Program logic bugs vs input/environmental errors

[Walter Bright via Digitalmars-d]

On 10/3/2014 5:16 AM, Jacob Carlborg wrote:
> I have no idea of airplane works but I think Walter usual says they have at
> least three backup systems. If one system fails, shut it down and switch to the
> backup.

It's a "dual path" system, meaning there's one backup.

Some systems are in triplicate, such as the hydraulic system.