
FAA issues emergency directive on 2,000 Boeing 737 NG, Classic planes - sjcsjc
https://www.reuters.com/article/us-boeing-737ng/faa-issues-emergency-directive-on-2000-boeing-737-ng-classic-planes-idUSKCN24P1KS
======
wwv25
One of the inspection checklist items:

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

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

~~~
adenozine
They've had calipers for literally thousands of years.

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

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

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

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

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

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

[edit] remove a word

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

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

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

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

------
benrrio
The official PDF document is here:
[https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf...](https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/ec0e5875b315e9e5862585ae007cd448/$FILE/2020-16-51_Emergency.pdf)

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

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

~~~
0xffff2
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.

~~~
FDSGSG
> I rather doubt there are any examples of an off airport landing of a 737
> without significant damage to the aircraft.

[https://en.wikipedia.org/wiki/TACA_Flight_110](https://en.wikipedia.org/wiki/TACA_Flight_110)

~~~
0xffff2
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." :)

~~~
elliottkember
The famous Gimli Glider wasn't badly damaged either, although it was a 767!

------
leetrout
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.

~~~
dehrmann
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
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.

------
JanSolo
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.

~~~
matt-attack
And this may be obvious to some, but why does use inhibit rust?

~~~
t0mas88
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.

~~~
aunty_helen
According to the AD this is the 5th stage bleed air valve, so on the
compressor side and cold air.

>Bleed air is tapped at the 5th and 9th stages of the HP compressor and from
the fan.

[https://hursts.org.uk/airbus-
technical/html/ar01s19.html#idm...](https://hursts.org.uk/airbus-
technical/html/ar01s19.html#idm45838557976224)

(Airbus doc but the engines are CFM56 on the 737 models listed) Also according
to this page the CFM56 doesn't have a turbine bleed air takeoff.

~~~
benhurmarcel
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.

~~~
aunty_helen
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.

------
cameron_b
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.

------
jacquesm
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.

------
raverbashing
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
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
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.

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

------
deeblering4
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.

------
WalterBright
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.

------
meristem
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.

------
MrBuddyCasino
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?

~~~
perardi
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…

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

------
jefftk
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".

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

[https://en.wikipedia.org/wiki/Headline#Headlinese](https://en.wikipedia.org/wiki/Headline#Headlinese)

~~~
myself248
And it's dumb. We're not setting hot lead into column inches anymore.

~~~
azernik
But we are trying to fit summaries into a limited screen real estate

