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# Question re FAR 25 structural requirements

## Question re FAR 25 structural requirements

(OP)
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 25.305 specify limit cases, i.e. does it require that all structural loads calculated for those conditions must be multiplied by a factor of safety to provide a margin of safety beyond Vd?

### RE: Question re FAR 25 structural requirements

i'd use limit loads for vibration.

Quando Omni Flunkus Moritati

### RE: Question re FAR 25 structural requirements

I agree with rb1957. Vibration is usually of short duration in airframes.
It would be interesting to hear other opinions.

Andries

### RE: Question re FAR 25 structural requirements

funny this ! Were just doing some work with an antenna in the wake of an obstacle which is pretty continuous. Im not applying the ultimate factor (just realised that !) as this is a fatigue loading (so Im not comparing to ult strength, but fatigue allowables, well fatigue lives); and inside that analysis is the safe life factor`.

### RE: Question re FAR 25 structural requirements

Will...

Suggest taking a peek at following MIL specs for another perspective...
MIL-A-8861 AIRPLANE STRENGTH AND RIGIDITY - FLIGHT LOADS
MIL-A-8865 AIRPLANE STRENGTH AND RIGIDITY - MISCELLANEOUS LOADS
MIL-A-8866 AIRPLANE STRENGTH AND RIGIDITY - RELIABILITY REQUIREMENTS, REPEATED LOADS, FATIGUE AND DAMAGE TOLERANCE
MIL-A-8870 AIRPLANE STRENGTH AND RIGIDITY - VIBRATION, FLUTTER, AND DIVERGENCE
MIL-A-8892 AIRPLANE STRENGTH AND RIGIDITY - VIBRATION

And, maybe...
FAA AC25.613-1 MATERIAL STRENGTH PROPERTIES AND MATERIAL DESIGN VALUES

Regards, Wil Taylor

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