To Prime Or Not To Prime? That is the question!

[G-Foxtrot Oscar 69]

I was recently party to a discussion on whether or not to prime. In particular the standard plants found in Piper 28 series etc.

I have always been told that you should prime when the engine is cold, 5 squirts and if not sure give it 2 for a good measure.

I have now heard the view that you should not as it may cause an engine fire!

I know that pumping the throtel when cranking the engine on start as this may cause a carb blow back and subsequent fire. That is why you should prime.

Can any one cast light experience or anecdote on this?

[tom775257]

I use the rule that if the oil temp is showing nothing 5 squirts, if it is showing a low temp 1 or 2, and if it has just run I do not prime. I’m not sure about 7 though…if 5 doesn’t work I just stop turning the engine over, pump the throttle once or twice (gives a bit of fuel from the accelerator pump) and usually it fires. I have yet to flood the engine. Good luck starting a cold Pa-28 without priming!

[IO540]

What (if anything) priming actually does varies greatly with the engine model, the fuel plumbing around it, ambient temperature and engine temperature.

[WelshFlyer]

I use the system; if the A/c has been flying in the last hour I give it 2 squirts, if it has been flying in the last half hour I don't give it any, and if it is the first flight of the day I give it the full 5.

That's on the PA28-140. It starts every time:)

WF.

Just "getting into" the pa28 series - much better than Cessnas!

[Tall_guy_in_a_152]

Hmmmm. I've never primed an engine in 8 years of flying (C-152s and PA28 Archer). Never had a problem starting either. Admittedley, I have always taken a winter break (hoping not to this year) but have done early morning starts on chilly mornings the rest of the year.

My only contact with the primer has been to ensure that it was locked.

Have I been doing damage, or just lucky to have easy starters?

TG.

[mad_jock]

I use crank engine over and prime at the same time. when the engine fires stop priming.

But this works for the machines i fly. Always use the POH and local know how. Remember all planes are female so they all have different needs first thing in the morning

MJ

[kabz]

If it's fuel injected and been running in the last couple of hours, just turning it over normally works for me.

If it's a first start of the day, 4 secs on the fuel pump or until some pressure shows, then shout clear prop and crank it. Works for the injected travel air I fly.

For carburetted 152 and 172s on a first start of the day, I pull the primer out really slowly and give 2 squirts. Note that you want to feel that some actual fuel is being delivered by the primer. This is not always the case.

If a 152 or 172 has been used that day, then 3 pumps on the throttle (accelerator pump) is invariable enough to get it going on the first crank.

Fuel injected 172s have been my worst experience so far :{

[Mike Cross]

A normally aspirated engine needs a richer mixture when starting. Those of similar age to myself will remember the need to use the choke when starting a car from cold. Nowadays it is automated on a car and you never see it. Some aircraft, e.g. those with Rotax engines have a choke but most have its less efficient fore-runner, the primer.

The primer is a hand pump which squirts neat fuel into the air intake to enrich the mixture.

Many carburettors also have an accelerator pump which squirts additional fuel in to the carburettor throat when the throttle is advanced, this is because sudden opening of the throttle can cause a "lean cut" as additional air is allowed in by opening the throttle and the fuel supply hasn't yet caught up. Not what you want on a go-around and the reason why you are taught to open ther throttle progressively, not suddenly. Some, like the Luscombe I fly do not have an accelerator pump and are therefore more prone to the problem.

There are therefore two ways of primimg on most a/c, using the primer and pumping the throttle.

Over-priming puts too much fuel into the air intake. You will get a smell of fuel around the engine and some may drip from the air intake on to the ground. The most likely result is that the plugs will get wet with fuel and the engine will refuse to fire. However if you were to get a backfire it could ignite, in which case the remedy is to set the mixture full lean and open the throttle as wide as is safe so the excess fuel is sucked into the engine. Once it has all been consumed the engine will stop.

The Luscombe (Continental C-85) always needs 4 squirts when cold and a couple when re-starting after a taxi to the pumps.Remember all planes are female so they all have different needs first thing in the morning is absolutely right.

Mike

[Sensible]

I never use the primer either, just turn the engine over whilst sharply advancing the throttle if it doesn’t start within the first couple of turns, sharply advance the throttle again.

I developed this technique flying Arrows, if the Arrow gets too much fuel via the primer, it's a real pain to get it to start and involves hand turning the prop backwards to clear the excess fuel.

The primer-less start works so well for me that I use it on the Archers as well now.

I know of at lest one fire caused by over use of the primer and I guess that's a bit embarassing for the pilot! :{

[shortstripper]

Of course all these primers were fitted at great expense as ornamental exentricities ... weren't they? no?

