Cessna Fuel vent system

Jaybird180

Final Approach
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Jaybird180
Reading the POH and I’m wondering since the fuel caps are vented as well as the vent near the strut, how does it prevent rain water or condensation entering the fuel system during flight in rain or IMC?
 
Reading the POH and I’m wondering since the fuel caps are vented as well as the vent near the strut, how does it prevent rain water or condensation entering the fuel system during flight in rain or IMC?

Umbrella style caps with a pressure sensitive valve (usually a rubber membrane) that only opens when negative pressure exceeds the design limit. The vent on the Cessna has a flapper valve. If the vent is working properly, the caps remain sealed.
 
Reading the POH and I’m wondering since the fuel caps are vented as well as the vent near the strut, how does it prevent rain water or condensation entering the fuel system during flight in rain or IMC?
Like said, they use "pressure vacuum" fuel caps. Just like cars they are only used if the main vent line get's blocked. They will let air in and pressure escape if the main vent is blocked. So they are normally closed and no water can get in.
 
The little rubber (silicone) umbrella gets old and it loses its tension and sags. Now a pounding rain, or melting snow, find its way into the tank past that open valve. The little valve is not available separately. Got to buy the whole cap. At least it's not at Piper fuel cap prices.

Cap gaskets get old and don't seal well. They, too, let water seep in. The steel fuel inlet neck corrodes with age, and gets pitted so that the gasket has trouble sealing. The Monarch fuel cap people did a bunch of tests on Cessna caps, on Cessna tanks, and the results were dismal.

The vent tube under the left wing has a closely defined position behind the strut to deflect rain and inhibit ice formation. They're often found set lower, possibly by mechanics thinking that ram pressure is the deal, and then they'll scoop up rain and bugs and get iced over.

The vent check valve in the tank, that controls the air that comes in or out of that vent tube, has a two-way function. You would not want a totally-sealing valve, or you'd rupture the tanks. It has the spring-loaded flapper that lets air in quickly if necessary, but has a tiny hole that lets air out slowly due to the pressure drop with altitude or thermal expansion.
 
There is usually (always?) a small drain hole I the vent tube behind the left strut. It’s right near the radiused bend, at the bottom (obviously) and helps prevent any of that rain water the OP asked about from being siphoned up into the tank upon sitting post flight, or freezing and blocking the vent.
 
The little rubber (silicone) umbrella gets old and it loses its tension and sags. Now a pounding rain, or melting snow, find its way into the tank past that open valve. The little valve is not available separately. Got to buy the whole cap. At least it's not at Piper fuel cap prices.

Cap gaskets get old and don't seal well. They, too, let water seep in. The steel fuel inlet neck corrodes with age, and gets pitted so that the gasket has trouble sealing. The Monarch fuel cap people did a bunch of tests on Cessna caps, on Cessna tanks, and the results were dismal.

The vent tube under the left wing has a closely defined position behind the strut to deflect rain and inhibit ice formation. They're often found set lower, possibly by mechanics thinking that ram pressure is the deal, and then they'll scoop up rain and bugs and get iced over.

The vent check valve in the tank, that controls the air that comes in or out of that vent tube, has a two-way function. You would not want a totally-sealing valve, or you'd rupture the tanks. It has the spring-loaded flapper that lets air in quickly if necessary, but has a tiny hole that lets air out slowly due to the pressure drop with altitude or thermal expansion.

on the ol Cessna 140 w it’s very “primitive” vented caps, I travel with Tuna cans and lightly electrical tape them on if she’s tied up overnight. Get some real looks the few times I’ve parked at fancy GBOs :)
 
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