Narrative:

I was approximately 10-15 mi west of ZZZ flying VFR at 3000 ft talking to approach for VFR flight following. This was approximately 2 hours into the flight which had been normal up to this point. The first indication was a quick jump in cylinder head temperature. The cylinder head temperature went from normal to redline in a matter of mins. The other temperature gauges (oil temperature and exhaust temperature) were in the normal range. I immediately opened the cowl flaps and reduced the manifold pressure to try and reduce the cylinder head temperature. This did not have the desired effect and at this point the cylinder head temperature gauge started bouncing around erratically. The gauge was bouncing from maximum over redline back to normal and back to maximum again and again. The other temperature gauges remained normal. At this point I was approximately 10 mi from ZZZ and since I was on the radio with ZZZ approach; I decided to make a precautionary landing. I called up ZZZ and told them I had an indication of an overheated cht and that I would like to make a precautionary landing. Given my location; altitude and direction of flight; they suggested runway xx at ZZZ. I felt this was a good option and prepared for a landing on runway xx at ZZZ. ZZZ asked me if I wanted them to roll equipment. I said yes and that I was declaring an emergency. ZZZ asked me for number of people on board and fuel remaining. I told them 2 people on board and 2 hours of fuel remaining. I completed my pre-landing checklist and the approach and landing on runway xx was uneventful. ZZZ tower gave me taxi instructions to an FBO. Once parked at the FBO; I performed a normal engine shutdown and we exited the plane. We were met by a fire truck at the FBO. With the fire truck still there; I checked the heat of the cowling. It was not overly hot to the touch so I opened the flap for the oil to look in. Everything looked normal. At this point the fire team felt everything was ok and they left. The FBO did not have a mechanic on site on sep/xa/08 evening. I called my mechanic back at home airport and gave him a description of what happened. He felt given the normal reading on the other temperature gauges and the uneventful landing that it was probably a faulty indication on the cylinder head temperature and not a problem with the engine. In the meantime; the staff at the FBO had located a mechanic for me to talk to. The mechanic took the cowling off and did not find anything wrong with the engine. He did feel that the wire to the cylinder head temperature probe was routed in such a way that would allow it to bounce around and he used zip ties to secure it. He put the cowling back on and I decided to fly back. The flight home was uneventful with the cylinder head temperature remaining in the normal range for the entire flight. Even though it turned out to be a gauge problem; I believe I made the correct and prudent decision in declaring an emergency and landing as soon as practical so I could investigate more thoroughly on the ground.

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Original NASA ASRS Text

Title: M20 PLT DECLARES EMERGENCY AND LANDS DUE TO HIGH CYLINDER HEAD TEMPERATURES.

Narrative: I WAS APPROX 10-15 MI W OF ZZZ FLYING VFR AT 3000 FT TALKING TO APCH FOR VFR FLT FOLLOWING. THIS WAS APPROX 2 HRS INTO THE FLT WHICH HAD BEEN NORMAL UP TO THIS POINT. THE FIRST INDICATION WAS A QUICK JUMP IN CYLINDER HEAD TEMP. THE CYLINDER HEAD TEMP WENT FROM NORMAL TO REDLINE IN A MATTER OF MINS. THE OTHER TEMP GAUGES (OIL TEMP AND EXHAUST TEMP) WERE IN THE NORMAL RANGE. I IMMEDIATELY OPENED THE COWL FLAPS AND REDUCED THE MANIFOLD PRESSURE TO TRY AND REDUCE THE CYLINDER HEAD TEMP. THIS DID NOT HAVE THE DESIRED EFFECT AND AT THIS POINT THE CYLINDER HEAD TEMP GAUGE STARTED BOUNCING AROUND ERRATICALLY. THE GAUGE WAS BOUNCING FROM MAX OVER REDLINE BACK TO NORMAL AND BACK TO MAX AGAIN AND AGAIN. THE OTHER TEMP GAUGES REMAINED NORMAL. AT THIS POINT I WAS APPROX 10 MI FROM ZZZ AND SINCE I WAS ON THE RADIO WITH ZZZ APCH; I DECIDED TO MAKE A PRECAUTIONARY LNDG. I CALLED UP ZZZ AND TOLD THEM I HAD AN INDICATION OF AN OVERHEATED CHT AND THAT I WOULD LIKE TO MAKE A PRECAUTIONARY LNDG. GIVEN MY LOCATION; ALT AND DIRECTION OF FLT; THEY SUGGESTED RWY XX AT ZZZ. I FELT THIS WAS A GOOD OPTION AND PREPARED FOR A LNDG ON RWY XX AT ZZZ. ZZZ ASKED ME IF I WANTED THEM TO ROLL EQUIP. I SAID YES AND THAT I WAS DECLARING AN EMER. ZZZ ASKED ME FOR NUMBER OF PEOPLE ON BOARD AND FUEL REMAINING. I TOLD THEM 2 PEOPLE ON BOARD AND 2 HRS OF FUEL REMAINING. I COMPLETED MY PRE-LNDG CHKLIST AND THE APCH AND LNDG ON RWY XX WAS UNEVENTFUL. ZZZ TWR GAVE ME TAXI INSTRUCTIONS TO AN FBO. ONCE PARKED AT THE FBO; I PERFORMED A NORMAL ENG SHUTDOWN AND WE EXITED THE PLANE. WE WERE MET BY A FIRE TRUCK AT THE FBO. WITH THE FIRE TRUCK STILL THERE; I CHKED THE HEAT OF THE COWLING. IT WAS NOT OVERLY HOT TO THE TOUCH SO I OPENED THE FLAP FOR THE OIL TO LOOK IN. EVERYTHING LOOKED NORMAL. AT THIS POINT THE FIRE TEAM FELT EVERYTHING WAS OK AND THEY LEFT. THE FBO DID NOT HAVE A MECH ON SITE ON SEP/XA/08 EVENING. I CALLED MY MECH BACK AT HOME ARPT AND GAVE HIM A DESCRIPTION OF WHAT HAPPENED. HE FELT GIVEN THE NORMAL READING ON THE OTHER TEMP GAUGES AND THE UNEVENTFUL LNDG THAT IT WAS PROBABLY A FAULTY INDICATION ON THE CYLINDER HEAD TEMP AND NOT A PROB WITH THE ENG. IN THE MEANTIME; THE STAFF AT THE FBO HAD LOCATED A MECH FOR ME TO TALK TO. THE MECH TOOK THE COWLING OFF AND DID NOT FIND ANYTHING WRONG WITH THE ENG. HE DID FEEL THAT THE WIRE TO THE CYLINDER HEAD TEMP PROBE WAS ROUTED IN SUCH A WAY THAT WOULD ALLOW IT TO BOUNCE AROUND AND HE USED ZIP TIES TO SECURE IT. HE PUT THE COWLING BACK ON AND I DECIDED TO FLY BACK. THE FLT HOME WAS UNEVENTFUL WITH THE CYLINDER HEAD TEMP REMAINING IN THE NORMAL RANGE FOR THE ENTIRE FLT. EVEN THOUGH IT TURNED OUT TO BE A GAUGE PROB; I BELIEVE I MADE THE CORRECT AND PRUDENT DECISION IN DECLARING AN EMER AND LNDG AS SOON AS PRACTICAL SO I COULD INVESTIGATE MORE THOROUGHLY ON THE GND.

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.