McDonnell Douglas C-17 Globemaster III C-17A Block 21

Block 21 is a C-17A Globemaster III series upgrade devoted entirely to avionics. It was applied to the whole fleet so that the aircraft would meet new airspace requirements and modernize its military identification system. Like the earlier blocks, it changes neither the aircraft’s designation nor its performance: the aircraft remain C-17As, and what changes is the equipment and software on board.

The main source is an [article by the Air Force Life Cycle Management Center (AFLCMC) dated 28 January 2020](https://web.archive.org/web/2021/https://www.aflcmc.af.mil/News/Article-Display/Article/2068589/c-17-fleet-completes-block-21-upgrade/), written by Brian Brackens. According to that text, the program completed Block 21 upgrades on the entire fleet of 275 C-17 aircraft operated by the U.S. Air Force and eight allied countries, thereby meeting the mandate that set 1 January 2020 as the deadline. The article describes it as a two-year effort.

The core of Block 21 is the hardware and software of the ADS-B Out system (Automatic Dependent Surveillance-Broadcast Out), with which the aircraft broadcasts its own position. That system was the subject of the 1 January 2020 mandate, so the timetable of the upgrade was set from outside the program. In addition, according to AFLCMC, Block 21 included an Identification Friend or Foe (IFF) modification and other communication and navigation capability software updates. The article explains that the work touched seven or eight different sub-systems of the aircraft, which gives an idea of its scope even though it did not affect the structure.

The Air Force Association’s magazine, [Air & Space Forces Magazine](https://www.airandspaceforces.com/weapons-platforms/c-17/), specifies in its C-17 entry that Block 21 included Mode 5 IFF and airspace compliance, and that it was completed fleetwide in 2020. In other words, the IFF modification mentioned by AFLCMC is the adoption of Mode 5.

One feature of Block 21 is that it was not limited to American aircraft. By including the C-17s of eight allied countries, the upgrade left every operator of the fleet with the same airspace and identification avionics configuration, something particularly useful for an aircraft that often flies joint operations and through airspace all over the world.

Block 21 came after a long series of blocks. Boeing ties Block 12, in 2001, to the centre wing fuel tank (the configuration informally known as the C-17A ER); Block 16, introduced in 2005, brought the APS-150 weather radar and the OBIGGS II inerting system; and Block 20, according to Air & Space Forces Magazine, brought together some 60 programs to bring early production aircraft to a common configuration.

Modernization did not end with Block 21. The same magazine notes that in fiscal year 2024 the fleetwide replacement of the head-up display (HUD) was continuing, a task planned through fiscal year 2028. That replacement is not part of Block 21 and is mentioned here only as a continuation of the aircraft’s updating.

The AFLCMC article gives no performance figures: there are no changes in weight, range or speed associated with Block 21, and this page does not attribute any. Its data are what it adds, the fleet it was applied to and the date on which it was completed.