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Rules for deciding what cumulus cloud to fly beneath


RobertE

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I flew to Bozeman, MT this weekend, which is a town surrounded on all sides by peaks that vary from 9K to 11K.  Most afternoons thunderstorms form so I timed my arrival and departure in the morning.  But what if, say, I had to arrive or depart at noon when there are no thunderstorms but there are towering cumulus clouds with 10K-15K feet of development and bases a few thousand feet feet from the peaks.  Would you fly beneath them?

Basically, regardless of your answer, I’d love to learn what rules you observe in making that decision.  This is an area in which I could use some practical experience.  And keep in mind that these clouds don’t span hundreds of miles (in other words, it’s not as if 2 hours later when the thunderstorms form I’ll still be beneath them).  They’re a 20-40 mile phenomenon.

Thanks.  

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I love deliberately flying under them.... when I am in my glider!!!

 

when in other aircraft I "cheat" around where I think the core of the thermal is.  Remember, You have a few miles left or right of course to play with.  The core are typically only 300-400 feet wide at the most.

Some thermals can be 1500 feet a min, that can be pretty jarring in a powered aircraft with some speed.

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2 hours ago, RobertE said:

I flew to Bozeman, MT this weekend, which is a town surrounded on all sides by peaks that vary from 9K to 11K.  Most afternoons thunderstorms form so I timed my arrival and departure in the morning.  But what if, say, I had to arrive or depart at noon when there are no thunderstorms but there are towering cumulus clouds with 10K-15K feet of development and bases a few thousand feet feet from the peaks.  Would you fly beneath them?

Basically, regardless of your answer, I’d love to learn what rules you observe in making that decision.  This is an area in which I could use some practical experience.  And keep in mind that these clouds don’t span hundreds of miles (in other words, it’s not as if 2 hours later when the thunderstorms form I’ll still be beneath them).  They’re a 20-40 mile phenomenon.

Thanks.  

In flying small GA airplanes there can be no such thing as "have to" unless you have a potential death wish.  Plan your flights to be done flying by noon in mountainous areas. Enjoy the afternoons watching the clouds develop into thunderstorms---while you are on the ground.  That's my "practical experience" from flying since 1967.

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My preference would be early AM/late evening as you planned so I don’t get knocked around.  If I had to get in at 1430 then I would be looking at what the vertical development was and their color.  A cumulous that is 10-15000 feet might knock you around a bit but it isn’t going to kill you.  Where possible stay VMC, you can navigate any weather you can see.  

Thunderstorms are serious business for sure but a lot of what people fret about are just clouds.  If it is high and dark with lightening rippling through it, probably not good.  

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Our last flight into Bozeman stripped paint off the tail descending thru non radar painting cumulus out of 17 k and very rough ride.My wife was not amused to put it mildly.Last week ,same thing descending into Missoula...in and out of cumulus at 15 k and again every cloud penetration saw us grabbing the center glareshied tube !Am or total avoidance probably best.

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Thanks for the advice, everyone but I think some are addressing the wisdom of flying through such clouds.  I don’t fly through them because the likely turbulence isn’t worth it to me.  But my question concerns flying beneath them. Might I expect the same flying beneath them?  If so, how thick a cloud is too thick?  5K feet?  10K?  I’m assuming there is no hard and fast rule, which is why I’m really curious how others decide.  Thanks again.

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4 hours ago, ArtVandelay said:

If you have to get close, generally the weak part is the back side, so go behind the TS, added benefit is the storm will be moving away from you.

I avoid them like the plague.

 

 

Tom

 

Ohh by backside you mean UPWIND!

 

Edited by Austintatious
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Looking at the storm scope along with ADSB or xm to give some more info along with what you eyes tell you generally keeps you safe, expect some bumps or worse depending on the size and surrounding wx condition. My wife and eye generally grab the center bar simultaneously, I guess we’ve been flying to long together. She’s my ice watcher especially when topping clouds in the summertime. Robert I assume and your wife are similar same as mike and carol, Bob  and Nancy David and Deb me and Fran and so on, our copilots know us better than we know us 

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40 minutes ago, Becca said:

tops under 10K light turbulence, usually. Tops over 10K it going to bump you, make sure lap belt is tight. . Tops of 18K or more, I do not fly thorugh those.

