[wwv25]

One of the inspection checklist items:

Measure the clearance between the flapper bushings at both locations on each engine bleed air 5th stage check valve. If the clearance between the flapper bushings is a minimum of 0.004 inch (0.102 mm) at both locations, the engine bleed air 5th stage check valve at that location has passed this inspection.

I knew that aviation engineering dealt with tight tolerances, but boy, it must be difficult to make a call like that when you're dealing with thousandths of an inch. Do field technicians have dependable tools and methods of measuring with such high degrees of precision?

[rlpb]

If it's a gap between two parts that they're measuring, that's as simple as a feeler gauge: https://en.wikipedia.org/wiki/Feeler_gauge. The set pictured there looks like it goes down to 0.05mm.

[numpad0]

With that in mind the phrasing sound to me like “the bushing is supposed to be moving freely, if it’s stuck to the surface you have to replace it” in aviation lingo without using ambiguous words like “freely”

[tonyhb]

Yep. Also, fun fact, these are used in dentistry and orthodontics to check spaces between your teeth. We use the same information in our 3D treatment planning environment.

[elliekelly]

Perhaps an odd question from someone who only just found out that feeler gauges exist - do they periodically calibrate or confirm the measurements of the gauges? If someone is sticking the same gauge into tight spaces all day wouldn't it wear down, even slightly, and alter the reliability of the measurement?

[xnyan]

Can’t answer for aviation but I worked in the general testing and certification industry (Underwriters Laboratories) with tons of rules and externally audited standards about calibration of measurement and test equipment.

Yes, things like rulers and gauges are tested. The schedule depends on the use case and frequency of use. For relatively light lab duty stuff, rulers and gauges were checked 1-2 times a year. Things that are used more frequency or more aggressively are checked more often.

[bronson]

In theory, yes. In practice, they get dirty and bent faster than they wear out.

They're cheap. Shops that care have them on a replacement schedule.

[Gibbon1]

In aerospace everything like that is traceable and calibrated on a schedule. Traceable means that there is paper work that shows what it was calibrated against. Which itself is traceable to a NIST standard. The difference between traceable and ordinary measurement equipment is a couple of extra $.

Put is this way one time I found a set of hex torque drivers with cal stickers on them at a surplus place. So when I say everything I do mean everything. So yeah a feeler gauge should be traceable.

[a2tech]

Yup. Traceable all the way back to the batch of material the part is made of. For every part in the plane. One of my customers makes parts for Rolls-Royce engines for the aviation industry and have to store certs and manufacturing records for even the smallest part. That includes certs on each step of manufacturing and material. Its pretty intense! During crash investigations they can be called upon to provide the full audit trail. Now imagine the amount of paperwork that is produced during an investigation when every nut and spring in a plane has the same amount of associated paperwork produced.

[aunty_helen]

>The difference between traceable and ordinary measurement equipment is a couple of extra $.

This idea again. I don't know why people are saying this. It's not true. Aviation you're allowed to use any tool you want unless the workshop manual says to use a specific tool.

OEM calibration certs are useless to an aviation company because it's someone else promising accuracy that could see you thrown in jail if it's wrong. Before tooling is used the first time it will be calibrated. Then calibrated again at set intervals.

>hex torque drivers with cal stickers

It's most likely the part had stickers so it was in their system for visual inspection for good condition. I have a pair of lockwire pliers that were pulled from the shop floor due to them being indicated worn during a visual inspection. I still have them 7 years later and they're still the sharpest pliers I own.

As for the traceability from birth, this applies to parts that go on the aircraft not tooling. Tooling traceability you need to know what tool was used if it's required to be calibrated. ie torque wrenches.

[refurb]

They are typically harden steel, so wear minimally and when you test a gap, you’re not really applying much force.

If the feeler gauge doesn’t slip in with minimal force, then that tells you the gaps is smaller than the gauge.

[teh_klev]

> If the feeler gauge doesn’t slip in with minimal force, then that tells you the gaps is smaller than the gauge.

The numbers of sets of comically bent out of shape feeler gauges I've encountered in my life demonstrates that there's a bunch of folks out there who don't understand how they're supposed to be used. As in "maybe if I just push harder this'll fit" :)

[refurb]

It’s a combination feeler gauge/pry bar!

[a2tech]

Yes. And places that deal with these types of tolerances will typically have a person who's job is to simply verify and replace gauging. If a gauge doesn't have a documented test verifying its accuracy, then it has to be assumed to be inaccurate and any parts or tests based on results provided by the instrument need to be redone or the part scrapped.

