some autopilots also use design diversity. in this safety feature, critical software processes will not only run on separate computers and possibly even using different architectures, but each computer will run software created by different engineering teams, often being programmed in different programming languages. it is generally considered unlikely that different engineering teams will make the same mistakes. as the software becomes more expensive and complex, design diversity is becoming less common because fewer engineering companies can afford it. the flight control computers on the space shuttle used this design: there were five computers, four of which redundantly ran identical software, and a fifth backup running software that was developed independently. the software on the fifth system provided only the basic functions needed to fly the shuttle, further reducing any possible commonality with the software running on the four primary systems.