The International Cessna^® 170 Association

Okay, were getting a fair amount of snow here in Edmonton, and since this is FDH's first winter outside (bummer ) in quite a few years, I'm concerned about how much snow load on the wings is too much. I can't fly much right now and I'm wondering if I need to go to the airport and sweep the wings off every time it comes down. Should I be wary of this, or is it a non-issue and I'm just being overzealous. Appreciate any input or experience you guys have. Thanks

Jody

I sent an email to my engineer son, and our aerodynamacist retired Historian, Cleo about this (and perhaps Miles might have an idea or two on this....?)
I'll let you know what their replies are...(except Miles of course.)

I wrote:"...at the Cessna website one of the members asked if he should be concerned about his airplane sitting out in the snow... with regard to how much snow accumulates on his unprotected airplane.
I've seen 10-14" of snow sitting on airplanes and never gave it a thought, but it's a good question and I wondered if you had any data on how much relatively dry snow might weigh.

The airplane weighs 2200 lbs and it's approved for negative G-s of 1.76 ... (I'm thinking here of a negative wing-bending moment against those wing struts which are not really designed for as much compression load as they are tension....the airplane being designed for a positive G-load of 4.4+. )
Would you have any thoughts on this? (Of course, it is a different issue than what amount of evenly spread weight might leave permanent deformation of the aluminum wing skins against their internal ribs/stiffeners, etc.... but it'd be a start to consider just the wing bending moment.)"


I did not mention associated concerns, such as stresses on the landing gear, tailwheel bracket, etc. (The lower doorpost carry-thru structure is designed to carry both tension and compression, of course, but I haven't any model/idea of how to calculate the effects of the tension/compression reversals which take place re: inflight/on-ground/snow-laden...but I'll bet that won't be the weak-link in either case, as long as the struts hold up to the compression-load.)

Having seen airplanes sit out with incredible amounts of snow on them ...I suspect it's not an issue for a small airplane... but it's a great question in my opinion. Minus 1.76 times 2200 would be 3850 lbs or so....evenly spread across the span....I suspect about 15-18 lbs per square foot would be tolerated by the wings... but of course, this does not address the loads on the landing gear.... I'd hate to see what 3800 lbs does to the gear. The question might be: What does a ton and a half of dry snow measure? The answer might be to remove the snow before it makes lady legs look like early gear.

(Caution: My math is subject to the accuracy of my abacus.)

here is my 140 at clow airport
located east of chicago
during the winter of '79.
100's of planes sat out in the snow
like this every winter!!
i never heard of anyone's wings damaged
as least from weight.
but the exposure to the elements
was hard on the whole airplane.
dave

Looks like a C-170 in the background behind your C-140
I suppose another factor on what load is on a plane would be whether the snow was the dry, fluffy stuff or the wet kind we sometimes get down here. If the snow melts and water seeps into vrs locations and then refreezes that could cause some problems also.

A preliminary reply from my step-son/engineer Tracy:
(Keep in mind that I did not give the actual measurements
of the wing to him, and he's spent almost no time around a 170.
(The wing area of a B-model is 174 square feet):

A couple of ways of looking at this...

As you pointed out what something is rated for in terms of transient G
load is distinctly different than a long-term static loading. This is
purely a gut call (nothing engineering about it) but I would say you
would not want a long-term static load on the wing in excess of 20% of
it's negative G rating. That may be overly conservative but it would be
my starting point.

This is essentially (from the standpoint of figuring out the load on the
wing) no different than roof snow loading. Here is a pretty good link:
http://www.extension.umn.edu/extensionn ... M1434.html

12" of snow weighs about as much as 1" of water; roughly 5 pounds per
SF. I don't know the surface area of the wing -- I am sure every pilot
knows this for his airplane though. For argument sake, I'll assume a
30-ft wing span by 3-ft. That is 90-SF of wing -- also 90-SF of snow on
the wing. If the snow is 12" deep that is 5 PSF or 450 pounds evenly
distributed across the wings. Well less than 20% of the designed
negative G load (11.6%).

Conclusion, snow would not seem to cause much of a concern. I would be
concerned about ice however. Through cycles of thawing, refreezing,
additional snow 12-inches of accumulated buildup could weigh
considerable more than virgin snow (only a single inch of ice with the
same 12-inches of snow on top of it would be 23% of the negative g-load
rating from the proceeding example). This is what happens to roofs (ice
dam forms near the edges or gutters, repetitive semi-thaw, refreeze
cycles allow the 12" of snow to be compressed to 1-inch of ice, another
12-inches of snow stacks up. A few cycles like this and you have 50-60
pounds per square foot and the roof collapses)

? back to you: with the general shape of the wing will a repetitive
thaw, freeze, precipitation cycle stay on the wing? Seems that it may
just slide off if the snow near the wing melts.

Tracy follows up with some more commentary after I responded to him:

What happens with roofs
(with a much greater pitch than that of even the taildragger wing) is that
instead of 12" of snow (5 PSF) you end up with 6" of ice and 6" of snow (32.5
PSF) through freeze-thaw-snow cycles. Thing is that from the edge it still only
looks like 12" of snow. If this happened on a wing, it would be "problematic"
(4,550 lbs).

