Many new Performance Based Navigation (PBN) Instrument Flight Procedures (IFPs) are being developed as the United States transforms its airspace to improve safety and efficiency. Despite significant efforts to prepare for operational implementation of new IFPs, the process does not always go smoothly. The primary goal of this study was to understand what makes IFPs difficult from the perspective of line pilots. We spoke to 45 professional pilots in small groups. The pilots reviewed, briefed, and discussed six IFPs in an office setting. We extracted a comprehensive list of subjective complexity factors by observing pilot briefings and gathering pilot feedback. Then we organized the list into a framework that captures a variety of types of complexity. We define a subjective complexity factor as one that requires an extra mental or physical step by the pilot. IFP design parameters (e.g., the number of transitions and flight path constraints) are a main driver for subjective complexity for line pilots. Unusual IFP designs can result in novel chart depictions that are unfamiliar and more difficult to use. In turn, novel chart formats may have inconsistencies that increase subjective complexity. Participants also mentioned factors that are outside the control of IFP designers, such as weather, fatigue, and aircraft performance or equipment. We separate out these as operational complexity factors. The broad nature of the pilot interviews also provided insights into how pilots use charts today, in the context of the modern flight deck. A full report on the study is in preparation.