of carb. ht. is made and then forgotten so lining up with heat still applied. I am insistent therefore that the hand must not be removed from the control until it is put back to cold when this additional check is done.

How many pilots know the minimum static RPM for their aeroplane with the throttle fully open. It is a much more valuable check when done against the brakes immediately prior to the T.O. roll.

[Pilot DAR]

The minimum static RPM for many fixed pitch propeller planes is stated on the type certificate data sheet as a limitation (meaning the plane is not fit for flight, if it cannot produce that RPM).

I am not a proponent of full power against the brakes, unless absolutely necessary, and personally, aside from maintenance power checks, I have never found it necessary. Propeller damage can be serious with full power runs over a loose surface. I like to do my flying power checks as a quick confirmation of the tachometer reading early in the takeoff roll - with the nose being held light.

[Fl1ingfrog]

I like to do my flying power checks as a quick confirmation of the tachometer reading early in the takeoff roll - with the nose being held light.

Knowing the RPMs is the surest check that the power required is being achieved and I agree that you can do this on the roll.

[ahramin]

One tip that I was given long ago was to leave the carb heat on for a minute or two (or even longer) during the runup so that the carburettor body would be warmer. The idea was that no ice would then be able to form during the subsequent minutes of the take off. I can't claim the idea or confirm that it works... any thoughts on this?
I had a look at some recent data and here's the longest application of carb heat I could find. It's in the air obviously as running carb heat on the ground longer than absolutely necessary is bad for the engine. You can see how quickly the carb t° drops after the carb heat goes off. Half in the first few seconds, the rest in less than a minute.
https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/1799x512/carb_heat_f594d3474d33fb9f5088cdd7de1c250b0d4e4de6.jpg


Carb heat associated rich cut? Definitely.
I’ve had two engine stoppages as a result of a too agressive application of power with carb heat on.
After that I started teaching the 3-second rule, 1-2-3 count from idle to full power or back to idle.
Carb heat OFF 1-2-3
Carb heat ON 1-2-3
I did not realize you were talking about a power application. It sounds to me like the culprit here is the aggressive application of power rather than the carb heat. Does the engine not have the same trouble if you slam the throttle open with the carb heat off?

[Fl1ingfrog]

Yes, the fuel provided from the accelerator pump will be the same. The fuel supplied by the pump bypasses the carburetor so it adds to that supplied by the carb. In fact, by utilizing the accelerator pump you can prime the engine for starting and this is often the preferred way. In some types it is the only way of priming the engine to start. Amazing to me is that the hand primers fitted to the 150/172 series may only provide fuel to just one or two cylinders. In this case I always find that using the accelerator pump by pumping the throttle is preferable.

[Maoraigh1]

"Y​​ou might have mentioned NOT using carb heat in very cold conditions, when doing so can put the inlet temp INTO the carb ice range, from a nice too-cold position."
Can someone explain this? "Too cold for carb ice" means the air at that temperature cannot contain sufficient water vapour to cause carb ice at the venturi temperature drop. Raising the temperature without a source of water will only reduce humidity further.

[Pilot DAR]

by utilizing the accelerator pump you can prime the engine for starting and this is often the preferred way.

I disagree. If the airplane has a hand primer, it should be used rather than the accelerator pump. The reason being that the hand primer will atomize the fuel to a much finer mist, being drawn in with the air better, greatly aiding combustion, and reducing draining back as a liquid. That's why they're hard to push, you're pumping the fuel through a tiny orifice to atomize it, instead of shooting a stream into the carburettor throat. And, the primer nozzle is further downstream in the inductions system, reducing draining back. Drained back fuel is a source for a carb fire. Pumping the throttle does help a stumbling engine at start, but is not preferable over a primer. The Cessna and Piper POH's do say to use the primer....

[MarcK]

Checking the certification requirements that might be applicable to a similarly equipped certified aircraft it requires the carb heat to be able of generating a 100°F rise. I've never seen a system that can deliver this much heat but we have a carb t° probe on the way and we'll see what it can do.The carb heat on a normally aspirated B model Aztec can give a 120°F rise. I've used that ability in icing conditions (long ago).

