May 11, 2020 – Left Engine CHT#3

I am ready to go fly again this morning. My last flight was on May 3rd, and you can watch some of that flight in the video clip below. The part I’m focusing on here is where I had flown down the  coast and turning northbound as planned. I had just turned out over the water to avoid being low and making noise over populated beach areas when an engine monitor alarm flashed by too quickly to be read. I believe I was at 1500′ when it happened, and had just started the turn east over the water. You can find the segment of interest at around 13 minutes and 30 seconds in the clip.

The flash warning came and went so fast that I missed it, but I knew something had just happened. I remained in the turn, looked for traffic, and then was back inside monitoring that instrument more closely. It didn’t take long before I saw that the CHT#3 on the left engine had shot up from 330 degrees to over 600 degrees in one second!! Then suddenly it was back down to 330 for a few readings, and back up to 500 for a few seconds more!!

My checkbook was quivering at this point. I KNEW out of the gate with a  98% certainty that a temperature movement of this magnitude was nearly impossible without blowing something up. The engines purred smoothly the entire time, and no change in prop synchronization could be heard. This had to be data.

Supporting my theory was the fact that my friend Mike and I had seen one single CHT data point on the same cylinder shoot up and cause an alarm using Mike Busch’s analytical tools on that can be found on his website.  We looked at it and discussed it, and agreed that it had to be data. Of course it is my sole investment, but I wanted my friends analysis to make sure I wasn’t blinding myself with wishful thinking.

The same data issue appeared on two subsequent flights, but this last one had several hits. I shortened my plan flight, particularly avoiding a low over the water leg to the NJ coast. I didn’t land short or rush anything, since I remain convinced it is bad data caused by yet a fourth loose connection. On the other hand, I wasn’t taking any chances with further flight until I talked with my A&P.

Getting hold of Paul last week proved a challenge. By the time I did, my wife and I were just leaving for a 3 day trip to visit with friends. We traveled by car, so the airplane wasn’t in play. Paul is the A&P who did the work, and suggested I pull the left nacelle panel and look for a loose connection on the CHT.

The Lycoming IO-320-B1A cylinders are numbered from front to rear, odd numbers on the right, even numbers on the left. That means CHT #3 will be found inboard and rearward on the left engine, so that is the panel I should remove. I found this information in the Lycoming IO-320 operating manual; confirmed it with Mike, and then confirmed it again talking with Paul.

Now all I have to do is figure out what it is that I’m looking for, and where to look for this mysterious loose connection. For some guidance, I downloaded the installation manual for the EDM 760 CHT probe and found this guidance:

5) CYLINDER HEAD TEMPERATURE PROBE (CHT), BAYONET
The Bayonet probe 5050-T has the 3/8-24 boss as part of the probe and is screwed into the base of the cylinder (See fig-2). The bayonet probe has a screwdriver slot to facilitate tightening.

Temperature indicating system with Fuel Flow (that’s me….)
6) SPARK PLUG GASKET CHT PROBE

Most factory installed cylinder head temperature gauges utilize a bayonet or screw-in resistive type probe that occupies one of the bayonet sockets. This probe is not compatible with the thermocouple probes required for the EDM-760.

The spark plug gasket probe, P/N M-113, replaces the standard copper spark plug gasket on one spark plug. The plug chosen, upper or lower, should be the one that provides the best correlation with the other temperature probes.

Due to the spark plug location, the gasket probe may read 25oF higher or lower than the factory probe. The probe is usually placed under the plug that receives the most direct cooling air. After many removals the probe may be annealed for re-use. Heat and quench in water. At additional cost an adapter probe (bayonet) is available that permits the factory CHT probe and a JPI probe to fit the same bayonet location.

I am not sure what I have, but will call Paul and discuss when I’m standing there with the panel off, and have gained some perspective.

If I find something obvious that can be tightened, I’ll go fly. If not, I’ll talk with Paul and maybe stop in on him with the airplane. Failing either of those options, I’ll wait until I talk with him again.

The only pressure point to flying is that I’d like to get to zero squawks again and be done with break-in before I take this airplane and my wife to Florida in July. Let’s just assume Florida will be open and pools will be open where we stay.

Fly safe – I’ll put out a video of my loose connection search later today.

Frank