As has been stated, they are there for a purpose ... to richen the mixture to aid starting. All engines are different and if yours starts without all well and good ... or is it? Perhaps the mixture is set too rich generally and you are wasting fuel? dunno?, but it's worth checking.

Cetainly most VW powered homebuilds need priming or they just won't start (of course some have carbs with chokes so may use that method instead)

An added bonus of a primer, though not one I've had to use ... is that you can keep an iced up engine running on one! If you have severely cocked up and choked up the card with ice to the point the engine wants to quit ... you could slowly squirt fuel via the primer directly into the inlet manifold and keep it going until you manage to melt the ice! ... theoretically:\

IM

[BigEndBob]

In 25 yrs flying i,ve always taught ..2 strokes prime on C150/2 and 4 on Pa28. The rest of the day generally the engines don't need priming to start.
The biggest problem to starting i have found are pilots who fiddle around with the throttle whilst the engine is trying to start.
Opening the throttle reduces the pressure drop in carb upsetting the mixture. At small throttle setting we get enough suck for the slow running jets to supply fuel.
All Archers i've come across would start warm with the throttle fully closed. Would start and tick over at 800rpm for a few seconds to get oil pressure up then set 12000rpm.

How many times have we heard engines scream away at 1700rpm whilst cold and these are from highly experienced instructors.

In the winter with frost and cold soaked engine i always gave the Cessna 2 primes and the third applied as the engine turns.
I always felt this would help to prevent engine fires, especially in the C150. Also with carb heat selected hot to bypass air filter which is usually covered in early morning dew.

[fireflybob]

As been said above it depends on the type etc. and I suppose we ought to be following the advice in the Pilots Operating Handbook/ Flight Manual shouldn't we?

If in doubt under prime since a) less fire risk and b) you know where you are starting from. That said use the oil temp as a guide. If Oil Temp is in the green (talking about PA28/38 here) then do not prime. Oil Temp below green then use 4 to 6 shots depending in conditions.

The PA38 (Mk2) POH says only to prime if engine does not start first time (I wonder whether this is liability in case the engine catches fire on start!) but it also says throttle set half inch for cold start or quarter inch for warm start. It also says leave primer out for cold start and push in whilst cranking.

Sorry but totally disagree with "pumping" throttle on start - quite unecessary and liable to cause many more problems that it will solve.

Whilst on this subject why to some pilots have to keep cranking when it is obvious the engine is not going to start? If you have primed correctly and set the throttle then the engine should start easily first time unless there is something mechanically wrong!

[A and C]

Reading the check list you prime the engine with 5 or 6 shots of the primer ...........back to the list and you check the mixture is set to rich ........back to the list and you check the throttle is set........ back to the list and you put the keys in the Mag switch you take a good look around and yell "clear prop !" and put the check list down and turn the starter.............the engine fires once and you release the starter switch the engine kicks back and the prop comes to rest..............I wonder were that smoke is coming from ?..........S**T ITS THE FRONT OF THE AIRCRAFT !!!!!!!.........maggs off......fuel off ......mixture off .......master off............F**K off !!!!!!!!!.


How often will this happen this winter ? and all because of the "check list" being used as a "DO LIST" .

The time between the last shot of primer and cranking the engine should be as short as you can make it , the reason for this is that the fuel that you have shot into the inlet ports will run back into the carb and then into the airbox and then most likely into the cowlings , the shorter the time between priming and cranking the less chance of this happening.

Remember a check list is to check what you have done NOT a list of things to do!.

[UV]

Great shame that no one appears to read their Flight Manual.

My PA 28 Flight Manual quite clearly states that the PA 28 MUST NOT be primed UNLESS it does not start.

I know this from:
A) Reading it and
B) Reading an American AAIB Report following a fire in the aeroplane was destroyed. They quote the same paragraphs.

[A and C]

Some of us know that when the engine is cold and the OAT is below 20c then you will need about two shots of the primer ,as the OAT drops more priming will be required untill with an OAT of -5c you will require a mimimum of 6 shots of the primer.

I see no reason to crank a cold engine on a cold day when the battery is at its least capacity just to prove what I already know ......the engine wont start without priming.

Most instructors try to teach this to the students so to save running batterys flat and over using starter motors but the bit that is usualy poorly taught is the use of the checklist .....see my post above !.

The advice in the flight manual is not practical advice based on years of starting engines , in is a get out writen in by the lawyers to get out of claims that are the result of overpriming or slow starting after priming.

[G-Foxtrot Oscar 69]

Thanks for all your comments. It is as I felt it is an issues that causes some split.