[Fl1ingfrog]

Pilot DAR, your comments are of course valid but not the whole story. I've given a description of many priming installations that are problematic. The accelerator pump also provides fuel downstream from the carburettor. As to both engine and carburettor fires I've seen too many and due to over-priming using the installed primer. Whatever the technique used if the engine doesn't start then stop after 3 - 4 rotations. Consider why the engine failed to start. It might have nothing to do with the fuel/air mixture. Don't keep turning and priming although I have seen this advised in some POH. If the engine (exhaust) is hot you have a high chance of an exhaust fire from over priming.

[Pilot DAR]

The accelerator pump also provides fuel downstream from the carburettor.

Well, the accelerator pump discharges right in the carb, where the primer nozzle may be just downstream of the carb, or actually at the cylinder intake. The point which is more important is that the product of the primer is a mist of atomized fuel, which ignites very easily, and is less likely to flow backward and accumulate as a liquid. The accelerator pump discharge is something like a water pistol squirt, just a stream of gasoline. If it is not taken up with the induction air, it will flow backward into the airbox, and reside there, being a potential fuel source for a carb fire (I've had two). Sure, the primers can get really stiff, that means that they should be maintained. Primer nozzles do clog, and the primer plungers need occasional lubrication. There's special gasoline proof grease for this.Allowing the "normal" use of the accelerator pump as a primer is setting the student up for a carb fire.

I had a carb fire last winter in my 150 on a very cold day, preheated start. It did not start the first time, and fired and stumbled during the the second attempt until it stopped again. I had been priming it as it stumbled to run, as the POH says. I saw the smoke coming up the windshield defroster vent, and realized what was happening. I moved the mixture to ICO, and cranked the engine until the battery was discharged. With that I jumped out, a fire extinguisher completed the job. My wife was phoning me to tell me that she could see the fire from the kitchen window - I had to ignore her just that once - I was really busy. It was still damaging and expensive.

[ahramin]

Yes, the fuel provided from the accelerator pump will be the same. The fuel supplied by the pump bypasses the carburetor so it adds to that supplied by the carb. In fact, by utilizing the accelerator pump you can prime the engine for starting and this is often the preferred way. In some types it is the only way of priming the engine to start. Amazing to me is that the hand primers fitted to the 150/172 series may only provide fuel to just one or two cylinders. In this case I always find that using the accelerator pump by pumping the throttle is preferable.
Bypasses the carburetor? I'm guessing you've never had the cowl off an aircraft let alone a carburetor apart? The accelerator pump is part of the carburetor.

Yes, there are a few engines out there without a primer and using the accelerator pump is the recommended way to prime them. It's not an ideal situation though and they didn't add primers to all the later engines because they wanted them primed with the accelerator pump did they?

Ask PilotDAR about carb fires, he's an expert :).

[Jhieminga]

We used to operate both 152s and 150s at a flying school I used to work for. By the time winter came around, we always had to do a briefing on the differences in the priming systems as the students were very much used to the 152s, where the priming fuel is deposited a lot closer to the cylinder than on the 150s. To make matters worse, someone thought it a good idea to have all the dual flights use the 152s as they had a bit more power, while the 150s were scheduled for students flying solo. Anyway, that was a whole different discussion.

The cold starting procedure on the 150 (if my memory is correct) has you turning the prop over by hand while priming, so that the suction will draw the fuel up, from the point down in the carb where it is injected, towards the intake manifold. On the O-200, the carb is of course down below the engine and you've got an airbox at the bottom, the carb directly above it and from that point the intake manifold goes straight up and turns outboard to end up underneath the cylinders. Gravity will of course always take over and fuel will inevitably drain down again towards the air box. Because of this sometimes the procedure was modified to turning the engine over on the starter while priming, but this was not in accordance with the POH. It did get the job done though. We also had the inevitable carb fire at times, one time when I was on board. The first clue was a burning smell and smoke coming up through the vents. My student was very quick and starting shutting things down before I could tell him to keep cranking so the sensible next thing to do was: get out. Fortunately, there was no big fire, but you could tell something had gone wrong, see below.


https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/2000x1504/afb291_c625bacf3d62914f9758c5658de84f371eeba1b3.jpg

[pulse1]

For 15 years or so I have never had a problem starting a cold soaked 0-200 with no primer (OK Southern England, so never that cold). I use a heater to warm the induction system for about 30 minutes, pump the throttle 6 times, turn the prop through 8 blades, another 3 throttle pumps for luck and it never fails to start instantly. Relating to the earlier discussion about carb icing, the engine sometimes stops again before there is enough heat to stop icing which, as I said earlier, tends to happen a lot on the ground unless one uses carb heat.