[_trampeltier]

For things to measure, where a person finally have to sign for an OK, tools allways have to be calibrated and tested (not just in aerospace industrie). Thats also the reason, why there is not just the persons name but also the toolnumber on the test protocol.

[olyjohn]

I use these on my car to check / adjust valve clearances. Every 30,000 miles.

[gonzo]

You have a fantastic engine.

When I was young, it was guaranteed that a lifter would let loose just before you picked up your date on Saturday night if you had not “run the valves” in the past month.

Clack-clack-clack-clack-...

Date: Why does your car sound like it’s broken?

Me: <mutter>

[linuxftw]

The only person here that knows what they're talking about. Everyone's talking about calipers :/

[nabilhat]

Who even uses calipers for critical precision? Calipers are for dimensions machinists append with -ish, as in four thousandths-ish. Important dimensions deserve a micrometer, especially the feeler guage that just said you need to tell your boss to scrap a 737. Decent machine shops even have different classes of micrometer; there's the one you carry in your pocket, and then there's the fancy set locked up in your boss's office that only comes out for special occasions, like when your boss has to tell their boss to scrap a 737.

[cartoonfoxes]

I see this in my own shop. Digital displays instill people with greater confidence in the measurement than is warranted. This is something of a UI fail, as the tool manufacturer (Mitutoyo) has very clearly defined measurement precision for all their metrology equipment (+- 0.001" and 0.0015" for the calipers).

[sgnelson]

That is a very interesting observation, thank you. I've never thought about the digital vs analog in terms of actual "oh, it's digital, it's better." measurement variation.

In my experience, I think you're probably right, it does seem like people may take them as "more accurate."

[kayfox]

You don't scrap a 737 for some worn out parts.

[doodlebugging]

This is why I still own most of the vehicles I've purchased over the last 30 years. It is much cheaper to replace worn parts, ideally before they fail, than it is to replace the entire vehicle. I have the skillsets needed to maintain my own equipment and the tools so I just monitor the operating condition of each and replace or repair things as needed.

These planes have regular maintenance checks and diagnostics to help identify issues before they become catastrophic failures. This whole thing is likely a normal maintenance issue, maybe unexpected due to the unusual nature of the scenario they face with so many idle jets, but not something that would have escaped their servicing routines. The system flagged a problem and now they can address it. If for some reason their diagnostics were not set up to detect this sort of issue then you have a problem.

[bluGill]

Airplanes worry about fatigue. They have to be replaced after so many years or the wings will fall off in the air. If the airplane is only a few weeks away from replacement when it fails an inspection you scrap it now, if it has years to go you replace parts.

[teh_klev]

Not that I want to be pedantic, but that's a rather sweeping statement. The decision to scrap would depend on things like the number of flight hours and takeoff/landing cycles on the air frame and the number of other worn parts, hitting meantime-before-failure on large numbers of parts/assemblies, such that it may be more economical to scrap.

[bamboozled]

I love that out of all the more complex ideas about what may have been used, it turns out likely to be a solid piece of metal with of a given thickness, couldn't get more simple than that.

[WrtCdEvrydy]

If you're into 3D Printing, feeler gauges are also a great way to level your bed... move the extruder 0.04 above the bed at four points, stick your 0.04 feeler under it and adjust the springs until it touches the feeler. Voila, one perfectly leveled 3D printer bed.

[serf]

that's a weird in-between two more popular ways of doing it.

The non-precise way you've already heard about, sheet of paper, check for dragging.

The better (more precise) way, is using a magnetic base micrometer attached to your extruder head. You can then watch the bed run-out in real time; if your 3d printer supports many configurable sections across the bed ( I know Prusa style cartesian units mostly all support quadrants) you can record the run out everywhere across the plane without any more physical work than watching the gauge and recording the results.

P.S. be careful using both the micrometer and the feeler gauges on a 3d printer bed. Most work plates now-a-days are using PEI coatings that'll scrape off easily with metal-on-metal contact.

[WrtCdEvrydy]

> using PEI coatings that'll scrape off easily with metal-on-metal contact.

Yeah, you're correct, it's a glass bed so I use it to do the 4 corners (every few months) and then use a BL touch to do a 3x3 grid on top of it.

[jrockway]

Most people use a sheet of paper for this. I wouldn't use it to check the tolerances of an airplane part, but it gets you 99% of the way there.

