Fortunately it wasn’t exactly flying through a thunderstorm, as it was clipping the top part of it. Here is what happened.
I was flying up to Bellingham Washington and was about 1/3 of the way up, over the Siskiyou Mountains. I was flying at around 15,000 feet over an endless cloud layer just below me with an occasional bulge of clouds sticking up. The air temp was a cold 0 degrees F, around -20 C, the point at which the rule of thumb is that at that temperature everything in the cloud is frozen, and there is little risk of icing (Airframe icing is when small unfrozen drops of water hit and freeze onto your airplane. Its not a good thing).
I knew that over mountains or in unstable cumulus clouds the icing risk can go on in much higher altitudes because of the lifting of warm(er) air in the clouds. So the question hit me, “Is there ice in those clouds?” Now its not stupid to go get a little ice, if there is a safe way out. My instructor and I went looking for ice one day and I know others have done the same thing. So I decided to peek inside a cloud to see. I didn’t want to drop down because it may take too long to climb back up and I’d have to deal with ATC for the deviation, so I decided to fly into one of the bulges of cloud that were sticking up from the layer of clouds.
I flew in at an angle so it wouldn’t take that long to turn back out. The first thing I saw was a lot of something flying at me. I couldn’t quite tell if it was rain, sleet or snow. Then, within seconds, the windshield went opaque and I could see water ice building on the wing. OK, that answered that question.
Then I felt the plane zooming upward. I looked at the VSI and I was climbing over 1,500 feet per minute. My airspeed was fine so I knew I was OK (I wasn’t inadvertently pulling up into a stall), but that’s when it hit me. The innocent looking cloud bulges? They were growing thunderstorm cells. Duh. I am so accustom to our western US thunderstorms where normally they are isolated and very obvious.
By that time I was already in a turn to get out, and within 15 or 20 seconds I was out of it, and in another hour the ice had slowly sublimated off the plane. The cloud tops grew, and I finally ended up at 21,000 feet to complete the flight – careful to ask ATC for deviations to stay out of the “bulges”, which now towered above me and were more obviously thunderstorms.
Some of the pilots reading this are saying, “Gee dude, that was kind of stupid”. Yeah, it was. I should add I have XM weather radar and there were no areas of heavy rain underneath me or in the area. I think that would have been a clue. So obviously it was a growing cell, and with a top of around 16,000 it wasn’t a serious cell yet. Still, not the place to be.
What I don’t understand, not being a heavy IFR pilot with de-ice capability, is what happens when you are flying through these clouds trying to get on top and you run into a cell like this? There is nothing on radar yet that would indicate its presence. It is hard to imagine de-ice boots or a glycol weeping system like TKS keeping up with that amount of ice buildup.
Below is photo of a real thunderstorm, near Placerville CA where we don’t usually see super-cells like this. Now if I flew into this I don’t think I would be coming out.
West Coast Flying Adventures 
Hi Ney,
What i can offer at the completion of my first full year, and four seasons, of flying in the great northwest is that ANYtime i fly thru visible moisture ANY time of year and the temp is anywhere close to freezing, i pick up ice. Even a few degrees above freezing will work in a cloud that has supercooled droplets bouncing around. That includes a hot July day coming out of the home strip in western Montana climbing thru 10k over Missoula. The temp dropped pretty rapidly into the 40’s and caught me a bit by surprise when it dropped to the 30’s right when i hit the cloud…maybe some evap cooling right there. Anyway, i was still climbing and the clouds were patchy so i was not in it for more than a minute or two, so no worries.
The t-storm part of this is the tricky bit here. I learned to fly in the southeast where the cells get big, but generally are pretty benign for the most part until they build past say 15k. Then they really blossom and you avoid like the plague…and it is almost always the same exact pattern day in and day out. Sort of the converse here in that they seem to build much more slowly, and often never top out and get the anvil look…although they do on occasion. Much more variable formation around here. They still have tons of moisture and updrafts although nothing like the big CN’s down south. Maybe it’s a case of lots more moisture at a slightly slower vertical, and still kicks like a mule. Lightening is pretty rare in the coastal area here. As you go east away from the coast, the storms are more like what i grew up with in the southeast…i surmise because of greater lifting visa vi delta-T’s.
Cheers,
Bob
Thanks for the reply, Bob.
Thunderstorms are truly fascinating from the outside, and a bit terrifying in the inside. – Ney