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?
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.
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”
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.
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?
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.
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.
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.
>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.
> 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" :)
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.
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.
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.
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.
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).
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."
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
That the tolerances are so precise is exactly why you can do that on most modern rifles. It depends on the design - specifically, how they achieve headspace. On an AR-15, or almost any firearm with a similar bolt locking arrangement, bolts - indeed, entire BCGs - are swappable in practice, and people do that routinely without bothering with gauges regardless of what the manual says. On AK and similar designs, yeah, that's a really bad idea.
> 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
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.
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... ;-)
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.
>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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
> 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.
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.
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.
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.
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-...
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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!".
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.
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.
Not necessarily. A "forced off-airport landing" could also be the result of an engine failure beyond gliding distance of an airport. Or engine fire, medical emergency in remote area, etc. There are many emergencies that could force an aircraft to the ground, off-field.
Not all off-field landings result in an accident, or "crash".
>Aircraft accident means an occurrence associated with the operation of an aircraft which takes place between the time any person boards the aircraft with the intention of flight and all such persons have disembarked, and in which any person suffers death or serious injury, or in which the aircraft receives substantial damage. For purposes of this part, the definition of “aircraft accident” includes “unmanned aircraft accident,” as defined herein.
So while some off-field landings could be classified as an accident, that would only be the case if the off-field landing met the above definition.
For small 4-6 seater general aviation aircraft it's hard (relative to a normal landing), but not at all unheard of. A modern 4 lane highway is more than wide enough, and very probably has a long enough straight section within range, to make for a relatively uneventful landing if you can get traffic to cooperate.
You're absolutely correct though when it comes to larger aircraft. I rather doubt there are any examples of an off airport landing of a 737 without significant damage to the aircraft.
Well, I stand corrected. That's both surprising and an incredible feat of airmanship by the pilots!
Since I love reading about these kinds of incidents and the best way to solve a problem is to claim it's impossible, I amend my claim to "I rather doubt there are two examples of an off airport landing of a 737 without significant damage to the aircraft." :)
https://en.wikipedia.org/wiki/TACA_Flight_110 made a dead-stick landing on a grass levee with minor damage - it was later flown out (following an engine change which caused the landing in the first place)
A deadstick landing,is a type of forced landing when an aircraft loses all of its propulsive power and is forced to land. The "stick" does not refer to the flight controls, but to the traditional wooden propeller, which without power would just be a "dead stick".
Indeed, but planes are quite repairable. I don't have a source for this (too lazy to look), but I briefly worked for boeing on a government contract to service C-17 aircraft for the air force. There was a story there of a C-17 landing on a dirt/sand field somewhere in the middle east (this is normal there) except in this case there was a small concrete barrier out in the middle of the field that wasn't seen. The front nose gear impacted the barrier and it torn the nose gear off along with a large chunk of the underside of the fuselage. As the front of the plane touched the ground, the hole scooped up sand and blasted a ton of it into the cargo bay. A crew was flown out there and they "fixed" it and it was flown back to the states.
Another popular instance was the Lockheed EP-3 flying the China coast in 2001 before impacting a Chinese fighter in the air and making a forced "rough" landing. It was taken apart and shipped back to the states where it was repaired in Waco Texas and continues to patrol the China coast to this day.
The Qantas A380 from Singapore to Sidney that had an uncontained engine failure was repaired and entered service again, at an estimated cost of about $150m, or about 1/3 of the price of a new one.
A forced off-airport landing is not a necessarily crash. It’s an action taken, usually because of engine failure or imminent engine failure, to mitigate risk to the occupants of the craft. The pilot acts with intention and glides or flys the craft to the ground. They slow the craft down enough to touch down safely. The incident could still be considered an accident if there’s damage to the structure of the plane or injury to passengers.
In contrast, a crash could be a lot of things: two planes colliding on the tarmac, loss of wings, or a controlled flight into terrain (such as flying into the side of a mountain).
I have been wondering how dangerous flying is going to be when when / if it comes back and all these planes come out of storage. Mechanical things just sitting around is some times far worse than them being in constant use.
Luckily, this is already pretty standard practice for airplanes.
I worry a little more about theme parks that don't do winter shutdowns or might not have done the full shut it down for the winter procedure with sudden lockdown orders. The regulatory environment is also sketchier.
Luckily for theme parks, the pandemic hit right as winter maintenance was wrapping up for seasonal parks so it shouldn't have many adverse effects if any. And since then, maintenance workers have been on site running and getting the rides ready to reopen. Actually, it seems like this might be a good things for parks since the extra time has given them a lot more time to do more long term projects and refurbishment they never had time for in the past.
There are manufacturer approved procedures for putting aircraft into long-term storage, as well as returning them back to service.
Yes, there is absolutely additional wear that happens just from sitting around. Repairing this (if necessary) is part of the return-to-service procedure.
Though, if the maintenance schedule items are mostly of the form "W needs to be inspected after X flight hours, Y landings, or Z days since last inspection", there's probably significantly less experience backing up the Z parameter.
