So I know next to nothing about aviation as many of you have probably figured out. But I am involved in the design and manufacture of some fairly complex equipment. Deciding what the acceptable specification for the final product in my industry comes down to a negotiation between the designers and engineers (who know it can do more), the manufacturing group (who want to cover their asses and produce product that will pass the spec so they always push to reduce the specifications to make their job easier), the support group (who don't want to deal with equipment support issues so would rather us be running somewhat detuned), the legal department (who are always concerned about product liability so also want to to run at a reduced operational level to minimize risks in their mind). The people who are trying to cover their asses always seem to win out in many companies which is a pity in my opinion.
So as I said I know nothing about aviation but there are some of the same elements there I assume, so looking from the outside in I strongly suspect that I can safely do things with my certified plane beyond the official spec.
I'm reminded of the fresh-out-of-college design engineer that I used to be and of all those other brand new engineers that I've worked with over the years. People who tend to quote the simulations, the math, the pure analysis, the prototype's performance (The prototype which was hand built by the engineers and not by the folks on the assembly line) and talk about what a great design and contribution to the state of the art they've made. These youngsters tend to go on and on about how if only those manufacturing folks would get their act together and tighten up the process, or if the reliability engineers wouldn't be so gosh-darn ANAL about lifetime stress testing, or if the field application support engineers would just realize that this design is perfect and pure and won't NEED support, or if those marketing dweebs would stop adding needless feature requests, or if the legal department wouldn't needlessly point out that if a certain bit of the design failed it could mean ruin to the company, then boy, what a great and amazing product could be created. All those other folks are just so LAZY, aren't they? They just want to make their jobs easier because they're just fundamentally jealous of the almost god-like design engineers!
Look, the reality is this. The purpose of engineering is to create a product which will make money for the company. Companies lose money if the field failure rate is too high or if someone sues them because a flaw in their product caused bad things to happen. A prototype is just that; building a one-off is nice, but one must account for the actual manufacturing tolerances. Yes, manufacturing processes can be made tighter, but it costs money. It's a trade-off which the executives (and more experienced, more senior engineers) get paid to make. Simulations and analysis depend on imperfect models which
attempt to model reality. Yes, the models can be made better at the expense of time and/or money. Again, a trade-off. Then too, things degrade with time, this includes electronics, mechanical mechanisms, etc. A company will go out of business rather quickly if their products don't meet some acceptable usable life. Yes, some products rely on early failure (Gillette would probably be out of business if their razor blades stayed sharp forever), but I doubt if Cessna would be in business if their planes just started falling out of the sky after some time limit. Paying for field support engineers to go support a fragile design costs money; it would be better to limit the number of trouble tickets they have to deal with.
Now, to airplanes.
Yes, there is margin in the design. It's there to account for things like manufacturing tolerances, fatigue, degradation, dumb or unlucky pilots who needed that margin to get out of a jam and over-stressed the plane, etc. You could probably safely get away with exceeding the limits once, twice, even several times, but each time you do, the plane's limits are exceeded, things start to build up (metal fatigues just a little bit more, bolt holes open up just a tiny bit, rivets become a little looser, cracks propagate just a little farther, etc.). At some point, there WILL be a catastrophic failure. Unless you've had the airplane brand new from the factory, you most likely won't KNOW its full flight history. How many times in the past did some previous pilot exceed the limit? How close are you to that bolt finally giving way, that crack suddenly propagating too far, that partially corroded spar to snapping? You don't know. THAT's why all those lawyers, manufacturing engineers, reliability engineers, senior designers, flight test engineers, etc. set the limits where they are. They do it so that the airplane will stay safe and reliable for a LONG time.
There's always the anti-authority, invincible, types out there who say it won't happen to me and why should I follow the rules. You don't want to be one of those types, and you don't want to make a habit of exceeding the limits.
. So you take three passengers each underweighing themselves by 10-15 lbs then bringing a bag which is really 45 lbs and not "about 30 lbs" as they told you. I know this from experience being a passenger on other flights. Add that all up and the pilot flying might think he is only at gross but could easily be 50-60 lbs over.
