useful/payloads

[aero99]

Can someone explain to me how aircraft manufactures come up with the max weights for their plans, i.e. max ramp weights, max take off/landing weight, useful ect.

Do they load the planes up and test at clean and dirty stall configurations and then magically come up with a figure that is far enough away from the criticle weight for slight mis-calculations or is there a mathematical formula.

I was talking to a caravan pilot last night that said he will fly 300-400 lbs overweight sometimes with no worries. His reasoning was that by the time he lands he has burned enough fuel to have a safe landing weight. Then he showed me an article in Plane and Pilot that had a caravan hauling lions in Africa and they were flying 1100 lbs overgross. ??

Did I miss something during weight and balance training?

 

[avbug]

Aero99,

Your friend in the Caravan is, while technically not an idiot, unwise for providing any encouragement to anyone to fly over gross. Yes, many airplanes will do it, but the proper answer to the question of flying over gross is, "No, I would never intentionally fly over gross."

The weights specified for any aircraft design are not put together haphazardly. These numbers represent a series of factors, and are related to aircraft performance requirements, loads on landing gear and aircraft structure, wing bending moments, potential air loads, etc.

It's not uncommon to hear people state that it's okay to push a little farther than the design limitation for an airplane because it already has "150%" strength built into it. Or that it's got plenty of performance. The party line when I flew 207's was that they would haul whatever went into them (more true of the 206 than the 207, but beside the point). While a pilot in the bush who knows his business can certainly work those airplanes, I don't find it productive or proper to advise others on flying outside published limitations.

Most airplanes have landing weights which are less than takeoff weights, and this is based on the ability to burn off the difference in fuel, or dump it, prior to landing. Most airplanes have ramp weights greater than takeoff weights, because the difference will be burned off in fuel during the taxi. This is acceptable, and within the limtitations of the airplane. However, if one is flying over gross in the hopes of burning off enough fuel to be at gross upon landing, one is engaging in unwise actions.

Published stall speeds and performance information are based on a known quantity, including weight and balance. Exceeding those limits places the prospective performance outside a known envelope. This includes glide performance in the event of power loss. It also places the pilot in jeopardy for his or her certificate if caught; a high liklihood in the event of a mishap. The loss of a pop star in the Carribean recently was publicised as having been caused by an aircraft loaded over gross. This was true, but the weight had little or nothing to do with the accident. However, that's the one thing the media grabbed and ran with. Hardly surprising.

Many aircraft have required inspections for any landings over gross, or hard landings (which can certainly occur with normal proceedures when flying over gross). Manufacturers take it seriously. I observed the failure of a landing gear trunion assembly in a spar box during an inspection once, caused by hard landings, most likely performed over gross (based on other information). Considering what I saw, I was very surprised that the landing gear hadn't failed prior to the inspection. The aircraft was immediately grounded.

Heavier than gross weights impose additional bending loads on wings that are outside the operational design limitations for the airplane. That is in normal flight. Any air loads or gust loads may increase this value beyond the design limitations, even to the point of failure. Depending on the aircraft, other factors may come into play, such as controllability, aerodynamic balance (buzz, flutter), etc. The loading of the aircraft will have a big play in this.

Many aircraft have the capability of being flown over gross, but that's beside the point. Some things are legal but not safe, and others are safe but not legal. As professionals, we strive to ensure that we are both safe, and legal, at all times.

The pilot of the airplane carrying the lions, in the picture shown you by your friend, later said he wouldn't do that again.

Toward your origional question, the weight values are engineering values specific to the design of the aircraft. Unless one is an authorized engineer and test pilot, with the approval of the FAA and the manufacturer, and with the intent and program to develop an STC or expand the origional type certification data, one should respect the published limitations for the airplane. I doubt your friend is paid enough to serve as a test pilot, and based on his comments, he hasn't the experience.

 

[aero99]

Thanks for the info avbug.

 

[sstearns2]

.....

Zero-fuel wight is critical to this discussion. It is the wieght of the airplane loaded with cargo/pax but no fuel. The structure of the wing is driven by the zero-fuel weight privided the fuel is in the wings*. The fuel in the wings is 'span loaded', meaning that the fuel just resting on the cushion of air under the wing and does not require any structure beyond the wing skin. The fuselage with the cargo/pax on the other hand is suspended under the wings and requires a lot of structure to bring the lift generated by the wings down the span to the fuselage attach points.

I would have no problem (except the legal ones) flying an airplane overgross provided that the zero-fuel weight was not exceeded, there was adequate performance, and the max landing wieght was not exceeded on landing. I guess that makes for a lot of qualifiers, but my point is that it is possible in theory fly over gross and not exceed the design limits of the aircraft's structure.

Also, I believe that it is legal to fly 10% overgross in Alaska.

Scott

*It's actually the zero-fuel wieght minus the weight of the wings themselves that drives the structure of the wing. Also, if there is a fuselage fuel tank it and the fuel in it must be added to the 'zero-fuel' weight when designing the wing structure.

 

[avbug]

Many aircraft are not certificated with a zero fuel weight, which only plays a part of the picture. Zero fuel weight is typically an accomodation to wing bending moments, rather than attach point moments or inner spar strength.

Often other factors come into play, such as balance between tanks, etc. Some aircraft have zero wing and tip fuel weights, or specific limits per tank, per location, etc. However, in no case should an aircraft be flown over gross knowlingly without manufacturer authorization or other approved data.

As you are aware, exceeding ANY design parameter or limitation nullifies the airworthiness certificate, and makes the airplane illegal to fly. The strict definition of airworthiness is "conforms to approved data," and exceeding any portion of approved limitations exceeds the airworthiness limitations of the airplane.

In any way shape or form to advocate doing so is irrisponsible.

Zero fuel weight is only one factor to consider. There are many others. It is also a mute point when not given.

 

[sstearns2]

If an airplane doesn't have a published zero fuel wieght then the situation is pretty simple. A 172, for example, is legal to fly at gross weight with slightly more than 30 minutes of fuel on board. In this situation there is the maximum amount of wieght in the fuselage. Now you could top the fuel tanks off with the same fuselage weight and the stress in the wing and fuselage structure would not change, but you would be a few hundered pounds over gross.

Of course the climb performance would be poor. I'm certainly not advocating flying airplanes over gross, but I think it is important to understand about where the limitations come from and how, why, and how much they can be pushed if fate forces ones hand.

Scott