Mixture Control Return Spring Problem

April 1987

There have recently been two cases of engine fa i I u re resulting in forced landings (luckily without damage) caused by failure of return springs in the engine mixture control linkage. These, we believe. are due to improper installation of the bracket supporting the push/pull cable at the carburetor. The springs as properly designed are intended only to snub the system and improve the fidelity of the mixture control by eliminating free play.

The springs should never be required to move the mixture control away from the idle cutoff position. In both instances, the springs had failed or lost their force due to fatigue and vibration. Properly installed, the swage at the cable end should sit very close (within 1/2") to the cable conduit clamp when the mixture is in the idle cut-off position. If your bracket allows excess exposed cable, then the mixture control cable may buckle rather than positively force the arm away from idle cut-off in the event of a spring failure.

Do not depend on the spring to bring the mixture control into the mid range, well away from idle cut-off. If you do, the engine can fail due to a spring failure. If your aircraft does not pass the test shown in the plans changes section of this newsletter, ground it immediately and rebuild your conduit clamp so that the exposed cable is short, allowing the mixture lever to force the arm to at least mid range without assistance from a spring. This is required on the throttle as well as the mixture control.

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