Narrative:

After holding at bdf for over 1 hour due to thunderstorms in the chicago area, we were cleared direct jot, direct mdw at 3000 ft MSL. We were second aircraft released from holding. Air carrier X volunteered to be a 'WX ship' in. We neither received nor were given any PIREPS by ATC from this aircraft at all. As we progressed just past jot, we observed 2 small remaining cells of round shape and shallow gradients on either side of our flight path. The cores were approximately 10 NM away with green up to about 2 mi away on radar. Visually, these areas appeared darker than the general overcast covering the area, the base of which was about 4500-5000 ft MSL. We could see the rain shafts under these showers and could see through them. We were in clear air with no rain or virga on or above us. Very suddenly and without warning, the aircraft began a series of very violent gyrations, going divergent violently and rapidly in all axes. The instruments were intermittently unreadable due to severe eye ball bounce and position and negative G forces. We were intermittently not in control of the aircraft during this min or so, and due to intermittently blurred vision, it was difficult to maintain an orderly xchk. During this time, the PF/first officer turned off the autoplt and was hand flying attitude as best as possible and did a fantastic job. I got on the throttles as the airspeed suddenly and instantaneously fell from about 250 KTS indicated to about 160 KTS or so and the vertical speed stall foot rising rapidly. It is at this point that I jammed the throttles up and the #2 engine overspd/overtemp apparently occurred. We received absolutely no windshear warnings during this entire event. Starting at 3000 ft MSL, at one point, we dropped down to about 2400 ft and toward the end, popped up to about 4100 ft almost instantly. I advised ATC of a severe turbulence encounter and that we were back under control and of our altitude excursions which were nearly instant and totally uncontrollable. I told them to not let any other aircraft through here and that we encountered severe turbulence. Landed normally at mdw, wrote up in the maintenance log, called dispatch and maintenance technician as required. Everyone was strapped in and it was after the 'preparation and landing' PA had been made. The #2 flight attendant reported that she might have strained her shoulders putting a cart back in the rear of the cabin while seated in her jump seat. Fortunately, no passenger were up or injured. I strongly suspect that this was an invisible tornadic vortex just forming in clear air versus a microburst. None of the classic wet or dry microburst symptoms were observed visually anywhere around us or above or below on the surface. We received absolutely no PIREP via ATC from the first aircraft through this area whatsoever. I have been flying since a young adult, and in all types of aircraft all over north america and I have never experienced anything this violent in over 14000 hours aloft.

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Original NASA ASRS Text

Title: F100 FLT CREW ENCOUNTERS SEVERE TURB WHILE ON VECTORS FOR APCH TO MDW.

Narrative: AFTER HOLDING AT BDF FOR OVER 1 HR DUE TO TSTMS IN THE CHICAGO AREA, WE WERE CLRED DIRECT JOT, DIRECT MDW AT 3000 FT MSL. WE WERE SECOND ACFT RELEASED FROM HOLDING. ACR X VOLUNTEERED TO BE A 'WX SHIP' IN. WE NEITHER RECEIVED NOR WERE GIVEN ANY PIREPS BY ATC FROM THIS ACFT AT ALL. AS WE PROGRESSED JUST PAST JOT, WE OBSERVED 2 SMALL REMAINING CELLS OF ROUND SHAPE AND SHALLOW GRADIENTS ON EITHER SIDE OF OUR FLT PATH. THE CORES WERE APPROX 10 NM AWAY WITH GREEN UP TO ABOUT 2 MI AWAY ON RADAR. VISUALLY, THESE AREAS APPEARED DARKER THAN THE GENERAL OVCST COVERING THE AREA, THE BASE OF WHICH WAS ABOUT 4500-5000 FT MSL. WE COULD SEE THE RAIN SHAFTS UNDER THESE SHOWERS AND COULD SEE THROUGH THEM. WE WERE IN CLR AIR WITH NO RAIN OR VIRGA ON OR ABOVE US. VERY SUDDENLY AND WITHOUT WARNING, THE ACFT BEGAN A SERIES OF VERY VIOLENT GYRATIONS, GOING DIVERGENT VIOLENTLY AND RAPIDLY IN ALL AXES. THE INSTS WERE INTERMITTENTLY UNREADABLE DUE TO SEVERE EYE BALL BOUNCE AND POS AND NEGATIVE G FORCES. WE WERE INTERMITTENTLY NOT IN CTL OF THE ACFT DURING THIS MIN OR SO, AND DUE TO INTERMITTENTLY BLURRED VISION, IT WAS DIFFICULT TO MAINTAIN AN ORDERLY XCHK. DURING THIS TIME, THE PF/FO TURNED OFF THE AUTOPLT AND WAS HAND FLYING ATTITUDE AS BEST AS POSSIBLE AND DID A FANTASTIC JOB. I GOT ON THE THROTTLES AS THE AIRSPD SUDDENLY AND INSTANTANEOUSLY FELL FROM ABOUT 250 KTS INDICATED TO ABOUT 160 KTS OR SO AND THE VERT SPD STALL FOOT RISING RAPIDLY. IT IS AT THIS POINT THAT I JAMMED THE THROTTLES UP AND THE #2 ENG OVERSPD/OVERTEMP APPARENTLY OCCURRED. WE RECEIVED ABSOLUTELY NO WINDSHEAR WARNINGS DURING THIS ENTIRE EVENT. STARTING AT 3000 FT MSL, AT ONE POINT, WE DROPPED DOWN TO ABOUT 2400 FT AND TOWARD THE END, POPPED UP TO ABOUT 4100 FT ALMOST INSTANTLY. I ADVISED ATC OF A SEVERE TURB ENCOUNTER AND THAT WE WERE BACK UNDER CTL AND OF OUR ALT EXCURSIONS WHICH WERE NEARLY INSTANT AND TOTALLY UNCONTROLLABLE. I TOLD THEM TO NOT LET ANY OTHER ACFT THROUGH HERE AND THAT WE ENCOUNTERED SEVERE TURB. LANDED NORMALLY AT MDW, WROTE UP IN THE MAINT LOG, CALLED DISPATCH AND MAINT TECHNICIAN AS REQUIRED. EVERYONE WAS STRAPPED IN AND IT WAS AFTER THE 'PREPARATION AND LNDG' PA HAD BEEN MADE. THE #2 FLT ATTENDANT RPTED THAT SHE MIGHT HAVE STRAINED HER SHOULDERS PUTTING A CART BACK IN THE REAR OF THE CABIN WHILE SEATED IN HER JUMP SEAT. FORTUNATELY, NO PAX WERE UP OR INJURED. I STRONGLY SUSPECT THAT THIS WAS AN INVISIBLE TORNADIC VORTEX JUST FORMING IN CLR AIR VERSUS A MICROBURST. NONE OF THE CLASSIC WET OR DRY MICROBURST SYMPTOMS WERE OBSERVED VISUALLY ANYWHERE AROUND US OR ABOVE OR BELOW ON THE SURFACE. WE RECEIVED ABSOLUTELY NO PIREP VIA ATC FROM THE FIRST ACFT THROUGH THIS AREA WHATSOEVER. I HAVE BEEN FLYING SINCE A YOUNG ADULT, AND IN ALL TYPES OF ACFT ALL OVER N AMERICA AND I HAVE NEVER EXPERIENCED ANYTHING THIS VIOLENT IN OVER 14000 HRS ALOFT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.