Correct operation of the primer - PA28

[number1pilot]

Hi,

Can someone please explain to me how to correctly lock and unlock the primer in a PA-28.

Thanks

[BackPacker]

Push it in and rotate so that it can't be pulled out.

[A and C]

The primer is quite simple in that the piston has a lug on it, this lug locks into a detent that is just beside the slot that allows you to pull the primer piston out to prime the engine, once you have primed the engine and the piston is fully in rotate it slightly and pull it out while lining up the lug with the small hole drilled in the retaining ring, the piston will come out wards a little and engage in a detent, you can tell it is locked into the detent as the piston will no longer rotate.

It is vital that you use the check list as a check list and not a "do list" when priming the engine, you should have the engine ready to start with all the controls in the start position ( except for the mags..... But the key should be in the mag switch) before priming, once the engine is primed the engine should be started instantly if you don't start the engine quickly you are setting in place all the ingredients for an intake fire.

The reason for this is that the priming fuel will run back down the intake track and collect in the intake box below the carburetor, should the engine backfire you will then set fire to all the fuel in the intake system, I have seen two PA28's badly damaged and a Robin destroyed by intake fires due to bad practice when priming. ( the Robin has a different way of priming but the principles are the same).

[number1pilot]

Thank you A and C and Backpacker for your helpful answers. The reason I was asking is I am doing a conversion to the PA-28 soon and the aircraft I fly at the moment doesn't have a priming device.

[Thud105]

Yup, read the book and/or ask an instructor. I would never ask for advice about anything on an anonymous forum, and am constantly astonished so many do.

[RTN11]

So your first flight in a PA-28 and your two questions are how to operate the primer, and if you can fly an ILS approach. These questions are at the opposite end of the spectrum!

The primer could not be simpler, and many people don't even bother to use it and just pump the throttle instead. Not saying that's correct, but many people do it.

[airpolice]

Depends on which PA-28 you are talking about.

On both the Archer IIIs that I fly, it is a spring loaded button on the overhead panel, which needs held in for a few seconds to prime, when priming is required.

So.... it is "locked" by the battery master being off.

[JW411]

I'm not keen about pumping the throttle on a PA-28 unless it is November the 5th and you want to get rid of it. On my own aircraft, I would use the primer for the first start of the day. From then on (assuming that the engine was warm) I would leave the primer alone. I would switch on the electric pump and slowly open and shut the throttle slowly twice.

Then switch the electric pump off, set the throttle and the engine would fire first time. Worked every time.

[NorthSouth]

C'mon people. It's a straight question.

In my experience lots of PPLs, never mind students, have real trouble working out how to operate the primer. After operating it they tend to push it in, wind it round and round lots of times then wonder why it's still not locked.

The procedure is simple:
1. rotate till it is able to be pulled out, then pull fully out
2. push firmly in for as many primes as required (e.g. none or one if engine warm or up to five if cold - depends on the particular aeroplane)
3. push fully home for last time and turn it through 90 degrees to the right or left AND NO MORE THAN THAT.
4. Give it a tug to make sure it's locked shut

NS

[Fly-by-Wife]

Be careful not to twist the knurled locking ring with the detent - it's perfectly possible to unscrew it from the panel and allow avgas into the cockpit (at least in a Warrior II).

FBW

[abgd]

Yup, read the book and/or ask an instructor. I would never ask for advice about anything on an anonymous forum, and am constantly astonished so many do.


I've never had any trouble priming PA28s or 38s, yet I learned something new from A&Cs response and clear explanation and will be a lot more careful with the primer in the future.

Nothing wrong with anonymous discussions so long as you keep your brain turned on, which is also a necessity for conversations in real-life.

[Thud105]

"Nothing wrong with anonymous discussions"
I never said there was. I draw the line at anonymous advice, as I quite clearly stated.

[A and C]

IF you turn the primer piston by 90 degrees you have NOT locked the primer as it is not in the detent and can me rotated by vibration.

The detent can be found by looking for the hole that is drilled knurled retaining ring.

[Big Pistons Forever]

There is nothing wrong with pumping the throttle to help prime the engine. In fact on a cold day the engine will sometimes start to die right after it starts. A few quick pumps will give the engine the extra fuel it needs to keep running.

Similarly a slightly under primed engine will start with a few pumps on the throttle. The only precaution is to only pump the throttle while the engine is being rotated by the starter or all ready turning on its own.

[A and C]

I compleatly agree but having seen a number of people set fire to aircraft by over priming I think that the last paragraph of your post above should be in BIG RED letters so that you leave the inexperienced on this forum in no doubt that the engine must be turning before pumping the throttle

[NorthSouth]

A and C:IF you turn the primer piston by 90 degrees you have NOT locked the primer as it is not in the detent and can me rotated by vibration.Not quite. You MAY not have locked it IF you have not turned the lug so that it's behind the detent. But you WILL have locked it if you first "push fully home", then turn it 90 degrees, then "Give it a tug to make sure it's locked shut".

