Hey guys!
Jon from fly8MA.com and today we're going to be going over everything underneath the
cowling of our Piper Cherokee, all the parts, what they do, where the wires and lines run
to.
Hopefully, that will give you a better understanding of what's actually going on under the hood
of your airplane.
So we can see we've got our cowling opened up here.
We have our engine cylinders, the valve covers are these pieces right here.
Those are our valve covers.
And going into our cylinders we see these lines running.
Well, we have spark plug wires, those are these wires running here that go down to our
spark plugs which go into our engine cylinder.
We have one for our front cylinder as well.
This little copper line running here that's going right into the intake side-- see this
is our intake pipe-- we have an exhaust pipe over there.
We have another exhaust pipe for our other cylinder and another intake pipe.
So that little copper line running here, that is a primer line.
So when you pump the primer on the engine, that actually squirts fuel right through this
copper line, right into the intake tube, so that when the engine first turns it pulls
fuel right directly into the cylinder to help it start up before it starts mixing the air
and fuel together in the carburetor, which is at the very bottom of these intake tubes.
The air-fuel mixture flows through the intake tubes, gets burned up in the cylinder, and
then the exhaust, of course, it goes out the exhaust side.
You can see these spark plugs have a little chalky outline, a bit of a carbon build up
on it, and that's just from a little exhaust leak that's been repaired here.
Some of these exhaust baskets were leaking a little.
So we have our exhaust pipe that runs out here to the shroud.
And this shroud is for our carburetor heat.
So there's actually a hole underneath it so we'll take a look underneath the shroud.
Notice there's that hole.
So the air flows through that hole, around the exhaust pipe, and then it's going to flow
through that shroud into this scat tubing here.
And this scat tubing definitely needs to be replaced, because it definitely has none of
the springs left in it-- none of the rusty springs in it.
It needs to be rigid, so that needs to be addressed by the mechanic for sure.
And as we come back here we can take a look and see that that two or three-inch scat tube
line flows right to our airbox and the carburetor where it says "Schebler" on it-- that is the
carburetor.
So that's our fuel bowl, and we see these little cables here.
This is actually our mixture and throttle cable.
So if we were to move the mixture and throttle in the cockpit, those cables move back and
forth to adjust the mixture and also the throttle plate on the carburetor.
Now that's our carb heat scat tubing.
Our normal air flow comes from up front here.
We'll see if we can get a good shot of that.
So from the front of the cowling, we have that other three-inch scat tube that is where
our normal supply of air comes from to the carborator.
Where they join together, that is the airbox, and there's just a little butterfly valve
that selects whether you're using carb heat or hot intake air that's unfiltered or air
that's flowing through the air filter.
If we come back here, walk around to the front of the airplane we can see that this is our
air filter, and that's where the air is flowing through as it goes to the airbox for our carburetor.
We're going to look here back at our cowling.
We have this orange flapper stuff, and that is baffling.
The job of the baffling is actually to keep airflow as its coming through the front of
the cowling-- airflow through our big cooling intakes here-- flowing over the cylinders
and actually forcing the air to flow down in between the cylinders through these cooling
fins, and forcing the airflow to flow around the cylinders, taking away as much head as
possible.
So the baffling's very important to ensure you have a cool running engine.
The next thing you can see is-- although the air might get a little warm as it flows over
the cylinders, it actually flows right into this little hole there, and as it goes through
that hole, it comes back to this three-inch scat tubing, which then feeds into our shroud
over our muffler, so another heating shroud, to heat air for our defrost, which is the
skinny little one-inch tube on the other side.
Right where I'm shining my light, that's the defrost hot air going into the cabin.
Then on the bottom side here, we can see a little two-inch line that's not connected,
that should be.
Right there is where our two-inch scat tube should be coming out to heat the cabin and
then go down to our cabin airbox down there.
That's where we actually are able to select cabin heat on or off.
If we turn it off, we simply just close a door and the hot air blows out the bottom
of that airbox.
If we take a look at the bottom of the airbox there, that's a little flapper valve there,
where the hot air-- where the scat tube should be hooked up to over on this side, coming
in.
if the cabin heat is off, it just blows out the bottom here.
If the cabin heat's on, the little door closes this bottom and forces the hot air into the
cabin.
Next thing we can see from this side here-- I'll shine my light on it, that little bicycle-valve-tube-looking
thing.
That is the Schrader valve, and that's at the top of the nose strut where you fill the
nose strut with 5606 hydraulic fluid and nitrogen to give it it's shock-absorbing ability.
That little piston-thing behind it right there is the shimmy damper to reduce the shimmy
in the nose gear.
Come around the front of the airplane.
