Primer In and Locked

[Docbrad]

Been reviewing my notes for an upcoming checkout in a 1976 Cessna 172M, and it reminded me of something I learned in my ppl years. The cause check for an engine failure in flight includes making sure the manual primer is in and locked. I remember being told that if it is unlocked, it could be a reason as to why the engine died.

So my Question is, why would having that primer punger unlocked (and therefore out a bit) cause an engine to quit?

[AirFrame]

I remember asking my instructor that once, after he chided me for leaving it unlocked before starting a 150(*). I forget the answer, but i've always locked it before starting since.

(*) I did it because my father always left it unlocked when starting the 190... Sometimes it needed another shot or two while cranking to get running. SOP was to lock it *after* the engine was running, so that's what I did on the 150.

[iflyforpie]

The primer when unlocked allows fuel to be siphoned from the fuel strainer to the primer nozzles because of the vacuum in the intake manifold. This causes the fuel mixture to become overly rich and will potentially cause rough running or an engine failure.

[Shiny Side Up]

Had a renter once who complained aboutthe poor performance of an airplane. Checked inside, turns out he had the primer out the entire flight. Since the airplane was topped up before he went, we topped it up again just to see. I figured the little 172 was running through about 13 GPH with that set up. So even if you didn't notice the engine running rough, if you found the primer out, you're likely to have used way more fuel than you had calculated and there's a chance you don't have any left in the tanks, or tank.

[pelmet]

The primer is one of many items that should be checked just prior to takeoff. After you have finished the checklist, check the killer items on your aircraft that if not placed properly can kill you. Primer locked is one of them, if installed.

Personal technique only.

[trey kule]

Ifp.

I think you answered the question very well. Not sure about all these other posts relevance.

[Shiny Side Up]

Ifp.

I think you answered the question very well. Not sure about all these other posts relevance.

So you lament irrelevent posts by making an irrelevent post?

[trey kule]

Yes, what's your point?

Actually, i found the OP question interesting. And IFP answer was good, I thought. But I had to wade through the usual ' watch me mom, this is what I do" posts, so my non relevant post hopefully will keep things on topic....though in considering your response, and my response to your response, it failed quite miserably.

[Shiny Side Up]

But I had to wade through the usual

What are you talking about? You had to "wade" through one post to get to IFP's "valuable" post.

Besides, if I can ignore your slight that my post was irrelevant, not all primer systems are made the same, so not all will result in a rough running engine. Or at least one that runs rough enough someone who's not listening for it (or caring about it) will notice. Either way, in all manual primer systems will result in higher fuel consumption since essentially you're letting fuel bypass the mixture control. The main problem being that you may use your fuel up quicker than you thought you were going to. Something to consider when it comes to the whole engine failure procedure as it is taught (especially since our OP is a new class 4, stuff he should think about).

[iflyforpie]

I found that quite informative, SSU.

I've had a few leaky primers on the field that have been enough to blacken the spark plugs and require you to shut off with the mags or higher RPM, and were otherwise unnoticeable in flight... but I'd never thought the fuel consumption would be that high.

[trey kule]

Besides, if I can ignore your slight that my post was irrelevant, not all primer systems are made the same, so not all will result in a rough running engine. Or at least one that runs rough enough someone who's not listening for it (or caring about it) will notice. Either way, in all manual primer systems will result in higher fuel consumption since essentially you're letting fuel bypass the mixture control. The main problem being that you may use your fuel up quicker than you thought you were going to. Something to consider when it comes to the whole engine failure procedure as it is taught (especially since our OP is a new class 4, stuff he should think about).

See, its workings!


Please forgive the slight I caused you to feel

[Shiny Side Up]

That was when it was flown with the plunger all the way out. While a leaky primer will let a little bit of fuel through, when that valve is all open, you essentially have the engine working like a suction pump drawing fuel right through that line.

If that's been done, its also noticeable by a lot more residue in the exaust pipe.

edit: also the spark plugs will undoubtedly be terribly fouled.

Back to the engine failure scenario, if you do your "restart" check, find the primer out, check the fuel guages. If they're "E" don't bother with trying anymore restarting.

[Shiny Side Up]

See, its workings!