Interesting that the vast majority of 737s were flying so frequently that this was not found earlier.
For their entire 20-ish year service life, those planes are never left idle long enough for this part to rust! Amazing.
Goes to show how popular and successful the 737 line is.
Yeah I'm sure most commercial airliners are the same. Apart from maybe the Super-Heavies; I hear that there are dozens (hundreds?) of 747s and A380s sitting around doing nothing currently because they're too expensive to fly.
Those 747s and A380s are only sitting around not flying because of the pandemic. And it seems likely that many of them will never return to flight again at this point, except maybe after being modified for cargo hauling.
Prior to the pandemic, if you had a flight-worthy plane of any size, you were using it continuously. Once a plane was no longer being used continuously it was likely about to be permanently retired.
From my cursory glance at the doc, the part in question is inside the bleed-air cutoff system. This is a flapper valve (a bit like in your toilet) that takes air from the engine and uses it for other systems in the plane (air conditioning, cooling, starting other engines, etc).
When it's in use, the movement of the parts will prevent rust from building up in one spot. A little rust might accumulate, but it will be rubbed/wiped off when it's next used. The problem occurs when enough rust builds up that the flapper cannot move anymore.
In normal use these valves take air out of the turbine stage, so they see very very hot air with almost zero moisture in it in normal use. If you park them long enough I can imagine they get wet from rain or even just condensation.
When you say "cold air" I guess you mean that it's bled before the combustion stage. It's not cold at all after the compression.
And bleed air is indeed always taken in the compressor, never in the turbine. First there's no point taking it later than the compressor, you just want compressed air. And it's ultimately used to ventilate the cabin, you don't want fuel vapors or smell.
Stage 5 would be pretty early on. However generally when talking about the HPC you're talking about cold air. At this stage, cold air that's sprayed on the turbine housing when engine power is reduced to thermally shrink the metal to keep the turbine blades and casing within tolerance for efficiency's sake.
All is explained in the link I posted, would recommend it as good introductory reading on the subject.
Not specific to this part, but most mechanical items that move get oiled as part of their operation. When they don't move for a long time that oil evaporates, and now the moisture in the air can attack the surfaces. This is a common situation in general aviation if they're not flown regularly and can lead to early replacement of the engine.
This, coupled with the idea that big planes simply aren't flying enough to keep the pilots certified has me concerned about commercial aviation. The scale-down and scale-up is incredibly costly and will result in huge write-offs of equipment that cannot be cost-effectively kept up. You can't just park these airframes and expect them to be ready when you need them.
Anything mechanical that you don't use is on the way out, airplanes, cars it doesn't matter. Use it or lose it very much applies and all it takes is to not use something for a while to find completely new failure modes. Case in point, and very timely: just today I had to replace the valve that allows for the pressurization of the fuel tank on my car simply because it wasn't used for three months. It had seized and no amount of brake fluid or other solvents would get rid of the gunk enough to unstuck the valve so it could be worked.
I think it wasn’t an issue before because most airlines aren’t going to keep a plane they have in storage for multiple days. They’ll sell it to another airline. Otherwise they still have to maintain it and pay storage fees plus whatever lease or finance costs are associated with the plane. Boeing runs a finance corporation which is how a lot of planes are “sold.” They have to plan for maintenance windows and unexpected failures but when your fleet is big enough these events are regular enough that you can tune your fleet size to absorb these events without needing idle capacity.
This is also why planes out of use for extended periods are typically stored in dry, desert climates - less moisture in the air to cause corrosion.
The massive increase in stored aircraft due to the pandemic meant that there wasn't enough room at these airports, and so aircraft had to be stored anywhere there was space available, often in less than ideal climates.
Your car will also start exhibiting random failures if you leave it parked long enough. Parts will corrode, stick, develop flat spots, cold weld themselves together, water gets in, bugs and mice set up shop, chew on the wires, etc.
Aircraft are meant to be in high usage, so decreased flight time and larger flight intervals will cause interesting problems. For example many part 135 planes currently parked need to be rolled to a different position daily because parked weight deforms tyres.
I’m guessing some other parameter was more important. Thermal expansion characteristics, ease of tooling, interactions with the materials around it, resistance to some other non-oxidative chemical corrosion…
bearings and bushings need to be hard to resist wear and maintain their dimensions. Stainless steels hard enough for the job are too brittle and those other metals (Al, Ti) aren't even close to the hardness of any category of steel.
Stainless doesn't corrode, but it tends to corrode any other metal it touches. Depends on alloys and a bunch of other factors, but all stainless isn't the answer either.
And why not just "NG and Classic"? Really curious, I see this replacement of "and" for a comma in a lot on headlines, and I don't know why or where it comes from (English is not my first language). Is there a name for this?
In English the comma would be used between non-restrictive modifiers. However, this is a designation, and the industry or manufacturer denotes how the designation is to be structured. This is further compounded by being in a headline. Editors, and headlines in particular, use abbreviated syntax.
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?