Narrative:

My aircraft had been severely damaged 3 months earlier by another taxiing aircraft while it was tied down. Since then it had undergone extensive repairs; including complete tear-down and rebuild of the right engine. I went down to check out the aircraft with two mechanics; and fly it back to [home base] for final repairs. I took off with one mechanic in the right seat for a test flight. On take off; the other mechanic watching from the ground reported hearing backfiring; which we did not hear. However; during climb-out to about 2;000 ft; we experienced major power surges; rpm irregularities; intermittent roughness and resultant yawing. I became very concerned for our safety and decided as an emergency to land immediately on the nearest runway; which I could see to the north. I set up for landing; and although I soon realized that this was the nearby [military airfield] I did not have time to switch frequencies and request landing clearance.on final for the runway; the engines began to run smoothly and normally; so I then broke off the approach and returned to [departure airport]. Lessons: after major mechanical work; I should have loaded or written down the frequencies for any nearby airport in case of possible engine problems. I could have called the tower at [the military airfield] in advance and notified them that we would be maneuvering in the area on a VFR test flight. In my fear for our safety; I was forced to accept the possibility of landing without clearance; which fortunately was (in the end) not necessary.

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Original NASA ASRS Text

Title: A Baron 58 pilot experienced a rough running engine following a rebuild and began a divert toward a military airfield. The engine smoothed out and he returned to the departure airport.

Narrative: My aircraft had been severely damaged 3 months earlier by another taxiing aircraft while it was tied down. Since then it had undergone extensive repairs; including complete tear-down and rebuild of the right engine. I went down to check out the aircraft with two mechanics; and fly it back to [home base] for final repairs. I took off with one mechanic in the right seat for a test flight. On take off; the other mechanic watching from the ground reported hearing backfiring; which we did not hear. However; during climb-out to about 2;000 FT; we experienced major power surges; rpm irregularities; intermittent roughness and resultant yawing. I became very concerned for our safety and decided as an emergency to land immediately on the nearest runway; which I could see to the north. I set up for landing; and although I soon realized that this was the nearby [military airfield] I did not have time to switch frequencies and request landing clearance.On final for the runway; the engines began to run smoothly and normally; so I then broke off the approach and returned to [departure airport]. Lessons: after major mechanical work; I should have loaded or written down the frequencies for any nearby airport in case of possible engine problems. I could have called the tower at [the military airfield] in advance and notified them that we would be maneuvering in the area on a VFR test flight. In my fear for our safety; I was forced to accept the possibility of landing without clearance; which fortunately was (in the end) not necessary.

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.