No its not! You caused me to reply to this with another irrelevant post!

Please forgive the slight I caused you to feel

I don't feel slights, I was being facetious. Everyone knows flight instructors don't have feelings.

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through.

[DonutHole]

Stupid pilot tricks:


Pumping the throttle on a cold day before starting the engine.

The ensuing panic is always entertaining.

Even more entertaining in the summer.

[Shiny Side Up]

Ok Trey, now you can complain about irrelevant posts.

[DonutHole]

You two guys are kinda jerks.

[photofly]

But loveable jerks.

[DonutHole]

Lol.

The stupid primer thing reminded me of other things people do with the gas during start... it seems everybody has their own ritual... half a primer shot? OK man...

It is related (albeit loosely)

Starting an engine can be a bitch, like trey said, pie nailed it down so let's progress. I venture to opine that pumping a bunch of gas into the cowl and lighting it off is more relevant than pointing out its irrelevance.. but I've seen that argument somewhere.

How many times have any of you witnessed a pilot suck up the fire? I've seen five fires, one guy got it right... that's pretty scary.

[Shiny Side Up]

Lol.

The stupid primer thing reminded me of other things people do with the gas during start...

Small bit of advice from a jerk: If you would have started your post with that, you would have tied it all together and it would have made sense. Otherwise it sounds like your post comes out of left field, its confusing. Its something my wife does in conversation, we could be talking about the mortgage and out of the blue she'll start mention that we need new drapes. The reader is left wondering what the hell they missed. Topic makes a sudden left turn. No sense of segue. Now I can roll with it with the wife, but I really need you guys to be a bit more sane.

Now back to your regularly scheduled aviation thread.

[DonutHole]

Small bit of advice from a guy who has to try really hard not to be a jerk.

I'm not saying this to piss on you, as a colleague, sometimes, a lot of the times, your posts come off as really aggressive and dismissive. You could have asked me where it came from, but you must have known, as left field as my post was, what it was referring to, but you chose to be a jerk..

I came out of left field, my bad, but you chose to be a jerk to me...

Already this site has a rep for running people off, I think it's time for some introspection, myself included, to determine what share of the responsibility we each take in that reputation.

[Cessna driver]

Stupid pilot tricks:


Pumping the throttle on a cold day before starting the engine.

The ensuing panic is always entertaining.

Even more entertaining in the summer.

This actually does work on a 152,

The 152 has an accelerator bump that gives an extra squirt of fuel into the carb, usually has 3 settings depending on OAT, adjustable on the carb

[DonutHole]

the accelerator pump is used to provide extra fuel for a short period of time when the power is increased.

Most, if not all carburetors (short of pressure carbs, been awhile but I think it's called an economizer that serves the same function) have some form of accelerator pump arrangement.

Its intended function is not to be an engine priming device. Yes, it can work, but with the updraught arrangement found in most light aircraft there is a risk to be weighed, and there is technique involved.

[iflyforpie]

This actually does work on a 152,

The 152 has an accelerator bump that gives an extra squirt of fuel into the carb, usually has 3 settings depending on OAT, adjustable on the carb

It may 'work' but it is incorrect.

No, there is no 'settings' for the accelerator pump. It pumps the exact same volume of fuel into the carb from the idle stop to the maximum stop. The only adjustment on the carb other than throttle and mixture is idle mixture.. which only sets the mixture at the idle stop when there is enough suction to draw fuel out of the idle nozzles (which is actually the same way fuel is drawn from an unlocked primer... with no mixture setting ).

No, the best way to start an aircraft engine is to follow approved procedures.... which is preheating it if necessary, priming it as required, and cracking the throttle. Most engines that have trouble starting with this method have other problems. Leaky primer orings that don't put enough fuel in the engine, intake leaks, or worn carb shaft bearings.... ...all of which will require more fuel to remedy if you don't plan on doing actual maintenance to your aircraft.

[Shiny Side Up]

I came out of left field, my bad, but you chose to be a jerk to me...

Sorry to hurt your feelings, you were actually a bystander in my attempt to poke Trey in the eye for being a bit of a Kenny G, you know, since I can't give him a Snickers.

[DonutHole]

it's not about hurt feelings. It's about a lack of respect.