Narrative:

The precurser to the actual 'event', an altitude excursion, was trying to solve the loss of the captain's #2 display unit (du). The inbound crew had written up '#2 deu failed, recycled circuit breaker.' he and I had confused the du with the deu as we resolved this problem the aircraft was leveled at FL330 and a mach .83 cruise established on the autoplt in the navigation mode. About 2-3 mins, later a violent airframe vibration began. The aircraft began a rolling pitch up to the right. Almost immediately I shifted my gaze from the operations manual procedure to the pfd, disengaged the autoplt and autothrottles and reassured myself that the flap/slat handle was up locked. The aircraft pitched up 7 degree and rolled 10 degree right before it was put on the horizon manually. At this time or shortly after an altitude excursion of about 300 ft occurred. Disengaging the autoflt system about 20-30 seconds after the beginning of the event seemed to be the cause of the slat retraction. The cause of the problem that created the event is unknown. The corporate operating manual used for this type aircraft has very little technical information for troubleshooting. A discussion with the aircraft tech writer at the factory indicated there was pressure there to keep the manual thin and bare. It was further reduced by my company consequently when it is necessary to troubleshoot an unusual problem in the air, the crew member has little to work with. Callback conversation with reporter revealed the following information. Reporter indicated that the problem solving involvement with the du and deu (digital equipment unit, meaning the display unit signal generator), and the uncommanded slat extension were unrelated. The reporter also noted that an uncommanded slat extension occurred shortly after takeoff, but at the low speed involved did not pose a safety hazard (at that time). Slat extension can occur as a result of movement of the slap/flap handle system, or by automatic computer actuation as a result of aerodynamic stall detection. The reporter asserts that there was no activation of the stall warning system. The reporter was also able to examine DFDR (digital flight data recorder) information subsequent to the event. That examination allegedly revealed that the uncommanded slat extension was most probably the result of a slat/flap misrigging problem, that is, a problem in the quadrant handle to actuator cable run. The reporter also feels that the slat retraction (which ended the control problem and emergency) occurred because the severe airframe buffeting 'jiggled' the cable run mis-rig back into proper alignment. The reporter also alleges that his investigation has revealed that there have been at least 11 other such events, with at least 2 resulting in structural damage to the aircraft, specifically loss of portions, or damage to, the acfts elevator. It should be noted that the slat extension appears to have been asymmetric. The air carrier was aware of the potential problem on this type aircraft prior to this event, and SOP pre-departure briefings include instruction to positively and forcibly ensure that the slat/flap handle is secured in the fully retracted position.

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Original NASA ASRS Text

Title: THE FLC OF AN ADVTECH WDB ACR ACFT EXPERIENCE AN UNCOMMANDED SLAT EXTENSION WHILE AT CRUISE.

