Boeing seeking to reduce scope, duration of some physical tests for new aircraft

[EEngr]

So, Boeing want to replace testing with analysis. That might work, but I'd be hard pressed to trust Boeing to maintain a design and analysis engineering staff of any experience level between new product release events. When I was there, engineering staffing cycled up and down pretty drastically between new model launches. And there was never any overhead funds available to keep skills and tools current in the interim.

[arearadar70]

Destructive testing. Developed following the Comet crashes in 1951/2. It has worked well since then. If it it ain`t broke don`t mend it, or in this case maybe you should.

[Turb]

Originally Posted by infrequentflyer789

To my mind the materials testing (although there are still a lot of questions in that area) wasn't the biggest issue with Grenfell. The real problem was grafting new-standard-stuff onto old-standard-design without fully understanding how the old and new interact ...........So, where, you ask, was that building control inspector, or his London brethren, when they did the Grenfell refurb (which added extra accommodation floors as well as cladding, and definitely involved more than four floors and no sprinklers)? Well he wasn't involved, because the big guys get to do "self certification" on building projects (which brings us right back on topic again)...

Very accurate post and very much on topic.

• Old thing with safety critical new stuff grafted on top which changed fundamental characteristics
• Inadequate testing (in the case of Grenfell no testing at all) and definitely no destructive testing
• Pretence that the new thing is the same as the old thing, just better performance
• Insufficient training ( in Grenfell's case the fire crews hadn't got a clue what they were dealing with or how to deal with it and they killed a lot of people by telling them to stay put when they should have bust a gut to get everyone out ASAP)
• Insufficient or compromised or just non-existent regulatory oversight

Is any of this going to change? Is society willing to pay the cost of doing things right?

[Euclideanplane]

Originally Posted by Turb

Very accurate post and very much on topic.

• Old thing with safety critical new stuff grafted on top which changed fundamental characteristics
• Inadequate testing (in the case of Grenfell no testing at all) and definitely no destructive testing
• Pretence that the new thing is the same as the old thing, just better performance
• Insufficient training ( in Grenfell's case the fire crews hadn't got a clue what they were dealing with or how to deal with it and they killed a lot of people by telling them to stay put when they should have bust a gut to get everyone out ASAP)
• Insufficient or compromised or just non-existent regulatory oversight

Is any of this going to change? Is society willing to pay the cost of doing things right?

In the case of Grenfell, decades long experience, i.e. "testing" of buildings actually occupied by people, and with varying number of casualties aquired, had already documented the problems with the concept of cladding a building using flammable material.
And the "stay put" principle had apparently been "tested in situ" with similar results at those same occasions as well. www.youtube.com/watch?v=kDdO-FVNfQI

[SeenItAll]

While I can't speak to the issue of whether computer testing should replace physical testing, what about the issue of design/test specifications? For example, wings are required to handle loads that are 1.5 times their typical expected load. Who chose the 1.5 figure? The fact that it is such a round number makes it suspect by itself. Indeed, over the past 60 years, how many planes have had their wings snap off in situations and attitudes that were otherwise recoverable? I think the answer is few to none. Thus, my hypothetical question: why should the number be 1.5? Maybe 1.4361 is more appropriate and economical? And while computer modeling may be less exact than physical testing, perhaps its margin of error is less than the difference between 1.5 and 1.4361 -- so using computer modeling and keeping 1.5 still results in appropriate safety.

Just my two cents.

[futurama]

Originally Posted by SeenItAll

While I can't speak to the issue of whether computer testing should replace physical testing, what about the issue of design/test specifications? For example, wings are required to handle loads that are 1.5 times their typical expected load. Who chose the 1.5 figure? The fact that it is such a round number makes it suspect by itself. Indeed, over the past 60 years, how many planes have had their wings snap off in situations and attitudes that were otherwise recoverable? I think the answer is few to none. Thus, my hypothetical question: why should the number be 1.5? Maybe 1.4361 is more appropriate and economical? And while computer modeling may be less exact than physical testing, perhaps its margin of error is less than the difference between 1.5 and 1.4361 -- so using computer modeling and keeping 1.5 still results in appropriate safety.

Just my two cents.

You're right. In engineering there are lots of "rule of thumb"s based on past experience. In the early days of flying, planes with 1.5x margin seemed to survive. Those with lower margins seemed to fail. And 1.5x margin is still economical from cost/weight perspective. And yes, it's a nice round number. Thus 1.5 became the standard safety factor -- not based on strict mathematical analysis but from engineering experience / observations.

Over many decades, newer analysis using more sophisticated methods seem to support this ~ 1.5 margin, at least for airplanes. Yes you can save weight by using a lower safety factor, but there are probably other ways to save weight -- and at the end of the day, the cost difference is marginal, so the industry just sticks to 1.5. There's very little incentive to change this.

(But for spacecraft, any weight savings equal big (big!) bucks, so they tend to use lower safety margins when building rockets, etc.)

[Aihkio]

1.5 is not really a "factor of safety" it is more properly a "factor of uncertainty" that cumulatively includes all the various imprecicions in assumptions made during the design.

[futurama]

A "factor of uncertainty" that cumulatively includes all the various imprecisions in assumptions made during the design == "factor of safety".

And in FAR Part 25, it is legally defined as such:

Sec. 25.303 — Factor of safety.

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. When a loading condition is prescribed in terms of ultimate loads, a factor of safety need not be applied unless otherwise specified.

[Aihkio]

Calling it FoS leaves out the origin. I seem to recall that in some space projects it was called FoU. Whichever, one should remember the origins.

[Smythe]

In structural engineering, the typical Factor of Safety is 1.5. You take the intended load, and design it to 1.5 times. This is a fundamental load factor for analysis.

Along those lines, when you test to ultimate (ie failure) you can then determine the actual safety factor of each part or the assembly. In ultimate, you may determine a factor of safety of 5 for the part or assembly. This is what is strived for, because in the 1.5 factor being the bare minimum, if ultimate is only 2, then you are always looking at cases where the load or conditions may be higher than your assumption. With a higher factor of safety, you know you can cover more conditions without restrictions.