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 172 Skyhawks.

The real shocker is that while the right tank is remaining full, the engine is actually using fuel from the right tank. What is causing 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 fuel tank to begin to collapse.

To avoid this in almost all fuel systems, whether they are in an aircraft, a car, or a lawn mower, fuel that is used from the tank is replaced by air from the outside. On most Cessna 172 Skyhawks, prior to 1997, 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 inter-connect 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 inter-connect 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 inter-connect line, the condition will continue as fuel sloshing in the tank periodically gets into the inter-connect 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. On some of the later 172 models of the 1980s, the right tank is vented directly with it’s own vent tube behind the right lift strut. This feature which became standard on all 172s beginning in 1997, reduces the uneven fuel feeding but does not eliminate it.

Cessna Pilots Association (6/18/07)

ENGINE STARTER DUTY CYCLE TIMES - Cessna 172/172RG

Crank the starter for 10 seconds followed by a 20 second cool down period. This cycle can be repeated two additional times, followed by a 10 minute cool down period before resuming cranking. Repeat cranking procedures one more time. If the engine still fails to start, do not attempt to restart and notify Dispatch. Dispatch will notify Maintenance.

This duty cycle time should be strictly followed to enhance starter life and assist in reducing the possibility of premature engine starter failure.

Cessna Service Newsletter

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