S-58/S-58T Choctaw/Seahorse
Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Commercial
Flight Crew Last 90 Days 150; Flight Crew Total 2500; Flight Crew Type 800
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy
Aircraft Aircraft Damaged; Flight Crew Diverted; Flight Crew Overcame Equipment Problem; General Maintenance Action; General Flight Cancelled / Delayed
Procedure; Company Policy; Aircraft
During cruise flight at 3;500 feet and 106 knots ground speed. Aircraft was in straight and level flight. I noticed a 3% torque spilt between the #1 and #2 engine; with the number 2 beginning to slowly decrease in torque. N1 remained steady at 93% which is normal reading with both engines at 80% torque. The number 2 motor rapidly declined in torque causing a larger torque split to occur. Once the torque split became 15% difference; the N1 on the number 2 engine began to decrease. It went from 93% down to 73% causing an even larger torque spilt by 60%. At that time I decided to transition to single engine flight (OEI) and make a mayday call requesting vectors to the nearest airfield. Approach vectored me to ZZZ. I kept the engine at idle; since it was still producing minimal power. I figured some power is better than no power. So I used what power the number 2 engine had coupled with my good number 1 engine. Preformed a low power roll on landing into ZZZ and taxied onto the ramp. Upon visual inspection of the engine and discussion with the director of maintenance it was decided that an attempt to start the #2 in manual fuel control mode was the next course of action. Thinking that the fuel control was the culprit. The engine was started and sustained power along with the number 1 engine. The flight was then resumed in manual mode to our hangar in ZZZ1 where maintenance was performed. After inspection of the engine a borescope revealed that the #2 had suffered a near catastrophic failure. With multiple hot section blades missing; broken off; cracked; and folded over. Along with significant sized holes in the combustion cans.
S-58T pilot reported a diversion after decreasing engine torque on #2 engine.  After a manual restart and ferry flight to a maintenance base; borescope of the affected engine found major damage to the internal components of the affected engine.