Fuel tank vents

[Alberta_Canada]

Here's a newbie question, but how do fuel tank vents work on small aircraft? Lets use something common like a C172 or a Piper Warrior as an example. Is there a risk of the vents plugging up in the right climate (freezing up) or becoming plugged by dirt or bugs? I've never actually been told anything about it but with what I've seen with cars/trucks through my work its something I'm thinking about adding to my own personal inspections when I fly.

As an automotive tech, I've dealt with these issues on vehicles (mostly field trucks) where the evap canister vent gets plugged with dirt. The result is the fuel tank will be drawn into a vacuum and can potentially collapse as the engine is running. It will even cause issues where the engine will stall or run rough from fuel starvation (since the pump won't be able to create suction when there is a vacuum in the tank).

Do you run into this with aircraft? I was thinking about gravity fed fuel systems and how that could potentially become a very serious problem if it can actually happen. Even an A/C running an actual pump could get nailed by this too. What would the tell-tale sign be with something like that? Aside from engine performance issues, you must be able to hear a massive rush of air be drawn into the tank when taking the fuel cap off, no? Have any AME's had issues with collapsed tanks? That must be a tough one to notice as its all hidden in the wings (not like when I deal with cars where I just take a look underneath to see if the tank has been permanently damaged).

I'm just thinking if its possible, and you didn't notice the tank was under vacuum when you removed the cap to fill with fuel, you could potentially be filling up a collapsed tank. If you didn't pay much attention to the actual volume filled and rather went by the visual of a full tank then you could really be in trouble when you depart since you could have far less fuel on board than what you thought, and also deal with potential fuel starvation issues......or am I just crazy??

[cgzro]

You are correct it can be an issue, some aircraft have vents, some have vented caps etc and things do occasionally plug them and its wise to check them.

[iflyforpie]

Fuel vents are often neglected on aircraft in everything from flight training to maintenance, but you are dead on in your concerns about their potential malfunction.

Fuel tank vents--like engine breathers--on aircraft are mind numbingly simply compared to automotive. They just vent straight to atmosphere with nothing like evap canisters or PCV valves to contain the toxic fumes. This is for simplicity, safety, and because the greenies haven't found out about it yet.

There are numerous types of fuel vents. The simplest ones are the open tubes on top of the fuel caps. That's it, nothing more to it.

Some use a tube coming from the top of the tank and going through a series of bends so that fuel will not spill out of it unless above a certain level but air will be allowed in, much like a water trap.

More complex ones have a check valve in them that lets air in but won't let fuel out unless there is an over pressure.

Also, most fuel caps have a small vent in them to let air in should the main vents fail.


A fuel vent failure is pretty much as you described. A light aluminum tank or a rubber fuel bladder will simply collapse from the vacuum. It's pretty unlikely that the aluminum tank would escape without getting fuel leaks, but the bladder can easily.

Lots of vents on the planes I fly will often lag behind the fuel flow, as you can hear gurgling for a while after shutdown.

Bugs, dirt, and ice can certainly plug them up, so they are good to check on a walk around.

[Colonel Sanders]

Mud daubers love to plug up fuel vents, pitot tubes,
etc. Many years ago, I had a fuel vent plugged up
badly enough one night that fuel simply stopped flowing
and the engine quit on final. The tank went BANG
after I removed the cap after we landed, and presumably
bounced back to it's original shape. I hope.

Well worth checking during the pre-flight. If air can't
get in, fuel can't get out. Can even happen to hangared
aircraft, but especially aircraft tied down outside.

I worry about fuel vents plugging up during icing conditions,
too.



Sometimes aircraft will have a tube running between tanks
to try to help equalize the venting. Check your POH/AFM
for the fuel system diagram. There are ancient SB's on
this, IIRC, as well, like this one:

http://wwwapps3.tc.gc.ca/Saf-Sec-Sur/2/ ... 0-14R1.htm

It is very common that on the "BOTH" fuel selection, one
tank will drain faster than the other. This is sometimes
caused by the ball not being centered, but more likely
uneven vent/pressure.

