On Monday, December 18, 2017 at 1:32:24 PM UTC-7, wrote:
Oh gosh ... that depends enormously on

Does the field slope?

WIND!

Elevation

What glider are you flying? (And how good are you?)

Some gliders are capable of exceedingly short landings ... in the hands of a really good pilot who knows the sailplane very well. These include the 1-26 (capable of really steep descent with combination slip and spoilers, flies slowly, has skid for for aggressive stop), and almost all of the sailplanes with full landing flaps ( Pik 20B, SGS 1-35, and of Dick Schreder's designs)

A 1-26 flown competently can safely land on a football field ... very few other gliders can do that, I'd say the 1-35 is the other.

If you have a conventional glass sailplane 500' over any sort of trees is very short -- I have landed a Grob 103 (with 2 on board!) in that distance over 50 ft trees in no wind (and at an altitude near sea-level) because I had to ... but absolutely would not want to make a habit of it!

Normally never pick a field less than 1000' ... and almost all sailplanes can land in 1000 ft over trees if you are reasonably competent. Note Tom Knauff's comment to this effect -- I agree.

Picking landing fields from the air is usually NOT primarily a problem of length. What is in the field is usually the main issue/worry ... what crop, how tall is that, and is there hidden junk in the field that you can't see? A fence-line or irrigation gear you don't spot is among the biggest worries.

You must know local agricultural practice ... if you are flying somewhere new this is among the first issues to enquire about.

It's also very difficult to judge slope from the air -- so many wrecked gliders have happened because a pilot didn't appreciate the slope and set up to land down wind on a sloping field.

Most really bad off-field landings occur either because the pilot waited far too late to start considering/choosing a field ... or because they chose one, got there ... and discovered as they got close that it was unsuitable. On that score, given choices ... always glide toward something that gives you more than one "decent" choice if possible, rather than one great-looking (from a distance).

That 500' landing I mentioned happened because it was my second choice, and the first was ruled out by a barbed-wire fence I couldn't see from two miles away ... but could when I got there at pattern altitude.

Be really-really wary about any line of coloration across a field -- from afar a fence will show up (maybe) as a faint line of color due to the crop/grass either being taller there, or not planted right up to it.

On the other hand, a lot of fields have harmless stripes in them due to moving farm equipment,. etc.

A warning to inexperienced sailplane pilots -- land on golf courses only in the worst desperation, and be prepared for great unpleasantness and whopping expense if you do. There are too many people about on a golf course -- your chances of hitting somebody are far too high ... and it is entirely fair of them to charge you the loss of green fees from the play you stall, and an extremely difficult time getting the glider off the golf course. I have never been in this position because I learned early from the stories of others.

If you fly where landout options are scarce (like my location in the desert southwest), it pays to visit and document possible landout options - fields, farm strips, etc. Years ago, a couple of us did this in the winter off season and was surprised by one. A disused WWII airfield had asphalt runways that looked quite landable - from the air it seemed there was just a bit of grass growing in cracks. On the ground, the "grass" turned out to be palo verde trees several feet high that would have ripped your wings off!

I echo previous remarks that hazards are very hard to spot from the air, so I prefer to keep a database of known landing fields in my flight computer.

Mike