Ugh
Given what we know about the P4, I seriously hope it's a P3 or below. The P4 simply added heat, energy required, and slowness. Many of my own calculation routines using standard ADD/SUB/MUL/DIV perform noticeably slower on a P4 than a P3. I haven't worked with floating-point, so I can't say anything there.
As for the people questioning why use old technology, I would counter with "Why not?" Just because a Core 2 Duo or Athlon 64 X2 is available doesn't make it the best tool for the job. If I need to light a candle, sure, a flamethrower would probably do the trick, but it's what we call "overkill".
re: Modernization?
"Why not a safer langue like eg. Eiffel with design by contract and automatic garbage collection? ... Or maybe the pilots...wouldn't hear of anything that might imply the use of the word safety?"
Are you actually implying that "automatic garbage collection" is "safe"? I don't work in C, but even accepting as true all the bad things people say about it, any "automatic" processes cannot possibly be considered "safe". When you're talking about keeping planes in the air and controlling weapons, you need 100% control, nothing more, nothing less. Executing any code that is not in the source is unacceptable, as is deviating from the source in any way. When lives are hanging in the balance, and the difference between life is death is measured in milliseconds, you can't trust that the "automatic garbage collection" won't release something at the wrong time, or that "automatic" processes won't be running and thus delay execution of the actual program code.