Towering cumulus (not sure what a cumbulus is) are by formed in areas of deep moisture and convection. Think back to your private pilot training days. There are indeed three elements required for a thunderstorm to form. A TCU is presents all three. Additionally, it's careless, reckless, and quite foolish to be so naive and assume that all will be OK when flying in such a condition.
Cumulus clouds, or fair weather cumulus, are as you said are generally an indication of a relatively small area of instability. However, they're still an indication of convective activity. Generally speaking, they're simply uncomfortable for passengers (or pilots alike who don't like turbulence). The last thing I teach and practice is being an airborne convective activity expert by gambling on which hazards a cumulus (fair weather and TCU) may/may not present.
The towering cumulus clouds are a textbook indication of the first stage of a thunderstorm and as some have described here, are hazardous to small and large airplanes alike. In my profesional opinion, it's an unwise practice to penetrate convective occurences such as TCU.
Quote: flight2000
So where did anyone say they penetrate convective activity in this thread? Both cloud sets below are cumulus are they not? One I'll fly through, the other gets a really wide berth or I'll land until it moves along.
My experience has shown me what my minimums are for flying through a cumulus cloud and I have not seen any major turbulence, hail, or lightning in any cloud that size (or smaller) in almost 15 years of flying with my IR ticket. If clouds (cumulonimbus) are in the flight levels I will simply go around it or alter my plans if it's to big to go around (i.e. a squall line that extends for a couple hundred miles). I have no need/desire to fly above 10,000 feet and I base my weather decisions on that starting point.
This thread seems to have morphed into flying through convective cumulonimbus clouds instead of answering what the original poster asked.
Brian