Monday, May 7, 2018

Special Cause Variations vs Common Cause Variations

Special Cause Variations vs Common Cause Variations

Post by CatalinaNJB

Before a corrective action plan is applied to a special cause variation, it must first be established if the variation itself is a special cause or a common cause variation. On the surface it might look like a simple task to identify a common cause variation since this variation is known to occur each time the process is applied. This could be in a manufacturing process, a service process, a training process, a safety improvement process or in any other processes. That a variation is common in the process does not necessary define that variation as a common cause variation. It could be a special cause variation disguised as a common cause and embedded into the process. It is conventional knowledge that new pilots are less qualified than long-time pilots. While this might be true, by accepting this as a common cause variation, there is no corrective action required for continuous improvements. There are opportunities for an increased return on investments by making minor adjustments to processes, but some operators don’t see the forest for all the trees.

A common cause variation may also be disguised as a special cause variation. By incorrectly assigning a special cause variation as a common cause, a finding could be applied to a common cause variation of that type of process. When a finding incorrectly is applied to a common cause variation it becomes a task in itself to develop a corrective action plan, since this variation is necessary for the process to function effectively. The weather process is a common cause variation. It commonly varies by the day or by the hour. If the weather is defined as a special cause variation, then a corrective action plan must be applied to the weather process.  

SPC of standard operating parameters and operating in a virtual reality.
It might not be obvious to discover when a common cause or special cause variations are incorrectly identified. There might be times when standard parameters are applied to simplify the flight planning process. A standard fuel burn is established and no matter what the weather and winds are that day, the operator does not change time enroute to compensate for a common variation and the variations in weather. Weather variations are common cause variations. When analyzing standard operating parameters in SPC it becomes obvious that something is not right with the process. It’s a straight line! Real life processes do not produce straight lines.

A standard weight may also be applied as a passenger weight. When a standard weight is applied, the actual weight of the airplane will not be correct and fuel burn deviates from standard. Passenger weight variations are also common cause variations. It is common for this weight to change. In a statistical process control, the process may show an out of control process when a common cause variation is applied as special cause variation. Should an airplane run out of fuel due to incorrect passenger weight when applying company accepted standards, the passenger weight might incorrectly be identified as a special cause variation. Applying standard inputs to processes is to operate in a virtual reality. Applying SPC as a tool to identify a virtual reality is a simple task and the control chart will show that there are no variations. When it has been established that a process is a virtual reality process, implementing corrective actions to that process becomes an impossible task, since the process itself is not applicable to any real data. A new process must be implemented.

Special cause variations identified.
Standard weights are still used in calculations as it has been since the first flight. Then, when there is an incident the weight could be assigned as a contributor factor to the incident, while it was known all along that the weight was manufactured.

This Fuel Burn SPC Chart shows that there is an out of control process. However, because of the red point, the process cannot automatically be scrapped, but needs to be investigated. The investigation revealed that another type of aircraft was utilized during the days of red points and it was determined to be a special cause variation. A CAP was implemented to utilize one type of aircraft only for these runs the next 30 days to monitor the process.

Variations are acceptable. Make sense that a different aircraft did not affect safety.
Special cause variations require that corrective action plans are implemented. Defining the special cause variations becomes a safety critical area in operations. Not everything is equally unsafe in flying. There is no need for an incident to occur before special cause variations are identified.  
The next safety question might be to identify if by using another type of aircraft affected safety in operations. Another SPC Chart was developed with the following result that for the process to be in control, it is acceptable to utilize another type of aircraft and still maintain safety in operations.

CatalinaNJB





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