Years ago I worked at Ames on the ACES system (minor but fixes and a Greenfield projects involving data/flight path visualization), but I felt the airplanes themeselves were these magical black boxes. I'd would appreciate having a deeper understanding about how they differentiate from one another in a non-superfical sense.
I worked specifically on the stabilizer trim system (like the one in the news on the 737MAX), and did some work on the elevator system. In particular on the latter, I did many calculations to prove it would not flutter (dynamic instability).
My very first assignment was to size the stabilizer trim jackscrew. I panicked and told my lead I had no idea how to do that. He laughed and said of course you do, it's a simple column buckling problem. Which of course it was, and I sized it.
A couple years later, and the first jackscrew gearbox assembly came off the line, and was doomed to be subjected to the ultimate load test. Any buckling, cracks, deformation, etc., would be a failure. The test guys told me they were gonna bust my jackscrew.
They hooked it up to this big ugly iron I-beam with a hydraulic ram to compress my green BMS10-11 painted gearbox and shiny chrome steel jackscrew (made by Saginaw Gear, who made the best kick-ass forgings).
They started cranking up the pressure, while I stood around anxiously watching it. Slowly, the I-beam bent into a nice curve. We didn't have to make any changes to any of the stab trim system due to test failures. Whaddya know, the math works! So I am not a bit afraid to fly on a 757. The seating can be cramped, but that's the way it goes these days.
As far as know, there have been no in-service failures of that system. The 757 itself has a fantastic safety record, of which I am proud to have contributed to. The last D conference I flew to on a 757 operated by Iceland Air, yay!
At the time I bought a bunch of Boeing stock as a result of my confidence in Boeing, and it has paid off handsomely.
One day, I got called into the stress group, where they told me they were unhappy with my parts. I asked why, they said there just barely strong enough for the ultimate load, only about 1% over. They said they'd feel more comfortable with 10% extra strength.
I said this was no coincidence, I used math to size the part to the load, rather than guessing at a size and checking to see if it was strong enough. The requirement was the ultimate load, which is 150% of the maximum load that could ever be expected.
I asserted that I designed to the design requirement, and an extra 10% would make it overweight. Brat that I was, I said if you guys were unhappy with the design requirement, increase it. They grudgingly signed off on it.
The reason it didn't break on the test stand is because all parts vary in size due to manufacturing tolerances. It was sized to pass the ultimate load test under the most adverse size allowed under the tolerances. Odds are, it'll be a little stronger.
For comparison, spacecraft have (I think) an ultimate load of 110% of the max load. The margins are awfully thin, but they have little choice. If you really want a math heavy engineering job, design spacecraft.
BTW, the stress group signed off on the jackscrew size. If it had failed the ultimate load test, both them and I would have had a black eye. As it turned out, the test guys were embarrassed by their bent rig :-) and fortunately that wasn't really a problem.
Did they want the extra 10% buffer for any particular reason?
Reason I ask, I always agreed with your take on not over-designing, philosophically. I had a couple of professors who made a big deal about being conservative/ round-up on calculating design elements, but once you tally up the different loading cases, you shouldn't ever bump the total required strength (again) beyond whatever the required multiplier is.
However, when faced with real life projects, I learned fairly early to ignore that advise, mostly because of the risk of having to redesign due to a changed design requirement. Of course I've also never worked for a company as structured as one like Boeing, and I've also encountered LRFD 99% of the time over ASD.
It's one of those life lessons that's never really sat right with me, but I still consider to be the rational choice.
They said they just were more comfortable with more margin. The stress group was an independent organization to avoid conflicts of interest with the design group. Our job was to design, their job was to verify. If parts broke, they got the black eye. If the airplane was overweight, design got the black eye. We both had to agree, and it was a system that worked well.
Most design engineers didn't work out the stresses themselves, they just hoped to pass the stress group. It was pass/fail for them. In my not so humble opinion, they were making overweight parts. I was interested in using math to sculpt perfect parts :-)
For example, the inside diameter of the jackscrew was specified to the ten-thousandth of an inch, something like 2.1834..2.2096. So I'd get the "why not make it a nice round 2.19..2.20?" I'd reply because if it came in at 2.205, it would get rejected, even though it was perfectly usable. And I got my way, because the jackscrew forging was a very, very expensive part and rejecting a usable part did not make anyone happy.
(Rejected expensive parts often got bounced back to engineering to find a way to salvage them. I just was doing the math in advance so they wouldn't have to.)
