In 2014 the USAF began upgrading block 30/35 E-3B/Cs into block 40/45 E-3Gs. This upgrade replaces the main flight computer with a Red Hat Linux-based system, as well as replacing the DOS 2.0-like operating system with a Windows 95-like system on the operator workstations.[29] In 2016, a three-week long cybersecurity vulnerability test revealed that the 40/45 block and its supporting ground equipment were vulnerable to cyber threats, and were thus deemed "not survivable."[30] This caused a delay of approximately two years.[31] Twenty-four E-3s are projected to complete this upgrade to 40/45 by the end of fiscal year 2020, while seven aircraft will be retired to save upgrade costs and harvest out-of-production components.[32][33]