Narrative:

While leveling off at 16;000 MSL I noticed the right prop RPM gauge was slightly fluctuating. I opened the aom to see if there was another checklist other than the erratic engine that would explain these symptoms; since torque and itt were steady and we were not experiencing 'large engine instrument fluctuations; specifically high and/or fluctuating torque'.at this time; the right prop RPM began fluctuating more severely and we could hear and feel changes in engine/prop operation. This is when we also noticed the right torque gauge starting to fluctuate slightly; in addition to steadily increasing itt. The captain called for the 'erratic/uncommanded engine operation' checklist as soon as we saw the right torque indication start to vary. I grabbed the emergency action card and started reading. As I started reading the checklist I looked up and saw the right engine overtemp light illuminating intermittently; but the captain was quick to pull the right power lever back to 20-30% as per the checklist and the light went out. The right engine overtemp light was illuminated for an estimated 5-10 seconds total. Torque indications at the time of the overtemp ranged from approximately 55% to 85%. I remembered the training video addressing np sensor failure and believe this reduced the severity/longevity of the overtemp and helped avert an inflight engine shutdown.we continued the checklist by locking-out the right torque motor and verifying the auto-coarsen switch was off. Power fluctuations did not occur after completing these first three steps so we continued the checklist by turning off the propeller sync and restoring normal power on the right engine. Both engines operated without any fluctuation thereafter. The captain locked-out the torque motor on the left engine as suggested by the pom. The captain notified dispatch while I flew the aircraft and coordinated our diversion to our departure airport. We did not declare an emergency. Maintenance control; the flight attendant and the passengers were also notified of the situation shortly after turning back towards our departure airport. After the captain was done talking to dispatch; maintenance and the flight attendant; I gave him back the airplane and started a descent checklist. The rest of the flight was uneventful.

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Original NASA ASRS Text

Title: The flight crew of an SF340 followed appropriate QRH procedures to tame an erratic/fluctuating right engine. They returned to their departure airport after the engine was stabilized.

Narrative: While leveling off at 16;000 MSL I noticed the right prop RPM gauge was slightly fluctuating. I opened the AOM to see if there was another checklist other than the Erratic Engine that would explain these symptoms; since torque and ITT were steady and we were not experiencing 'large engine instrument fluctuations; specifically high and/or fluctuating Torque'.At this time; the right prop RPM began fluctuating more severely and we could hear and feel changes in engine/prop operation. This is when we also noticed the right torque gauge starting to fluctuate slightly; in addition to steadily increasing ITT. The Captain called for the 'Erratic/Uncommanded Engine Operation' checklist as soon as we saw the right torque indication start to vary. I grabbed the Emergency Action Card and started reading. As I started reading the checklist I looked up and saw the right engine overtemp light illuminating intermittently; but the Captain was quick to pull the right power lever back to 20-30% as per the checklist and the light went out. The right engine overtemp light was illuminated for an estimated 5-10 seconds total. Torque indications at the time of the overtemp ranged from approximately 55% to 85%. I remembered the training video addressing Np sensor failure and believe this reduced the severity/longevity of the overtemp and helped avert an inflight engine shutdown.We continued the checklist by locking-out the right torque motor and verifying the auto-coarsen switch was off. Power fluctuations did not occur after completing these first three steps so we continued the checklist by turning off the PROP SYNC and restoring normal power on the right engine. Both engines operated without any fluctuation thereafter. The Captain locked-out the torque motor on the left engine as suggested by the POM. The Captain notified Dispatch while I flew the aircraft and coordinated our diversion to our departure airport. We did not declare an emergency. Maintenance Control; the Flight Attendant and the passengers were also notified of the situation shortly after turning back towards our departure airport. After the Captain was done talking to Dispatch; Maintenance and the Flight Attendant; I gave him back the airplane and started a descent checklist. The rest of the flight was uneventful.

Data retrieved from NASA's ASRS site as of April 2012 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.