FAA issues emergency directive on 2,000 Boeing 737 NG, Classic planes

https://www.reuters.com/article/us-boeing-737ng/faa-issues-emergency-directive-on-2000-boeing-737-ng-classic-planes-idUSKCN24P1KS

by sjcsjc • 6 years ago

216 162 6 years ago

reuters.com

benrrio• 6 years ago

The official PDF document is here: https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf...

I always wondered what the proper terms was for an airplane crash, and now I know: "forced off-airport landing"

rileyteige2• 6 years ago

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".

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.

Sebb767• 6 years ago

I think it would be rather hard to land a plane off the airport without substantial damage, especially when talking about larger ones.

leeoniya• 6 years ago

i also like crashing into a mountain:

https://en.m.wikipedia.org/wiki/Controlled_flight_into_terra...

and in space flight: "off nominal"

elil17• 6 years ago

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).

FabHK3• 6 years ago

And you know the checklist for forced off-airport landings at night?

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• 6 years ago

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.

frakt0x90• 6 years ago

You still have to keep the aircraft's normal maintenance schedule even when it's in storage.

KMag2• 6 years ago

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.

tjohns• 6 years ago

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.

dehrmann2• 6 years ago

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.

maverick2007• 6 years ago

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.

FabHK3• 6 years ago

I wonder what effect will dominate:

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?

tyingq• 6 years ago

This only applies to aircraft coming out of storage, so it's not going to create a disruption.

wwv25• 6 years 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• 6 years ago

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

rileyteige2• 6 years ago

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

ethagknight• 6 years ago

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

tesseract• 6 years ago

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

juskrey2• 6 years ago

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

rightbyte• 6 years ago

Micrometers are quite common tool in even hobbyist workshops.

adenozine5• 6 years ago

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.

CaptainZapp• 6 years ago

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.

LittlePeter3• 6 years ago

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

throwaway0a5e2• 6 years ago

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.

kube-system4• 6 years ago

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

nogabebop232• 6 years ago

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.

rlpb• 6 years ago

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.

linuxftw2• 6 years ago

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

numpad0• 6 years ago

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• 6 years ago

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• 6 years ago

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?

olyjohn• 6 years ago

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

kube-system4• 6 years ago

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.

cartoonfoxes4• 6 years ago

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-...

jcrawfordor• 6 years ago

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.

dehrmann2• 6 years ago

I'm sure car engines have similar tolerances.

myself2482• 6 years ago

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_proven4• 6 years ago

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.

aunty_helen5• 6 years ago

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.

ashtonkem• 6 years ago

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.

formerly_proven4• 6 years ago

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!".

esaym2• 6 years 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

Animats2• 6 years ago

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...

perl4ever• 6 years ago

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.

jacquesm2• 6 years ago

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• 6 years ago

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.

raverbashing• 6 years ago

That is surprising. Pandemic or no pandemic, you have NGs and classics going and coming out of storage with a certain frequency.

Funny this wasn't found before.

jschwartzi• 6 years ago

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.

kenny11• 6 years ago

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.

JanSolo3• 6 years ago

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.

CydeWeys2• 6 years ago

This is true of all airplanes, not just the 737. They're so expensive that they all need to be in use continuously.

JanSolo3• 6 years ago

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.

matt-attack• 6 years ago

And this may be obvious to some, but why does use inhibit rust?

t0mas88• 6 years ago

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.

JanSolo3• 6 years ago

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.

bfuclusion• 6 years ago

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.

cameron_b• 6 years ago

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.

MrBuddyCasino• 6 years ago

This may be a dumb question, but why use materials that can corrode on such an important part? Just use stainless steel (or AL or TI), problem solved?

enraged_camel• 6 years ago

Explain that to the MBAs and bean-counters who are in charge over at Boeing.

nemetroid• 6 years ago

The 737 Classic was introduced in 1984.

perardi• 6 years ago

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…

benhurmarcel2• 6 years ago

This particular valve sees temperatures above 600°C. You basically need to use steel or inconel.

AceyMan• 6 years ago

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.

bluGill2• 6 years ago

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.

jefftk• 6 years ago

The title here should have "737 NG, Classic" not "737 NG Classic". As in "Boeing 737 NG and Classic aircraft", not "Boeing 737 NG Classic aircraft".

abrowne• 6 years ago

Or even more clearly, "Boeing 737 NG and 737 Classic", since it wasn't obvious to me that they're all 737 variants.

Contax• 6 years ago

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?

mmanfrin• 6 years ago

Most english speakers also wouldn't be familiar with the term, but it is called 'Headlinese'.

https://en.wikipedia.org/wiki/Headline#Headlinese

IncRnd• 6 years ago

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.

dingaling3• 6 years ago

It confused me too when I first started working in the USA where it is common in headlines.

Using an ampersand requires only one more space than a comma but is unambiguous.

dang• 6 years ago

Ok, have a comma.

jacquesm2• 6 years ago

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.

meristem• 6 years ago

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.

WalterBright• 6 years ago

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.

Especially if you leave it out in the weather.

deeblering4• 6 years ago

At the beginning of lockdowns I was wondering what might break down on the fleets of planes that have been grounded, or under utilized.

I wonder what other bugs will be uncovered from not exercising machines that are designed to be used on a very regular basis.

FAA issues emergency directive on 2,000 Boeing 737 NG, Classic planes

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