Narrative:

After the run-up; while holding short of runway (engine at 1;000 RPM) the engine RPM suddenly dropped to approximately 600. Thinking it might due to the hot/humid conditions; I placed the throttle at 1;300 RPM while we waited. About 30 seconds later; the engine speed dropped again to 900 RPM. I adivised tower that we were going to do another run-up. All indications during run-up were in the green and there was nothing abnormal; as they were during the first run-up. We were cleared for takeoff with a right; downwind departure. During the takeoff roll; the engine took longer than usual to reach peak RPM and the engine sounded a little deeper than usual. I questioned whether or not to abort but the engine speed was increasing. During climbout at approximately 700 ft AGL I noticed the engine speed begin to drop from 2;400 to 2;050. I lowered the pitch to try to increase engine speed at which point it increased to 2;300 RPM. I notified tower as soon as we turned to downwind that we had lost power during the climb and wanted to return for a landing. They asked if I needed assistance to which I replied no. ATC told a plane on final to go-around and cleared us for a short approach. We landed without incident and taxied back to the ramp. Maintenance met us as we pulled up. I explained what had happened and squawked the airplane. Maintenance was not able to start the engine while we were there. We were able to get another plane and continued the lesson without incidence. The first indication that the engine was not producing full power as quickly as it usually does and did not reach full power by rotation speed should have been the moment I chose to abort the takeoff. The drop in RPM during idle/waiting to depart did not seem unusual as it had happened to me before when it was hot. I seem to remember feeling some external pressure to get the lesson done and keep the students progressing but can't say to what extent it may have played in my decision not to abort. The event provided a good learning experience to the student pilot and student observer in the back seat; who both noticed the engine discrepancies; but wish I had relied on my gut and aborted the takeoff when I realized all was not right during takeoff roll.

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Original NASA ASRS Text

Title: PA28 instructor pilot experiences a mysterious RPM drop while waiting for takeoff and elects to perform a second run-up. The run-up is normal and the flight takes off but RPM begins to decrease almost immediately. The Tower is informed and the reporter is cleared for a short approach to the departure runway.

Narrative: After the run-up; while holding short of runway (engine at 1;000 RPM) the engine RPM suddenly dropped to approximately 600. Thinking it might due to the hot/humid conditions; I placed the throttle at 1;300 RPM while we waited. About 30 seconds later; the engine speed dropped again to 900 RPM. I adivised Tower that we were going to do another run-up. All indications during run-up were in the green and there was nothing abnormal; as they were during the first run-up. We were cleared for takeoff with a right; downwind departure. During the takeoff roll; the engine took longer than usual to reach peak RPM and the engine sounded a little deeper than usual. I questioned whether or not to abort but the engine speed was increasing. During climbout at approximately 700 FT AGL I noticed the engine speed begin to drop from 2;400 to 2;050. I lowered the pitch to try to increase engine speed at which point it increased to 2;300 RPM. I notified Tower as soon as we turned to downwind that we had lost power during the climb and wanted to return for a landing. They asked if I needed assistance to which I replied no. ATC told a plane on final to go-around and cleared us for a short approach. We landed without incident and taxied back to the ramp. Maintenance met us as we pulled up. I explained what had happened and squawked the airplane. Maintenance was not able to start the engine while we were there. We were able to get another plane and continued the lesson without incidence. The first indication that the engine was not producing full power as quickly as it usually does and did not reach full power by rotation speed should have been the moment I chose to abort the takeoff. The drop in RPM during idle/waiting to depart did not seem unusual as it had happened to me before when it was hot. I seem to remember feeling some external pressure to get the lesson done and keep the students progressing but can't say to what extent it may have played in my decision not to abort. The event provided a good learning experience to the student pilot and student observer in the back seat; who both noticed the engine discrepancies; but wish I had relied on my gut and aborted the takeoff when I realized all was not right during takeoff roll.

Data retrieved from NASA's ASRS site as of July 2013 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.