Your question is a very good one. Please spend more
time learning and thinking about your aircraft systems.
You will be surprised what you will find out. For example,
99% of the time, you will want to start only on the
left magneto.

[PilotDAR]

The Teal, whose photo appears with my "name" has tip float tanks. When I pumped them into the mains one flight, to my horror, I noticed that the sides had collapsed, like sucking your cheeks in. I stopped transferring, and landed as soon as I could. When I opened the caps, a big whoosh of air allowed the sides to pop back normally. Plugged vents - those little mud bugs.

I have since drilled four tiny holes, and threaded fine lockwire through so it forms an "X" across the vent tube bore - problem solved.

Though plugged vents are a less common problem, it can be serious. This one reason for liking bladder tanks. The tank will collapse, and probably unsnap inside the wing, but allow you to use most of the fuel without damage.

On the subject of vents, some people ask why some certified aircraft with two wing tanks do not have a "both" fuel selector setting, only left and right. The design requirements require that to be permitted a "both" position on the fuel selector, the airspaces of the tanks must be interconnected, and vented. Most aircraft with low wings, or any amount of dihedral, cannot accommodate such a vent - so no "both".

Some vents are located in unexpected places, but for good reason. The Cessna twins with the teardrop shaped tip tanks, have the vents on the inside of the tank, so the pilot can see the vent. Thus, if you have selected the aux tanks, but done so with full mains (the tips), fuel returned to the tips, which is unused by the engine, would vent, and you could notice the overflow, if you were looking periodically.

[CpnCrunch]

In the 172 there is generally a tube vent on the port wing, and a vented fuel cap on the starboard wing (or perhaps on both wings). So if the vent plugs up you should (in theory) be okay as long as you have at least one vented fuel cap. The fuel caps weren't originally vented, but an AD was issued a few decades ago after some accidents caused by a plugged vent:

http://www.airweb.faa.gov/Regulatory_an ... enDocument

[pelmet]

Regarding the vented caps on the high wing Cessna's, it seems like in the old days, it was obvious when you had a vented cap and if I remember correctly, there had to be a vented cap on the fuel tank side without the tank vent. But getting checked out recently in some Cessna's, I was told that the caps were vented but when I looked at them, I could not see where there was a vent. I was assured by the instructor that they were vented caps.

Is it just me not seeing the obvious or is it difficult to see these vents on newer caps.

Only had one venting problem which was on a 337 which came out of maintenance with a small ball of rolled up tape way up in the vent after they had been doing some work on the fuel system. Heard a bit of a ding in the tank after shutting down. I did hear a bit of a whoosh when opening up a DA-40 fuel cap recently as well and mentioned it to the instructor to check it out. Never heard any more about it but it seems like a good idea to look into it by maintenance if you hear a whoosh when you open a fuel cap.

I'm not sure if icing conditions would cause a fuel tank cap to become blocked. Rime usually forms mostly on the leading edges of things. I suppose clear icing in freezing rain could be a problem. I wonder if the certification for known icing considers this stuff?

[Colonel Sanders]

Plugged vents - those little mud bugs

Many times I have used a piece of lock wire to
check and unplug fuel vents and pitot tubes.

If sand pours out when you run the lockwire
through it, you have a problem.

[Alberta_Canada]

Wow, thanks for the excellent replies. With so many of you having experienced plugged vent issues, it's really amazing that instructors don't drill this check home during pre-flight inspections. I know I will definitely be adding it to my check.

Your question is a very good one. Please spend more
time learning and thinking about your aircraft systems.
You will be surprised what you will find out. For example,
99% of the time, you will want to start only on the
left magneto.

I think next time I go to the flight school I am going to bring a notepad and spend some serious time just poking around the A/C and write down every possible question I can think of and then offer the AMEs coffee and donuts or bring in lunch to see if I can pick their brains.

