Another stall spin

On Tuesday, August 28, 2012 1:53:21 AM UTC-4, Ramy wrote:
Tom, easy said than done. We all agree that circling too close to the ground is not a good idea, but there is no always a clear indication when it is too close. I don't know why many of you insist that there is nothing to learn from this accident without knowing the most important fact: how low was he circling?? If we find out it was 800 feet, will you go and claim that it is dumb to circle below 1000 feet? This is why we need to know the details, so we can make more informed conclusions. Ramy

Jim was a bit above the sight line of the trees bordering the airport and I would estimate maybe 3/4 of a mile away. Based on this, I would estimate his altitude as being on the order of 300 feet or so. That the glider rotated only a portion of a turn before impact would support this estimate.
Many time people decide it is OK to circle low because they have the airport, or maybe a really good field below. That makes the likely outcome of the landing somewhat better, IF the set up of the landing doesn't get messed up by a sudden loss of altitude. The possible consequences of a spin, however, are still deadly.
UH
UH

One point not reiterated yet here -- the atmosphere down low is very
different from what you're used to at 2000 feet and above. It's easy
to say "I haven't unintentionally stalled /spun in a thermal in a
thousand hours. How much of a dope do you have to be?"

A short list of what's different down low: The atmosphere is much more
turbulent. Thermals, such as they are are much smaller. In this layer,
many small punchy thermals will start. Many will die. The ones we use
up higher consist of many little parcels of hot air that have
coalesced. Most thermals are either short lived, or basically
unworkable to a modern glider. You're in the boundary layer where wind
is being affected by the ground, so there is wind-induced turbulence.
Punches of strong lift/gust followed by sink when you make a half turn
will be the norm.

The ground picture will be totally different to the pilot. If you turn
downwind at altitude, you don't notice that much. If you turn downwind
at 300 feet, all of a sudden the ground will rush by and, this being a
high stress moment, you may pull back. Just as the gust you turned in
fades, or the thermal turns to sink. And when the canopy fills with
trees going by at 70 mph, the urge to pull back will be really strong.
You may push forward to recover at altitude, but it's really really
hard to do with the ground coming up fast.

So, just because you've never unintentionally spun at altitude does
not mean your chances at 300 feet are the same.

Not to raise a tired subject, but why we give out contest points for
thermaling at 300 feet or below remains a puzzling question to me.

John Cochrane

On Tuesday, August 28, 2012 11:08:01 AM UTC-6, John Cochrane wrote:
snip

So, just because you've never unintentionally spun at altitude does

not mean your chances at 300 feet are the same.

snip


John Cochrane

Excellent point John, and for the same reasons I think the spinning intentionally at altitude and being confident you can recover may provide a false sense of security when thermalling low. It just isn't the same when done done at 300 feet.

Brian

And when the canopy fills with trees going by at 70 mph, the urge to pull back will be really strong.
You may push forward to recover at altitude, but it's really really hard to do with the ground coming up fast.

This reminds me of personal aerobatic experience and being inverted at 45 deg nose down at 1000ft. The normal pushing and rolling then goes against
everything your gut tells you to do.

It also reminded me of this crash in the UK, by an aerobatic pilot who was obviously very familiar with spinning this particular glider (although not at
gusty weather I suspect):
https://www.youtube.com/watch?v=zxbulrrQVig

Roel

On 8/28/2012 11:08 AM, John Cochrane wrote:

One point not reiterated yet here -- the atmosphere down low is very
different from what you're used to at 2000 feet and above.
Snip...

A short list of what's different down low: The atmosphere is much more
turbulent. Thermals, such as they are are much smaller. In this layer,
many small punchy thermals will start. Many will die. The ones we use
up higher consist of many little parcels of hot air that have
coalesced. Most thermals are either short lived, or basically
unworkable to a modern glider. You're in the boundary layer where wind
is being affected by the ground, so there is wind-induced turbulence.
Punches of strong lift/gust followed by sink when you make a half turn
will be the norm.

The ground picture will be totally different to the pilot. If you turn
downwind at altitude, you don't notice that much. If you turn downwind
at 300 feet, all of a sudden the ground will rush by and, this being a
high stress moment, you may pull back. Just as the gust you turned in
fades, or the thermal turns to sink. And when the canopy fills with
trees going by at 70 mph, the urge to pull back will be really strong.
You may push forward to recover at altitude, but it's really really
hard to do with the ground coming up fast.

So, just because you've never unintentionally spun at altitude does
not mean your chances at 300 feet are the same.
Snip...

John Cochrane

Science now can create movies & pictures of the accuracy/reality of what John
asserts above. Roughly 10 years ago I attended a presentation that included
LIDAR movies and pictures of thermals from ground to ~1500' agl. I expect
atmospheric imaging technology has significantly advanced since then.

