Boeing Satellite Systems (El Segundo California) uses Tcl extensively in its Ground and Test Equipment operations. Large and complex products for both internal and external customers use Tcl as the core language, with extensions written in C/C++.
Ease of integration with existing legacy applications facilitated its initial use.
[incr tcl/tk] is the primary language extension used, and without it, continued use of Tcl for the large projects would be questionable.