There's no such thing as a "zero day backdoor". You're conflating "backdoor" with "zero day exploit" which are entirely separate things.

And its not a question of whether or not the NSA is capable of doing that. It's whether they're capable of doing it in a way that they would absolutely 100% certain could never be discovered.

But more importantly, as I pointed out elsewhere, in order for it to even be possible for such a backdoor to exist, the entire aircraft would have to be designed in a way that was hilariously, outrageously and inconceivably unsafe to operate. You simply do not link mission critical system to external communications systems that are in operation while a vehicle is airborne. Such a design flaw would be immediately obvious to the people whose job it was to approve the purchase, because there's no way you connect up systems like that in secret. While the US might supply the parts, it's still our guys who maintain them and integrate them into the vehicle.