Mick
well im not quite sure exactly what to say, but when you have a closed fleet and you can manage and use the information that you can get from trending your engines you can make changes and monitor your changes in a positive or a negative. So by changing your plugs what else did you do..
Trending your engine wether turbine or piston is such an important tool. You are able to develop a knowledge of your machine and make changes to your fleet accordingly, use properly you will have a much more reliability.
I will just say well done
cheers.