[WrtCdEvrydy]

I like the sheet of paper method, but I have gotten a lot more use out of a $0.15 feeler gauge and I get consistently better prints... but I don't have the tools to scientifically quantify it so it could be placebo.

[cartoonfoxes]

They're deceptively simple. If you want to get into the history of modern manufacturing, a lot can be written about the development of gauges of accurately known dimensions.

[nitrogen]

I went down a rabbit hole of videos last yearish about the origins of precision (that should be a searchable phrase to get started).

[cartoonfoxes]

Go find a PDF of "Fundamentals of Mechanical Accuracy" by Wayne R. Moore, if you haven't already seen it. That book, along with George Daniels text "Watchmaking" are some of the top inspiring works on the mechanical arts in my collection.

[noir_lord]

Somewhat sadly a lot of the drive for that tolerance was weapons, until we could accurately make very high tolerance (for the day) parts you couldn't use parts from one rifle on another without a smith, it certainly wasn't field serviceable - this imposed a lot of maintenance overhead until they solved it properly.

It seems like we really like to push the limits when it comes to shooting each other.

[linuxftw]

> Somewhat sadly

Weapons can be used for defensive purposes. There's nothing inherently wrong with them, only how people use them.

> parts you couldn't use parts from one rifle on another without a smith

You cannot swap bolts in the field on most rifles. At minimum, you require a go/no go gauge. The tolerances on modern firearms are very precise.

[bilbo0s]

couldn't get more simple than that.

Believe it or not, that's actually a lot of the idea that drives commercial aviation design.

Or used to anyway.

[KMag]

... and most engineering in general. Simple solutions tend to be very robust and often cost-effective.

[noir_lord]

and easier to troubleshoot/replace.

> A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over, beginning with a working simple system. - John Gall

[na85]

That's what real engineering is about: the simplest tool or process that will get the job done.

Despite what we see on the web more and more, this principle applies to code too.

It shouldn't be about stacking up layers of complexity for no good reason.

[Gibbon1]

Somehow your comment reminds me of a pissing match I saw between a young aerospace engineer and an old retired one. The old guy was right in his argument that it doesn't matter what the constraints on materials and dimensions are, it that they come from somewhere and are documented.

His example was designing brackets for a Indonesian construction company that was using locally sourced tropical softwoods for constructing buildings. The brackets were designed to be made with 'steel' and the tolerances were compatible with a hungover guy running a drill press.

[bewaretheirs]

Yep. To check tolerances, you'd need a go/no-go pair -- one that should fit, and a slightly bigger one which shouldn't.

[lisper]

This is a thing that non-aviators often don't realize about aviation in general: the hard part is designing the artifacts and processes so that by the time you get around to operations, everything is routine and nothing is hard. Flying, building and maintaining airplanes seems hard, but that's mainly because of the volume of stuff you need to know, not because any single item is particularly difficult. All aircraft are built, maintained, and flown by mere mortals. (The designers, on the other hand... ;-)

[int_19h]

It's not just aviation. Our entire industrial civilization works like that, and this is the only reason why it can work. We are surrounded by high-precision machinery even in daily life, and are completely oblivious to just how much technomagic it is - and how much historical effort went into building the industry up to the point where it can produce such things so easily and cheaply.

[esaym]

>Do field technicians have dependable tools and methods of measuring with such high degrees of precision?

Yes, it is basic stuff in most mechanics. For instance, the valve clearance on most car engines is going to be .002-.005.

I worked aviation maintenance for years. Most larger planes are held together with 'hiloks' [0] They are an "interference" fit.

For a quarter inch hilok, you'd first pilot drill a hole around 3/32" (#40 drill bit), then up drill it to .242 (C drill bit), then swap out to a .242 to .247 "reamer" and ream the hole to .247. The hilok itself is around .248-.249" so you can't just "push" it in but tap it in with a hammer or rivet gun. There was a "go-no-go" gauge. The "go" end was .246 and the "no go" end was .248. So yes, tolerances of 0.001" are quite common.

[0] https://duckduckgo.com/?q=hilok&ia=web

[Animats]

Adjustments to tolerance are something aviation still has, but which has disappeared from consumer products. It's too labor-intensive.

Here's the "Adjustments" manual for a Model 15 Teletype, the 1930-1958 model. I've restored two of those. Making those adjustments isn't that difficult, just time-consuming. Adjustment to 0.002 in. is sometimes required, but it's not that hard. Most Teletype adjustments can be set with feeler gauges. You don't need a micrometer.

