While reading this question, I ran across an interesting unanswered comment. Why did Boeing choose to use some of the spoilers as mechanical backup controls rather than the ailerons?
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The B777 is a high subsonic aeroplane, and at high airspeeds the ailerons remain stationary because wing twist may cause aileron reversal.
By using inboard spoilers as mechanical backup roll control surfaces, there is roll control available at high airspeeds as well. Roll authority would be limited at approach speeds due to the smaller moment arm than the aileron provide and due to negative lift on one side only.
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$\begingroup$ By transitory measure, is this meant to serve the time between dual engine failure (fuel starvation or APU off) and RAT deployed? The batteries cannot tie over this transient? $\endgroup$– JZYLAug 17, 2019 at 22:52
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$\begingroup$ @Jimmy Cannot find an exact reference and have amended. It looks like a last resort measure. $\endgroup$– KoyovisAug 18, 2019 at 0:45
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1$\begingroup$ Re item 2 - the spoilers are also hydraulically-powered on the 777. The 'mechanical' does not refer to actuation, but to signalling the surface to move. $\endgroup$– RACAug 19, 2019 at 8:12
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$\begingroup$ May involve a new question, but what failure mode is this addressing (referring to "last resort measure"? $\endgroup$– JZYLAug 19, 2019 at 13:31