If it's a rabbit hole you're interested in, then check out the history of geodesy / geodetic surveying & the design of mechanical / optical surveying equipment- especially theodolites & auto-levels (and even chronometers and astronomic observatories, if you want to go 4-dimensional).
It was the "industry" that had a large hand in innovating this technology, which led to the smaller versions used in manufacturing (made even more interesting due to the fact it was all funded to better artillery, and ultimately ballistic missle targeting, sans-GPS.)
I'd be happy to steer you in the right direction, if interested. It's a personal favorite.
Going beyond that, to tens of millionths or sub-micron, is where things get nutty with special climate controlled rooms. Digital indicators can easily read with such precision, but controlling for factors like radiated body heat become important.
Reference measurement is usually done with gauge blocks and pins. The development and popularization of these sets is basically how mechanical parts became standardized.
If you're interested in that sort of thing - bootstrapping precision and the like - check out "The Foundations of Mechanical Accuracy".
https://www.youtube.com/watch?v=dzIsR4Mg158
"ball screw" and "lead screw" are your search terms. For aerospace it's gonna also be center drilled to save weight as well.
It's forged slightly oversized, then machined down to spec.
Forgings are about 3x stronger than a casting, and are of a more consistent quality, which is why when I increased the HP for my Dodge, I spent the extra bucks to get forged spinning parts (usually they are cast).
For example, if you're testing the integrity of a pressure vessel, the standard is to test it to 110% the rated maximum allowable working pressure if the medium is a compressible fluid (like air) or 150% if it's a 'non-compressible' fluid (like water, although everything is compressible to a certain extent). This is because the compression of a fluid stores a lot more internal energy if suddenly released. (Don't quote me exactly on these numbers because it's been a looooong time since I've had my nose in ASME standards)
https://en.wikipedia.org/wiki/National_Airlines_Flight_102
A loose armored vehicle crashed into the jackscrew snapping it. There is no possible recovery from that. I cringe every time I see the video of that one. Horrible.
A few days ago I'd asked how the NTSB gets good co-operation from the airlines in these cases (thanks for the answers at the time!)
The interesting point to me was the contrast between the 2 cases.
In the Aeroflot case:
> Three engineers who signed the jackscrew drawings were convicted.
In the Alaska Airlines case:
> The investigation then proceeded to examine why scheduled maintenance had failed to adequately lubricate the jackscrew assembly. In interviews with the Alaska Airlines mechanic at San Francisco International Airport (SFO) who last performed the lubrication it was revealed that the task took about one hour, whereas the aircraft manufacturer estimated the task should take four hours. This and other evidence suggested to the NTSB that "the SFO mechanic who was responsible for lubricating the jackscrew assembly in September 1999 did not adequately perform the task". Laboratory tests indicated that the excessive wear of jackscrew assembly could not have accumulated in just the four-month period between the September 1999 maintenance and the accident flight. Therefore, the NTSB concluded that "more than just the last lubrication was missed or inadequately performed".
I couldn't find any mention of personal sanction.
Just surprised by the difference in treatment between the 2 cases...
> In 1998, an Alaska Airlines mechanic named John Liotine, who worked in the Alaska Airlines maintenance center in Oakland, California, told the Federal Aviation Administration that supervisors were approving records of maintenance that they were not allowed to approve or that indicated work had been completed when, in fact, it had not. Liotine began working with federal investigators by secretly audio recording his supervisors. On December 22, 1998, federal authorities raided an Alaska Airlines property and seized maintenance records. In August 1999 Alaska Airlines put Liotine on paid leave, and in 2000 Liotine filed a libel suit against the airline. The crash of AS261 became a part of the federal investigation against Alaska Airlines because in 1997, Liotine had recommended that the jackscrew and gimbal nut of the accident aircraft be replaced, but had been overruled by another supervisor.In December 2001 federal prosecutors stated that they were not going to file criminal charges against Alaska Airlines. Around that time Alaska Airlines agreed to settle the libel suit by paying about $500,000; as part of the settlement, Liotine resigned.
The deficiencies of the FAA regulator regime should be obvious from this case, but it seems that no-one was held accountable for Alaska Airline's lack of safety culture in its maintenance operation.
Fatigue failures like the one in the crash are from multiple sub-maximal loads (look up "S-N" curves for more info). In other words, fatigue comes from cycling lower loads many times on a part. Buckling calculations look for maximal load failures on a column (i.e. a single larger load event that can make it fail).
