A C172 pilot experiences a rough running engine and decides to land as a precaution.

Attributes
ACN	601391
Time
Date	200311
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : nvq.airport
State Reference	CA
Environment
Flight Conditions	VMC
Light	Dawn
Aircraft 1
Controlling Facilities	tower : nvq.tower
Operator	general aviation : personal
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	cruise : level descent : approach
Flight Plan	None
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : instrument pilot : commercial
Experience	flight time last 90 days : 50 flight time total : 400 flight time type : 350
ASRS Report	601391
Person 2
Affiliation	government : military
Function	controller : local
Events
Anomaly	aircraft equipment problem : critical
Independent Detector	other flight crewb
Resolutory Action	flight crew : landed as precaution
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

I was flying a traffic watch C172 in the vicinity of moffett field when my engine began to run very rough. I could feel an abnormal vibration through the yoke and could see that the instrument panel was shaking much more than normal. I initiated emergency procedures with my checklist. Already in contact with moffett tower, I requested a precautionary landing. After being cleared to do so, I landed without incident at moffett federal. I requested a precautionary landing instead of declaring an emergency because I was unsure of the severity of my problem. Thinking about it afterwards, I find that the term precautionary landing is somewhat ambiguous. If faced with a similar situation in the future, I will declare an emergency.

Original NASA ASRS Text

Title: A C172 PLT EXPERIENCES A ROUGH RUNNING ENG AND DECIDES TO LAND AS A PRECAUTION.

Narrative: I WAS FLYING A TFC WATCH C172 IN THE VICINITY OF MOFFETT FIELD WHEN MY ENG BEGAN TO RUN VERY ROUGH. I COULD FEEL AN ABNORMAL VIBRATION THROUGH THE YOKE AND COULD SEE THAT THE INST PANEL WAS SHAKING MUCH MORE THAN NORMAL. I INITIATED EMER PROCS WITH MY CHKLIST. ALREADY IN CONTACT WITH MOFFETT TWR, I REQUESTED A PRECAUTIONARY LNDG. AFTER BEING CLRED TO DO SO, I LANDED WITHOUT INCIDENT AT MOFFETT FEDERAL. I REQUESTED A PRECAUTIONARY LNDG INSTEAD OF DECLARING AN EMER BECAUSE I WAS UNSURE OF THE SEVERITY OF MY PROB. THINKING ABOUT IT AFTERWARDS, I FIND THAT THE TERM PRECAUTIONARY LNDG IS SOMEWHAT AMBIGUOUS. IF FACED WITH A SIMILAR SIT IN THE FUTURE, I WILL DECLARE AN EMER.

Data retrieved from NASA's ASRS site as of July 2007 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.