Graviman,
For any aircraft, the primary structure must be designed to show that the probability of catastrophic failure is "extremely remote" or 10e-9 This is done by design analysis, and test, and is inherent in the certification of any aircraft. Additionally. for engine failures, helicopters must be safely autorotated to be certified (this is the underlying method to show that a power loss is not a catastrophic failure).
I speculate that for a flying machine that is held up and controlled by engines like the Xhawk, the safe completion of a flight must have this same integrity, so the engine power needed to maintain control and to make an emergency landing must have the same integrity (probability of failure) as the structure, or 1 to a billion. If not, then the FAA would be allowing a catastrophic failure (power loss) at a much higher probability - not at all likely.