In China,
First, it is good of you to post your experience. I'm sure many will learn from it
My experience is Classic, so I should think we have more drag and slow quicker than the NG of which I have no experience. But maybe the similarities are similar enough. Also I think Kosmo is quoting a company manual.
I then entered some descent information on the FMC.
Did you exec the changes? One can get massive profile changes.
At 40 seconds after beginning decent vertical speed was 5400 feet/Min (we know its fast)
Yes, but even getting to 6,000 fpm hasn't presented us with problems. So that I doubt that in it's self is a problem.
Then the aircraft started to pitch up, but due to momentum the air speed still increased, but at a slower rate.
I have found that one needs to assert a zero pitch attitude to get any kind of meaningful decellerration near the limit. As others have said, the most expeditious and easiest way to accomplish this is gentle back pressure on the column to trip it into CWS P. My previous near MMO incursion I disconnected completely and although not an issue in anyway, required more chatter, more work generally so I don't see what is going to be gained by fully disconnecting the A/P.
t 50 seconds after the decent began , at 29740 feet we had the Mach over speed warning sound at Mach 8.26, with a decent rate of 3400 feet per minute
On the Classic, 3400 fpm is not enough to decelerate unless you intervene much earlier.
We think the wind change contributed quite a lot to the overall situation.
Unless you exec'd your FMC changes I don't think the FMC would calculate such an abrupt profile change.
In your defence the PF did extend the speedbrake but it was ineffective. Also, there is no specific guidance on what kind of pitch attitude or V/S would actually result in adequate decellaration. So you effectively had to suck it and see from which a minor incursion doesn't seem to be too bad a result. I would have done it differently, but then my company is a strong advocate of disconnecting the A/P if it's performance in insufficient. What is your company's written policy and how strongly is this reinforced in sim and line checks?