[Jhieminga]

I use a heater to warm the induction system for about 30 minutes
That is called cheating! ;)

[Fl1ingfrog]

The cold starting procedure on the 150 (if my memory is correct) has you turning the prop over by hand while priming

This was a similar technique shown to me by the engineers of a very prominent UK school. They were required to start and warm 30 PA28 aircraft every morning throughout the cold mid winter. The technique was: set a piston at the top of compression and then using the accelerator pump give two strokes of the throttle and pull through. Do this for each cylinder plus two more for luck and start without delay. This worked every time; no flattened batteries, no flooded engines and no fires. I've done this successfully for over twenty years.

An engine fire is only a risk if you over prime. If you do not over prime then you will not have an engine fire, be it exhaust or carburettor, whatever starting technique you employ. If the engine is hot you probably do not need to prime. Never assume, in the first instance, that the fuel mixture is too lean but rather it is too rich.

A common reason for a difficulty when starting a warm engine is that the previous flight has not been shut down correctly. A common fault is to treat the mixture as if a switch. Rich to shut off; in a milli-second. Shut down with care allowing the engine to burn off all excess fuel and to leave the spark plugs clean

[EXDAC]

https://www.lycoming.com/content/tips-start-your-engine

"Lycoming engines of more than 118 HP have a throttle pump which can be used for priming under moderate ambient temperature conditions while turning the engine with the starter."

Lycoming says it's ok and it works very well. Don't knock the technique if you have not actually evaluated it.

[Maoraigh1]

Anyone familiar with this?
For Lycoming 160, in very cold air, I was told:
5 primer pumps.
Leave primer open.
Throttle set.Turn over X times, close primer to lock and open throttle as normal when it starts. (X~≥10? - memory)
If it doesn't start, delay then repeat.

[Fl1ingfrog]

From the same publication that EXDAC has provided is the following bulletin:

Article-Knowledge BaseService Instruction No. 1148 C (https://www.lycoming.com/content/service-instruction-no-1148c)Use of Carburetor Heat ControlSupersedes Service Instruction No. 1148C October 12, 2007 MODELS AFFECTED All Lycoming engines using float type carburetors. TIME OF COMPLIANCE During engine operation. Under certain moist atmospheric conditions (generally at a relative humidity of 50% or greater) and at temperatures of 20° to 90°F it is possible for ice to form in the induction system.

[Pilot DAR]

Lycoming says it's ok and it works very well.

I take a different meaning from the same Lycoming instruction;

Priming can be best accomplished with an engine priming system, as opposed to use of the throttle.

And goes on to [correctly] advise about engine fires resulting from excessive "throttle" priming. And, it's worth noting that where there is a conflict between the engine manufacturer's published information, and the airframe manufacturer's flight manual, the flight manual shall prevail. Lycoming has no design control over the airplane manufacturer installing a hand primer as a part of the type design, so something is better than nothing, but they are saying that a primer is the best way to prime. The POH's for a Lycoming powered 172 and 177 do not mention using the throttle to prime at all, though give instruction to use the primer.

It is possible that a Lycoming engine could be installed in an approved airplane type, for which there were no dedicated primer system, and then, yes, throttle priming would be better than ruining a starter motor trying to start an unprimed engine, but the fire risk would still be there. That'd probably be why every Lycoming carburetted engine I've ever flown has a dedicated priming system.

[EXDAC]

And, it's worth noting that where there is a conflict between the engine manufacturer's published information, and the airframe manufacturer's flight manual, the flight manual shall prevail.

I suggest you read the 1975 Piper PA-28-180 starting procedure. It is total garbage. It would have me crank for up to 10 seconds with no prime before attempting to use the primer. I can let my airplane sit a month and start it in less than 2 seconds with one or two cycles of the throttle lever. I've been starting it this way for 20 years. I know the airplane and I know what works.

There is zero risk of an induction fire if the throttle is pumped one to three times while cranking. All the fuel is ingested by the engine. In contrast, excessive prime with the engine not turning will pool fuel in the induction system.

The local flight school probably instructs the student to use the Piper procedure. The continuous grinding of starters is painful to hear. Some people never develop any feel for engines. They blindly follow a procedure with no understanding of what is happening or why it doesn't work.

