Powell D, Spencer MB, Holland D, Petrie KJ. Fatigue in two-pilot operations: implications for flight and duty time limitations. Aviat Space Environ Med 2008; 79:1047–50.Objectives: Two-pilot operations make up the majority of commercial flights. Fatigue is an important consideration in these operations as there is little opportunity for in-flight rest. We investigated the role of duty length, time of day, and whether one or two sectors were flown on reported fatigue at the top of descent in two-pilot regional operations. Methods: Pilots flying two-pilot operations ranging from 3-12 h completed Samn-Perelli fatigue ratings prior to descent at the end of each rostered duty over a 12-wk period. We collected 3023 usable ratings (72% of rostered duties) comprising 26% single and 74% double sector duties. Results: We found that time of day has a marked effect on the pattern of fatigue at the start of the duty and on the rate at which fatigue levels increased, with the highest levels in the window of circadian low (0200–0600). Fatigue also increased with the length of duty and was 0.56 higher at the end of a two-sector compared with a single-sector duty. Discussion: The results imply authorities should consider increasing existing limits for daytime duties and reducing those for nighttime two-pilot operations.