[1] http://aetherltd.com/public/model15manuals/138_Model15_Adj_O...

[BinaryRage]

I was staggered by how much adjustment was required when watching CuriousMarc's excellent Model 19 restoration series, seems incredibly finicky.

https://www.youtube.com/watch?v=_NuvwndwYSY&list=PL-_93BVApb...

[Animats]

I just put up a few stills of my own restorations, some short demo videos, and Github repositories for the interface board and the software. I had no idea anyone would want to watch four hours of Teletype repair videos.

Model 15 and 19 Teletypes are not hard to work on. Everything comes apart easily; it's all screws and lockwashers. They're human scale; it's not like building surface mount electronics or repairing an iPhone, where you work under a microscope. The adjustments aren't that finicky, except for a few near the selector magnet. It also helps that the whole thing is unidirectional - there's a straightforward path from input signal to typebar hitting the paper, and you can work through problems in order. As a nice feature, movement is powered in one direction and spring-loaded in the other, so if something gets stuck, it's just stuck in the operated position and doesn't get bent or broken.

Aircraft are built like that. Some parts require careful adjustment, but there's almost always an easy way to check that you got it right. Because, after all, you can't fix it in flight.

Mechanical design has a design philosophy embedded in it. If you work on complex mechanical systems, you can sometimes get a feel for how the original designer thought. Good machinery design is not a common skill. All the good Teletypes were designed by only two people - Howard Krum and Ed Kleinschmidt.

Very few people study this any more in the US, which hurts when you need to design production machinery.

[perl4ever]

Tolerances and path dependence are the fundamental reasons why I've always gravitated towards software and programming rather than building/repairing physical stuff.

It's interesting to me how getting the right physical configuration of something so it's reliable kind of encodes the complex procedure that gets it there.

It's so much harder than a program where you can identify all the parts and modify them in any order. And whenever you make a mistake, you can generally just reach in an fix it, rather than redoing a long disassembly or something.

[adenozine]

They've had calipers for literally thousands of years.

I used to be involved in some intermediate gunsmithing work, and we had these tools called 'dial calipers' which have a little gauge you read and you can measure extremely precise things. We used it for getting cylinder diameters or clearances that needed that degree of precision.

I can only imagine that in aviation, there are even more advanced tools, especially with a well-respected corp like Boeing.

I would think also, that when there are these clearance issues in parts, part of the concern that makes it worth raising flags over is the fact that it's consistently off amongst different aircraft. If someone were to bring me a rifle that had some important gas clearance off, I wouldn't be that concerned, just do the work and get it back in the field, but if guys are bringing me the same rifle with the same issue over and over, it makes me really uneasy because it's inefficient for my shop, and it points to carelessness from the vendor, which raises suspicion when dealing with orthogonal issues from the same vendor.

If these flapper bushings are bad consistently, and they had a spec and didn't meet it, what other corners did they cut?

Just my two cents. I don't know shit about aviation, but I can appreciate the systemic concern.

[throwaway0a5e]

This is basically the FAA putting everyone on notice by saying "yo, we found a few that were real bad so y'all gotta make sure you check yours real good." It's a bushing on a control surface, of course it wears over the course of normal use. If it sat for months and corroded slightly the corrosion would cause it to wear faster. This is all perfectly normal. These things are meant to fly so the engineering more often favors dainty little parts that can barely do the job with maintenance tacked on over heavier overbuilt parts that you don't need to check as much.

>I can only imagine that in aviation, there are even more advanced tools, especially with a well-respected corp like Boeing.

Lol. "More advanced" often being something a long the lines of a $100000 dollar go-nogo gauge because you don't trust your employees to read a measuring tool.

>If these flapper bushings are bad consistently,

Bushings are probably (i.e. "almost certainly" but I wasn't in that engineering meeting so I don't know for sure) a wear item and of course they could build beefy ones but the OEM has to balance between weight and maintenance hours. Doesn't surprise me that a little extra corrosion grit in there makes them go out of spec fast.

>if guys are bringing me the same rifle with the same issue over and over, it makes me really uneasy because it's inefficient for my shop, and it points to carelessness from the vendor

The whole value proposition of a Hi-point or a Chinese pump shotgun is that the manufacturing tolerances are wide opens so that your wallet doesn't have to be.