And challenge accepted on the left magneto Colonel!! I spent some time today reading about it and the impulse coupler. So am I right that the magnetos on an aircraft are installed with a fixed timing (no form of ignition advance for higher RPMs?) so they pretty much have to be installed at a base setting that ends up being more of a compromise than anything throughout the RPM range right? ........and the impulse coupler offsets this by retarding the timing to allow for better starting. If you have the mags set to "both" you could potentially have 1 mag with proper timing (retarded so its closer to TDC) for starting at the low cranking RPM via the impulse coupler, and the right magneto would be too far advanced and the flame front created by it will hit the piston before TDC and end up with a possible kickback? I read up on the 0-320 for example and found some generations had both mags that had impulse couplers and the rest were the left only, so I guess I'd have to see which version is in the aircraft and that will determine the proper selection right? Would a kickback potentially damage the carburetor like it would on some auto applications??? (using a holley carb as an example that may experience power valve blowout during a backfire through the intake).

[iflyforpie]

Yes, aircraft have fixed ignition timing except for the hot late spark provided by the impulse couplings at starting (<300) RPM. Timing is based on full RPM and full power... on some engines with proper modifications you can advance the timing to get more power. 20-25 BTDC is typical. At lower power settings, we aren't as concerned with power (which is why we reduced it!), and the engine only has to accelerate the prop, not the whole vehicle like a car engine

The easiest way to check the impulse couplings is to slowly pull the prop through and listen for the clicks. Even on engines without staggered mag timing, you will be able to hear two clicks if there are two impulse couplings.

As for starting on the left mag. If you have a dedicated start button and you know there is only one with an impulse coupling, absolutely. But of course, some planes cannot be started on just one mag (with a mag-start switch) in which case they will either have a second ground for the other mag when you go to start, or nothing and the other mag simply makes a very weak early spark.

Remember, the impulse coupling not only retards the spark but snaps the rotor through at a high speed to allow a good spark at cranking RPM. I've only ever seen a kickback a couple times, and no damage was done... it just looked weird.

[Colonel Sanders]

Alberta_Canada: You nailed it!

Two differentiators of great pilots:

- systems knowledge (you are on your way)
- stick & rudder skill (comes with time)

Starting with magnetos is important enough
that I wrote an article about it:

http://www.pittspecials.com/articles/Magnetos.htm

IFP wrote:

I've only ever seen a kickback a couple times, and no damage was done

I don't like kickbacks. I worry that the starter
might be damaged, if it's still engaged. Ever
seen teeth sheared from the flywheel of a
Lycoming?

And I don't even want to think about what a
kickback would do to a TCM, with that horridly
expensive 90 degree drive into the accessory
housing (shudder).

PS Some definitions:

Kickback: engine rotating wrong way briefly
during start, generally due to unretarded spark

Afterfire: fuel burning as it exits the exhaust

Backfire: fuel burning out the intake manifold

N.B. Afterfire is perfectly harmless. You can
hear it at 0:23 of this video, as I pull the throttle
all the way back:

http://www.youtube.com/watch?v=dHE503RzueM

Backfire is common during starting. Main problem
is damage it might do, for example to Brackett
foam intake filter. Our biplanes have no filter, only
screen. Occasionally I will see flames shoot a foot
forward out of the air intake, generally during a hot
start. Doesn't seem to damage the prop.

[iflyforpie]

You're right CS, a kickback in a TCM would be terrible--especially if you've got a geared starter like a Skytec on your engine.

But most (all?) TCM engines I've seen have impulse couplings on both mags.... probably for that reason. I've only ever seen Lycomings kick back, and only on shut down.

Usually it is overeager pilots trying to start the engine twice or a lack of lube on the Bendix that is the bane of Lycoming ring gears.

[Colonel Sanders]

TCM engines I've seen have impulse couplings on both mags

Exactly!

lack of lube on the Bendix

Careful ... oil on the Bendix shaft attracts dirt
which can jam up the mechanism. You can use
silicon lube, I suppose. Mostly what I use on
sticking Bendix drives is electrical contact cleaner
to try to clean them. Brake cleaner would work,
too.

[Colonel Sanders]

Pop Quiz: (mostly for IFP)

Why do some engines not use impluse couplings
on their conventional magnetos to retard the spark?

Forget about electronic ignition ... this question is
solely about the impulse coupling mechanism.

