As some of you may know, the CAF B-29 "Fifi", still the only flying B-29 in the world, has been grounded since 2006 due to engine problems.
"Fifi" was rescued from China Lake in 1971 to become a CAF flying museum exhibit. With the plane came its original WW II-vintage Wright R-3350-57 radial engines. These engines were unreliable and problematic in field operations, due to design weaknesses (many to do with inadequate cooling) and lack of proper debugging caused by the B-29 being rushed into service in 1944. As a result, "Fifi" has always been a PITA to operate because of engine problems. The problems are fundamental with the design specification of the engines. When metal was found in the oil of the engines in the Fall of 2006, "Fifi" was grounded, and a big and financially mind-boggling decision was made to not put any more flying time on what was clearly a dangerously worn set of fundamentally unreliable engines. The decision was to re-engine the plane with a brand new hybrid specification of R-3350. The main challenge for the project was folding green, the final cost will probably exceed $4m.
The progress of the re-engine project has been documented here. The good news is that all four engines have been replaced with the brand new hybrid specification models, and "Fifi" is awaiting the right moment to begin its flight test program with the new engines.