Narrative:

Left outboard flap failed without indication in cockpit. Part of flap went to 20 degrees (normal takeoff) the other half was stuck in the 0 degree position. Because it's a single actuator/composite flap it twisted like a pretzel. The flap indication is measured at the actuator and does not measure actual flap displacement. Thus the flap indicator and takeoff confign system do not, and cannot detect the asymmetric condition. This could be very dangerous. The failure/condition is very misleading during takeoff because of the lack of cockpit information. At takeoff the failure first appears to be like a fuel imbalance with about 50% control wheel deflection required to hold wings level, then as airspeed increases the asymmetry gets worse, similar to an aileron power control unit runaway, but slower. Critical flap indication and takeoff confign warnings should measure actual flap position, not actuator displacement. Callback conversation with reporter revealed the following information: reporter captain explained that the flap system is single actuator composite flap and that each actuator drives 2 sections of the flaps. In this case the left outboard and inboard flaps. Basically the flaps are positioned by an actuator driving cables that literally twist the flap into position. The difficulty with that system is that asymmetry is detected only through the monitoring computer and gets its inputs from the position of the actuator and not the flap position itself. Thus, as in this case, if one of the sections is off track and remains at 0 degree, but the other section extends normally to the selected position, the actuator is properly aligned with position selected, but in fact one part of it is stuck. When the flaps were retracted on schedule in this report, they came up, but due to the twisted material, they would not retract completely and then the computer sensed that failure and sent the 'flaps fail' warning. Reporter says after rotation, the aircraft commenced a slow roll and it took 50 degrees of aileron to counteract the roll. The roll became worse as speed increased, though, once the flaps were retracted the roll almost disappeared. One of the primary problems for this flight crew was they didn't know what was causing the roll. When they looked at EICAS pages, the flight controls were all normal as well as the flap position. When they got the 'flaps fail' indication there is no procedure to correct it, just land, which they did. Reporter says that he knows of one other case where the same type aircraft, CL65, had a similar problem and others wherein the split condition was discovered prior to takeoff by an observer outside the aircraft such as another flight crew taxiing behind the aircraft.

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Original NASA ASRS Text

Title: CL65 ACFT ON INITIAL CLB EXPERIENCED SLOW BUT SIGNIFICANT ROLL REQUIRING 50 DEG OF AILERON TO COUNTER THE ROLL. ALL INDICATIONS HAD BEEN NORMAL, FLAPS AT TKOF, 20 DEGS, ALL EICAS PAGES NORMAL. FLC RAISED FLAPS AT RETRACTION SPD AND THE ROLL DECREASED, BUT THEN A 'FLAPS FAIL' WARNING WAS DISPLAYED. FLC RETURNED TO LAND AT DEP ARPT.

Narrative: L OUTBOARD FLAP FAILED WITHOUT INDICATION IN COCKPIT. PART OF FLAP WENT TO 20 DEGS (NORMAL TKOF) THE OTHER HALF WAS STUCK IN THE 0 DEG POS. BECAUSE IT'S A SINGLE ACTUATOR/COMPOSITE FLAP IT TWISTED LIKE A PRETZEL. THE FLAP INDICATION IS MEASURED AT THE ACTUATOR AND DOES NOT MEASURE ACTUAL FLAP DISPLACEMENT. THUS THE FLAP INDICATOR AND TKOF CONFIGN SYS DO NOT, AND CANNOT DETECT THE ASYMMETRIC CONDITION. THIS COULD BE VERY DANGEROUS. THE FAILURE/CONDITION IS VERY MISLEADING DURING TKOF BECAUSE OF THE LACK OF COCKPIT INFO. AT TKOF THE FAILURE FIRST APPEARS TO BE LIKE A FUEL IMBALANCE WITH ABOUT 50% CTL WHEEL DEFLECTION REQUIRED TO HOLD WINGS LEVEL, THEN AS AIRSPD INCREASES THE ASYMMETRY GETS WORSE, SIMILAR TO AN AILERON PWR CTL UNIT RUNAWAY, BUT SLOWER. CRITICAL FLAP INDICATION AND TKOF CONFIGN WARNINGS SHOULD MEASURE ACTUAL FLAP POS, NOT ACTUATOR DISPLACEMENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CAPT EXPLAINED THAT THE FLAP SYS IS SINGLE ACTUATOR COMPOSITE FLAP AND THAT EACH ACTUATOR DRIVES 2 SECTIONS OF THE FLAPS. IN THIS CASE THE L OUTBOARD AND INBOARD FLAPS. BASICALLY THE FLAPS ARE POSITIONED BY AN ACTUATOR DRIVING CABLES THAT LITERALLY TWIST THE FLAP INTO POS. THE DIFFICULTY WITH THAT SYS IS THAT ASYMMETRY IS DETECTED ONLY THROUGH THE MONITORING COMPUTER AND GETS ITS INPUTS FROM THE POS OF THE ACTUATOR AND NOT THE FLAP POS ITSELF. THUS, AS IN THIS CASE, IF ONE OF THE SECTIONS IS OFF TRACK AND REMAINS AT 0 DEG, BUT THE OTHER SECTION EXTENDS NORMALLY TO THE SELECTED POS, THE ACTUATOR IS PROPERLY ALIGNED WITH POS SELECTED, BUT IN FACT ONE PART OF IT IS STUCK. WHEN THE FLAPS WERE RETRACTED ON SCHEDULE IN THIS RPT, THEY CAME UP, BUT DUE TO THE TWISTED MATERIAL, THEY WOULD NOT RETRACT COMPLETELY AND THEN THE COMPUTER SENSED THAT FAILURE AND SENT THE 'FLAPS FAIL' WARNING. RPTR SAYS AFTER ROTATION, THE ACFT COMMENCED A SLOW ROLL AND IT TOOK 50 DEGS OF AILERON TO COUNTERACT THE ROLL. THE ROLL BECAME WORSE AS SPD INCREASED, THOUGH, ONCE THE FLAPS WERE RETRACTED THE ROLL ALMOST DISAPPEARED. ONE OF THE PRIMARY PROBS FOR THIS FLC WAS THEY DIDN'T KNOW WHAT WAS CAUSING THE ROLL. WHEN THEY LOOKED AT EICAS PAGES, THE FLT CTLS WERE ALL NORMAL AS WELL AS THE FLAP POS. WHEN THEY GOT THE 'FLAPS FAIL' INDICATION THERE IS NO PROC TO CORRECT IT, JUST LAND, WHICH THEY DID. RPTR SAYS THAT HE KNOWS OF ONE OTHER CASE WHERE THE SAME TYPE ACFT, CL65, HAD A SIMILAR PROB AND OTHERS WHEREIN THE SPLIT CONDITION WAS DISCOVERED PRIOR TO TKOF BY AN OBSERVER OUTSIDE THE ACFT SUCH AS ANOTHER FLC TAXIING BEHIND THE ACFT.

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.