Some very good replies so far, however the one thing that hasn't been mentioned so far is that for the static analysis there is both ultimate and proof / limit assessment.
A lot of you have mentioned the 1.5 Ultimate factor, that is only for the ultimate static load case (limit case x 1.5), in this instance the component is assessed against the material ultimate strength (that is the strength at which rupture will begin to occur).
It is also normal to consider the limit load (ie: no safety factor applied) or a reduced safety factor (proof) against the materials yield point (the point at which a material starts to deform and the deformation remains permanent).
Typically this is where the civil and military regs are different, in the civil world the proof factor is 1.0 (therefore the same as the limit load), the military have typically used a proof factor of 1.125.
So on the proof load check the aim is to show that no permanent deformation of the structure occurs.
So for aluminium structure, deformation is expected well before an out right failure in most circumstances.