Ten of 13 upgrade projects for the aircraft are experiencing cost, schedule or performance challenges, the GAO said. The report highlighted a Rolls-Royce Holdings Plc engine upgrade and a radar project as facing all three types of problems. The upgrade program aims to keep the aircraft operational through the 2050s alongside B-2 and B-21 stealth bombers. The engine program faces risks from concurrency, which is the overlap between development testing and starting production, the GAO said. Related articlesBoeing's $21 billion B-52 upgrade faces cost overruns, delays, GAO saysCiti says investors have shifted to 'buy-the-dip' mode on KospiTech specialist explains why China memory fears are overblown