Aeronautical Engineering/Standard Design Safety Factor?
I've just been watching a documentary on aircraft design, entitled “Terror in the Skies," (Presenter: Brendan Walker) who addresses in it the question of, "Has aircraft design has reached the limit of how safe a commercial aircraft can ever actually be made?"
As far as I am concerned, this effort in populist perverted thrill seeking was wholly disappointing—because it failed even to go anywhere near perhaps the most critical question in aircraft design, namely: that of the standard safety factor. Of course, I know you had nothing to do with the program, but as you yourself are an aero engineer I thought you might be able to throw some light on the issue of what was there so obviously avoided.
Way back in the day when things were designed with slide rules, buildings, (for instance) were designed with a saftey factor of 2.0. (For the uninitiated who may be reading this question, this means that each and every structural component of a building was designed to be literally twice as strong as was thought by the engineers involved would ever be required of it.) As for aircraft, and owing to the ever-critical Weight issue, the safety factor of all structural was then held to a mere 1.15—i.e., each part was designed to be only fifteen percent stronger than its maximum anticipated loading. (We were lead to believe that that was enough.)
Along with the dawn of the computer age, however, and as software and testing methods get ever-more sophisticated and powerful, the temptation has arrived, and thus grows, for manufacturers of all types to cut ever-closer to 1.0, the safety factors involved. (Again, for the uninitiated and in short; most everything these days is designed not to perform, but merely to "Get past the till.") As most everything purchased seems designed to withstand only the most immediate loads, and with less and less being allowed for the possibilities of mishap and abuse, far too many things break at the slightest provocation.
Accordingly, I wonder about today’s aircraft. “Commercial” aviation is just that: a business—and in that business, and whereas strength means weight and weights means money, strength means Overhead; and as everybody in the business world knows, overhead is everywhere and at all times to be cut.
Accordingly, I continually wonder (especially now as I am shortly to get on one of the infernal machines) if the question that Brendan Walker was really asking—but which he was afraid to ask—was, "Have the commercial aircraft conglomerates cut the design safety factor as close to 1.0 as they dare?"
And so to get to my actual question to you:
What is/are the actual standard structural design safety factor(s) involved in commercial aircraft design, today? Is it still 1.15?
Dan - Sorry for the late reply, I have been involved with a wind tunnel research study for a week.
The U.S. Federal Aviation Agency established design safety factors and load limits that are somewhat complex because of the many aircraft types, components, and possible maneuver conditions. However, the basic structural design safety limits can be summarized as follows:
"The FAA establishes two kinds of load conditions:
Limit Loads are the maximum loads expected in service. FAR Part 25 (and most other regulations) specifies that there be no permanent deformation of the structure at limit load.
Ultimate loads are defined as the limit loads times a safety factor. In Part 25 the safety factor is specified as 1.5. For some research or military aircraft the safety factor is as low as 1.20, while composite sailplane manufacturers may use 1.75. The structure must be able to withstand the ultimate load for at least 3 seconds without failure."
In other words, you must design commercial transport aircraft to survive a load 1.5 times the maximum expected load. If that ultimate load is encountered, no permanent deformation of the structure is allowed.
I hope this answers your question.