[adenozine]

Suffice to say, nobody in my unit was rolling out with Hi-point.

Thanks for your post, you are clearly much more aware of aviation than I am, and what you've explained makes perfect sense.

I could've clarified that while I wasn't in control of ordinance, I had some level of trust in those who were, as I'd expect the engineers at Boeing have some level of trust in the designs and blueprints they are working on.

I'm curious to know more about the attitude that an engineer might have about their role in Boeing. It often seems that Boeing is being dragged through the mud, and rightfully so in some part due to the 737 MAX. However, I know enough to know that shit often rolls downhill, even onto those who never ate in the first place. I imagine there could be some disgruntled vibes going around in the shops, far from the boardrooms.

[nogabebop23]

It's not necessary to use calipers to measure these tolerances. In this case talking about clearance between two parts you just use a feeler gauge which is a calibrated thickness of metal. If the documentation says no more than X mm, you grab your X gauge and make sure it doesn't fit. It's the same if you're working on a 737 or your John Deere.

[kube-system]

They're measuring it to ensure corrosion hasn't thrown it out of spec, right? Not that it wasn't manufactured to spec?

[adenozine]

Excellent point, shows you how much of an engineer I'm NOT. That's probably a big point too.

Surely there's some sort of chart they've made for all these parts and how fast they're allowed to decay, given some number of flight-hours?

[kube-system]

I believe there are procedures/checklists for placing aircraft in and out of storage which are different than those used for normal operation, because the conditions are different. It is my understanding that this directive is an additional check that would be performed during the typical storage procedures.

[steffan]

> given some number of flight-hours?

I think the issue here is the lack of flight-hours. These parts were designed and specified for an in-use plane, not one that sits around.

An airplane is an extremely expensive asset and they are typically almost constantly in service. It's very rare for one to be idled for months at a time.

[nogabebop23]

yes - this is related to non-routine maintenance. I assume operation prevents moisture forming that can corrode. Storage for several weeks or months means this is like a more stringent used car "road worthiness" inspection.

[CaptainZapp]

Boeing used to be a well respected company. At least until the merger with McDonnell Douglas, which turned it from an engineering driven company to a financial engineering company.

[kitteh]

Eh there were several phases of stupidity there.

757 to replace the 727 when the 727 was doing quite fine. That said the 757 is a great plane and proved out well, just had a high price tag that really held it back.

The delay and screwing around with getting the 767 out there for fear it would mess with 747 sales. Let airbus get ahead with wide-bodies and took longer than expected to get the ER variants out.

Post McDonnell Dougals:

The handling of the 717 (basically shutting it down when there was demand). Could have had an impact if they wanted to bring it to the regional space (to go against the crj900+ and EMB ejets).

Oh so much in the planning and execution of the 787.

The continued extending of the 737 well beyond what it should be (see the 900, the MAX).

[dingaling]

And in turn the same had happened with Douglas; McDonnell's investment was reluctantly accepted in order to save the company but utterly changed the culture from engineering to finance.

[LittlePeter]

I really struggle to understand how you could have used the term "well-respected" here. It ain't muscle memory, this was done deliberately. Could you explain how you can refer to Boeing being well-respected after the 737 MAX fiasco? Maybe I am missing something.

[edit] remove a word

[TallGuyShort]

They've had some big screw-ups that were well covered on HN. But from 2014 to 2018 they were consistently the biggest and most profitable aerospace company in the world[1]. You don't get there by being disrespected and all-around shitty.

[1] https://en.wikipedia.org/wiki/Aerospace_manufacturer#Largest...

[quarterdime]

As a long time lurker I finally had to create an account to reply to you. Boeing was indeed extremely large and profitable. Building such a successful enterprise came from lifetimes of work with dedication to excellence in engineering, manufacturing, safety, etc. Unfortunately we are seeing that a decade or so of attacking these values is enough to destroy even a giant. I do not believe that we are seeing just a streak of bad luck. Off the top of my head, let's review a few of Boeing's recent big projects.

