Let's say we are flying a piston engine high wing airplane (Cessna 172) that uses a carburetor.

What would happen when the fuel tank of a Cessna 172 became (partial) vacuum during horizontal flight?

  • $\begingroup$ C172 has two tanks. Are you asking what happens if only one stops flowing? Probably depends on model year also, as I think the venting methods changed over the model years. $\endgroup$ – CrossRoads Feb 5 '20 at 15:36
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    $\begingroup$ I don't understand your question. Can you tell us exactly what it means if a fuel tank "becomes vacuum"? $\endgroup$ – Pondlife Feb 5 '20 at 17:07
  • $\begingroup$ By vaccum, do you mean just empty or even without air (in depression)? $\endgroup$ – Manu H Feb 5 '20 at 17:26
  • $\begingroup$ By becoming vacuum I refer to the pressure inside the tank that drops below atmospherical pressuure. $\endgroup$ – Julian Feb 5 '20 at 21:27

The 172S POH, the current version in production indicates:

FUEL VENTING Fuel system venting is essential to system operation. Blockage of the system will result in decreasing fuel flow and eventual engine stoppage. Venting is accomplished by an interconnecting line from the right fuel tank to the left tank. The left fuel tank is vented overboard through a vent line, equipped with a check valve, which protrudes from the bottom surface of the left wing near the wing strut. Both fuel filler caps are also vented.

FUEL SELECTOR VALVE The fuel selector valve should be in the BOTH position for takeoff, climb, landing, and maneuvers that involve prolonged slips or skids of more than 30 seconds. Operation from either LEFT or RIGHT tank is reserved for cruising flight.

NOTE When the fuel selector valve handle is in the BOTH position in cruising flight, unequal fuel flow from each tank may occur if the wings are not maintained exactly level. Resulting wing heaviness can be alleviated gradually by turning the selector valve handle to the tank in the "heavy" wing. It is not practical to measure the time required to consume all of the fuel in one tank, and, after switching to the opposite tank, expect an equal duration from the remaining fuel. The airspace in both fuel tanks is interconnected by a vent line and, therefore, some sloshing of fuel between tanks can be expected when the tanks are nearly full and the wings are not level.

  • $\begingroup$ So if the pressure inside the tank drops below atmosferical pressure then fuel may not reach the carburetor? $\endgroup$ – Julian Feb 5 '20 at 21:30
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    $\begingroup$ Yes. Take a jug of water with a small hole in the cap and tip it upside down. Water stops flowing as the suction is created in the top. The 150/172 didn't originally have vented caps. They were were added later (under an AD IIRC) to provide a back up venting method if a wasp fills your vent line with mud. So the vent tube itself is kind of redundant. $\endgroup$ – John K Feb 6 '20 at 4:57
  • $\begingroup$ @John K That sounds like a struggle for the last piece of ketchup with a squeeze bottle. $\endgroup$ – Julian Feb 6 '20 at 13:18
  • $\begingroup$ Except: fuel flows much more readily then ketchup, and fuel tanks aren't squeezable. $\endgroup$ – CrossRoads Feb 6 '20 at 15:25
  • $\begingroup$ Interestingly, I saw recently on a youtube doc on ketchup that it has a unique physical property where its viscosity drops sharply when subjected to shock of a certain severity, which is why you have to whack the bottom just so to get it out. $\endgroup$ – John K Feb 7 '20 at 19:08

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