Saturday, 30 July 2011

Uneven Fuel Feeding

Have you been flying along and watched your fuel gauges show that your left tank is going down while the right tank remains full even with the fuel selector on "Both"? This is a common problem with Cessna 182 Skylanes before the 1979 model year. And the real shocker is that while the right tank is remaining full the engine is actually running off of fuel from the right tank!
What causes the situation is the way Cessna designed the fuel tank venting system. When fuel is used from a tank it must be replaced with something, otherwise a vacuum is created which will either cause interruption of fuel to the engine or cause the bottom of the bladder tank to be "sucked" up. To avoid this in almost all fuel systems, whether they are in an aircraft, a car or a lawnmower, fuel that is used from the tank is replaced by air from the outside.

In the Cessna 182 Skylane this venting occurs by connecting the upper outboard portion of the left tank to the "L" shaped vent tube underneath the wing behind the left wing strut. This allows air into the left fuel tank as fuel is used. To vent the right tank, a vent inter-connect line is run from the upper inboard area of the left tank to the upper inboard area of the right tank thus, in theory, venting the right tank to the vented airspace of the left tank.

Unfortunately, wing dihedral, where the wing tip is higher than the wing root, was not sufficiently considered. When the wing tanks are full, the vent interconnect line is actually submersed in fuel and thus as fuel is used from the left tank, the air coming in from the vent pushes fuel from the left tank through the vent interconnect line into the right tank, thus replacing fuel that is used from the right tank. And even after enough fuel is used from the left tank to bring the fuel level below the vent interconnect line the condition will continue as fuel sloshing in the tank periodically gets into the interconnect line and pushed through to the right tank.

In really severe cases fuel usage from the right tank might not be indicated on the gauge until the fuel level in the left tank is as low as 1/3 capacity. The positive thing to keep in mind when experiencing this condition is that fuel is actually being used from the right tank and that fuel being used from the right tank is merely being replaced by fuel from the left tank. This means that even if the left fuel tank should go to empty you will not experience fuel flow interruption as long as there is fuel in the right tank and the fuel selector is on "Both".

This condition can be minimized somewhat by adjusting the position of the fuel vent behind the lift strut on the left wing, making sure that fuel caps seal tightly so that the "head pressure" in one tank is not altered by a leaking cap, and assuring that the wing strut fairing is sealed against the strut, thus avoiding burbling air right in front of the vent. However, in the end the design of the system does not allow for complete resolution of the problem. The Cessna Pilots Association has a handout available to its members that discusses this situation in even greater detail.

Beginning with the 1979 model year the Cessna 182 Skylane went to an integral bay "wet wing" fuel system with vents under both wings which went a long way to reducing the problem.

1 comment:

  1. A little uneven fuel feeding is not a big problem, but does anybody know why a significant difference in left/right feeding would suddenly develop after a new left tank is installed? The mechanic has adjusted the L-shaped vent tube under the left wing several times and that hasn't helped. Any ideas?