787 - Historically over budget and delayed. Failed static wing deflection test, showing how much we can trust the FEA work. In cockpit battery fires led to a historic worldwide grounding of Boeing commercial aircraft. A NTSB report blamed (in part) Boeing engineers for not considering worst case scenario for a lithium battery in a cockpit compartment that contained no fire suppression system. A grounding like the one OP linked is not unprecedented, it is of the 'the plane is grounded until it passes an inspection' type. The battery fire issue led to a 'all planes are grounded until Boeing has a fix.' I believe that this is the first time that Boeing had an aircraft grounded in this way. I believe it was 3 months for Boeing to have an FAA approved fix. Incidentally the 'fix' was a heavy duty sheet metal box around the battery, with a vent to outside the aircraft. I suppose time will tell how reliable a fix this is. Finally there have been issues with debris being left in fuel tanks, metal shavings in wire bundles, etc. Allegedly in aircraft delivered to customers.

737 MAX - A half-baked software bodge has left hundreds dead and all these aircraft grounded worldwide. This is the second time Boeing had a commercial aircraft grounded worldwide with no end in sight. As with the 787 there are issues with debris being found in "complete" aircraft, with foreign object material being found in fuel tanks AFTER the aircraft has left assembly and passed inspection.

KC-46A (Air Force Refueling Tanker) - Years late, over a $1B over budget. Egg on face issues like not having the (required) FAA approval for fuel pods and drogue system. Repeated issues where Air Force refused delivery because of... debris in fuel tanks.

Starliner Crew Capsule - multiple critical software errors that meant the capsule never docked with the ISS and was nearly lost.

There are some common threads here. Bad software for 737 MAX and Dreamliner. Foreign object material ending up in wings and fuel tanks over and over.

Finally there is good reading to be had about quality issues in assembly, parts being rejected as defective and then "disappearing." Whistleblowers are reporting that the "disappeared" defective parts such as tail assemblies are ending up on aircraft and being delivered to customers. It seems there is pressure from management to sacrifice safety for profit, pressure to approve designs (thanks to some really good lobbying, Boeing essentially gets to approve its own designs with minimal FAA oversight), pressure to keep you mouth shut about safety concerns (and retaliation if you don't).

These are not signs of a healthy company. By now I think that Boeing has slid into "all-around shitty." The above is my best recollection of news stories from years of watching Boeing, if I have made a mistake then I am happy for any corrections. Let me know if you would like links to any of the particular stories or believe that a [citation needed] is in order.

[adenozine]

Your post seems to imply that a bad product negates the respect earned by leading an industry, nay, multiple industries, for several decades.

Do you really truly think that just because the MAX is a POS, that everything that comes out of Boeing can be evaluated under that same lens? Surely you aren't so blind to the reality that engineers do occasionally produce quality work.

To your credit, I wouldn't fly myself or my family in a MAX, but I'm not uneasy about getting into a Boeing aircraft across the board, at the same time. That'd just be irrational.

Perhaps you could clarify your stance, as you may know more than your post reveals.

[LittlePeter]

> Do you really truly think that just because the MAX is a POS, that everything that comes out of Boeing can be evaluated under that same lens?

No I do not. I still wouldn't ever refer to Boeing as well-respected nowadays. Granted, it may be irrational quirk of mine.

To clarify, do I respect Boeing? Yes. But I would not make a post on the Internet and write out "well-respected". I understand Boeing's cultural shift from being an engineering-first company and its MAX fiasco would not warrant "well-respected" in many people's eyes nowadays.

Was Boeing well-respected in the past? Yes. Is Boeing well-respected nowadays? Yes. But to deliberately write it out to me feels like trolling and stoking fire. You surely must understand that a lot of people lost a bit of respect for Boeing in recent years.

[adenozine]

I can understand why it may have sparked those feelings because of the way I wrote it.

I'm a pretty forgiving person, I've spent some time in and around rehab and I've learned personally the value there is in giving people a chance they don't deserve. I know how hard some of the engineers at any big industro-* corp are working, and I choose to have hope that those hard-working folks' ideas and values are represented in the product line that they serve on.

I didn't mean to say that recent events should be scrutinized any less critically, and I disagree with that, especially in the case of passenger aircraft, the utmost care should be taken.

I've read a little bit about 737 MAX, and it strikes me as one of those things where too many boardroom cowboys got to run off and make deals, and the brains and engineers and designers were left with the scraps of an impossible task.

I don't know. I just have a soft spot because I can imagine what it's like for a lot of those guys, going to work and doing their best, and the project is so large that there's just not much any single person can do when it all begins to fall apart at the seams and catch fire. I feel bad that all those people have this terrible mark on them because of the product being a huge, public, terrible failure.

