Am interested in people’s opinions of how much vertical extent cumulus cloud needs to develop before they consider entering it will likely be too rough for a light single.
Does the threshold vary depending on whether you have passengers?
Graham wrote:
it will likely be too rough for a light single
I think this bit is going to be very subjective.
To me it comes down to 1) does the cloud look like it is developing vertically – tightly defined upper surface, appearance of ‘boiling’ upper surface, is it growing vertically, how dark is the interior.]
What I like to see if an upper surface which is diffusing away, anvil structure, the energy looks to be gone, the interior is same colour as other benign clouds, not darker, it is not higher than other clouds in the area.
Having said all that, it often surprises me how much turbulence one encounters in even a small (60 second transit) clearly ‘benign’ cloud. In my 15 years of IFR flying I swear that airline pilots are more willing to demand diversion “due weather ahead” than they used to be.
Buckerfan wrote:
In my 15 years of IFR flying I swear that airline pilots are more willing to demand diversion “due weather ahead” than they used to be.
With better Doppler radar capabilities they see turbulence more clearly and they tend to avoid it for the passenger comfort.
In a light single with inexperienced pax, I try to avoid all clouds. Unless it’s a very small “fair weather cumulus” and I ask them ahead, “hey guys you want to see what it feels like to fly through that little cloud?”
In my little experience, turbulence is difficult to predict just by looking at the cloud. Sometimes the least intimidating are very bumpy and vice versa.
While TCU with tops at FL100 might be fair weather Cu in the ITCZ (tops punching into the tropopause), they can give a proper jolt. Orographic effect will also boost convective energy. Embedded CBs are a no dispatch for me, and I guess am not keen to entertain building TCU with CB offspring. Arguably Cumulus extending beyond 5,000’ is developing into TCU.
The other consideration is that you might enter the top of a TCU and find moderate/severe SLD icing.
How rough a flight is depends on wing loading.
If you go into almost any IMC in a Maule, passengers are likely to be throwing up within 10 minutes – I’ve seen it 
In fact they are likely to be throwing up in VMC on a hot summer day… the price you pay for an extreme-STOL aircraft.
In a TB20, which has one of the highest wing loadings in piston GA, it takes a really dangerous turbulence to have the same effect.
GA types with a high Vs e.g. Lancair 4 (Vs maybe 80kt in reality) will work even better.
So the answer depends on the type and the passengers. I think most people can be scared so they will never fly again and you don’t want to do that if it is your wife/GF otherwise you will spend the rest of your flying days looking for a passenger, and will give up flying a lot sooner.
Airliners fly through almost anything; consequently I have been on loads of airline flights which were really horrible and much worse than anything in the TB20, but most passengers will accept that because they know up front are two supermen with 40 years of experience and just the right amount of grey hair Airliners fly fast so get chucked around much more but OTOH have much higher wing loadings than GA.
I avoid proper buildups i.e. TCU and larger, but if flying in VMC you can avoid most things visually. VMC on top is the best way by far because you can see it. But if you have to climb/descend through a layer then you can’t. If descending, pick a spot with smooth cloud.