None of this problem arises though if, having unlocked the primer in the first place, you pull it straight out and push it straight in without twisting it, because by definition the lug remains aligned with the hole through out the process.
NS

[stevelup]

I compleatly agree but having seen a number of people set fire to aircraft by over priming I think that the last paragraph of your post above should be in BIG RED letters so that you leave the inexperienced on this forum in no doubt that the engine must be turning before pumping the throttle

What if you haven't got a primer ;)

[A and C]

Just by turning the primer through 90 degrees does not result in locking the primer, there is a locking detent in the knurled ring, the primer is locked by pushing the primer all the way in, rotating the piston until the lug in the primer piston alignes with the small hole in the knurled ring and then pull the piston out to ensure that the lug in engaged in the detent.

You can check that the primer is locked into the detent as it should no longer be free to rotate. I know that if you just rotate the piston by 90 degrees it is unlikely to rotate due to vibration on the average GA flight but if you are flying a long way it could happen........ And that is just the sort of flight that you can't afford to have the extra fuel consumption due to the primer becoming unlocked !

Stevelup

Primer or no primer ( a la Robin ) it is far better if the engine is turning before you prime with the throttle, it is very easy to set fire to a Robin if you prime with the throttle and dont have the engine turning as the carburetor is only just above the air box and the fuel will quickly return southbound to collect in the air box. If memory serves me correctly the London Gliding Club lost a Robin to an intake fire on starting, the most likely cause being overpriming.

Gypsy engines are the exception to this as you can't prime the (4 cyl) while the engine is turning as the priming is done from outside the aircraft. ( some six cylinder Gypsiys have a primer or Ki-gas system as it was known )

[tmmorris]

Interesting, thanks for the Robin tip which I didn't know. I've always primed before cranking. Got away with it thus far...

Tim

[JW411]

My son set fire to my PA-28 one night. He was starting up at the fuel pumps after refuelling. This ensured that he got a lot of attention quickly for he was not a stone's throw away from the fire station.

He was pumping the throttle whilst cranking the engine.

He did not do it again.

[david viewing]

Years ago when the plane (Warrior) was on rental a pilot set it on fire while starting and the aircraft was only saved by another pilot who used his own extinguisher (£300). Damage was limited to scorched hoses, wiring, etc.

My late partner Trevor Shaw was a race car mechanic and he did some tests on the priming options. We knew that the pilot was in the habit of pumping the throttle and we found that only three pumps was enough to have liquid fuel running out of the air changeover box beneath the carburettor. Any blowback on firing would set this off and because the fuel was running over the fibre glass bottom cowling which is rich in paint, oil residue, etc. etc. and fire would take hold quickly.

On the other hand, it took a great deal of priming to get any fuel to run back out of the carb. (I remember 15 strokes being mentioned but please don't rely on that!)

The problem with pumping while cranking is that there probably isn't enough airflow to guarantee that the fuel is drawn into the engine. In my opinion it's only safe to pump the throttle once the engine is actually firing.

Many instructors, and also some engineers that I have discussed this with, appear to be confused about this and continue to advocate throttle pumping. My experience says: NO!. Do not pump the throttle until the engine is firing and only then if you have to to keep it 'alive'.

Underlying all this is the poor starting which besets many rental aircraft and encourages 'innovative' starting techniques. The best insurance against fire is to get the engine fixed so that it starts properly.

[cockney steve]

saved by another pilot who used his own extinguisher (£300). ]
They must have seen him coming!...Unfortunately, BCF is now unobtainable to all intents and purposes ( though I believe there are exemptions)...just leaves you with CO2 as a practical alternative. The small ones are an alloy bottle with a plastic "megaphone"...not much opportunity to save weight there!...and change out of £50.

I once despatched an extinguisher salesman from my garage-business, with a flea in his ear. - Upon being told that I'd give a £1000 for one of his products, if I had a fire,- I waved the keys, said no, I'd lock the door, cross the road (to the Pub) and call the Brigade and Insurance Co.....but anyway, I'd an Aerosol-style BCF extinguisher from the car-shop ~£1.50....when he countered that such "cheap rubbish" couldn't be relied on, I pointed out that a dozen-case would be cheaper than ONE refill of his company's product, so I'd 12 chances of success...whereas if HIS didn't work...:eek:...Also , Redex (remember them?) would be interested in his comments re-their quality -control.

In the event, i DID have a major fire, the CO2 extinguisher burst with the heat and even the brass valve melted..

Petrol (Avgas) is dangerous and the capacity of the heavier-than-air fumes to collect and "puddle" in still air , should not be underestimated...I endorse everything said about pumping an UPdraught Carburettor....side or down is a different ballgame.
Abie-"sorry about your warehouse fire"
Hymie,-"Shutup, it's tomorrow. :p

Priming will leave a puddle of fuel in the inlet-TRACT incoming air to the cylinders will evaporate this and the "fumes" will thus enrichen the mixture in the cylinders....over-priming will just result in longer cranking to deplete the surplus puddle....remember , the air-fuel ratio is quite critical and more is NOT better.