Through the front here we can see our ring gear, basically our flywheel, or flexplate
as some people might call it if they're familiar with automotive parts.
That is where our starter is going to engage to turn the propeller.
Here is our alternator belt, and that's connected basically to the crankshaft and to the flywheel
of the motor, so as the engine spins it spins that belt which drives the pulley on the alternator,
which charges our battery.
We can actually come back around the side here.
Look up inside the engine, and there's our alternator, there are our big battery wires
going to it.
And that's a relatively new shiny alternator.
We'll come back out here, and coming around the other side, we'll be able to get a better
view of our starter.
So, now we can see where the starter pinion gear when we press the start button, will
come out and that will engage with the ring gear and turn the crankshaft, turn the propeller,
turn the engine and get it fired up.
Then once it's running the pinion pops back in place.
So it pops out to start, and then goes back in once the engine's running.
On the front side, we have our oil cooler.
That's responsible for taking in some cool air from out in front of the engine and keeping
the oil cool.
So, the engine is not just air-cooled, it's also oil-cooled.
It relies on cool oil to take the heat from the metal parts inside the engine.
If we come back around to the backside here, we can look at our oil cooler and we notice
two orange hoses coming off of it.
Those orange hoses are oil-cooler lines.
We can kinda come back here and take a look at where they flow to.
Those two oil cooler lines go up and into the motor right there.
So, we have one going in there, and one in just below the oil filter, and that white
thing with the safety wire on it, that is our oil filter.
Much like an oil filter on a car, except on these ones we actually right the tail number
of the airplane, the tach time it was done, so if you ever want to know when the last
oil change was done on the airplane, you should be able to look right on the oil filter and
it'll be written in Sharpie on there.
That's where our two oil cooler lines go to.
So oil gets pumped out to the oil cooler, gets cooled, and then gets pumped back into
the engine to keep the engine cool.
On the back here we see this big black box.
There's another one on the other side.
If we trace where these wires are coming from, out of that black box, well those are spark
plug wires, so this is our lefthand magneto.
We can see the "LH" there.
The other black box on the other side, on the opposite side of that oil filter there,
that would be our righthand magneto.
We follow those spark plug wires and we see that they come to our spark plugs on our motor.
We can also follow the wires out of it that go up to the top here and they go to the top
spark plugs.
Each cylinder, of course, has two spark plugs.
One on the top, one on the bottom.
The bottom one is typically a little lead first, and with any given cylinder the top
plug is going to be controlled by a different magneto than the bottom plug, so you'll never
have the lefthand magneto controlling both the top and bottom plug for this cylinder.
One looks like the bottom plug here is controlled by the lefthand magneto and the top one is
fired by the righthand magneto.
At that's just for redundancy, so if you lose one magneto, all four cylinders will still
get spark.
You'll notice a small reduction in power, but hopefully nothing too huge or dangerous.
Other things to note on this side of the motor.
We can look here, we have our air coming in, we have our baffling and we see that we have
another little two-inch tube there, a two-inch hole going to a two-inch scat tube.
And that scat tubing comes down to this little box in here, this little shroud.
That shroud is around our engine-driven fuel pump.
How do we know it's our engine-driven fuel pump?
We can follow the orange lines here, and we can find out that these are fuel lines.
If we follow where they go to we see it goes right to the carburetor down there.
I'll shine a light to make it a little easier to see.
So those fuel lines go right to our carburetor.
We can see that we have an inline, and also a return line.
The return line comes back to feed our primer.
We can see the return line is T'd in with a little fitting right there.
If I can tilt this we'll see if I can show you guys.
This fitting right here by my finger where it turns into a copper line.
That goes back into the cockpit, these copper lines here.
Those are primer lines going to the primer for the intake for the primer and the skinny
ones are the output for the high-pressure fuel coming from the primer being pumped back
into the engine cylinders when you're trying to start up.
Now, we notice another two-inch scat tubing down here at the bottom going to this box-shaped
thing.
That box, or shroud, is actually what houses our electric fuel pump.
So, we have some electric wires here as well as fuel lines going into that box.
Obviously, the fuel lines come in for our electric fuel pump, come out right there.
That's a fuel line, this little skinny one-inch one.
The two-inch scat tube, the job of that is to blow cool air into there to keep the electric
fuel pump cool when you're operating on the ground or even in the air to keep it from
overheating.
We come down to the bottom of the cowling here, we notice this big air scoop.
That air scoop is what feeds that two-inch scat tube right there through the bottom of
the cowling, and if flows into the airbox for the electric fuel pump.
Other things to note here: we've got our starter solenoid because well it looks like a relay
of some sort of skinny wires going to it.
Those would be the wires going to your starter switch.
