Priming - How?

[silvereagle]

Having spent the vast majority of my flying in turbine engined aircraft, I now find myself flying piston. I was wondering if someone could advise me on the whole issue of priming. I fly a 150 with an O-200 engine.

My key question relates to the position of the throttle during the priming sequence. Is it open (fully) or closed? No check list I have read indicates where the throttle should be during priming. It does say mixture rich but no mention of the throttle.

Also, why does the 152 manual offer very different advice on priming? The O-200 manual suggests up to 6 primes while the O-235 needs only 3.

Finally, if you over prime, where does the fuel go? Does it remain in the carb or in the cylinders? And reading the 150 manual, why, with a flooded engine do you crank it with the throttle fully open and the mixture at cut off?

God, turbines are so much easier!

[snips]

Start with two primes for a cold engine. The add and one/two per attempt at a start. You soon get to know the peculiarities of the individual aircraft you fly.


Give the primer time to fill up before you push it back in.
Throttle position doesn't matter. The fuel is squirted in close to the inlet on the cylinder.

To clear a flooded engine, throttle wide open gives maximum airflow over the fuel to evaporate it.
Mixture to ICO stops any more fuel being added to worsen the problem.
When the air/fuel mixture is correct hopefully the engine will fire then the mixture can go rich.

Over priming usually results in the fuel entering the cylinder washing the oil off the bores then out through the exhaust and onto the floor underneath the a/c.

[Duchess_Driver]

Snips mate.....

Take exception to the blanket one/two per attempt at a start.....

Over priming usually leads to engine fires rather than the engine firing!

Your POH should be clear about the number and sequence of events.

My advice, use the primer as little as possible - but when you do prime - give your warning as you start the priming process, then immediately after the 'pumping' stroke turn the switch to start, don't give the vapour time to evaporate.

Happy starting.

[silvereagle]

OK, so if you have over primed, with the fuel in the cylinders, where does the fire start?

[A and C]

Just give the throttle a few pumps and that should be enough to get the engine primed!

[snips]

Duchess Driver, I suppose I should have reiterated the max 6 primes as silvereagle had posted.

If that doesn't work treat it as a flooded engine and follow the relevant FRC's Checklists

The fuel needs to vapourise to burn. No point rushing, prime the engine, have a look and shout your warning. Then engage the starter.

If you start the fire in the cylinders usually means the engine runs:}

Coat door etc !

However the fuel that has entered the exhaust system, dribbled all over the floor is ripe for catching fire. (Fuel Oxygen and Heat all together)

Some engines prime on all cylinders some on just two hence the different number of prime/max primes.

[modelman]

Sunday was pretty cold I had the first start of the day on a club 150:

Throttle 1/4" open(don't think it matters much at this stage),mixture full rich.Keys in (so you can start cranking immediately after priming),5 full primes (allow time for primer to refill),2 full pumps of the throttle,1/4" throttle and instant start.

I also like to pull the prop through a few blades to break the stiction that can form before starting during walkaround (check keys out,throttle shut,ICO and park brake on,chocks if poss).Try to finish with the prop pulled back against compression so the starter motor has less initial resistance (do not turn engine opposite to usual direction as this can knacker the vac pump vanes).

Warm start then just 2 primes,2 throttle pumps.Works ok for me these days.
I got quite hung up about starting after a couple of failures but this turned out to be poor battery/plugs at the end of their life/mags due overhaul.
MM

['Chuffer' Dandridge]

OK, so if you have over primed, with the fuel in the cylinders, where does the fire start?

Depends where the fuel prime is directed within the engine......And depends on the type of engine. I've seen induction fires after overpriming, and also seen a pool of fuel catch fire that had dripped out after the guy was pumping the throttle.

However, one aircraft I fly (0-540 x 2) will not start in this icy weather unless you pump the throttle as you crank the engine over.

I've also seen a PA28 (0-360, not an injected engine) that would only start when warm with the mixture control in 'ICO'

As with most things aviation, know your aeroplane...:ok: And if in doubt, start from lean and work up to rich rather than the other way.

[Mark1234]

I have found opinion divided between use of throttle pumping and primer to prime, however, there appears to be more risk of causing fires with throttle pumping - particularly if the carb is upside down on the bottom of the engine - where does the fuel go? Some A/c however do not have a primer, and the POH specifies throttle pumping (robin)

The primer is designed for the job.. and usually deposits the fuel in the induction manifold, directly upstream of the cylinders. It's operation is independant of the throttle, it's just a sliding pump that chucks a slug of fuel.. This may account for different instructions - different size of priming pump?

[stevef]

[Try to finish with the prop pulled back against compression so the starter motor has less initial resistance (do not turn engine opposite to usual direction as this can knacker the vac pump vanes).]

Sorry - this thing about not turning the engine backwards by hand is a popular misconception. I have an e-mail from Parker Hannefin's Service Dept which states that it will cause no harm whatsoever. What will ruin the vacuum pump carbon vanes is oil/solvent contamination or fitting the wrong handed item to the engine and running it (some pumps are designed to run CW, others CCW).

