On 12/8/2017 10:57 PM, FZ wrote:
I'm driving past the nice grass field. Looks big, but it is surrounded on
all sides by the 80 ft high trees. What should be a minimum length of this
field, for a modern glider to safely land there? How to calculate it?

That last question - 'How to calculate it?' - is one many glider pilots down
the years likely wished they'd given some active - as distinct from
'hope-based' - thought to in the wake of an attempted off-field landing gone
wrong. Here's what has never failed to work for me...

Multiply the (nominal/guessed-at/known/etc.) height of the closest downwind
obstruction to the intended landing field by 10, and assume that distance
UPwind along the flight path from the obstruction simply doesn't
exist...because you're not going to be able to use it. 6-foot high fence? At
*least* 60 feet beyond unusable. Thirty feet high power lines? At *least* the
first 300 feet beyond it can't be used. Etc.

N.B.: the determining obstruction may well NOT be the one closest downwind to
the field under consideration, e.g. a field-bordering fence, itself in the
shadow of trees might as well not exist; the trees will be the
field-length-required determining obstruction.

Undoubtedly some will disagree with the above, basing claims on a ship's
(occasionally, even measured) glide angle with full drag devices in use. The
HP-14 I used to fly had somewhere between a 4:1 and 2:1 L/D glide angle in
even the slightest landing headwind (breeze), its successor perhaps 7:1. It
was while flying these ships that my 10:1 "rule" got developed. How willing
are you to bet your ship - and perhaps your life - on judging your wheel's
height above some obstruction - which you can no longer see - at the instant
you cross it, your skills (of course) being so well-honed that you can
*always* hit your intended spot from a max-steep-descent-angle approach? How
often do you actually *practice* - as distinct from imagine you *could*
practice - such approaches? How often do you actually *measure* such
practice's results? Truly thoughtful pilots will likely prudently conclude
such things - clearance margins, speed control, OFL-related mental stress,
etc. - are real, far from 'black-and-white', and never provide you a second
chance. Safety cushions are simple prudence.

In my experience, off-field-landings are generally great fun (a self-inflicted
adventure - woo hoo!)...but (duh!) only if the actual landing goes well.

Too bad RAS any more has so few of these sorts of "How ought I, as Joe Glider
Pilot, be thinking about such-n-so aspect of actually committing safe XC
soaring?" anymore...

Bob - your mileage WILL vary - W.

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