On 2012-08-20, Gilbert Smith wrote:
It is quite simple. You identify a point on the runway where you quit
if not off the ground.

It can be more complex than this. About four years ago I witnessed
a serious stall/spin crash. It was a rather marginal microlight being
tested at gross weight by an experienced pilot and the owner.

Where I live it's not much above sea level, but this aircraft didn't
have great climb performance. Off the end of the runway in use are some
low hills (actually, a glacial terminal moraine left by the last Ice Age)
probably only 150 feet high at most. They don't look in any way imposing
from the airfield, they are a good 3/4 mile off the end of a runway that's
the best part of a mile long, and probably not really high in the thoughts
of the pilot.

However, the "clutching hand" from these hills goes way above their tops
and can require good climb performance to escape. The hills are much more
sheer sided than they look from the far end of the runway, and the
sink can be very strong. I was actually on downwind at the time and
watching his position as I was planning a stop-and-go landing, and he
broke ground well before the halfway point of the runway. After doing my
stop-and-go, I lowered the nose just to check on his progress - knowing
I would probably be faster - only to see the wreckage on top of the
hill. (Fortunately they both survived, the owner was just bashed and
bruised but the pilot was more seriously injured. Another thing in
their favour was that there happened to be a Royal Navy Sea King in the
air on routine exercises only 20 minutes flying time away, fully
crewed and equipped for transporting casualties)

From his point of view, the takeoff was nominal, he was off before the
halfway point and climbing, apparently enough to clear the hills.