Any "somewhat experienced" glider pilot would instantly recognize the 1,500'
images as being a thermal. However, in the absence of previous exposure to the
presentation, it required an explanation of what one was looking at, before
Joe Average Glider Pilot might recognize the rising air patterns from ground
level to the base of "a recognizable thermal" as being a coalescing thermal.
(For doubting-Thomas readers, there were multiple thermal examples, so we
weren't looking at the notorious "sample of one".)

Aficionados of tornado photographs might have a glimmer, because the closest
visual wavelength pictures I've seen that kinda-sorta mimic what the LIDAR
imagery showed, have been tornadoes with multitudes of thin, ropy,
mini-twisters feeding into the main funnel well above ground level.
Near-ground-level organization of some "multiple rope twister" photos I've
seen is scanty to non-existent. It doesn't take too much imagination to equate
"plenty of low-altitude garbage" I - and probably many RAS readers - have
tussled with striving for a low-altitude save, with what LIDAR and tornado
photos suggest (to me, anyway) isn't uncommon low-level thermal organization.
"Dynamic" is a pale descriptor of what goes on between ground level and the
agl level a modern sailplane can effectively use.

Unless your ship has the thermaling radius of an insect or a small bird, "what
John C. said" is likely to be in your low-level future. Is it worth betting
your life on?

For the record, the lowest I ever thermaled away from was 650' agl (Dalhart,
TX) on a day with 15-20 knot ground winds (not uncommon there). Yeah, right
above the launch airport. It took me 20 minutes and multiple low points, and
despite being both on vacation and on top of my game at the time, I was
sufficiently wrung out by the process that it also required some decompression
time once I'd established myself, before I could talk myself into heading out
on-course. The save was right after launching, when I was fresh.

Bob W.

I also have wondered how an experienced pilot can spin in from
low thermalling. It must be a full spin, with the resulting steep
recovery dive, that causes a spin-in accident: a stall doesn't lose
much height. Even the most spin-eager gliders I've flown (Dart 17
and Puchacz) always signalled an approaching stall in plenty of
time to stop an unintended spin developing. I've never
accidentally spun while thermalling at normal altitudes, not once in
my several thousand hours of gliding, so I wonder why it would be
more likely to happen low down.

One explanation might be that pilots are very stressed when
circling low, and simply don't fly as well as usual. Or maybe they
are circling unusually tightly, perhaps in a small thermal. I suspect
experienced pilots would not make these mistakes.

So I think that John Cochran's comment of August 28th may be
right. There may some unknown, unexpected risk when low-and-
slow that catches pilots out, even the best ones. Maybe small,
strong, bubbly thermals exist low down, that can perhaps
suddenly stall one wing? That would produce an immediate,
uncontrollable, violent roll, somewhat like a flick manoever,
without any advance warning signals. Not technically a spin, but
probably ending the same way: a steep dive with insufficient
height. If true, that's a risk that no amount of pilot skill can
prevent, except by adopting the sensible rules I was trained with:

1. never thermal below pattern altitude, and
2. always fly at approach speed below pattern altitude.

I have had one personal experience that supports John's
suggestion: after a normal thermal flight in the midwest, on a day
with light winds, I was on a normal final approach with wings level,
at normal approach speed (60kts). At about 100ft agl, without any
warning, my starboard wing was pushed rapidly and smoothly
upwards, and despite immediate full opposite control input, I was
put into a steep bank, I'd estimate close to 45 degrees. The surge
vanished as fast as it had arrived. After it stopped, I was able to
level the wings, correct the heading, and made a normal landing
further down the runway. It totally surprised me. I assume a
narrow thermal bubble lifted off under the starboard wing just as I
passed. I estimate the surge lasted about three seconds, so at
60kts it must have been about 300ft long. I would not have
believed it, except that it happened to me. I'm sure it would have
been much harder to cope with, if I had not been flying at
approach speed. Perhaps if I'd been flying slowly, at a higher
angle of attack, the surge might have stalled the starboard wing.
This roll event was also seen by an experienced pilot observer on
the ground, who said he was astonished to see it, and inquired
about it after I landed.

A roll upset like this has only happened to me once, so
(thankfully) its clearly a very rare occurrence, and maneageable
at approach speed. If such bubbles are baby thermals, they are
probably only in small areas, miles apart, and short lived, so
would also be rarely encountered. However a pilot who is
attempting to thermal low is presumably intentionally over an area
where baby thermals are forming, so may have a higher chance
of encountering such an effect.



At 17:08 28 August 2012, John Cochrane wrote:

One point not reiterated yet here -- the atmosphere down low is
very
different from what you're used to at 2000 feet and above. It's
easy
to say "I haven't unintentionally stalled /spun in a thermal in a
thousand hours. How much of a dope do you have to be?"

