How do you judge correct acceleration?
I'm not for a minute pre-judging the cause of this incident, however for a llong time now I've held a view that very few pilots have any way of judging whether acceleration is correct for the runway in use.
In my former life, we used to have a speed we expected to achieve by a point on the runway (it was a fighter jet, and we expected to achieve a minimum speed by 2,000ft down the runway..otherwise you'd abort the take-off).
I've always been surprised there is nothing similar on bigger acft (despite the complexities involved).
I ask many of my FOs (and other Capts) what they use to assess correct acceleration. Most people reply "just a feeling" That doesn't work for me! I've watched the trend vector on the 777 speed tape on just about every takeoff I've done in the last few years. At 100kts or so, it's a minimum of 30kts..sometimes more if you're using higher than normal thrust on a shorter runway.
But given a normal 3500m+ runway, heavy jet, normal derate (or indeed full thrust if needed...it takes about the same acceleration rate to get to rotate speed on the appropriate distance, in particular if you're using balanced field figures). So, if you're taught awareness of what you expect to see on the speed tape for acceleration, you may be better placed to pick up abnormal (reduced) acceleration. To me, its far more likely to pick that up than "sensing" that the aircraft is not accelerating fast enough..especially in the myriad conditions we operate in (the Potomac river incident comes to mind...slow acceleration in low vis ops)
Curious to see what others think of my theory? (again..not saying this is necessarily relevant to this incident, but it brings up the subject I've been passionate about for some time.)