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why cant regular ngk spark plugs be used on airplanes?

on my box of NGK br8es's, it says not for use in airplanes. why cant you use these plugs in airpanes?

Components used in safety critical applications like light aircraft engines have to be very specially tested and certified. It is an expensive process, which, for NGK is not worth their while in your market.
Just like a lot of microchips' datasheets will have a disclaimer - "not to be used in medical equipment"  - simply because it is a specialty chip that they did basic testing to ensure it works, but they're not in the 99.9999% confidence that it will not malfunction, ever, causing harm. 

Like in fight club:  Liklihood of failure * number of units out there * cost of a court settlement...
which costs more, the testing to alleviate the above scenario, or simply put a warning "not for x use"