Solid advice. 

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I find that if you give the rain shaft 2-3 miles you will be OK 5-10 is better if you can. I have flown under the black underbelly of a building TC and had a smooth ride and I have been tossed around. 

I commuted to Tucson for 20 years. During monsoon season I would have to dodge storms every day.  You kind of have to read the clouds. What are the other clouds in the area doing? What do the underside look like? What is going on down on the ground? If you see a dust circle the storm has turned and is about to let loose. Strangely, the area between the rain shaft and the downburst dust circle is often very smooth. Also the very evil looking dust front is pretty benign.

If there is clear sky around the TC fly in the clear sky. If not aim for the brightest line between the storms.

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47 minutes ago, Becca said:

tops under 10K light turbulence, usually. Tops over 10K it going to bump you, make sure lap belt is tight. . Tops of 18K or more, I do not fly thorugh those.

I assume these are AGL altitudes?  Out west 10K MSL may very well just be 4-5k layer, and in some places 10k MSL tops are in the valleys.  

In the east these are probably good rules of thumb.

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4 hours ago, RobertE said:

Thanks for the advice, everyone but I think some are addressing the wisdom of flying through such clouds.  I don’t fly through them because the likely turbulence isn’t worth it to me.  But my question concerns flying beneath them. Might I expect the same flying beneath them?  If so, how thick a cloud is too thick?  5K feet?  10K?  I’m assuming there is no hard and fast rule, which is why I’m really curious how others decide.  Thanks again.

Flying underneath just as rough or worse.My Vfr advice is if planning pm arrival into kbzn...plan non direct flt to Idaho falls and Than one of three valley routes...Madison river ,hiway 287,Yellowstone ,emigrant Valley ,Bozeman pass or most lengthy Dubois ,red rock river valley,Dillon to Whitehall.Our problem seems to be we are incapable of getting airborne before 9am.Thus we arrive Bozeman arround 1 pm just in time for maximum bumps

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4 hours ago, RobertE said:

But my question concerns flying beneath them.

My rule for flying around mountains is the higher the better.  Not the place I would want to be trying to scrape the bottom of clouds down low. 

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2 hours ago, N201MKTurbo said:

I find that if you give the rain shaft 2-3 miles you will be OK 5-10 is better if you can. I have flown under the black underbelly of a building TC and had a smooth ride and I have been tossed around. 

I commuted to Tucson for 20 years. During monsoon season I would have to dodge storms every day.  You kind of have to read the clouds. What are the other clouds in the area doing? What do the underside look like? What is going on down on the ground? If you see a dust circle the storm has turned and is about to let loose. Strangely, the area between the rain shaft and the downburst dust circle is often very smooth. Also the very evil looking dust front is pretty benign.

If there is clear sky around the TC fly in the clear sky. If not aim for the brightest line between the storms.

Finally, a response to the question.

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4 hours ago, N201MKTurbo said:

I find that if you give the rain shaft 2-3 miles you will be OK 5-10 is better if you can. I have flown under the black underbelly of a building TC and had a smooth ride and I have been tossed around. 

I commuted to Tucson for 20 years. During monsoon season I would have to dodge storms every day.  You kind of have to read the clouds. What are the other clouds in the area doing? What do the underside look like? What is going on down on the ground? If you see a dust circle the storm has turned and is about to let loose. Strangely, the area between the rain shaft and the downburst dust circle is often very smooth. Also the very evil looking dust front is pretty benign.

If there is clear sky around the TC fly in the clear sky. If not aim for the brightest line between the storms.

Good details, thanks. 

I have often found, too (as recently as last week) that the desire to remain VMC sometimes calls for lower altitude.  At FL230 we were in and out of the tops of “benign” cumulus and stratiform clouds so  spotting the building Cumulonimbus spires was tough.  

Descended to 6,000’ and all cloud was above us.  Picking our way through the rain shafts was then straightforward.  We still had NEXRAD and Stormscope to help pick a path.   

It is ever so comforting to be able to see outside and confirm what the wx tools are showing.  

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