For a given airspeed, an aircraft may consume less fuel at a higher altitude than it does at a lower altitude.  In other words, an aircraft may be more efficient in flight at higher altitudes as compared to lower altitudes.  Moreover, bad weather and turbulence can sometimes be avoided by flying above such weather or turbulence.  Thus, because of these and other potential advantages, many aircraft are designed to fly at relatively high altitudes. 
As the altitude of an aircraft increases, the ambient pressure outside of the aircraft decreases and, unless otherwise controlled, excessive amounts of air could leak out of the aircraft cabin causing it to decompress to an undesirably low pressure.  If the pressure in the aircraft cabin is too low, the aircraft passengers may suffer hypoxia, which is a deficiency of oxygen concentration in human tissue.  The response to hypoxia may vary from person to person, but its effects generally include drowsiness, mental fatigue, headache, nausea, euphoria, and diminished mental capacity. 
Aircraft cabin pressure is often referred to in terms of “cabin pressure altitude,” which refers to the normal atmospheric pressure existing at a certain altitude.  Studies have shown that the symptoms of hypoxia may become noticeable when the cabin pressure altitude is above the equivalent of the atmospheric pressure one would experience outside at 8,000 feet.  Thus, many aircraft are equipped with a cabin pressure control system to, among other things, maintain the cabin pressure altitude to within a relatively comfortable range (e.g., at or below approximately 8,000 feet) and allow gradual changes in the cabin pressure altitude to minimize passenger discomfort. 
To maintain aircraft cabin altitude within a relatively comfortable range, cabin pressure control systems may be equipped with one or more outflow valves.  An outflow valve can assist in controlling cabin pressure by regulating air flow out of the cabin.  One particular type of outflow valve that may be used is a butterfly outflow valve.  A butterfly outflow valve typically includes a rotatable flapper or gate as the control element to regulate the flow of air out of the cabin.  The flapper is coupled to a shaft that is rotationally mounted to the outflow valve body.  An electromechanical actuator, which is coupled to the shaft, positions the flapper element in response to commands from a controller to thereby regulate the air flow out of the cabin. 
Although the above-described type of outflow valve is believed to be generally safe and reliable, is fairly simple to design and construct, and thus fairly inexpensive, under certain circumstances it may exhibit certain drawbacks.  One particular drawback is that some of these types of outflow valves may not automatically move to the closed position in the unlikely event that the actuator becomes inoperable.  Although some outflow valve designs do include a closure mechanism to automatically move the valve to the closed position in this unlikely event, the closure mechanisms are typically relatively complex and expensive, and do not address backlash that may occur in the actuator. 
Hence, there is a need for a relatively inexpensive and/or relatively non-complex outflow valve closure mechanism that automatically moves the outflow valve to the closed position in the unlikely event that the actuator becomes inoperable and/or reduces or eliminates outflow valve actuator backlash.  The present invention addresses one or more of these needs.