Hi Dave: If AH was off, then Ben would have had manual control and would doubtless have increased the throttle appropriately -- the FC board would have allowed him to do it. I think what this episode is telling us is that, when you first enable AH, you watch the copter very carefully to see if it's heading for the ground or the stratosphere -- and quickly turn off AH if it's doing either one of those and fly it manually. I also watch the copter very carefully when I take AH off -- depending on the throttle setting I've seen some sudden scary descents. Andy.
The Smart OSD may have provided some insight as well. If you watch the Virtual OSD Playback on the GPX tool, you will see that the Vertical Speed Indicator (The Bouncing Carrot Arrow, Left of the Altimeter Bar) is always showing a a decent even though the Altitude showing an increase in the altimeter box. This is what set off the alarm bells to me that something was wrong to investigate further. With AH on it will be normal to see some fluctuation in the VVI. But those fluctuations should not be much more than + - .1 to .5 feet/sec. If the VVI before takeoff is being erratic than that also is an indication that the Baro maybe off. I did a test recently where I sprayed some compressed air at the Baro, with the copter on. I found that the Velcro loops can get loose over time allowing for a air to enter causing a response in the VVI and Altimeter. I took the FC board off and re-affixed the Velcro and resprayed air on the baro and the altimeter did not significantly respond. So something to consider, when doing a periodic check of the system. I am even considering bringing compressed air with me as a pre-flight check before I fly.
Interesting data (and technique), Shaun. Thanks for posting that. Loose Velcro. Add that to the Great List of Things That Can Go Wrong, I'd say. Andy.