The link is a great example of a real world case study (and thank you for finding it!), and no doubt somebody was doing fatigue calculations on the jackscrew at Boeing as well. I just wanted to add a bit of nuance
I'm always surprised by the general publics' lack of awareness and understanding of fatigue. And then seeing the figurative lightbulb illuminate after asking them how they would break a metal clotheshanger, without any tools.
Anyway, it may not have been his personal responsibility, but certainly fatigue must have been a significant consideration during design, since (afaik) it's most often the controlling factor in lifetime estimates for structural components in airframes.
I would strongly assume the same would apply to a component who's expected failure mode might be buckling.
(and of course there is the case of the part being damaged somehow, where some of the other comments have linked cases where this happened catasropically).
I'm not real familiar with that mass production qaqc.
Is there a term that describes a minimum design spec (or similar aspect), that takes into account the actual manufacturing (and other) variances?
I'm thinking that in construction, a rough equivalent would be a designs constructability / bidability review.
For critical parts on the aircraft, there is the backup of inspecting for aspects like accelerated fatigue just for variation of manufacturing, of environmental contributors, etc. It's a lot like security defense in depth - many layers of protection for lives (and aircraft industry trustability linked to profitability too) at stake.
Yes, they were (the stress group). The airframe was designed for 62,000 hours of operation, then scrapped due to fatigue considerations. Fatigue was a bit of a black art at the time, with rules based on extensive testing and experience. It's much better understood today.
Take a paper clip and bend it back and forth. Eventually, it will break. That's fatigue.
When I studied this it seemed to just be empirical, as in you calibrate some law to some measurements and use that to tell you when it is likely to fail. Has this changed?
That's refreshing. I have a friend who worked on the A380 and she says she'll never set foot on one.
https://www.aviationpros.com/aircraft/commercial-airline/pre...
My answer was usually to mock them and say "so just because you are looking from too close and have lost perspective you prefer to fly an ageing aircraft or trust some random Boeing contractor more than yourself".
And fast forward ten years, the "deathtraps" they were working on - A340, A380 and A350 - turned out to be extremely safe (with fatality-free records).
The 757 was the last airplane designed using paper drawings and wooden mockups. The end of an era :-) Some of the engineers still used slide rules. My lead engineer once said "let me show you how it's done" and pulled out a very nice slide rule in a leather pouch. I made a little show of blowing the dust off of it, and he laughed heartily. He started at Boeing on the B-47 design, and even he had already moved on to calculators.
But nobody used computers yet. I found a PDP-11 in a neighboring building and sweet talked the sysop into giving me an account on it. I'd write my own numerical analysis programs using Fortran IV.
My other lead was an old salt who told me he didn't trust computers, they always produced unreliable answers. But he was a good egg, and said he'd give me a chance. There was some long complex geometrical calculations to be done to prove the elevator linkage wouldn't bind up under load. The usual method was to use drafting tools to figure it out, and he put his best draftsman on the job. I wrote a program to figure it out, and came back with my column of numbers to 6 places. His draftsman compared it with his column, accurate to 4 places.
One of them was off. My lead said see, I told you computers were garbage. I asked the draftsman to recheck that one, and he did, and came back and said I was right.
Ever since then I got the calculation jobs, and my lead would defend me. One day, the CS dept head discovered I'd been stealing unauthorized time on the PDP-11, and I got in trouble. My lead went to bat for me, it went up 3 levels of management, and the top guy told the CS mucky-muck to piss off and give me whatever computer time I needed.
I was only the vanguard of what was to come in using computers to design airplanes.
My experience with that motivated me to ensure D had a first class foundation for numerical programming.
I later bought a Heathkit H-11 so I could have my very own PDP-11. You can see it on my twitter picture https://twitter.com/WalterBright
I’m quite sure that if you started a blog today with the stories that you can remember, that you’d have a ton of HN’ers who would read it.
Thanks again - your posts are great to read!
> One day, the CS dept head discovered I'd
> been stealing unauthorized time on the
> PDP-11, and I got in trouble.
I didn't want the sysop who let me in to be sorry he did so. Besides, I was engrossed in my models.
When I got my own 11, I wrote a game for it:
https://github.com/DigitalMars/Empire-for-PDP-11
> I recognize your user name
Is that 11 still in operation? Wow! DEC sure makes a tank for a computer!
Have a great week!
I’d say you’re in decent company.
:-)