I recon that GA pilots need a cheap Turbo Shaft then we could save a fortune on service intervals and burn cheap fuel.

Oh sorry I guess that aint going to happen in a hurry!

[FlyingForFun]

I can't recall ever flying two aircraft which liked the same amount of priming. Even two aircraft of the same type. And the other the aircraft, the bigger the difference between individual aircraft of the same type.

The POH is, as always, the place to start looking for hints. But you will find a technique which works for your aircraft, based on the POH guidelines.

Even for the same aircraft, different people seem to prefer different techniques. I can't start my Europa from cold without some choke (the amount depends on the outside temperature). Other group members can only start it without choke. Doesn't matter, so long as it's a safe technique which works.

FFF
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[A and C]

All engines of the same type will start with the same amount of fuel , the problem is getting the fuel into the engine.

It is quite common for the primer nozzels to become blocked with carbon so the number of shots of primer required will depend on the state of the nozzels.

[FlyingForFun]

That sounds like an extremely plausible explaination for what I've noticed, A&C.

FFF
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[Spikeee]

wow, I almost totally forgot how much I usually prime!

I usually do 3-5 depending on when it was last used.


Last time I flew I gave it 5 - when it fired up it was running very slowly; so advanced the throttle and it kicked in, the primer was very sticky though! Almost pulling the engine onto my knees! (slight over exaduration :P)

PA38 by the way, as a far as I can remember even on my QXC where it was only like 20-30mins before the next leg I gave it 3 primes.

Is this wrong? Should I be priming less? I've never had a problem or flooded the engine before.


Spike

[dmjw01]

If the aircraft flew only 20-30 minutes ago, I'd be astonished if it required priming. You'll usually get away with a certain degree of over-priming, but one day you won't! IMHO it's a good idea to use the absolute minimum amount of priming you can get away with.

Also, in my (limited) experience, you'd be surprised how many engines like being started with the bare minimum of throttle. A quarter-inch is often too much... try just a couple of millimetres. This will also mean that you need less priming, because you're letting less air into the engine - hence you need less fuel to achieve the correct mixture. You'll also avoid the 2000rpm scream ;)

[david viewing]

I've had a bit of experience of starting troubles on a PA-28-161 (O-320-D3G) that highlighted the 'black art' approach to starting engines that seems so common in the aviation community and is illustrated in places on this thread.

This particular PA-28 became obstinate to start over a period of time and various pilots evolved 'home brew' approaches to starting. Unfortunately one of these appears to have involved throttle pumping and one day the aircraft caught alight, damaging hoses, cables and, thank goodness, emptying the fire extinguisher of a nearby pilot who knew what to do!
Without that quick action, the aircraft might have been a total loss.

Throughout this period, various engineers protested that there was nothing wrong and that the starting problems were 'normal'. Each engineer also had a 'home brew' starting philosophy.

After the fire, we investigated the possible causes and found that throttle pumping introduces fuel directly into the carburettor bore and runs straight back down the throat after only 2 or 3 pumps, flooding the air changeover valve box (where the fire started).

The primer introduces fuel into the inlet manifold and seems much less likely to result in fuel running out of the carb. I was told (by yet another engineer) that there is a bowl shaped depression in the manifold that holds the primer fuel but I have not seen this with my own eyes.

The carburettor and airbox were replaced on the engineers theory that worn slides were the cause of the starting problems but not so. Next, the primer nozzles and pipes were replaced with no apparent change. All this time the aircraft was tempramental to start with on average 2 or 3 attempts at cranking before it would go. Even the CFI volunteered this was to be expected with an engine of 'this age' (1500 hrs).

Eventually, it was discovered by a new engineer that the mags which were shown as overhauled at last annual had in fact not been opened and required replacement. Voila! Instant starting without funny proceedures - 4 or 5 primes from cold, none when hot, goes every time.

So everything that went before - agonising cranking, back firing, over priming, pumping, near disastrous fire - seems attributable to weak or mis timed ignition.

Now this might not be the case with your aircraft. But if starting is poor, needing non-standard proceedures, please find out why before something nasty happens. Engines are repeatable mechanical creations that behave consistently when properly maintained. Otherwise we'd all be walking (or swimming).

[WelshFlyer]

Is that eninge still running in said aircraft?

It seems to me (Back on the subject of new technology) that a choke would be far more efficient, and in some cases a lot safer than the primer? Why don't they use chokes? The Lycoming philosophy of using 30's technology at work again?:confused:

WF.

[IO540]

This thread has really shown the bizzare ways of engine starting... I bet a lot of them are unnecessary!

A question worth asking is whether priming is EVER needed. The engine ought to start eventually if cranked for long enough. I have certainly seen this "procedure" with fuel injected engines; it tends to start after a lot of cranking but no more cranking than the "textbook hot start procedure" normally requires.

