Lot of misinformation regarding what a "hack" actually is. The ability to secure root access is largely a function of a true operating system allowing you to reach a command prompt that will accept an executable file. On a system like an avionics "computer" you have a fundamentally different purpose and architecture entirely. As mentioned by those with an intimate working knowledge even with full access and source code the system is not easy to manipulate.
Certainly not an expert but I'd wager very few entities have the combination of specific system knowledge and expertise to accomplish this, basically it would take major resources at a governmental level.
The real question is would access enable someone with lesser experience to degrade or destroy the system or interfere with the genuine inputs in such a way as to render the system inoperable. This is entirely different then actually hacking the system.