A lot of people don't really understand why low density alloys are useful in airplanes aside from "it's lighter". I am an engineer who has spent a good deal of time working with aircraft structures. I am also a pilot and work on my own airplane. If you look at the BEST available alloys for steel, aluminum, and even titanium, you will find out that the strength to weight ratio is surprisingly similar for all three materials. In general aluminum has about 1/3 the density, and 1/3 the strength, and 1/3 the stiffness of steel. Titanium alloys are in between. So, given a similar strength to weigh ratio, why is aluminum better? Looking at an aircraft skin for example you have a sheet surface supported intermittently by ribs underneath and loaded by a distributed aerodynamic pressure. This aircraft skin is primarily loaded in bending. I won't to through all the math, but the strength and stiffness of a beam loaded in bending depends on the thickness of that beam CUBED. So, if you replaced a steel wing skin with an aluminum wing skin, and used the same mass of material, the aluminum will be three times as thick. Using the more "flimsy" material has cost you a factor of three in both strength and stiffness, but tripling your section thickness has gained you a factor of 3^3. The skin is, in fact, 3^2 as strong at resisting the load meaning that in this application it's 9 TIMES STRONGER (but not, unfortunately, 9 times lighter). Note that this advantage is not gained in every application. Struts, flywheels, fasteners and lots of other parts are still made from steel in many areas of an aircraft.