Another stall spin

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