Boeing has done incredible things for the field of aviation, aerospace, maritime, rescue, you name it, they've made a flying vehicle to do it. That can't be washed away because a bunch of guys fucked it up, because it wasn't the people that worked the hardest. So, while I perceive that critically, I respect the name Boeing for what it's given the world in the past.

Anyhow, I wasn't trying to start anything by writing that, I guess it was just part of my thought process that didn't get edited into words very well.

Thanks for your perspective.

[mberning]

They are a respected company with many respected products, despite your personal feelings towards them. They did screw up big time with the 737 MAX, but they still have a huge portfolio of well engineered products and will surely launch more well respected products in the future.

[LittlePeter]

In my opinion, a big screw up at least warrants that you lose respect, at least temporarily.

They can regain respect by building new good products. Or by admitting mistakes, surely not by laying blame on the pilots.

I am less forgiving. If you want to forgive them, that's your choice.

[dingaling]

> they still have a huge portfolio of well engineered products

Mostly obtained by acquisitions.

In current production only the 737, 747, 777 and 787 and T-7 originate solely from Boeing.

[kube-system]

This tool would be capable of performing that measurement with 0.001" accuracy. https://www.harborfreight.com/feeler-gauge-32-pc-63665.html

Someone performing maintenance in their garage on an internal combustion engine would encounter the need to measure the same tolerances, for example, adjusting valve clearances. This is a common and accessible measurement task.

[cartoonfoxes]

Strictly speaking, it wouldn't. One of the interesting things the aviation industry deals with is a need for traceability of the metrology all the way back to NIST standards. That extra test, certification, and paperwork has a price. Compare the harbor freight example with this one: https://forneyonline.com/feeler-gauge-set-26-leaves-0015-to-...

[aunty_helen]

I'm not sure where you got this idea from, sounds like a lot of work.

It would be much easier just to calibrate the tool before putting it into service. And given that there is a routine calibration schedule for measuring equipment, there's no need to have some NIST paperwork certifying the metallurgy of the brass rivet holding your feeler gauge together.

The two tooling engineers I sat next to when I worked at a jet engine MRO would buy any tool they wanted from their parts catalogue. I can't remember the name of it now but it was yellow covered and about 6 inches thick.

They would buy the same tools you could get at any respectable hardware store. Snapon, Koken, TengTools were common. They bought good quality so it lasted, not so they could get NIST certificates with them.

In the engine manuals, for specific jobs, there would be specific tools you would need to use. These would be listed by part number. If you used any old torque wrench when the manual specified a particular part, you weren't in compliance. This could be picked up by an aviation authority when your paperwork was audited. Or of course, if you have an 'escape' where an engine gets put on a plane and then stops mid flight (second worst case scenario).

The same goes for the calibration of the tool, each time you torque a procedure you need to fill out the paperwork with the tool # that the engineers will have engraved on the side of it. If the records show that tool is overdue for it's calibration when it was used, best case scenario, engineer doing the procedure gets told off, tooling guys get a 'finding' and have to improve their system of tool tracking. Worst case scenario, people die. Which could result in the Quality manager getting thrown in the clanger.

[kube-system]

I just meant it's 'capable' in context of the above question of engineering capabilities. But you're very right that aviation requires expensive safety and compliance procedures on top of that.

[ryandrake]

Yea, they are both capable of doing the same job, but one comes with a $104.01 piece of paper that proves that a clerk in an office stamped another piece of paper and filed it away in a cabinet, which makes it legal to use with a certified airplane.

[SAI_Peregrinus]

You can take just about any measuring tool to a calibration company and get the certificate. Indeed, you MUST do this on a regular basis: calibration is a set of measurements of a system over time. Single-point-in-time calibration is worthless for showing drift. You can also get things tested at a range of temperature and humidity levels, high-grade lab standards may get calibrated at multiple temperature & humidity levels every 6 months.

[myself248]

4 thou isn't a particularly high degree of precision.

For about five bucks you can get feeler gauges more accurate than that, and for about thirty you can get a micrometer that's accurate to 0.0005" over a whole inch. (I checked mine against a set of grade 2 gauge blocks at their calibrated temperature.)

Normal CNC machines, not designed for extreme precision but the sort of thing an exceptionally serious hobbyist or decent makerspace might have just sitting around, will happily hold 0.001" across the entire working envelope, which may be the volume of a couch cushion or larger. The one I'm familiar with (which dominates one corner of the local makerspace) has thermal sensors scattered about the large castings that make up the machine frame, so it can compensate in software for the estimated warpage of the frame depending on how the HVAC has been blowing on it. And that's mid-90s tech, things have only gotten better since then.

