AN ACR SAAB 340B LOST PWR ON BOTH ENGS AFTER TKOF.

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THE FO WAS FLYING. AFTER A NORMAL TKOF AT 100% PWR THE GEAR WAS RETRACTED. AT APPROX 200 FT AGL; THE L ENG TORQUE DROPPED SUDDENLY ACCOMPANIED BY A LOUD HAMMERING SOUND FROM THE L ENG. THE TORQUE DROPPED TO 20% AND THE FO BEGAN THE MEMORY ITEMS FOR ENG FAILURE AFTER V1. JUST BEFORE SHUTTING DOWN THE L ENG; THE R ENG BEGAN TO LOSE PWR AND A LOUD HAMMERING NOISE BEGAN ON THAT SIDE. AT THIS POINT WE HAD CLBED TO 500 FT AGL. I LEFT THE L ENG RUNNING AS IT WAS. THE FO BEGAN TO LOOK FOR A PLANE TO LAND STRAIGHT AHEAD. ABOUT 10 SECONDS LATER; THE R ENG BEGAN TO DEVELOP PWR AGAIN. WE OPTED TO MAKE A L TURN AND RETURN TO THE ARPT. THERE WAS A FOG BANK AT THE DEP END OF RWY 29 WHICH LIMITED OUR OPTIONS. BY DOWNWIND; BOTH ENGS WERE PRODUCING PWR AND WE LEFT THE PWR AND CONDITION LEVERS IN THEIR POS. THE LNDG WAS ACCOMPLISHED WITHOUT FURTHER INCIDENT. SUPPLEMENTAL INFO FROM ACN 368622: I CALLED OUT 'LNDG STRAIGHT AHEAD.' THE CAPT SLOWLY MOVED BOTH PWR LEVERS WITHOUT ANY RESPONSE FROM EITHER ENG. HE TURNED THE CONTINUOUS TORQUE ON TKOF SWITCH OFF WITH NO RESPONSE FROM EITHER ENG AND TURNED IT BACK ON. THE #1 ENG BEGAN TO COME BACK ON LINE AND PRODUCED ENOUGH PWR TO MAINTAIN ALT. BOTH ENGS BEGAN TO RUN SMOOTH AT APPROX MID-FIELD DOWNWIND. I HAD SUFFICIENT AIRSPD; AND PWR TO CLB WITHOUT A PWR CHANGE. I BELIEVE THERE WAS A MECHANICAL FAILURE OF BOTH ENGS DUE TO AN INTERCONNECTED SYS MALFUNCTION. PERHAPS THE CONTINUOUS TORQUE ON TKOF. I AM TOLD THAT THE ENGS ARE SUSCEPTIBLE TO COMPRESSOR STALLS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE SAAB 340B ACFT WITH GENERAL ELECTRIC ENGS. THE NTSB IS INVOLVED IN THE INVESTIGATION OF THIS INCIDENT. FUEL CONTAMINATION HAS BEEN RULED OUT. THERE WAS A VERY SEVERE TEMP INVERSION ON TKOF; WITH 9 DEGS C ON THE GND AND 22 DEGS C AT 500 FT WHICH MAY HAVE CAUSED PART OF THE PROB. THE ACR HAS A 'DERIVATIVE PWR PROGRAM' FOR THESE ENGS WHEREIN THE OVERHAUL PERIOD CAN BE EXTENDED; BUT THE ENG MUST BE OPERATED AT REDUCED THRUST AFTER A CERTAIN POINT. THE #1 ENG ON THIS ACFT WAS ON THIS PROGRAM. THE ACR HAS HAD 6 ENG FAILURES IN THE LAST FEW YRS; 5 OF WHICH WERE ON THE 'DERIVATIVE PWR PROGRAM.' A BROKEN WIRE WAS FOUND IN THE THROTTLE QUADRANT THAT INDICATES PWR LEVER ANGLE. THIS MAY HAVE HAD A BEARING ON THE PROB. THE FLT DATA RECORDER SHOWED THAT; FOR ABOUT 14 SECONDS; #1 ENG WAS PRODUCING ZERO TORQUE AND #2 ABOUT 40%; AND BOTH ENGS WERE STALLING FOR ABOUT 30 SECONDS. BOTH ENGS WERE 'TOASTED.' WHEN THE #1 ENG WENT TO AUTOCOARSEN (THE SWEDISH TERM FOR AUTOFEATHER); THE AUTOMATIC PWR RESERVE SYS WORKED PERFECTLY AND BOOSTED THE #2 ENG'S PWR TO 107%; THEN THE #2 ENG BEGAN TO FAIL. THERE HAS BEEN NO CRITICISM OF THE FLC TO DATE.