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Fuel Tank Air Vents

Thu Aug 25, 2005 3:59 am

Hey, I work around alot of Cessnas and I've noticed that on the 172, the fuel tank air vent is a little pipe hanging under the wing on the pilot's left side. On the right side the vent is built into the fuel cap. Why are these two different? I know it's for air to get into the tanks to take the plce of the used fuel, but why aren't the vents the same on both sides? Wouldn't that make more sense? What about airliners? Are they built into the cap or is there a different system?

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RE: Fuel Tank Air Vents

Thu Aug 25, 2005 4:09 am

i think most larger aircraft have a vent/surge tank which is on the outboard section of the wing, it also saves against damaging the tanks by overfilling as it goes into the Vent/surge tank!

not sure bout the cessna tho!
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RE: Fuel Tank Air Vents

Thu Aug 25, 2005 5:19 am

On the C-150/152 and the C-172 the fuel vent is located behind the strut under the left wing, this vented the left fuel tank. Internally a vent line went across the upper fuselage above the windshield to the right tank so both tanks were vented through this single vent pipe.

There were a few cases of the internal vent pipes clogging up or collapsing which caused the right tank to be unvented. This meant that fuel could not drain from the right tank by gravity to the engine, or possibly collapse the tank.

The FAA issued an AD note years ago requiring the right tank to be vented separately by using a vented cap.

On my C-150 I installed the vented caps on both fuel tanks, which is legal and I now have a second source for venting. In the Northeast mud wasps love to build nests in pitot tubes and fuel vent pipes blocking them. More than once I have had to clean out a mud wasp nest from my airplane.
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RE: Fuel Tank Air Vents

Thu Aug 25, 2005 5:26 am

What's with the 172 using more fuel from one of it's tank?

IIRC it was the right tank that would consume more then the left when flown with a topped off fuel tank and fuel selector is put in BOTH position.

Even if an equally # and timed banks and a level flight cruise still causes this.
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RE: Fuel Tank Air Vents

Thu Aug 25, 2005 1:53 pm

On the DC-9/MD-80, the fuel vents are on the outter ends of the wings, about 2-3 feet inboard of the tips....rectangular in shape. If I were to overfill, say, the left tank (assuming the overfill protection was bypassed), fuel would vent out of the right vent, and vicaversa.....always looking at the ground near the wingtips when I get the volume over 9100/9300ish  silly 

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RE: Fuel Tank Air Vents

Thu Aug 25, 2005 4:23 pm

On the B737s they are located on the Undersurface of the Wings almost near the Tips,Venting into the Surge tank & to all the Three tanks thru channel vents providing all Altitude venting.
In case the Fuel SOV does not close on reaching tank capacity vis cutout sw,excess fuel passes thru to the surge tanks & then to the vents & out the port.
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RE: Fuel Tank Air Vents

Wed Aug 31, 2005 2:39 pm

The way I had the difference explained is that it is a redundancy issue. Something likely to cause a blockage in the left-hand vent is unlikely to also block the right-hand one, and vice versa. If one of the vents is blocked, the engine will keep running. If both are blocked, it won't...

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