On Saturday, September 1, 2012 6:15:03 PM UTC-6, Andrew wrote:
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.





May have been a dust devil, without the dust, or a thermal plume (see the Angevine article linked elsewhere in this thread), or a burst.

Over three decades ago I witnessed an SR-71 on short final get banked left to nearly 90 degrees. It was arriving from the US to an overseas location. The sky had several high based virga in the area. By the time the pilot got level, the Habu was several hundred feet left of the runway center line, over the fuel depot. After that event, Blackbirds returning from missions would do a high-speed pass down the runway to check the local air. Of course this was well before micro-burst and wind shear detectors.

The air can be fickle at any altitude, but down low it can be fatal. Which is reason 39 why I don't paraglide.

Frank Whiteley