What are the factors that determine Gross Weight when an aircraft is certified? What allows them to increase it? Is it calculated? What happens when you go 1 pound over?
The companion to this question is how do they determine the CG range?
Weilke's got a great answer... I'm going to reorganize it and add a few bits:
- Power, to achieve the 'climbs with flaps' criteria
- putting a turbonormalizing system on a A36 Bonanza yields a gross increase
Limiting the flaps on the C172/C182's to 30 degrees allows a higher gross weight - Cessna did this themselves after a while. Also, adding a bigger engine can do the same thing.
- Strength of the landing gear
- replacing the landing gear arc on ea early DA40 with a thicker one will increase allowable weight
- replacing some brackets in the landing gear in some Socata planes will give a landing weight increase.
- reducing elevator travel (---> lower max g-load) can provide an increase
Those early DA40's had a MGW of 2535 and MLW of 2407. With newer landing gear, they got the MLW up to 2535 as well, and increased performance as part of the bargain, too.
Later on, another mod became possible that raises the MGW to 2645. It's a collar that limits elevator travel. That might have something to do with the G-load, but I know of at least one airplane where the same thing was done (limiting up-elevator) to give it friendlier (ie recoverable) spin characteristics.
- Airframe strength (g-loads)
- Putting wing extensions with or without Flint tip-tanks will give a increase on some Cessnas (as the fuel is not stressing the spar and the airfoil creates enough lift for the additional fuel at the tips)
Same with the Comanche/Twin Comanche. Oddly enough, in the Twin Comanche the tip tanks raise the MGW by 125 lbs, but that extra weight must be fuel in the tips (so, kind of like a zero-fuel weight, but specific to those tanks). However, on Ed's single Comanche, there's no such limitation on the extra weight.
- changing aircraft category from 'utility' to 'normal' will yield an increase in some aircraft (lower required design loads)
I haven't seen a pure-utility category aircraft, but this is why the Cessnas and other such birds that can be flown in either normal or utility category have the separate envelope with lower MGW for the utility category.
Well, in reality, nothing happens with 1 pound. In Alaska, some charter operators are allowed to operate 15% above gross routinely, ferry pilots fly planes 10,15 and 30% above gross after obtaining flight permits. You go over and you either bend the aircraft or run into a guy in green corduroy pants, you may have some unpleasant experiences with 'the goverment'.
Well, things DO happen - But it's not an instantaneous change with one pound, and those same things usually change within the envelope as well. For example, changes in weight change nearly all of your V-speeds. You know how your stall speed goes down when you're lighter (and in turn, your landing speed, at least if you don't want to float)? It goes up when you're heavier. Many of the things published in your POH are based on the published maximum gross weight and they'll change when you're over gross. Vx, Vy, Va, Vg, etc. will all change, as will anything else that's based on angle of attack.
One interesting read on this subject is the TCDS for the C172 or C182. There's a specific note in them about operating up to 30% over gross for ferry flights - It still requires a permit, but they at least put the limitations in there so you're not guessing. In addition to what's already been mentioned, Vne goes down IIRC.
So, when you're operating over gross, even if the structure is strong enough to take it, you need to be aware of what else is changing as well. In addition, while it may be fairly clear what the *first* limitation is in terms of gross weight (that'd be the one that everyone modifies for the MGW increase on a particular type), you don't know what the *second* limitation is. So if you decide you're going to operate a 172N over gross and just not use the last notch of flaps, how do you know that the wing is designed to take the extra weight?
While the planes will handle a small difference, our calculations aren't as accurate as we'd like. Did you weigh the change in your passengers' pockets? How about the lunch they just ate? Most folks' weight is measured when they're mostly naked, shoeless and nothing in their pockets, and before they've had anything to eat or drink for the day. (Not to mention, they'll usually tell you their most optimistic number.) Most airplanes have gained some weight in dirt in the carpets/seats, stuff that got thrown in the baggage compartment and forgotten that ended up under the seats (and hopefully nowhere else), POH supplements, wiring under the panel, etc. that hasn't been accounted for in its normal empty W&B numbers. So, let what little extra the plane may handle make up for those differences, and don't purposely load the plane over gross unless you have a permit to do so.
