What scale is the speed trend line on a PFD's ASI? A primary flight display (PFD) in a glass cockpit has an airspeed indicator (ASI) that doesn't just display the speed, but also a vertical arrow or bar that shows the "speed trend" i.e. the acceleration. But there's no extra "acceleration scale" shown behind it like there is for speed. What's the scale of this trend arrow and how do you read it?
I found some training material on Airbus PFDs which explains:


  A vertical arrow, called the speed trend arrow, indicates the aircraft's
  predicted speed in 10 seconds if the current acceleration or deceleration rate remains constant.


Put another way, if you measure the acceleration in knots per second, and then multiply it by ten, it's on the same scale as the speed tape.

If you want to reach a particular speed in ten seconds' time, adjust pitch or power to put the head of the arrow on that tick on the speed tape, then keep adjusting to keep it on the same tick as it approaches.