A short list of what's different down low: The atmosphere is
much more
turbulent. Thermals, such as they are are much smaller. In this
layer,
many small punchy thermals will start. Many will die. The ones
we use
up higher consist of many little parcels of hot air that have
coalesced. Most thermals are either short lived, or basically
unworkable to a modern glider. You're in the boundary layer
where wind
is being affected by the ground, so there is wind-induced
turbulence.
Punches of strong lift/gust followed by sink when you make a
half turn
will be the norm.

The ground picture will be totally different to the pilot. If you turn
downwind at altitude, you don't notice that much. If you turn
downwind
at 300 feet, all of a sudden the ground will rush by and, this
being a
high stress moment, you may pull back. Just as the gust you
turned in
fades, or the thermal turns to sink. And when the canopy fills
with
trees going by at 70 mph, the urge to pull back will be really
strong.
You may push forward to recover at altitude, but it's really really
hard to do with the ground coming up fast.

So, just because you've never unintentionally spun at altitude
does
not mean your chances at 300 feet are the same.

Not to raise a tired subject, but why we give out contest points
for
thermaling at 300 feet or below remains a puzzling question to
me.

John Cochrane

Jim was a bit above the sight line of the trees bordering the airport and I would estimate maybe 3/4 of a mile away. Based on this, I would estimate his altitude as being on the order of 300 feet or so. That the glider rotated only a portion of a turn before impact would support this estimate.

snip

UH

Thanks UH,
That is the information that 99% of us were missing from this accident.

Would be nice to have some flight recorder evidence to confirm it if available, but understand it might not be available.
Would also be nice to have similar information about the other accidents.

Brian

On Monday, August 27, 2012 10:53:21 PM UTC-7, Ramy wrote:
Tom, easy said than done. We all agree that circling too close to the ground is not a good idea, but there is no always a clear indication when it is too close. I don't know why many of you insist that there is nothing to learn from this accident without knowing the most important fact: how low was he circling?? If we find out it was 800 feet, will you go and claim that it is dumb to circle below 1000 feet? This is why we need to know the details, so we can make more informed conclusions.



Ramy

Ramy,

I listened to one pilot explain how he is confiddent to thermal at an altitude of 100 ft: do you think that is high enough?

Minimum thermalling altitude would reasonably be high enough to recover (demonstrated) from a stall/spin plus (at least, triple that for lower time cross country pilots) a 200 ft safety margin. This is REALLY not that difficult to figure out.

Tom
2G

At 18:35 28 August 2012, 2G wrote:
On Monday, August 27, 2012 10:53:21 PM UTC-7, Ramy wrote:
Tom, easy said than done. We all agree that circling too close to the
gro=
und is not a good idea, but there is no always a clear indication when it

=
is too close. I don't know why many of you insist that there is nothing
to
=
learn from this accident without knowing the most important fact: how low
w=
as he circling?? If we find out it was 800 feet, will you go and claim
that=
it is dumb to circle below 1000 feet? This is why we need to know the
deta=
ils, so we can make more informed conclusions.
=20
=20
=20
Ramy

Ramy,

I listened to one pilot explain how he is confiddent to thermal at an
altit=
ude of 100 ft: do you think that is high enough?

Minimum thermalling altitude would reasonably be high enough to recover
(de=
monstrated) from a stall/spin plus (at least, triple that for lower time
cr=
oss country pilots) a 200 ft safety margin. This is REALLY not that
difficu=
lt to figure out.

Tom
2G
This is so similar to the others that I can only say the same things ,was

there a situation in the cockpit we don't know. Insect bite,medical
problem,PDA fell off in the chop???
The only thing I know it is depressing me to keep loosing people from our
small community .
These days I fly a duo discus turbo and our rule is 1000ft AGL and the
engine
comes out or we set up a circuit into our chosen field.We don't fight
about it
but when the gear goes down you know your partner is making the point.
We are only a small voice but I would suggest changing the rules to the
affect
that if you are less than 600ft AGL that is your GPNS land out position
This
would mean there are no points to be gained from low scrapes.

snip
We are only a small voice but I would suggest changing the rules to the
affect that if you are less than 600ft AGL that is your GPNS land out position
This would mean there are no points to be gained from low scrapes.

This is one of those good ideas that gets worse as you try to implement it.

The problem is how do you enforce it?
How does the pilot know if he is or was less than 600ft? If he doesn't know he has to assume he wasn't and keep racing?
Does coming in at 90kts at 500 ft and pulling up to 650 in a 10kt thermal disqualify you?
Does passing over a Ridge at 500 (or 300 or 200) feet disqualify you?

I don't think a rule will work, plus it only addresses contest flying. However I have gotton on the radio and told a pilot he was setting a bad example and it was time for him to land. We do need to do more of this and teach pilots to apply this kind of peer pressure.

Brian