Primer in and locked

[Dude~]

Another rainy day and another thing that I've been meaning to look into after a primer unit came away in my hand during priming recently:

If the primer syringe was deliberately extended, or worked its way out in flight, would it really cause the engine to stop?

And if so why?

:confused:

[FlyingForFun]

I have experienced very rough running on a PA28 during the power checks once. A sweep of the cockpit to check everything was in order revealed that the only problem was the primer, which was almost but not quite right the way in, and was not locked. Once this was rectified, we repeated the power checks with no problem.

FFF
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[foxmoth]

Try it during the power checks sometime - certainly on the Pa28 you will get a good indication of why it needs to be locked - even if only out a fraction!:uhoh:

[hardcase]

we have a solo-hirer of our PA28 who complained of rough running when doing the power checks and called it a day saying the mags were playing up. when i went out and had a quick look at it, i noticed that the primer was unlocked. i started it and had no problems once the primer was locked.

with same hirer on a different day, we had to go out to the apron and explain why it sounded so bad when he was taxiing. some people just like learning the hard way....he just didn't listen the first time. :rolleyes:

don't want to think what would of happened on take-off to him if he had not done proper power checks....we are reviewing his situation as i write this

[Lister Noble]

I'm learning to fly on a PA28 and am suprised that the primer lock is not more positive, you can turn it too far and it pops out again,but I always check it thoroughly just to make sure.
I'm also suprised that the mixture lever does not have a detent (like a gear lever) to prevent accidental operation, as I have heard of low hours pilots inadvertantly operating mixture instead of carb heat on a PA28.
Lister:)

[Dude~]

Thanks for the replies - sounds like conclusive proof.

Can anyone explain exactly why it causes problems though?!:8

[OpenCirrus619]

I believe it can allow fuel to dribble through the primers - causing very rich running / misfiring.

OC619

[A and C]

The primer lock is positive, the pin in the primer piston should be engaged into the slot in the primer outer case.

The position of this locking detent is indicated by a small hole in the primer outer case, the only problem with seeing this is that due to the way that the primer is fitted into the panel the hole might be at the bottom and not visable unless you lay on the floor and look up !.

When you push the primer piston and turn it to lock the piston so soon as the piston locking pin is engaged apply a LIGHT pressure on the piston towards you and rotate it , you can then feel it when it engages in the locking detent.

[Send Clowns]

It allows quite a lot of fuel through, causing a rich cut. There is too much fuel for the air going in, so preventing efficient burn. I've never found the lock a problem, I try to pull it out every check, to make sure it won't come out.

[Dan Winterland]

It allows more fuel into the cylinders by the vacuum sucking fuel through the primer which needs to be locked to seal the priming line. The PA28 primes directly into the cylinder on three pistons only, can't remember which. This makes the mixture too rich and you get rough running much the same way as if you climb too high without leaning.

Also, when you try to shut the engine down using the ICO, you may find enough fuel is being drawn into the cylinders to keep the engine running.

I have seen this one one more than one occasiion and it should be the first thing to think about if a pilot reports a higher than normal idle, rough running at power or an inability to shut down an engine on the ICO.

[Lister Noble]

A and C ,thank you,I will check it out this weekend.
Lister:)

[shortstripper]

It does have a use other than it's primary one, that can work in your favour too. I've never tried it, but apparently if you have serious carb ice or other carb related power failure, it is possible to keep the engine going by gently pumping the primer to feed it fuel.

SS

PS .... The reason I've never tried it is that IMHO it is an action very much reserved for dia emergency use only! Normal engine failure over landable fields is not such a dia emergency and 100% focus should be given to putting down, not playing with dubious remedies ... Over the channel and running out of ideas may induce me to try it though!

SS

[Benix]

might possibly work, but u would have to very, very gentle with it. Just to see what would happen (and probably very foolishly :O) i operated the primer very slowely whilst at 1000rpm on the ground and it very nearly killed it as you would expect from a well over rich mixture. Well u have to try these things :rolleyes:

[runway16]

The old Primer debate. It seems to be happening more in recent times. Maybe checks are declining.

If not locked in the primer will allow the engine to run like a Fergy tractor and will make the pilot think that an engine failure is about to happen.

When this subject comes up it always reminds me of the over-confident instructor at the flight school I was working with at the time. Said instructor ways knew more about everything than anyone else and was quick to let everyone know that he knew more than them.

One day said instructor brought a Warrior back from the run-up bay and complained to the engineers that the engine was running extremely rough and that he felt it had a major problem. An engineer checked out the aircraft and returned it to service after he had written up the paperwork as follows; 'Aircraft reported to have a rough running engine. Fuel primer
found unlocked. Primer locked in position. Engine operated normally. Aircraft returned to service'.
For ever afterwards the engineers always had a sly smile when said instructor was around. I know why.

[G-ANDY]

Having the primer unlocked allows to much fuel to enter the cylinders causing a rich cut. But having the primer lines open, can air also get into the cylinders having an adverse effect?

Ta.

[BEagle]

Dan Winterland explained the symtoms of an unlocked primer to me once and I NB'd it for future information..... It wasn't something I'd heard about before.

A couple of years later, one of my a/c was snagged for rough running. The FI had aborted the trip. As soon as I climbed into the a/c I tried the primer.....

....it was unlocked. When resecured, the engine ran perfectly happily.

Because we don't run a 'blame culture' or embarrass people with patronising tech log entries, we made sure that all our members learned from the incident - since then there have been no further instances.

Thanks, Dan!

[sander82]

I usually ask my students during flight what would happen when I pull the primer out and push it back in. Usually I get a big question mark, sometimes they tell me the engine will quit.
The truth is that nothing will happen when I pull the primer out, and I mean absolutely nothing, also in a warrior. The problem comes when I push it back in, since now I have excess fuel going to the cylinders.
I mean try it, and you will see, just be carefull when you push the primer back in, so you wont flood the engine.

[Pink skies]

Sander 82

I think you will find that if the primer is left unlocked for a while it will cause rough running. But yes you are correct pushing the primer in makes it worse as you are pushing fuel straight into the cylinder.

[Dan Winterland]

I once read an account of a WW2 P51 pilot who got a hole shot through his cooling system and his engine was starting to overheat. Remembering from his training that a rich mixture aids cooling, he started pumping the primer to push more fuel into the cylinders. It kept the engine cool enough for him to limp over the North Sea back to Blighty. However, his left hand was a bloody mess from his exertions.