The dynamic is somewhat different for a roof because radiant heat from inside
the home is melting the bottom of the snow on the roof. Water runs to the edge
of the roof (forming the icicles we all love). At night the weather cools
sufficiently to freeze this water at the edge of the roof (since it is not
insulated by the snow above). A lip of ice forms at the edge of the roof. The
next day everything repeats; each day the ice dam gets thicker allowing snow to
stack up higher than would normally occur (normally it would slide off after a
certain height).

Since there is no heat source within the wing, I can't see this happening. At
least not on a scale that permitted significant accumulation.

One BIG thing to look for when preflighting an airplane sitting out in such conditions: Inspect the aileron counterweight area for ice. Melted snow can refreeze in the aileron counterweight area and flow into the hollow aileron and drastically upset the balance. Elevators have also gotten ice refrozen within them occasionally. LOOK FOR THIS on preflights!

George brings up a good point with the aileron counterbalance and ice build up. There have been incidences where the ice has caused a flutter situation at normal cruise speeds and ripped whole control surfaces off aircraft. Flutter is a nasty aerodynamic phenomenon because it can be divergent (gets worse) even if the pilot notices and the aircraft is slowed...
I knew the aero degree would come in handy someday

Also check the elevator trim tab. There's not a lot of clearance there and it's easy for just a little ice to cause it to jam if it's not clear.

At my airport the only airplanes that seem to have trouble with the snow are the training (nose) wheel airplanes. The heavy snow on the tail makes them aspire to be "real" airplanes ... so they sit on their tail. Their problem is the damage on the bottom of the empennage (sp?) if the tail hits too hard.

As a practical matter I don't get excited with a foot of dry snow. Had 13 inches on the plane the other day and cleaned the bulk of it off, a couple hours later it started raining for a few hours then cleared off and froze hard. 1/2 foot of heavy wet snow will make the plane squat quite a bit and will cause me to drive a long way to clean it off. Have heard of a 172 being left out all winter with little or no cleaning and come spring it was a pile of bent metal. A foot of dry snow means clean it off before the next snow, 3 inches or more of heavy wet snow means clean it off before the next or continuing snow. Clean your neighbors off if he-she is not able and they will do the same for you if you can't get to the plane in a timely manner, ( like clearing the driveway and road to get out ). Have had a foot + of medium wet snow on the plane and was real careful to take the snow off a little at a time alternating sides to prevent a wing from hitting the ground. Did have a broke tail spring one time also from snow load.

I don't want anyone to clean off my plane and I won't clean anyone elses. A friendly old geezer cleaned off my wifes plane one day. By the time I got there the tail had dimples where he tried to knock off some ice. Good thing it was her's and not MINE!!!

The object is to get the weight off, not clean enough to fly. Around here it can and does snow heavy snow at a rate of a foot to two feet every eight hours sometimes. And everyone is digging out and the first to get to the airport does the planes if nobody else can. We don't have many out in the weather. A few years ago a hanger colapsed on several planes when it had about six feet of ice snow, ice, snow etc. and then rain. This a coastal area and most inland people can't understand how heavy mixed rain and snow can be, and if it freezes it will be hard as cement and stick like glue. But I'll garantee if somebody doesn't want their plane touched, it won't be touched. Some might wonder what it will take to salvage and rebuild the plane afterward though.

We sure don't have much snow up here yet

GOSH IT'S GREAT TO LIVE IN LOUISIANA WHERE SNOW LOAD, SNOW AND ICE REMOVAL AND EXTREME WEATHER FACTORS ARE ONLY HEARD ABOUT FROM FELLOWS NORTH OF 35* LAT.

THE DAY WE SET OUT TO BRING '93D HOME WE WANTED TO FLY THE 60 OR SO MILES TO THE AIRPORT WHERE SHE WAS BASED. WE WENT TO OUR HOME FIELD IN ATCHISON KANSAS AND FOUND OUR '49 TRI-PACER'S BELLY WAS NEARLY ON THE GROUND. THERE WAS ABOUT 1" OF CLEAR ICE ALL OVER THE UPPER SURFACES AND THE MAIN GEAR WAS SPREAD OUT TO AN EXTREME ANGLE! I DON'T KNOW WHAT WAS KEEPING THE BUNGEES FROM FAILING COMPLETELY. IT WAS A MYSTERY TO ME HOW THEY HELD AS THEY WERE STRECHED TO THE LIMITS! I DONT KNOW WHAT THE ICE LOAD WEIGHED BUT IT MUST HAVE BEEN SEVERAL HUNDRED POUNDS.

NEEDLESS TO SAY WE REMOVED THE ICE VERY CAREFULLY THEN DROVE OURSELVES TO THE OTHER LOCATION.

BY THE WAY JODY OUR 170'S SERIAL NUMBER IS 20845 WHICH IS VERY CLOSE TO YOURS. AND ALSO HER HISTORY INCLUDES A HANGAR ROOF COLLAPSING IN ON HER DUE TO AN ENORMOUS SNOW LOAD, I THINK IT WAS IN 1954, AT ATCHISON KANSAS. SO WATCH OUT FOR THE ABOMINABLE SNOW MAN

yup, I think that the coastal guys that get the ice storms have more to worry about than most of the rest of us due to the weight

wet snow can be a problem too, I've seen many times up at South Tahoe all the tied-down planes sitting on their butts after a storm

it even happened to one of our C340's in Reno a couple of winters ago, 24" of wet snow and it sat on the tail stinger, $2500 for a used one for replacement