What we are referring to isn't even severe turbulence, its considered extreme. Anytime anything enters conditions that are described as extreme, all bets are off. However, one needs to keep in mind that extreme turbulence pretty much means the aircraft is completely out of control, anything unsecured in the cabin is being violently tossed around, things are breaking, etc. Most pilots will never see this in their careers (mostly because we do our best to avoid anything that even looks like it could be that bad).

Furthermore, in my experience, turbulence is usually over reported. Take a look at the following descriptions (from the FAA) and think of how many times you were in what you thought might be severe, when in fact, it would only be moderate at best.


Light


Turbulence that momentarily causes slight, erratic changes in altitude and/or attitude (pitch, roll, yaw). Report as Light Turbulence;

or

Turbulence that causes slight, rapid and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude. Report as Light Chop.


Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered in walking.

Moderate

Turbulence that is similar to Light Turbulence but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed. Report as Moderate Turbulence;

or

Turbulence that is similar to Light Chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude. Report as Moderate Chop.


Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult.

Severe

Turbulence that causes large, abrupt changes in altitude and/or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control. Report as Severe Turbulence.

Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food Service and walking are impossible.

Extreme

Turbulence in which the aircraft is violently tossed about and is practically impossible to control. It may cause structural damage. Report as Extreme Turbulence.

Severe turbulence will almost always been encountered inside areas of convective activity. So yes, I can point to an airliner that crashed because of flying into a severe thunderstorm and its associated severe turbulence and conditions, Southern Airways 242.

http://en.wikipedia.org/wiki/Souther...ays_Flight_242

The crew did not understand the concept of radar attenuation, where the target is so dense (ie so much rain) that it absorbs most of the radar signal. On an radar screen this looks like a thin area and an attractive area to fly through when deviating around storms, when it actually is the worst place to be.

This is highly stressed during radar training and most radars have a setting that will display any areas that are being attenuated and thus the returns or lack there of in those areas cannot be trusted.

Checko