As I said earlier, don't knock the technique if you have not used it. Well, have you used it?

(This thread is starting to look like a re-run of a recent SuperCub.org thread)

[Pilot DAR]

Well, have you used it?

Yes, I have, in three of the four planes I have owned, and others I have flown. It works. But, with a reduction in effectiveness and safety. My present carburetted Lycoming 360 starts best when cranked for 5 to 10 seconds, and then a primer prime introduced. I have perfected this technique over the 550 hours I have flown this plane since 2008, as there is no stated factory procedure for starting it (that was actually mine to develop for it, but the project was not finished to STC approval, so no flight manual was ever developed for it). I have a particular interest in consistent good starts, as some of them are floating. I have certainly flooded it with the primer, and tried pumping the throttle during other experimentation. Using the primer sparingly after the engine is already cranking works best. Certainly, running the battery flat, or ruining a starter motor is to be avoided, as hand propping this plane would be impossible, and help may be a long way away at some destinations.

I certainly agree that it's possible for a pilot to get to know a plane so well, that even a non approved technique results in consistent success. I used to have the 310R I flew down pat. Anyone reading this, who knows their plane that well, knows this. But, to suggest that a student pilot should start developing their own starting techniques invites carb fires, or overheated starter motors.

[EXDAC]

My present carburetted Lycoming 360 starts best when cranked for 5 to 10 seconds, and then a primer prime introduced.

You use the starter more in one start than I would in 5 starts and it seems that's for the same or similar engine. How can you assert that a technique that requires about 10 seconds of cranking is more effective that a technique that requires about 2 seconds of cranking? You offer no justification for the claim of an increase in safety and I believe that claim is nonsense.

13 hours short of 1,000 hours in my PA-28-180 and I consider my starting technique to be well proven. I would only consider using the primer if operating in much colder conditions than I normally operate in.

[Pilot DAR]

How can you assert that a technique that requires about 10 seconds of cranking is more effective that a technique that requires about 2 seconds of cranking?

The starter has a 30 second duty cycle. I'm within that - as long as I don't flood it. A ten second crank, with priming as I have established gives me reliable starts. A reliable first start prevents the need for a second attempt, and greater risk of something going wrong with that start. Were I to have a carb fire in my 360, I would have no way whatever of knowing that, so preventative technique is the best for me. I had a carb fire on my other plane last year, having flown it more than 3000 hours over 34 years (so I kinda knew it). It was a surprise to me, I thought I had that plane figured. It was an expensive fix. If I run the battery down with failed start attempts, I have a bigger problem to solve. So, I've learned to be patient, start the way it works, even if it take a few second longer, get a predictable first start, and go flying. Typical ambient flying temperatures for me range from 90F to -15F, so varied preheating and priming techniques are appropriate. One technique will not cover that range. The safest start is the start which worked smoothly the first time, even if it required a little more patience.

Certainly starting my O-200 was a very different priming technique from my O-360. The O-200 was generally purring after a 2 - 5 second crank. No so the O-360 as installed in my plane, believe me, I've tried!

You do what works for you, it's your plane. As a person who writes flight manual supplements and issues STC's on GA planes for a living, I respect the techniques provided in existing approved flight manuals and POH's, as they are a product of testing and approval. They are not always the only way, they are the way preferred by the manufacturer of the plane. I can hardly advocate to deviate from the type of documents I create and approve, using methods I have tested myself, and understand. But yes, I have tried other ways....

[ahramin]

You use the starter more in one start than I would in 5 starts and it seems that's for the same or similar engine. How can you assert that a technique that requires about 10 seconds of cranking is more effective that a technique that requires about 2 seconds of cranking? You offer no justification for the claim of an increase in safety and I believe that claim is nonsense.

13 hours short of 1,000 hours in my PA-28-180 and I consider my starting technique to be well proven. I would only consider using the primer if operating in much colder conditions than I normally operate in.
EXDAC the increase in safety comes from a reduced risk of fire as already pointed out. No one collects stats on carb fires but the engine and aircraft manufacturers have knowledge of them and a few people here discussing this have knowledge of them. Personally with your aircraft I would just use the primer (that's why it's called a primer) but pumping the throttle will work just as well or better under your conditions with little risk. No one is telling you that your technique is not acceptable for starting your aircraft in Arizona.

