Narrative:

For the past 3 days my crew and I had been flying saab 340B models. On the last leg of a 2 day trip at night after 3 aircraft swaps for 4 legs, we experienced an uncontrollable torque fluctuation between 55% and 70% torque on the left engine drive. In other words, advancing the power lever would incur normal torque output until 55% at which time the torque would increase to 70%. Further advancement of the power lever would cause normal operation above 70%. A reduction in power level would cause a constant 70% with a subsequent sudden drop to 55%. Operation below 55% was normal. We entered the discrepancy in the maintenance logbook upon landing and reported the problem to maintenance control. I learned the following day that the corrective action was to turn the left ctot switch off. Ctot is an acronym for 'constant torque on takeoff' which is an automatic torque setting system that provides performance benefits by quickly and evenly setting the engine torque via a dial switch and once arming switch in an SF340B and 2 arming switches in the SF340A. On the climb checklist a challenge and response item in for the ctot switches to be turned off and the dial switch to be reset at its lowest setting. It is apparent that both the first officer and myself checked the settings and confirmed the position but due to reasons unknown, we both subconsciously selected only the right switch to the off position. While the 'B' model switch is of a different locking type with 3 position and the 'a' has 2 throw switches with 2 position, we believe that flying a 'B' for 3 days then switching to an 'a' model, coupled with departure from the primary airport in class B airspace, a short flight, 3 aircraft swaps, and the time of day, contributed to this mistake.

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Original NASA ASRS Text

Title: AN SAAB 340A DURING PREFLT CHK HAD ENG TORQUE SWITCHES CONFIGURED FOR A SAAB 340B RESULTING IN UNCTLABLE ENG TORQUE FLUCTUATIONS BTWN 55% AND 70%. ENG OP NORMAL ABOVE AND BELOW THIS RANGE.

Narrative: FOR THE PAST 3 DAYS MY CREW AND I HAD BEEN FLYING SAAB 340B MODELS. ON THE LAST LEG OF A 2 DAY TRIP AT NIGHT AFTER 3 ACFT SWAPS FOR 4 LEGS, WE EXPERIENCED AN UNCTLABLE TORQUE FLUCTUATION BTWN 55% AND 70% TORQUE ON THE L ENG DRIVE. IN OTHER WORDS, ADVANCING THE PWR LEVER WOULD INCUR NORMAL TORQUE OUTPUT UNTIL 55% AT WHICH TIME THE TORQUE WOULD INCREASE TO 70%. FURTHER ADVANCEMENT OF THE PWR LEVER WOULD CAUSE NORMAL OP ABOVE 70%. A REDUCTION IN PWR LEVEL WOULD CAUSE A CONSTANT 70% WITH A SUBSEQUENT SUDDEN DROP TO 55%. OP BELOW 55% WAS NORMAL. WE ENTERED THE DISCREPANCY IN THE MAINT LOGBOOK UPON LNDG AND RPTED THE PROB TO MAINT CTL. I LEARNED THE FOLLOWING DAY THAT THE CORRECTIVE ACTION WAS TO TURN THE L CTOT SWITCH OFF. CTOT IS AN ACRONYM FOR 'CONSTANT TORQUE ON TKOF' WHICH IS AN AUTOMATIC TORQUE SETTING SYS THAT PROVIDES PERFORMANCE BENEFITS BY QUICKLY AND EVENLY SETTING THE ENG TORQUE VIA A DIAL SWITCH AND ONCE ARMING SWITCH IN AN SF340B AND 2 ARMING SWITCHES IN THE SF340A. ON THE CLB CHKLIST A CHALLENGE AND RESPONSE ITEM IN FOR THE CTOT SWITCHES TO BE TURNED OFF AND THE DIAL SWITCH TO BE RESET AT ITS LOWEST SETTING. IT IS APPARENT THAT BOTH THE FO AND MYSELF CHKED THE SETTINGS AND CONFIRMED THE POS BUT DUE TO REASONS UNKNOWN, WE BOTH SUBCONSCIOUSLY SELECTED ONLY THE R SWITCH TO THE OFF POS. WHILE THE 'B' MODEL SWITCH IS OF A DIFFERENT LOCKING TYPE WITH 3 POS AND THE 'A' HAS 2 THROW SWITCHES WITH 2 POS, WE BELIEVE THAT FLYING A 'B' FOR 3 DAYS THEN SWITCHING TO AN 'A' MODEL, COUPLED WITH DEP FROM THE PRIMARY ARPT IN CLASS B AIRSPACE, A SHORT FLT, 3 ACFT SWAPS, AND THE TIME OF DAY, CONTRIBUTED TO THIS MISTAKE.

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.