Date: 201306
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: Golden Eagle 421
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Cylinder
Component problem: Malfunctioning
Reporter organization: Air Taxi
Reporter function: Single Pilot; Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Training / Qualification; Troubleshooting
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Maintenance; Person Flight Crew
When detected: In-flight; Aircraft In Service At Gate
Result: General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Flying direct in a Golden Eagle 421 at about 80 miles out from my destination the right engine started running a little bit rough; it wasn't continuous; [and] it would come and go. Everytime that the engine would run rough; engine analyzer would show cylinder Number 4 running between 120 to 150 colder on the EGT compared to the other cylinders. No other indication of engine malfunction was present; oil temperature; oil pressure; manifold pressure; fuel flow; cylinder head temperature (CHT); everything was OK; just engine running a little bit rough and the EGT on cylinder Number 4 lower. At that point I suspected that one of the spark plugs had frozen or failed on that cylinder. We landed without any problems. As soon as we landed I did a magneto drop check at 1500 RPMs; that is the recommended for those engines; and then at 1800 RPMs under load.  Magneto check was perfect with the drop about 75 RPM per magneto per engine on both. I shut the engines down; off loaded the passengers; and called my DOM explaining to him what I saw. He made questions about; engine parameters in descent; magneto drop in run-up; etc. I was staying the night so he told me that next morning we would do another run-up and he would decide what to do. Next morning me and my DOM tried to find a maintenance facility at that airport but [there] were none so I got to the plane I did a walkaround not noticing anything out of the ordinary. Oil levels were between 10 1/2 and 11 quarts both engines. I finished with the walk around; started the engines no problems at all. Engines running perfect; my DOM is on the phone all the time. I go to the run up area; do another run up this time leaning the engines since we were suspecting of a fouled spark plug; checked magneto drop; everything perfect. DOM tells me on the phone that since engines are running OK to take the plane to a nearby airport. Plane is doing perfect on takeoff roll; climb; [and] cruise. On descent the same thing happens; cylinder Number 4 running a little bit colder on the EGT. I landed without any problems; head direct to the shop and explain them that the DOM wants them to inspect the spark plugs on cylinder Number 4 because they seem to be fouled or failed. They open the cowlings and they started pulling the spark plugs out. After removing the spark plugs they see that 2 studs holding the cylinder down had sheared. After inspection they found a small crack on the case coming from the front top stud into the case. I don't really know what caused the problem and I believe that is pretty much impossible to know that a cylinder is running a little bit rough because it has lost one or two studs holding it down. How to correct the situation is pretty much the same since it's really hard to know what really happened and where.




Synopsis: A C421 pilot noted the Number 4 cylinder running significantly cooler than normal. Maintenance subsequently found two Number 4 cylinder hold down bolts sheared in addition to a small crankcase crack.