The objective of this work was to identify the cognitive skills needed in flight planning and evaluate how they degrade over time. Cognitive skill degradation has been identified as a potential issue in information automation systems that manage and present relevant information to the flight crew. Much as physical piloting skills can degrade over time due to lack of practice, the cognitive skills associated with many aviation tasks may degrade over time if these skills have been automated and pilots no longer have a chance to practice them. To further evaluate cognitive skill degradation in information automation, two studies were conducted. The first study was an Applied Cognitive Task Analysis to find what decision points and skills are involved in flight planning. The second study examined the effects of skill degradation on performance, workload, and completion time, as a result of differing levels of reliance on automation. The first study determined that the skills found to be particularly vulnerable to skill decay were those that involved calculation and estimation. The second study found that automation as an aid did not suffice as a method for maintaining skills for flight planning tasks. It also showed that after a period of nonuse for the manual method, completion time and workload increased. The results of these studies provide insights into cognitive skill degradation in regards to aviation. Calculation and estimation were found to be particularly vulnerable to skill degradation. It was shown that after nine weeks the cognitive skills of calculation and estimation degraded for flight planning tasks. Additionally, it was found that using an automation aid did not suffice as a method for maintaining skills. By understanding which cognitive skills degrade as a result of reliance on automation, designers can develop mitigation techniques to counter cognitive skill degradation.