Cessna 152
Flight Crew Commercial; Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor
Flight Crew Last 90 Days 90; Flight Crew Total 445; Flight Crew Type 390
Human-Machine Interface; Troubleshooting
Aircraft Equipment Problem Critical
Flight Crew Landed in Emergency Condition; Flight Crew Landed As Precaution; General Declared Emergency; General Maintenance Action
Aircraft
I was performing a maintenance test flight in an aircraft that had had a rough running engine the prior day. The previous day's event occurred in cruise about 10 minutes after departure when the engine started fluctuating back and forth about 250 RPM at about 4 second intervals. I landed at the nearest suitable airport.Maintenance replaced all 4 of the lower plugs stating that they were so bad when pulled out of the cylinder head the carbon/lead build up could be heard hitting the lower cowling. Maintenance returned it to flight pending a test flight which I did today. I did a runup with no issues and after departure stayed in the pattern and climbed to 6;000 MSL prior to turning for the aircraft's home airport and advised Approach I would be descending East of the airport. I was handed off to Tower about 6 miles out. Prior to this I had noted slight RPM fluctuations as in the day prior; but this time only about 50 RPM. After flying over midfield; however; the fluctuations had grown to 250 RPM drops. I advised Tower of the issue and that I needed to spiral over the left downwind leg for Runway 25 but they asked for me to spiral to the South. They asked if I was going to declare an emergency. I did so; they cleared me to land and I made a descent with power and carb heat on as the fluctuations were less at higher settings; I made an uneventful landing and was able to taxi in under my own power.
A C152 on a maintenance test flight suffered fluctuating RPMs; declared an emergency and landed safely.