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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
by wwv25 6y ago
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?
- McDev 6y agoI supposed they'd be using something like this: https://en.m.wikipedia.org/wiki/Micrometre https://en.m.wikipedia.org/wiki/Micrometre
- rileyteige 6y agoyou mean https://en.m.wikipedia.org/wiki/Micrometer https://en.m.wikipedia.org/wiki/Micrometer ?
- ethagknight 6y agoIs that just being able to slide a piece of thick paper into the gap?
- tesseract 6y agoOr a "feeler gauge" meaning basically a 0.004in thick piece of spring steel.
- deleted 6y ago[deleted]
- juskrey 6y ago0.1 mm is a pocket calipers grade tool. Of course they have better ones but nothing extraordinary I think
- adenozine 6y agoThey'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.
- CaptainZapp 6y agoBoeing 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 6y agoEh 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).
- rlpb 6y agoIf 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 https://en.wikipedia.org/wiki/Feeler_gauge. The set pictured there looks like it goes down to 0.05mm.
- linuxftw 6y agoThe only person here that knows what they're talking about. Everyone's talking about calipers :/
- bamboozled 6y agoI 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 6y agoIf 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.
- jrockway 6y agoMost 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 6y agoI 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.
- 6y ago
- kube-system 6y agoThis tool would be capable of performing that measurement with 0.001" accuracy. https://www.harborfreight.com/feeler-gauge-32-pc-63665.html 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 6y agoStrictly 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-025-w-nist-cert https://forneyonline.com/feeler-gauge-set-26-leaves-0015-to-...
- kube-system 6y agoI 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 6y agoYea, 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 6y agoYou 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.
- jcrawfordor 6y agoThat'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.
- dehrmann 6y agoI'm sure car engines have similar tolerances.
- myself248 6y ago4 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 6y agoThere'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.
- wjdoge 6y agoEDM?
- formerly_proven 6y agoLaser
- SAI_Peregrinus 6y agoNanotech 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-Brochure-Rev.0818.pdf https://nanotechsys.com/wp-content/uploads/2019/12/HDL2600-B... [2] https://www.youtube.com/watch?v=sFrVdoOhu1Q https://www.youtube.com/watch?v=sFrVdoOhu1Q
- ashtonkem 6y agoWhile 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.
- formerly_proven 6y agoI 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!".
- esaym 6y ago>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 https://duckduckgo.com/?q=hilok&ia=web
- Animats 6y agoAdjustments 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_Oct41.pdf http://aetherltd.com/public/model15manuals/138_Model15_Adj_O...
- BinaryRage 6y agoI 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-_93BVApb5-9eQLTCk9xx16RAGEYHH1q https://www.youtube.com/watch?v=_NuvwndwYSY&list=PL-_93BVApb...
- Animats 6y agoI 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.
- jacquesm 6y agoThousandths 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 6y agoOne 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.