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.
>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.
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.
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.
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.
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.
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.
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.
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.
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.
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?
[+] [-] wwv25|5 years ago|reply
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|5 years ago|reply
[+] [-] esaym|5 years ago|reply
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
[+] [-] adenozine|5 years ago|reply
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.
[+] [-] kube-system|5 years ago|reply
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.
[+] [-] myself248|5 years ago|reply
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.
[+] [-] jcrawfordor|5 years ago|reply
[+] [-] ethagknight|5 years ago|reply
[+] [-] formerly_proven|5 years ago|reply
[+] [-] juskrey|5 years ago|reply
[+] [-] ashtonkem|5 years ago|reply
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|5 years ago|reply
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|5 years ago|reply
For this kind of measurement, they could almost certainly just use a feeler gauge and make sure that the 4 thou gauge fits.
[+] [-] unknown|5 years ago|reply
[deleted]
[+] [-] McDev|5 years ago|reply
[+] [-] dehrmann|5 years ago|reply
[+] [-] benrrio|5 years ago|reply
I always wondered what the proper terms was for an airplane crash, and now I know: "forced off-airport landing"
[+] [-] rileyteige|5 years ago|reply
Not all off-field landings result in an accident, or "crash".
From our own regulations, NTSB 830 "NOTIFICATION AND REPORTING OF AIRCRAFT ACCIDENTS..." https://www.law.cornell.edu/cfr/text/49/830.2
>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.
[+] [-] elil17|5 years ago|reply
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).
[+] [-] leeoniya|5 years ago|reply
https://en.m.wikipedia.org/wiki/Controlled_flight_into_terra...
and in space flight: "off nominal"
[+] [-] FabHK|5 years ago|reply
1. Airspeed - SET
2. Fuel Valve - OFF
3. At 100 ft: Landing Lights - ON
4. If you don't like what you see: Landing Lights - OFF
[+] [-] leetrout|5 years ago|reply
[+] [-] dehrmann|5 years ago|reply
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.
[+] [-] tjohns|5 years ago|reply
Yes, there is absolutely additional wear that happens just from sitting around. Repairing this (if necessary) is part of the return-to-service procedure.
[+] [-] unknown|5 years ago|reply
[deleted]
[+] [-] frakt0x90|5 years ago|reply
[+] [-] FabHK|5 years ago|reply
1. BAD: The degradation of airplanes in storage (seems that's fairly well understood, though)
2. BAD: The loss of proficiency and currency by the crews
3. GOOD (maybe?): Less traffic?
[+] [-] JanSolo|5 years ago|reply
[+] [-] CydeWeys|5 years ago|reply
[+] [-] matt-attack|5 years ago|reply
[+] [-] cameron_b|5 years ago|reply
[+] [-] jacquesm|5 years ago|reply
[+] [-] raverbashing|5 years ago|reply
Funny this wasn't found before.
[+] [-] jschwartzi|5 years ago|reply
[+] [-] tyingq|5 years ago|reply
[+] [-] deeblering4|5 years ago|reply
I wonder what other bugs will be uncovered from not exercising machines that are designed to be used on a very regular basis.
[+] [-] WalterBright|5 years ago|reply
Especially if you leave it out in the weather.
[+] [-] meristem|5 years ago|reply
[+] [-] MrBuddyCasino|5 years ago|reply
[+] [-] perardi|5 years ago|reply
[+] [-] AceyMan|5 years ago|reply
[+] [-] bluGill|5 years ago|reply
[+] [-] enraged_camel|5 years ago|reply
[+] [-] jefftk|5 years ago|reply
[+] [-] dang|5 years ago|reply
[+] [-] abrowne|5 years ago|reply
[+] [-] Contax|5 years ago|reply
[+] [-] 23f2gg23|5 years ago|reply
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