[formerly_proven]

There's a "high tech" (i.e. they mostly don't use cutting tools) machining shop nearby; their proprietary machines (designed & built in-house, never sold or leased externally, only used for contract manufacturing) are repeatable within a few µm (they offer tolerancing below 5 µm on finished parts) and move about as quick as the rapid traverse on a more standard CNC milling machine.

[SAI_Peregrinus]

Nanotech Systems (Moore's current name) make a high-precision drum lathe with 1nm positioning precision, with a 2600mm working area [1].

1 part in 2,600,000,000.

Dan Gelbart built his own lathe with <1µm final part tolerance [2].

Within a few thousandths of an inch isn't very impressive. I can hold 1 thou on a cheap manual lathe (Grizzly G0602 10"x22" working area), with a decent amount of care. Holding to tenths (1/10000") is impossible with the equipment I can afford though.

[1] https://nanotechsys.com/wp-content/uploads/2019/12/HDL2600-B... [2] https://www.youtube.com/watch?v=sFrVdoOhu1Q

[formerly_proven]

That's resolution, not precision or repeatability, although such high resolution is very impressive. I'm not sure how exactly a glass scale is supposed to work to 34 pm, I assume these use analog interpolation between marks or perhaps some kind of interference mechanism.

[wjdoge]

EDM?

[formerly_proven]

Laser

[aunty_helen]

The most accurate measuring device at the jet engine MRO I worked at was a CMM. It had an accuracy of 2 micron, 0.00008"

It was a robotic arm that lived in a temperature controlled room. The parts it was to measure were left inside the room to acclimatise. When they were at the correct temperature, you would put a part on the largest granite block in the southern hemisphere that was so flat they used lasers to calibrate it. Then run the program for the part. The arm would move into position and then a little probe would touch the part to get a measurement. It was used to measure things like warpage of inner diameters etc.

Funny trivial, if you ran the wrong program the arm would end up colliding with the part causing the probe to break. There was always spare probes kept because this happened. $20k each.

[jcrawfordor]

That's actually larger than the valve clearance specified for many automotive engines, which is commonly checked and adjusted by mechanics using feeler gauges or gap gauges. It's fairly straightforward.

[ethagknight]

Is that just being able to slide a piece of thick paper into the gap?

[tesseract]

Or a "feeler gauge" meaning basically a 0.004in thick piece of spring steel.

[formerly_proven]

I know someone who used to be in the business of installing high precision machining equipment. I relay the following anecdote: When he came on site once, the workers who put the machine down on its bedding told him, he didn't need to level it, since they already levelled it "to a thou" (Indian workers). He laughed and said: "Good, someone already did the coarse work for me!".

[juskrey]

0.1 mm is a pocket calipers grade tool. Of course they have better ones but nothing extraordinary I think

[rightbyte]

Micrometers are quite common tool in even hobbyist workshops.

[juskrey]

In fact every techie has one, sometimes passed from parents to children

[dt3ft]

Techie here. I have one.

[ashtonkem]

While machining down to thousands of an inch is a bit tricky, actually measuring that small isn't that hard. Vernier calipers are cheap (<$20), capable of measuring down to 0.001 inches, and typically have internal jaws for measuring the gap between two surfaces.

Typically the issue isn't the ability to measure to such tight tolerances, the problem is physically getting the calipers into position to do the measurement. Airplanes are full of tight spaces with weird angles, which makes measuring parts without extensive disassembly kind of hard.

[jacquesm]

Thousandths of an inch is not quite as precise as 100ths of a mm though they are at the same order of magnitude and top machinists using temperature corrections and all kinds of trickery can do better than that. Reproducibly.

Measuring with such high degrees of precision is easier than manufacturing by a very large margin. There are calibrated standards that fit in your pocket quite easily. Even ye olde feeler gauge was pretty precise if you were careful with it.

[inamberclad]

One of the first things I learned in my college shop class was how to mill a piece of metal to within a few thou of the target dimension.

For this kind of measurement, they could almost certainly just use a feeler gauge and make sure that the 4 thou gauge fits.

[McDev]

I supposed they'd be using something like this: https://en.m.wikipedia.org/wiki/Micrometre

[rileyteige]

you mean https://en.m.wikipedia.org/wiki/Micrometer ?

[dehrmann]

I'm sure car engines have similar tolerances.