Minimum field size.

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?

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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Big subject covered in the Glider Flight Training Manual beginning on page 248 through page 256.

Clear the trees by a safe margin. In this case round it off to 100 feet.

Full dive brakes (type certified glider) and no wind, the glide ratio is 5:1.

This results in 500 feet plus round-out, flare, touchdown, apply brakes and stop.

Being the expert pilot you are, make it 1,000 feet. Try it at your familiar home airport.

Tom Knauff

Lots more info in the book.

I will add, look for powerlines in the area.
I was at a central PA contest that had lots of off field landings one day. One pilot was coming into a HUGE harvested field (I did a retrieve from the other end that same day) over a tree line and house. This was going into Altoona, PA.
He was low over the trees, but NEVER saw the power lines that went through the tree tops.
End result, he hit a top line, it slid under the nose to the main gear and pitched him straight down from about 60'.
Broken glider, broken ankles, took a few years before he flew solo again.

As stated before, do you practice "spot landings" on pretty much any landing?
Have you done "max performance" decents recently?

What some replies make seem simple, others will be nowhere close.

How to calculate it?

Hmmm, complicated problem with lots of variables. Needs a computer. How about use the one you happen to be sitting in.

If you are operating on a grass strip, how many lights do you go by before wheel stop? They seem to be about 200 feet apart.

I figure if I can measure it, I should be able to improve it and adjust as needed. So far, so good.

Since there has grown a real adversion to landing-out prevelent in the soaring community, this is a good question to hear.

Answer: totally dependant on your bird and your skills. A great experienced pilot in a kestrel can land shorter than an inexperienced newbi in a 1-26..

How to calculate: take your ship and invest in about a dozen back to back pattern tows and simulate coming in over a 50 ft obstacle, have someone note your touchdown point and your stopping point. Thats the only way to know what will be realistic for your compination of ship/experience. Be sure to practice using maximum rate of sink approach, (full spoiler and learn to slip, yes glass ships can slip).

Rule of thumb: A beginner should practice marking out a 1,000 ft minumum usable landing space. Thats a good goal to shoot for. Then as he gets better, he can get that down quite a bit shorter.

Returning to our field, I caught a thermal to give two other gliders time
to land
and clear the runway. I was tired out from the bumps; so decided to land
straight in from 491 m 2.1 km out, approximate 1:4 average slope ≈ 14°
at
maximum 70 kt with landing flap (limit 80) and full spoilers.

Took 91 seconds.

No sideslip. Drag varies with the square of the true airspeed. For
example,
increasing airspeed from 60 to 70 increases drag by 36%.

Not long ago another club seriously crunched a glider that did not come out
of
a sideslip before contacting the ground. Possibly there was a side load on
the
gear as it got bent.

I've dealt with power lines on three outlandings. From over a high voltage
line
you do end up a long ways in the field.

You want to be clearing the tower or pole with a fat margin. I fly almost
directly
overhead where I can look down on it. In between it's harder to judge and
the
wires are pretty much invisible.

At 14:26 12 December 2017, wrote:
How to calculate it?

Hmmm, complicated problem with lots of variables. Needs a
computer. How
about use the one you happen to be sitting in.

If you are operating on a grass strip, how many lights do you go by
before
wheel stop? They seem to be about 200 feet apart.

I figure if I can measure it, I should be able to improve it and adjust
as
needed. So far, so good.


with respect, I don't think there is room in the cockpit for my box and
monitor so I have always made a calculation in my head. It is a very
simple formula, find another field :-)
If you get to the point where the field you describe is the only one now
available to you, calculating if it is long enough should not be on the
list
of your priorities, and you are adopting the right approach to field
selection.

lauantai 9. joulukuuta 2017 7.57.41 UTC+2 FZ kirjoitti:
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?

I would add 10 times the height of trees to required field length. Inexperienced pilots fly over treetops usually with too much altitude (yes it feels safer, but at same time is kind of dangerous if the field is short). Experienced pilot who aims for treetops (and with normal landing speed), 6 times tree height to touch-down should be doable.

At 13:17 13 December 2017, krasw wrote:
lauantai 9. joulukuuta 2017 7.57.41 UTC+2 FZ kirjoitti:
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?

I would add 10 times the height of trees to required field length.
Inexperi=
enced pilots fly over treetops usually with too much altitude (yes it
feels=
safer, but at same time is kind of dangerous if the field is short).
Exper=
ienced pilot who aims for treetops (and with normal landing speed), 6
times=
tree height to touch-down should be doable.

Having landed in a field with tall trees on the far boundary and 15kts of
wind, I was amazed by the extra wind gradient the trees caused. If there is
any appreciable wind then give the trees a lot more room. A ground loop at
the end of the field is a lot less painful than a glider/tree interface
moment at 80ft in the air!

The better option is select a better field.