Uneven Fuel Feeding

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Uneven Fuel Feeding

Read the Pilots Operating Handbook!

Bottom line is the Pilots Operating Handbook states that "The fuel selector valve should be in the BOTH position for takeoff, climb, landing, and maneuvers that involve prolonged slips or skids. Operation from either LEFT or RIGHT tank is reserved for cruising flight." There is also a NOTE: "When the fuel selector valve handle is in the BOTH position in crusising flight, unequal fuel flow from each tank may occur if the wings are not maintnined exactly level. Resulting wing heavinesss can be alleviated gradually by turning the selector valve handle to the tank in the "heavy" wing.

1. "To understand what causes uneven fuel feeding we have to go through some basics of the fuel system...one force that everyone is aware of is Gravity...The air pressure in the tanks will also be a force acting to push fuel down the fuel feed lines. Ideally this pressure will be slightly above ambient or outside air pressure. This pressure is often referred to as 'head pressure' meaning the pressure above the fuel level is pushing fuel down the lines...Head pressure is necessary to hold fuel bladders in place but too much pressure can push fuel around and cause uneven feeding therefore it is sensitive. ...pressure must be equal for the tanks to flow at the same rate... tank with lower head pressure will flow slower. As fuel is used, it must be replaced by something or else the 'head pressure' would be less than the ambient air pressure outside. To prevent dire consequences, the fuel tanks are vented to the outside and air is allowed to replace the fuel by ... single "L" shaped vent tube conected by hosing and tubing to the outboard portion of the left fuel tank. As fuel is used.. replaced by air. This takes care of the left tank, but what about the right? ...In venting the right tank Cessna engineered a not very satisfactory solution. Knowing that the right tank had to be vented and that the head pressure had to be equal between the two tanks, Cessna thought good idea to connect the top of the left tank to top of the right tank and let right tank vent from left tank and equalize head pressure at same time. What wasn't considered was dihedral of wing and fuel slosh (and turns). Long, shallow tanks make fuel feed very sensitive to tank, or head pressure. Wing dihedral places the outboard portion of the fuel tank above the inboard portion which means that at times the interconnect line is below fuel level and is full of fuel. When air enters left tank, the pressure forces fuel to move through interconnect line from left tank into right tank, even replacing any fuel which has been used from the right tank which causes right tank to remain full even though fuel is flowing from it to the engine, while left tank level drops at a rate even faster than it is feeding the engine. (notice this flow is NOT through the same line as feeds the engine. GW)...transferring of fuel wil continue until level in left tank drops below level of interconnect line and can even continue..."

2. "However the misconception in the field is that these caps (fuel caps mandated by AD 79-10-14) provide primary and constant venting for the tank on which they are installed. The vented caps have secondary vents in them that are normally closed and will only open when there is a significant difference in external and internal pressure. In other words these cap tank vents only open when the pressure differential is so great that the tank is in imminent danger of collapse. This makes sense in that the caps are mounted on the top surface of the wing, a low pressure area, and if the tank vents were open all the time both erratic feeding and fuel loss could occur"

3. "After hearing complaints from owners Cessna tried to do several things to imporve situation...made changes to fuel system. Having suspicions that problem was related to unequal pressures in the tanks, made attempt to equalize by venting forward fuel feed lines to the vent interconnect line running across the top of the fuselage(Did any of you know and consider this? Does this sound like it might be a major cause of fuel flow between tanks? I would certainly think so). What this actually does is allow fuel to move from one tank to other at any time there is a difference in head pressure...idea was to 'hide' vent tube behind lift sturt to gain ice protection and have just a small amount of ram air by protuding... positioning of vent is SO CRITICAL that Cessna calls out the dimension in hundredths of an inch... can be adjusted by moving behind or from behind to change ram air pressure...also problem caused by lift sturt fairing positioned in front of 'L' shaped vent tube causing airflow to burble..."(I wonder what happens here when the fairing is torn or cracked?)

4. ...fuel tank cap that does not seal tight will have a lower head pressure in the tank that one with a good sealing cap and will lead to uneven fuel feeding...Simple test to see if fuel caps are leaking air is to attach rubber hose to vent tube and blow while having someone using spray bottle with soapy water observe fuel cap..."(my thought is that this might be a problem on many planes)

AGAIN