Max Zero Fuel Weight and G Force

[Lumps]

Is the maximum zero fuel weight in aircraft calculated as the forces acting at the max design load limit?

If this is so, and this is purely theoretical and I would be sure entirely illegal in the real world, then if the aircraft was rated to +4G then if one was to overload the aircraft past its max zero fuel weight (but still within MTOW) and the flight was a silken ride with no more than say 1.5G being applied then what are the actual mechanical / structural implications?

Google has let me down on this one

[NigelOnDraft]

Landing loads are a factor... Fuel in wings v Load in Fus an advantage due being OB of gear.

NoD

[rudderrudderrat]

Is the maximum zero fuel weight in aircraft calculated as the forces acting at the max design load limit?

Most can carry centre tank fuel & some stab tank fuel, which must be considered because it's part of the wing / fuselage loading.

I would be sure entirely illegal

- Oh yes!

then what are the actual mechanical / structural implications?

The structure would survive in your example, but you will have no idea of the affect on the fatigue life unless you measured & recorded "delta g" constantly and redo the design maths.

[Lumps]

THanks guys

Understood. Although if an aircraft is rated to 4G, and it spent its whole life flexing between 2G and 3G, rather than the more realistic 1G - 2G, then it is still within its designed limits and therefor should theoretically be as safe as the realistic example. Or does one have to redefine 'delta g' for this too?

Simple stuff for an engineer I guess, but for a pilot provided with only absolute limits with nothing in between the consequences are harder to forecast.

[john_tullamarine]

then it is still within its designed limits and therefor should theoretically be as safe as the realistic example

.. not necessarily so. There are two "strengths" to consider -

(a) static - this is what the normal G-loads relate to and prescribes an isolated requirement for the structure to withstand the proof load without any failure and the ultimate load without any catastrophic failure

(b) fatigue - this is a reference to the structural bucket of strength. One makes a withdrawal as one flies .. and the harder one flies, the bigger the withdrawal each time. The OEM makes reasonable presumptions as to the sort of flying the Type will do and does the sums and tests accordingly. Although there is a fair fudge factor in the figures, if Captain Pilot routinely does unexpected things .. then Captain Pilot's specific aircraft might find itself with an empty fatigue bucket of money earlier than expected and face the prospect of a premature significant failure .. not good.

So, while MZFW and the other limits are based on maximum loads, that is only part of the story. One is presumed to fly appropriately.

Occasionally, an operator will seek the OEM's revision of fatigue analysis and certification lives based on a review of the specific operation. Indeed, some OEMs prescribe multiple lives depending on the sort of operation conducted.

[Lumps]

aha!

thanks John and all, it is the logical answer but has never been explicitly spelt out in my (relatively) short time around aircraft.

Basically flog a car and eventually it starts falling to bits, drive it gently and that Corona will keep going and going. Makes sense.

Cheers!