You may be surprised at the answer!

[iflyforpie]

Well, I've heard about separate hand cranked magnetos on old engines (circa Spirit of St. Louis) that provided a hot late spark independent of the main magnetos. Of course there are booster coils on the R-985/R-1340 and the like.

I suppose that you could retard the timing Ford Model T style as well, but I never heard of that for aircraft.

[Colonel Sanders]

Aerobatic aircraft don't use impulse couplings -
they use a retard breaker (points) on the left
mag and a solid-state "buzz box" - just like
the shower of sparks on an old Ford, or rag-wing
Piper.

Here's the buzz box:

http://www.aircraftspruce.com/catalog/e ... ibratr.php

Impulse couplings work fine for straight and level,
but if you toss the engine around, they can vary
the spark timing. Not good.

[pelmet]

So am I right that the magnetos on an aircraft are installed with a fixed timing (no form of ignition advance for higher RPMs?) so they pretty much have to be installed at a base setting that ends up being more of a compromise than anything throughout the RPM range right?

Gipsy Major engine with original BTH mags has spark advance based on throttle position. They did it back then, why not now? Too complicated?

Back to fuel vents. Always good to check the vents after maintenance. I had that little ball of tape in the vent but big aircraft such as C-130 Hercules, 707, and C-141 Starlifter have literally had the fuel tank explode when being refuelled with pressure fuelling with blocked vents.

The guys in the article below had their problem while refueling in flight.
http://defensetech.org/2012/01/27/a-bas ... -a-jstars/

Check out page 13 of this Lockheed Service News to see an exploded fuel tank out the bottom of the wing.
http://www.lockheedmartin.com/content/d ... /V15N3.pdf

This is the most spectacular result....
http://www.google.ca/search?q=c-141+win ... B793%3B424

[cgzro]

Careful ... oil on the Bendix shaft attracts dirt
which can jam up the mechanism.

And when that happens two very bad things occur, you loose power and you starter becomes a generator.

[Colonel Sanders]

Peter, I can never remember: When the starter remains
engaged when the engine is running, do you see a large
charge or discharge on the ammeter?

[cgzro]

In that instance I was doing aerobatics at a contest over a runway and was only looking at the ground with the odd glance at oil press/temp/airspeed and altitude so i did not look at ammeter, what i did notice was a reduction of a few hundred feet in vertical performance and a wicked quick prop stop on engine shut down. Probably affected idle too but i did not notice, but i suppose it could stop your engine in a spin or ts.

If you are doing low level acro it would be good to double check amps once before committing to that first pull, or a careful check of expected mp/rpm and abort if you are missing a bit.

Like you always say, systems knowledge can save your ass, just one more example.

[pelmet]

Peter, I can never remember: When the starter remains
engaged when the engine is running, do you see a large
charge or discharge on the ammeter?

I believe that you will see a large draw from the starter as it is still being operated. That is why when you check your ammeter prior to takeoff(which we always do of course), if it has a large charge which is not decreasing as it normally should on a charge/discharge meter(or a larger than normal current on the other type of ammeter), time to consider taxiing back in and shutting down the engine as your starter contactor may be faulty. That being said, if the starter is still engaged due to gumming up instead of it having an electrical fault keeping it engaged, perhaps you would not have any indication on the ammeter. I guess that is the situation that you were asking about.

There was a King Air that had an incident over Montreal a few years back where it was IFR and he lost all electricity. In that case you have starter/generators. They act as the former for engine start and the latter for the rest of the flight. On the Twin Otter, the start switch was spring loaded back to the off position when released but for some reason, Beech decided to make their start switches as a lever lock type. Always wondered why as you can forget to select them from the start position once the start is complete. Anyways, that is what happened to this guy with both engines. I'm not sure if the starters are actually still engaged in this case but he had no generators and soon ran out of electrical power once airborne. He had to do a bit of fancy maneuvering to get back down.

Checking gauges for improper indication prior to takeoff can save some grief. Sort of like if you see high CHT's during the engine runup, perhaps it means that there is some jet fuel mixed in with your avgas and further investigation is a good idea.

