The world's only flyable Boeing B-29 Superfortress is not quite airworthy — yet. It's been four years since the Boeing last flew, and the efforts of the Commemorative Air Force (CAF) to return "FIFI" to the sky are close. But a "scheduling conflict" has led the FAA to delay issuing a new airworthiness certificate. FIFI was to have flown on Tuesday, but will have to wait at least another week for paperwork to be completed. The CAF grounded FIFI in 2006 when metal shavings were found in oil analysis of its original Wright R-3350-57AM engines. Since the -57AM radials have had problems dating back to World War II, the CAF decided to replace them with a hybrid version of Wright R-3350-95Ws and -26WD variants. New engine mounts and cowling modifications were necessary to make the switch, thus the four-year hiatus. CAF President and CEO Steve Brown said, "This has been an extensive project and the CAF is proud of the member-volunteers who worked tirelessly to get FIFI back in the air."