Originally Posted by
tdracer
While I didn't work the MAX, I have worked uncontained rotor burst analysis.
The position of the engine is largely immaterial to the risk to the flight control cables - first, if the uncontained components head inboard towards the fuselage, there simply is not any wing structure in the way on any modern wing mounted engine installation. In fact, that's one reason why the engines get hung so far forward - moving the engine forward relative to the wing lessens the risk of catastrophic damage to the wing.
Burst components are broken down into different energy states - in brief, blades and the like are 'low energy' - you can take credit for shielding - rotors (assumed 1/3 disc) are 'high energy' - no credit for shielding. For purposes of rotor burst failure analysis, the fan hub is assumed to have infinite energy - wing spars or floor spars offer no protection.
The Advisory Circular for rotor burst calls for what's known as a 'one in twenty' analysis - in short, since the trajectories are unpredictable, you're allowed a 5% chance that the debris will take out something flight critical (e.g. a wing spar). Even if you meet 1 in 20, you're supposed to provide the maximum amount of 'practical' protection. However there is a proviso for how 'practical' providing additional protection would be - if it's judged to be impractical then you don't need to provide the additional protection even if it might mean not meeting the 1 in 20. One example I'm familiar with is you could potentially provide additional protection by putting critical engine components (FADEC, oil tank, that sort of thing) always on the outboard side of the engine - if the cross engine debris goes through the engine to take out the component, that engine is dead regardless. But it was judged to be impractical to mandate different engine builds for left and right hand engines relative to the small amount of additional protection in would provide.
In short, they only thing that would make the MAX worse than the NG for rotor burst would be the larger fan disc on the Leap - the wider path of a 1/3rd disc would slightly increase the risk to flight control cables relative to the CFM56-7.
I can see how they might conclude that the considerable effort (and risk - making major changes to a flight critical system has it's own risks) required to mitigate the small increase in risk due to the larger fan disc wasn't justified.
Your explanation seems thorough, and I'm inclined to agree with you, but I'm equally compelled by the NYT article, citing 5 engineeres at the FAA, who were over ruled:
"F.A.A. managers conceded that the Max “does not meet” agency guidelines “for protecting flight controls,” according to an agency document. But in another document, they added that they had to consider whether any requested changes would interfere with Boeing’s timeline. The managers wrote that it would be “impractical at this late point in the program,” for the company to resolve the issue. Mr. Duven at the F.A.A. also said the decision was based on the safety record of the plane."
They conceded, not that there was a very minor increase in risk, but that it did not meet FAA guidelines for flight control protection. That's a strong statement, and hard for me to reconcile.