A C172 training flight returned to base when the engine started running rough.

Attributes
ACN	801424
Time
Date	200808
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	msl single value : 2000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : zzz.tracon
Operator	general aviation : instructional
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	climbout : intermediate altitude
Flight Plan	None
Person 1
Affiliation	other
Function	instruction : instructor
Qualification	pilot : commercial pilot : multi engine pilot : instrument pilot : cfi
Experience	flight time last 90 days : 50 flight time total : 500 flight time type : 250
ASRS Report	801424
Person 2
Affiliation	other
Function	instruction : trainee
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other flight crewa
Resolutory Action	flight crew : landed as precaution other
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

Instructional flight (specifically private pilot recurrent training). Aircraft preflight inspection was unremarkable; with no deviation or issues. Aircraft start; taxi and engine run-up was unexceptional and conformed to standard tolerances. Takeoff seemed somewhat sluggish; but there was significant density altitude (1800 approximately); so it in itself was not totally unexpected. On climb out the aircraft seemed to perform better; operation was within guidelines. As the aircraft reached 1500 ft; a slight vibration started to become apparent; and remained after the recommended leaning procedure was followed. After leveling off at 2000 ft; the vibration became worse and the engine (at full throttle) would not operate past 2150-2200 RPM and cruise speeds of 100-105 KTS. After troubleshooting the problem I made the decision to return to ZZZ. I contacted departure (having never been released; and just reaching the outer boundary of the class C airspace) and asked for a return heading. I was offered the chance to declare an emergency; but chose not to because while operating at 2000 RPM or less; then engine was significantly smoother. I allowed the student to land the aircraft and taxied back without incident. In retrospect I should have thought more about returning to the airport sooner given the indications of a problem developing instead of troubleshooting for the problem. Flts like this serve as a great reminder of why we focus so much attention on emergency operations.

Original NASA ASRS Text

Title: A C172 TRAINING FLT RETURNED TO BASE WHEN THE ENGINE STARTED RUNNING ROUGH.

Narrative: INSTRUCTIONAL FLT (SPECIFICALLY PVT PLT RECURRENT TRAINING). ACFT PREFLT INSPECTION WAS UNREMARKABLE; WITH NO DEV OR ISSUES. ACFT START; TAXI AND ENG RUN-UP WAS UNEXCEPTIONAL AND CONFORMED TO STANDARD TOLERANCES. TKOF SEEMED SOMEWHAT SLUGGISH; BUT THERE WAS SIGNIFICANT DENSITY ALT (1800 APPROX); SO IT IN ITSELF WAS NOT TOTALLY UNEXPECTED. ON CLBOUT THE ACFT SEEMED TO PERFORM BETTER; OP WAS WITHIN GUIDELINES. AS THE ACFT REACHED 1500 FT; A SLIGHT VIBRATION STARTED TO BECOME APPARENT; AND REMAINED AFTER THE RECOMMENDED LEANING PROC WAS FOLLOWED. AFTER LEVELING OFF AT 2000 FT; THE VIBRATION BECAME WORSE AND THE ENG (AT FULL THROTTLE) WOULD NOT OPERATE PAST 2150-2200 RPM AND CRUISE SPDS OF 100-105 KTS. AFTER TROUBLESHOOTING THE PROB I MADE THE DECISION TO RETURN TO ZZZ. I CONTACTED DEP (HAVING NEVER BEEN RELEASED; AND JUST REACHING THE OUTER BOUNDARY OF THE CLASS C AIRSPACE) AND ASKED FOR A RETURN HDG. I WAS OFFERED THE CHANCE TO DECLARE AN EMER; BUT CHOSE NOT TO BECAUSE WHILE OPERATING AT 2000 RPM OR LESS; THEN ENG WAS SIGNIFICANTLY SMOOTHER. I ALLOWED THE STUDENT TO LAND THE ACFT AND TAXIED BACK WITHOUT INCIDENT. IN RETROSPECT I SHOULD HAVE THOUGHT MORE ABOUT RETURNING TO THE ARPT SOONER GIVEN THE INDICATIONS OF A PROB DEVELOPING INSTEAD OF TROUBLESHOOTING FOR THE PROB. FLTS LIKE THIS SERVE AS A GREAT REMINDER OF WHY WE FOCUS SO MUCH ATTN ON EMER OPS.

Data retrieved from NASA's ASRS site as of May 2009 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.