Narrative: THE PRECURSER TO THE ACTUAL 'EVENT', AN ALT EXCURSION, WAS TRYING TO SOLVE THE LOSS OF THE CAPT'S #2 DISPLAY UNIT (DU). THE INBOUND CREW HAD WRITTEN UP '#2 DEU FAILED, RECYCLED CIRCUIT BREAKER.' HE AND I HAD CONFUSED THE DU WITH THE DEU AS WE RESOLVED THIS PROBLEM THE ACFT WAS LEVELED AT FL330 AND A MACH .83 CRUISE ESTABLISHED ON THE AUTOPLT IN THE NAV MODE. ABOUT 2-3 MINS, LATER A VIOLENT AIRFRAME VIBRATION BEGAN. THE ACFT BEGAN A ROLLING PITCH UP TO THE R. ALMOST IMMEDIATELY I SHIFTED MY GAZE FROM THE OPS MANUAL PROC TO THE PFD, DISENGAGED THE AUTOPLT AND AUTOTHROTTLES AND REASSURED MYSELF THAT THE FLAP/SLAT HANDLE WAS UP LOCKED. THE ACFT PITCHED UP 7 DEG AND ROLLED 10 DEG R BEFORE IT WAS PUT ON THE HORIZON MANUALLY. AT THIS TIME OR SHORTLY AFTER AN ALT EXCURSION OF ABOUT 300 FT OCCURRED. DISENGAGING THE AUTOFLT SYS ABOUT 20-30 SECONDS AFTER THE BEGINNING OF THE EVENT SEEMED TO BE THE CAUSE OF THE SLAT RETRACTION. THE CAUSE OF THE PROBLEM THAT CREATED THE EVENT IS UNKNOWN. THE CORPORATE OPERATING MANUAL USED FOR THIS TYPE ACFT HAS VERY LITTLE TECHNICAL INFO FOR TROUBLESHOOTING. A DISCUSSION WITH THE ACFT TECH WRITER AT THE FACTORY INDICATED THERE WAS PRESSURE THERE TO KEEP THE MANUAL THIN AND BARE. IT WAS FURTHER REDUCED BY MY COMPANY CONSEQUENTLY WHEN IT IS NECESSARY TO TROUBLESHOOT AN UNUSUAL PROBLEM IN THE AIR, THE CREW MEMBER HAS LITTLE TO WORK WITH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. RPTR INDICATED THAT THE PROBLEM SOLVING INVOLVEMENT WITH THE DU AND DEU (DIGITAL EQUIP UNIT, MEANING THE DISPLAY UNIT SIGNAL GENERATOR), AND THE UNCOMMANDED SLAT EXTENSION WERE UNRELATED. THE RPTR ALSO NOTED THAT AN UNCOMMANDED SLAT EXTENSION OCCURRED SHORTLY AFTER TKOF, BUT AT THE LOW SPD INVOLVED DID NOT POSE A SAFETY HAZARD (AT THAT TIME). SLAT EXTENSION CAN OCCUR AS A RESULT OF MOVEMENT OF THE SLAP/FLAP HANDLE SYS, OR BY AUTOMATIC COMPUTER ACTUATION AS A RESULT OF AERODYNAMIC STALL DETECTION. THE RPTR ASSERTS THAT THERE WAS NO ACTIVATION OF THE STALL WARNING SYS. THE RPTR WAS ALSO ABLE TO EXAMINE DFDR (DIGITAL FLT DATA RECORDER) INFO SUBSEQUENT TO THE EVENT. THAT EXAMINATION ALLEGEDLY REVEALED THAT THE UNCOMMANDED SLAT EXTENSION WAS MOST PROBABLY THE RESULT OF A SLAT/FLAP MISRIGGING PROBLEM, THAT IS, A PROBLEM IN THE QUADRANT HANDLE TO ACTUATOR CABLE RUN. THE RPTR ALSO FEELS THAT THE SLAT RETRACTION (WHICH ENDED THE CTL PROBLEM AND EMER) OCCURRED BECAUSE THE SEVERE AIRFRAME BUFFETING 'JIGGLED' THE CABLE RUN MIS-RIG BACK INTO PROPER ALIGNMENT. THE RPTR ALSO ALLEGES THAT HIS INVESTIGATION HAS REVEALED THAT THERE HAVE BEEN AT LEAST 11 OTHER SUCH EVENTS, WITH AT LEAST 2 RESULTING IN STRUCTURAL DAMAGE TO THE ACFT, SPECIFICALLY LOSS OF PORTIONS, OR DAMAGE TO, THE ACFTS ELEVATOR. IT SHOULD BE NOTED THAT THE SLAT EXTENSION APPEARS TO HAVE BEEN ASYMMETRIC. THE ACR WAS AWARE OF THE POTENTIAL PROBLEM ON THIS TYPE ACFT PRIOR TO THIS EVENT, AND SOP PRE-DEP BRIEFINGS INCLUDE INSTRUCTION TO POSITIVELY AND FORCIBLY ENSURE THAT THE SLAT/FLAP HANDLE IS SECURED IN THE FULLY RETRACTED POS.

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.