It depends on the type of aircraft, it's intended purpose, and how mature the design is. My current work involves developmental testing of aircraft, and one of

It depends on the type of aircraft, it's intended purpose, and how mature the design is. My current work involves developmental testing of aircraft, and one of the programs I'm working on is using spiral development. The iteration isn't continuous, and it's not nearly as fast as software, but it's pretty fast compared to other programs. Also, even the more stable and mature aircraft designs get relatively frequent software updates, and there is a lot of software in aircraft these days. The interesting part is where the faster iteration process of software development meets the slower process of aircraft development. The software developers are constantly iterating just like you would expect them to, and then every so often spin off a version for release on the side. That release version goes through extensive testing (mostly regression testing to make sure nothing that used to work is now broken) before it ever makes it onto an aircraft. Usually a few fixes are required as well. While all of that is happening, there are still people working on the next version.