Seaplane Useful Load

[matthewhinmann120cm]

Hi all,

I’m a new pilot and new to the Cessna 120/140 gang, so bear with me.

According to the TCDS, the useful load for a land 120/140 is less than the seaplane version, showing 1450 lbs for land, and 1550 lbs for the seaplane? Why the 100lbs difference? Why does the float version have higher useful load when it actually is carrying more weight due to the floats?

My 120 has the V-brace installed on the glare shield. Does this increase the useful load (because this is required for seaplanes) ?

Thanks!

[6298]

Those figures are Gross Weight Limits. You should have a weight and balance sheet with the airplanes records. The seaplane modifications add to the empty weight, so the Gross limits are a little higher.

[a64pilot]

Long time ago it was noticed that by the time you added floats to an airplane it had pretty much no useful load, not when you added a pilot and fuel.
So it’s very common for a float equipped airplane to have a higher gross weight limit, one theory which is pretty spurious is that floats generate lift and carry themselves.

No you don’t get a gross weight increase for the windshield brace.
In my opinion and that’s all it is, is my opinion, but a C-85 equipped 140 or 120 is performance limited more than aircraft structure limited, meaning that if you can get off of the water or ground and have a decent climb rate, how much you weigh is irrelevant. On a hot day at altitude you probably can’t at gross weight anyway.
an opinion only

[6643]

The floats actually do act as air foils and off-load some of the weight from the structure specific to the land plane (wings and lift struts.)

[6930]

I have often wondered why the extra MGW allowance for the seaplane version. Obviously, the intent is to provide a reasonable useful load with the added weights of the floats, but how do they "get away" with that? If the airplane can handle 1550lbs MGW, why can't we all just fly that way normally?

I think, perhaps, it has to do with load limitations. The 140 land plane is certificated with load limits of +4.57/-2.26 G's at 1450 lbs. At 1550, the load limits must be reduced...but I can't find reference to specific values anywhere.

Yes, I would agree that there is some gain in lift do to the airfoil effect of the floats, but I've not seen that quantified in any way either. In the end, I think the load limits are the limiting factor. The standard values for normal category airplanes are +3.8/-1.52. I'll bet the addition of floats reduces the 140 limits to something between +4.57/-2.26 and +3.8/-1.52.

That's my spin on it in any case.

[a64pilot]

As much as we would like to think that all these limits are computed to perfect extent, they aren’t. There is almost always quite a bit of “fudge”
The reason is you test to a specific load, you either pass or fail the test, you could just barely pass, or you could easily pass. Most especially older less complex designs easily pass, to design to a specific point is tough and may take several tests until you barely pass, and every test takes money and time.
So it’s far easier, cheaper and faster to design something that there is little doubt will pass, so there is a lot of margin.

But I believe this margin has saved us, it’s why our 75 yr old aircraft aren’t having inflight break-ups.


The downside is weight, strong usually means heavier.
So unless you get stupid with overloading, your almost always safe.

In my opinion a well designed aircraft is HP limited, not structural,meaning you run out of power before you run out of structure. At least my C-85 C-140 is power limited, I don’t believe you can structurally overload it, it just wouldn’t fly. Flown within limits of course, airplanes just don’t fail in level flight, and if your heavy that’s about all you should be doing.

However having said all that no CAR3 aircraft has any consideration for aging or fatigue built into those limits, they are for brand new zero time components, so think about that our old aircraft can’t be as strong as they were when new.

Unless things have changed the AK FSDO would easily grant a 10% overload for 135 aircraft, like 180’s, 185’s Supercubs etc.

My opinion is that way back in the day when the FAA was a whole lot easier to work with, Gross weight increases were given for float planes because they weren’t viable without it and the FAA knew it was less likely for people to be out “hot dogging” in a floatplane.
Since then it’s become standard practice.

[8224]

I find this discussion on MGW comparing regular wheel equipped planes with those on floats fascinating. Every type certificated plane has a “critical” portion of the structure that will fail if the Vne, usually by a considerable margin, is exceeded. For example on a Beech Bonanza it is the tail and for most Cessnas it is the wings, or at least that is the way I understand it. I have restored two Bucker Jungman aircraft, and am currently flying the last one I restored. The unique aspect of the Jungman is that EVERY part of the airplane is critical!! By that I mean that there is no extra weight being carried that is not critical to the integrity of the airframe. That is one of the main reasons the Jungman has been noted as the finest flying airplane ever designed. I am restoring a 140 now and make every effort to keep it as light as legally possible.
Steve Hawley