[silvereagle]

I am still no clearer on the pumping of the throttle business. Clearly, opinion is divided. That said, the guidance in the 150 pilot's guide states:

'Pumping the throttle, especially during starting, should be avoided as the accelerator pump in the carburettor can cause a fuel pool in the intake, leading to a fire risk.'

Views?

[Fright Level]

Views?

Follow the POH, it's what's it there for.

If you did make an insurance claim for a burned out plane, you can bet the insurers will check how closely you followed the manufacturers instructions for starting the engine.

[snips]

Pumping the Throttle makes the accelerator pump squirt and extra few cc's of fuel into the airflow through the carb. This is to enrichen the mixture.

If theres no airflow the fuel runs down hill back through the carb and air intake filter and into the engine cowling. This all depends on the amount of pumping you do.

Just need a source of ignition and you've got a nice intake fire!!

Some a/c do not have accelerator pumps, some a/c have no primers and rely on the accelerator pump to prime. Know your a/c

Another common problem is one of the conventional primer nozzles is blocked and the cheat fix is to pump the throttle to make up for the difference

['Chuffer' Dandridge]

Of course, even some modern aircraft are not fitted with primers and pumping the throttle is the ONLY way of doing it....Which is even more reason not to overdo it..

[Robin400]

Regarding over priming see Robin S/B 160 and 161

[Mark1234]

Would be very interested in a link to those..

I would also presume that the intake design and carb position on the throttle primed robins is slightly different to that in the C150 with a primer.. It's trite, but the answer should be follow the POH.

[hatzflyer]

The primer on the Continental shoves fuel in just above the carb throat(150).Overpriming allows neat fuel to dribble out of the carb onto the floor and is a recipe for a carb fire.If a fire starts dont stop cranking or the whole shebang will go up.Keep cranking and it will suck the fire into the carb and actually put it out!
The primer on the Lycoming (152) is injected direct into the cylinder head behind the inlet valve which is safer but more prone to flooding hence the different technique.
Also be aware that the Continental system will lead to a pool of raw fuel laying in the inlet spider if overprimed,this then leads to the engine sucking in a huge gob of raw fuel when it fires which makes the engine rich cut.This then confuses the hapless pilot who thinks it stopped from lack of fuel and proceeds to prime some more and make the whole situation worse!
If you suspect it is overprimed,open throttle fully,stand on the brakes,mixture FULLY lean and crank.Close the throttle SLOWLY as it fires.(If you shove it in it injects fuel via the accelerator pump in the carb).
The position of the throttle has no bearing on priming on either engine.

[silvereagle]

Thanks to all.

To summarise what I have learned here:

Priming is fine, but do follow the perceived wisdom in the POH.

Pumping the throttle is bad (on an O-200); it doesn't help matters and following priming, may introduce too much fuel, in the wrong place, at the wrong time.

If the ambient temperature is very cold, prime according to the POH, but be very careful about doing any more if it doesn't start first time.

In cold weather therefore, if it doesn't go first time (or cuts out almost as soon as you release the key) what is the best option? Go for a 'flooded engine' scenario and get rid of the unburnt fuel, then wait a while and re-prime?

Any thoughts?

[Whiskey Papa]

That just about sums it up, but I caution against turning the engine over by hand (modleman's post), no matter what precautions are taken - it's simply not worth the risk.

WP

[RTN11]

You should only ever need to prime on a cold start, and for that you should use the primer. If they wanted you to prime with the throttle, there wouldn't be a primer (eg robin)

on a warm start, very little or no priming is usually required.

[RatherBeFlying]

Starting below -10C is where it starts to get interesting; -20C was even more fun when I visited Sudbury, Ontario on a cold March weekend. Taxiing in I noticed that all the a/c in tiedown had on engine blankets and an electric cord leading away. I felt rather naked.

Starting began with a preheat; after a POH prime with six shots did not do the job, the mechanic took over to work the prime while the lucky apprentice pulled the prop.

Engine blankets help keep the heat in between flights, but a couple shots of prime doesn't hurt in -10C in a C-172 that has been flown in the last hour.

The most important trick is to conserve the battery and not burn out the starter motor with repeated attempts. If it doesn't fire up on the first or second try, find somebody who knows the airplane.

[A and C]

The usual reason for intake fires on starting is the time between the priming and turning the start switch.

I have to lay the blame firmly at the feet of instructors for the way that the majority teach the use of check lists, these lists are used as "do lists" that students slowly and deliberatly follow.

Most of these lists result in the tyro pilot taking a long time between prining the engine and turning the starter, this as a number of people above have rightly pointed out results in the priming fuel running down into the airbox risking an intake fire.

Staring the engine should be a memory drill so that the very minimum time is taken from the first prime untill the engine is stable at the recomended idle RPM and the engine T's & P's have been checked, only then should the check list be used to check that the correct checks have been done. This is why it is called a check list.

Starting a light aircraft engine is not rocket science so why do instructors try to teach it like it is?

[Pugilistic Animus]

Not meaning any thing by this but I'd really love to hear SNS3Guppy's take on the issue;)

PA