Question about limit and ultimate loads

[WilliamSeger]

FAR 25.301(a) says, "Strength requirements are specified in terms of limit loads (the maximum loads to be expected in service) and ultimate loads (limit loads multiplied by prescribed factors of safety). Unless otherwise provided, prescribed loads are limit loads."

FAR 25.303 says, "Unless otherwise specified, a factor of safety of 1.5 must be applied to the prescribed limit load which are considered external loads on the structure."

FAR 25.305(e) says, "The airplane must be designed to withstand any vibration and buffeting that might occur in any likely operating condition up to VD/MD, including stall and probable inadvertent excursions beyond the boundaries of the buffet onset envelope."

Question: Does that mean that Vd is the limit case, i.e. structural loads at Vd are multiplied by a factor of safety to provide a "realized margin of safety" beyond Vd?

 

[AlephZero]

No, 25.305(e) means what it says. If VD/MD was always the worst case condition, why would 25.305(e) say "including stall", etc?

 

[WilliamSeger]

Are you saying that the limit case is higher than Vd, to include those effects?

Edit to clarify the question: Does 25.305(e) specify the limit case or the ultimate case?

 

[AlephZero]

I am saying the maximum external loads which cause vibration and buffeting do not necessarily occur at VD/MD.

They might occur at different flight conditions on different parts of the aircraft. For example deploying full flaps and speed brakes and lowering the undercarriage is probably not a "likely operating condition" at VD.

The worst loads in 25.305(e) may or may not be the limit case for all operating conditions of the aircraft.

 

[WilliamSeger]

Thanks, AlephZero, I think that does answer my question. I recognize there are complications, but the question came up in discussing the "impossible" speeds recorded for the 757s and 767s used in the 9/11 attacks -- specifically the claim that the speeds were impossible because there is no "margin of safety" in the structural design for exceeding Vd/Md. Just to be sure I'm not misunderstanding or misrepresenting your answer, do you disagree with that claim?

 

[AlephZero]

If the plane was designed according to the regulations, with a margin of safety of 1.5 at Vd//Md, then clearly you can exceed Vd/Md by some amount before the margin of safety becomes 1.0.

A margin of safety of 1.0 doesn't necessarily mean something will instantly break, or that every plane will break at exactly the same loading conditions.

The other unknown is how "accurate" the design margin of 1.5 actually was. The factor of 1.5 only exists because of the uncertainty. The more accurately you can do the designs, and verify that they are accurate, the margin of safely will tend to reduce over time.

On the other hand, you can do some fairly severe things to aircaft structures without breaking them. For example look for videos of the Boeing 777 static wing loading test, which actually broke with a margin of safety of 1.54, which is a pretty good shot at a target of 1.5.

I'm an engine guy not an airframe guy so I can't comment on specific plane designs, and in any case Physics Forum doesn't allow discussion of conspiracy theories.