Gary Nuttall wrote:

As a glider pilot I act within both my and the glider's
limits. I trust my instincts that if something doesn't
feel right, I abandon the launch while it's still safe
to do so. Maybe it's a US vs UK thing

Very likely - how often does a pilot in the UK have to decide if an
airport at a 10,000 foot density altitude that has never seen a towplane
is safe to use? Here in the USA, I"ll bet we have this problem much more
frequently.

but here in
the UK we take personal responsibility for our actions.

I think it's the same here, too. After all, the tow can go bad for
several reasons besides a high density altitude tow at an airport that's
never been used for towing! And, of course, it's not dependent only on
the glider pilot: the towpilot should notice HE'S not off the ground
early enough, and let the glider go so the towplane pilot can deal with
the towplane's problem. In fact, the combination can still be in trouble
even if the glider has taken off "in time", because the critical element
is the towplane taking off in time. It's hard for the glider pilot to
assess this.

What I think Kilo Charlie and the others are trying to determine is if
it's even worth taking a towplane to this potential site. Without
experience at a similar site, looking for pertinent numbers seems like a
better idea than just showing up and trying it.

If you're not sure that you have sufficient distance
to take-off then why would you trust a set of numbers
that say otherwise?

Perhaps because you've verified the table or equation in other
situations, and added a margin for safety, and because you are using a
towplane, towplane pilot, and glider pilot you trust to handle the
situation, even if things go wrong.

I think the concept of calculating
takeoff runs is actually quite interesting but the
sheer number of variables involved make it an impracticable
exercise.

Nonsense. You aren't trying to precisely determine takeoff runs, but
decide if the situation is "safe enough". They know how the towplane
operates compared to it's POH values, and the addition of the glider can
be calculated (it's just drag and weight, not a huge number of
variables), so a sensible estimate can be determined.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org