Re a choke - if this (being simply a mixture enrichment) worked then the process of squirting liquid fuel into parts of the engine (which is what "priming" is) would not be required. (BTW I am NOT saying that squirting liquid fuel into parts of the engine IS required :O ) I don't think that a choke would help because the carb (or a fuel injection system) in an aircraft engine is already set up for a relatively (relatively to a car engine) rich cut, about 150F rich of peak with all 3 levers fully forward. Once an aircraft engine starts, there is normally no problem running it, so I don't think a "permanent" richer mixture would help.

[A and C]

A choke would be just another place for ice to form !.

[Mike Cross]

A choke would be just another place for ice to form !.
You need to bone up on your theory a bit.

Ice forms because the reduction in pressure inside the venturi causes the air temperature to drop. This causes water vapour to to condense out and form ice crystals, which are then deposited on the throttle butterfly, which is downstream of the venturi.

A choke butterfly is upstream of the venturi and would therefore not suffer from the problem.

Mike

[FlyingForFun]

I think there are lots of ways of designing a choke. The choke on my car works by moving the jet and the needle up and down - there is no butterfly at all, but the choke is alongside the venturi, not upstream of it.

No idea what type of choke my Rotax engine has, but I've never heard of any icing problems with it.

FFF
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[RichyRich]

Mike

Assuming we're talking about a conventional choke mechanism (i.e. a second butterfly, similar to the throttle one), then I would have said that the butterfly itself causes a reduction of pressure, and thus can contribute to icing. Both butterflies come to think of it.

How the restriction is achieved for the Venturi effect to take place, surely can be either by reducing the diameter offered by the walls, or by introducing a restriction 'from the centre out' - a butterfly for example. Either way, for a given volume of air to move past the restriction, the pressure and temperature will drop. Boyles' law P=VT? Was that his name?

Rich

[Spikeee]

Interesting and good to know.

I think i'll check with one of the instructors what they recommend as there are a lot of mixed views.

I've never had a 2000rpm scream before, it usually starts up ok. If anything I usually under prime as I can always prime more.

__

I'd hate a choke on an a/c like the one in the car i drive. Of a morning its like cruise control.

[FlyingForFun]

:D @ Spikee - I know what you mean about cruise control! But I don't even taxy my aircraft, let alone fly it, until the choke has been pushed all the way back in. In fact, unlike the choke in a car, it's a sprung knob which needs one hand constantly on it to hold it out, so it wouldn't be practical to move the aircraft whilst having to hold the choke out.

FFF
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[dublinpilot]

An engine for the PA28 I fly costs about €25,000. A new plane €200,000???

My Toyota car cost €24,000 in total!

Now the airplane is a bitch to start.

The car? Well, you just make sure it's not in gear, the hand brake is on, and do nothing more than just turn the key. Does't matter if it's hot or cold, up a mountain or down a valley. Just turn the key, and it starts effortlessly, every time. Time after time. It's so straight forward, I don't even need a checklist! I don't have to think about it, it just works.

Can't help but think that there is something wrong here. The cheap one is the one that's not a problem to start......not to mention, quiter more fuel efficient, more comfortable.....but then again it's ground based :)

dp

[Mike Cross]

In a conventional carburettor the choke butterfly is at the intake end of the venturi and the throttle butterfly is at the engine end.

The throttle butterfly therefore controls the volume of fuel/air mixture while choke only controls air. The effect is that if you close the choke the suction on the jet is greater and proportionally more fuel flows, creating a richer mixture. The choke is called the choke because it chokes off the supply of air.

There are other methods of enrichening the mixture, e.g. by opening up an additional jet or by using some sort of tapered needle valve to adjust fuel flow. The "tickler" on an old motor bike carb and the primer are other ways.

Re the icing issue:-
If ice were to be forming on the choke butterly it would also be forming on the airframe it's not the airflow hitting an obstruction that causes the ice to form, it's the temperature reduction as it expands through the venturi. The ice crystals that have already formed then hit the throttle butterfly and accumulate. The butterfly forms a much larger obstruction at low throttle openings which is why it is more prone to icing at low power settings. At higher power settings the ice crystals are swept into the cylinders as part of the mixture.

Try these two exercises.
1. Pump up the tyre on your bike. The pump will heat up because you are compressing air.

2. Give a prolonged spray from an aerosol can. The can will grow colder in your hand as the gas in it expands.

Mike

[A and C]

You have totaly mis-understood carb icing.

I would recommend that you go back and read the post by richyrich three or four times it is very well written and the theory cant be faulted.