You on the other hand are suggesting that PilotDAR's technique for starting an engine consistently in a range of temperatures from Arizona like down to double digits with a minus in front of them while drifting downriver are not as good as your technique.

I have found in aviation that the best results come from knowledge of both the system and the manufacturer's procedures coupled with experience. The problem with pilots finding their own homemade procedures that work under certain conditions is that they do not know the risks of these procedures under different conditions. It's a common cause of accidents.

My Lycoming IO-540 manual has a starting technique that works consistently whether the engine is warm or cold in a wide range of temperatures. Having seen many pilots following their own perfected technique fail to get that engine started in less than perfect conditions I follow this technique with the exception of cold starts on normal temperature days. I have a slightly different technique that works well under those conditions and start the engine faster. 95% of my starts are done this way but I don't go running around telling everyone that I'm smarter than Lycoming and my technique works better. When my partner started flying the aircraft, I taught him the Lycoming technique first and then once he had experience with the engine, taught him the modified technique.

[Piper.Classique]

The way that always worked for a cold start for my 0- 320 in a Super Cub was three strokes of primer, pull through eight blades by hand, one or two more primes depending on how cold it was, lock the primer, pump the throttle once, set one finger width of throttle and turn the key. (Yes, it had a key start) Keep it running with delicate use of throttle.
I was at one point complaining of poor starting when I discovered the coil for the shower of sparks magnetos was dead. A chuffing miracle that it started at all, really.
There was nothing in the airframe manual at all about starting procedures. (Or about anything much else, really)

[cattletruck]

Nice thread drift. Never had any issues starting a Warrior (O-320) the standard way, from memory it was several primes if cold, mixture full rich, throttle at the 1500 rpm position, crank and when it kicks over retard throttle to idle rpm. A Hughes 300 IO-360 was another beast altogether, open throttle a tiny bit, prime if cold, fuel pump on, mixture full rich for 3 seconds then mixture full cut-off, fuel pump off, crank, when engine fires quickly move the mixture to full rich and set throttle at idle rpm. If you did it right it would start on the first crank, if you did it wrong you'd be cranking for ages with rest gaps in-between screaming "start ya' b@stard'.

Re carburettor icing, I had a situation once when practicing engine failures under instruction. The carby heat was confirmed working during run up and was applied during the engine failure simulation, however the simulation probably went longer than usual as a better field was selected. Upon reaching the safe lower altitude and applying full throttle the engine started shaking the plane quite vigorously with only just a tad over 1000 rpm indicated at full throttle. Still, you could climb a Warrior with that limited power, a little flap and pointing into wind, and we managed to gain enough altitude for a straight in approach (rather glide from the reporting point). The reason for the carby icing was not doing any engine revs during the prolonged simulated engine failure to bring back some heat into the exhaust manifolds that feed the carby heat unit.

[EXDAC]

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]

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

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

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]

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]

Please confine your justification of increased fire risk to throttle cycles while cranking!
No.

EXDAC there is something you are obviously missing here. Fl1ingfrog (https://www.pprune.org/members/472281-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,

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 VaporsGasoline 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 VaporsBecause 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:8
I've learned a lot here about Carburetors!

[EXDAC]

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.
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.

[Fl1ingfrog]

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.

And it should be with light aircraft piston engines. This is where I think it should be. Mandatory placards: 1. 'Over priming may cause an engine fire'. 2. 'Caution: the engine must be clear of residual fuel prior to priming'.

The effective fuel clearing procedure should be detailed in the Flight Manual/POH and with expanded explanations.

[Piper.Classique]

I was sitting in the cockpit while my friend ( who owned the aircraft) was starting the engine on a Taylorcraft. A continental, can't remember if it was 65 or 90 hp. It was being very reluctant to start, lots of cursing, mags on and off, sucking in, priming, the usual hand swinging stuff. Eventually we thought we were getting somewhere when it decided to cough a couple of times, so "one more prime" he says "mags on" and another swing.
Which was when it caught fire.
He informed me of this fact, quite calmly, so I did the obvious thing and got out. I then sacrificed what had been a perfectly good hat to stifling the not very impressive flames.
The aircraft didn't seem to take any great harm from this, which is more than I could say for the hat, the only flat cap I have ever owned that was comfortable with a headset.
OTOH, I once managed to start an O-360 straight out of the hangar, -15c, mixture idle cut off, fuel and mags off, (keys in my pocket) four blade prop, by pulling it through prior to what I anticipated would be a difficult start. Good job we had a standing rule at that club to always chock the wheels when parked. Yep, hot mag.
Things like that just don't happen with cars, do they?