[digits_]

On a sidenote: at the flying school where I used to fly, we had 3 robin aircraft. A normal 2 seater, an acro 2 seater and a 4 seater. One day, the fuel cap of the normal 2 seater went missing. Since this was the most booked aircraft, pilots started to look for alternatives. Turns out that the fuel cap of the 4 seater fitted perfectly. Without any problems at all. So this replacement was used for a while.

One day, the 4 seater was flying, and a pilot wanted to fly with the normal 2 seater. Not finding the 4 seater in the hangar, the tried the fuel cap of the acro 2 seater. He happily installed it on the normal 2 seater and went flying. What he didn't know, was that the acro 2 seater fuel cap was NOT vented, the other ones all were. Thus this pilot returned with a ruined fuel tank and taxied to the mechanic.

Bottom line: even a basic simple thing as a fuel cap (the vent was literally a small hole in the cap, nothing fancy, as mentioned above) can have serious consequences in an airplane.

Which made me think: what would happen if you exchanged fuel caps on a C172 where one of them has a vened cap, and one of them has a non-vented cap ?

[Colonel Sanders]

what would happen if you exchanged fuel caps on a C172 where one of them has a vented cap, and one of them has a non-vented cap ?

Absolutely nothing, most likely. If you look at the 172 POH,
the left fuel tank is vented outside, and the right fuel tank is
vented via the interconnection tube between the tanks.

However after AD 79-10-14 R1:

http://wwwapps3.tc.gc.ca/Saf-Sec-Sur/2/ ... 0-14R1.htm

as a backup, you were required to install vented "umbrella" caps
as an additional form of insurance, in case the aircraft fuel venting
failed for some reason. I suspect many Cessnas are driving around
with the wrong fuel caps that have been swapped as you described,
and that 34 year old AD is forgotten in the mists of time for many
young AME's that weren't even alive when it was issued.

The lesson here is that JUST BECAUSE IT WILL FIT, DOESN'T MEAN
IT IS THE CORRECT PART! You must check the parts manual and any
applicable SB's and AD's before moving stuff around.

PS You're correct about the aerobatic fuel caps - they are NOT vented:

http://www.aircraftspruce.com/catalog/a ... ckkey=7373

Every once in a while, pushing negative G, you will lose a fuel cap and
fuel will pour out, covering your windscreen. This is annoying.

[Colonel Sanders]

One last point about the humble fuel cap .... be sure
that the gasket or seal is nice and new and pliant and
springy and thick. The cap should be just a little hard
to put on, as you compress the seal or o-ring.

This is really important.

As the gasket/seal ages, it compresses and contracts
and the cap gets really easy to put on. But it will allow
water (rain) to get into the tank. Ask me how I know
this.

If it rains hard enough, and the aircraft is parked outside,
you can get enough water in the tanks that you drain
out straight water - no bubbles!

Water gets into the gas tank via the filler neck, either
via old fuel cap gaskets/o-rings, or via bad gas. I've
experienced both. Not condensation. Some brain on
the internet did a calculation and said that you would
need 100 years of wx changes and condensation to
get significant water in the tank.

tl;dr

Be suspicious of aircraft that are parked outside, for
water in the tanks. Change the fuel cap seals frequently
if the aircraft is going to be parked outside in the rain.

[Beefitarian]

I can never remember: When the starter remains
engaged when the engine is running, do you see a large
charge or discharge on the ammeter?

I am still wondering about this.


Since this is true.

Careful ... oil on the Bendix shaft attracts dirt
which can jam up the mechanism.

And when that happens two very bad things occur, you loose power and your starter becomes a generator.

As the engine is now driving the motor which would then act as a generator. I suspect you should see an over voltage from the large charge which would not have any form of regulator. Unless it trips a breaker, burns out a fuse or something else in the circuit.

The starter circuit might be fused in some large planes but typically will not be because the starter motor needs such a high current to function normally. In automotive applications there is a heavy gauge cable straight from the battery post to the solenoid on the starter. I presume small piston engine airplanes are the same.

Pie?