There are some big, heavy wires going to it, really big strong ones that carry a lot of
currents.
Well, that also feeds to the starter.
So, basically, you're able to control a big flow, a big electrical load, with a tiny little
switch in the cockpit.
So it's basically just a relay, and if you want to know more about relays you can Youtube
that, and we'll have another video to explain relays.
Over here we have our brake fluid reservoir.
I know it looks kinda like just a tin can but that's literally a brake fluid reservoir.
There's a steel line that goes to the firewall and goes down to your master cylinder and
your brake pedals.
Other things to note on this side of the airplane-- I think that almost covers it-- we do have
this one funny little pipe here coming out of the engine, and that is a crankcase breather.
If we follow that metal tube down we see that just dead ends on the bottom of the airplane.
We can kinda come around here and take a look and see if we can see it better.
That is just the crankcase breather for all the blow past the piston rings, so all that
gas doesn't hold up in the crankcase, it can be expelled overboard.
Along with your electric fuel pump in this little compartment, is your gascolator, that's
that little T-fitting down there.
There is a little steel bowl, so to speak, that we can kind of see by looking up in there.
That little steel bowl is supposed to catch all the water, and then you have that little
drain on that steel bowl that when we push up we drain some fuel, and hopefully, drain
whatever water is in the system out before we go fly.
Let's see.
From this side of the cowling, we can see this one wire here.
That little wire is actually connected directly to the crankshaft.
It measures engine RPMs.
So, it's a little wire on the side of that casing and it spins around and that goes to
our tachometer.
There's just a little metal wire spinning around in there, and that is actually what
feeds our tachometer, so as soon as that propeller turns, it'll start registering on the tachometer.
This little thing, with the red bands around it, looks like a pump of some sort.
If we follow those rubber tubes coming off of it-- here's one rubber tube coming off
back into the cockpit-- that is our vacuum pump.
We have, basically, the air that's pulling being expelled overboard, and then the low-pressure
suction being pulled through this tube right here.
So, that little guy back in there that is our vacuum pump.
These little thin wires here, right there, those little thin wires-- those are going
to your ignition switch, and we can follow them from the firewall forward here to the
magneto.
We can see that there on the magneto-- those are P-leads.
So, the job of the P-lead is to ground out the magneto and stop supplying spark to the
spark plug when you turn the ignition switch to "off."
So if that tiny, skinny, little, thin wire breaks or becomes disconnected, when you turn
the key off the engine, if it's already running, will continue to run.
Or if it's off, could accidentally start if you turn the propeller by hand.
So those skinny little wires, that is what you check when you momentarily turn the magneto
switch to "off," right before you shut down the engine, to make sure you don't have any
broken P-leads.
Coming back out here, we'll walk back around.
One last thing I want to show you guys, I don't think I went over it the first time.
We have this little line coming out here and that comes to the most important part of the
airplane, and that is a pressure switch.
It trips at 4psi as soon there's 4psi of oil pressure-- this is an oil line-- as soon as
there's 4psi in this oil line, we can see these two wires coming into it and going back
into the cockpit, that is just an open/close switch that runs the Hobbs meter.
So, if you want to save some money on your flight training, come out here before your
flight, undo that screw, and the Hobbs meter will not work, and that is the best advice
I could possibly give you during your flight career.
Other things here, that come out of this T-fitting-- this is basically, all these wires for your
oil pressure-- that's going to your oil pressure gauge and some of these other wires are going
to your temperature sensor inside the engine there.
So, this is your oil pressure switch which actually measures a numeric value of oil pressure
and displays it digitally in the cockpit.
That just about covers it.
I can't think of too many other things.
These are just great little things to check on your pre-flight, make sure they haven't
moved at all.
That connects the intake tube to the intake that flows to the oil sump.
This is where all the oil is kept, and this little blue thing down here is where the oil
drains when you do an oil change.
They just open up this little blue valve there, all the oil drains out.
And because the oil's hot, it actually kinda keeps the air-- or it keeps the carburetor
a little warmer than usual so that helps prevent carb ice on Cherokees.
Also, because the cool fuel-air mixture flowing through the carburetor into the intake pipes
is flowing through the hot oil sump, the air going into the engine actually acts to help
cool the oil.
That's actually part of the way that Piper designs to help keep your oil cool, and help
keep your engine cool so that the intake air flows from the carburetor, through the hot
oil, then to the intake pipe, then into the cylinder.
So the cool air coming into the front of the engine, mixing with the cool fuel, helps keep
the engine cool overall before it burns it and makes it very hot.
I think that's just about it.
If you guys have any other questions about stuff you saw here or anything I should have
pointed out and didn't, don't hesitate to leave it in the comments below.
I'd be happy to answer any questions.
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