[Pilot DAR]

'Caution: the engine must be clear of residual fuel prior to priming'.

One airplane I flew, a Siai Marchetti 1019, had just about this placard, it was even more severe; something to the effect that if you found the fuel control out of the cut off gate, you must do a dry run before a start attempt. But, this is uncommon in GA planes (and the SM1019 isn't really a GA plane anyway).

In general, placarding is a subject of varied certification discussion. It's possible to bury critical information as a placard, by surrounding it in many important information placards. Sometimes, it's an AD or the lawyers which require a placard, otherwise, it's those of us who certify the aircraft design. There are so many things I can think of which merit placarding, but there is no panel space. In an ideal world, the pilot would be so well trained and current on type, that no placards would be needed at all, the pilot would just know. So how much can we accept a pilot "forgetting" little things?

But, as I reminded my boss, and the certifying authority, for the five years I worked in GA airplane starter design and approval; if you can't get it started, it's really unlikely the plane will be at risk of an accident. The only time it was a discussion point was restarts in flight in twins. So, failures to start and unsafe conditions during start are way down the list of hazardous flight conditions. Sure, a carb fire is a bad thing, but the plane has a firewall, and exiting it is pretty straight forward. A start fire is the least possible safety of flight risk of things a pilot can get wrong. The only reason that the SM1019 had the start warning placard is the risk to the $150,000 engine of a hot start!

And, with placards, as flight manuals, we really try to quantify things, rather than subjective words like "over" [priming], or "must be clear of...", which are very difficult for the pilot to measure or confirm. I so dislike assembly instructions which advise for screws and bolts: "do not over tighten". What a silly thing to say. My wife can't turn it tight enough, but I could strip the hole tightening. If it matters, state a quantity, and a way to measure it if needed.

It's a real balance as to training verses flight manual information verses placarding. A lot of thought goes into what information where during approval. I encountered this philosophy while I was receiving type training on the turbine DC-3 (a very technique sensitive airplane!). The flight manuals, both Douglas original and Basler STC, are very brief. My mentor explained that for that airplane type, the main customers were airline and military, so training top heavy. Verbose manuals were less necessary, as you weren't flying it without a lot of training. But you go and buy a Caravan from Cessna, which is only a little less complex than the DC-3T, and you get a 500 page flight manual - because Cessna knows that a wealthy low trained pilot can buy a Caravan, and choose to fly it home with little or no type training, so they hope that pilot reads the manual (and they heavily placard the cockpit, so at least you read the placards when you're bored during a long flight!).

[Jhieminga]

The effective fuel clearing procedure should be detailed in the Flight Manual/POH and with expanded explanations.
I agree, but I'm sure you are familiar with the brevity of the Cessna 150 POH.... I mentioned earlier that this particular school (it is not operating anymore) would have their students fly dual flights on a C152, then switch them to a C150 for their solo flights. They were suddenly confronted with a different flap system, an ASI in mph rather than KT and a different engine and they were often on their own for this experience. I'm still very glad we only had a few carb fires, it could have been a lot worse.

[Fl1ingfrog]

Pilot DAR, I haven't quoted you because I agree with all that you have written in your last post.

[Pugilistic Animus]

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.
Pilot DAR, when you say turbines do you mean turboprop or turbojets?

[Pilot DAR]

P.A., In this context, I mean turboprop and turboshaft engines. The engine with the start warning to which I have referred is the Allison C-250, when installed as a turboprop in a Siai Marchetti 1019. I have flown the same engine as a turboshaft in a number of different helicopters, for which there is not a similarly worded placard.

I am a turbojet and turbofan neophyte! 'Never flown a jet.

[Pugilistic Animus]

Pilot DAR I'm on the flip side, I have never flown a big piston but it's on my bucket list to do that some how. The biggest thing that I noted going from pistons to Jets is Jets are more sensitive in pitch than props...
also, if you wanna go have real fun try to get a hold of any of the Learjet 20 series because they are fun and a little mischievous. Also, I'm sure you know about Handling The Big Jets...reading DP Davies you will realize that Jets aren't more difficult than props, just different in some ways Jets are easier than props.