Boeing discloses a new problem with the 787 Dreamliner
edition.cnn.com
edition.cnn.com
As far as I can tell, the problem is the gap between the bulkhead and the fuselage is larger than spec. This means that it'll take more airflow to pressurize the cabin.
There are always going to be leaks. The idea is that if all the joins are at the adverse end of the tolerance, meaning more air leaks, there must be enough pressurization air to keep it pressurized. But this is rarely the case, so one part being larger than spec should not cause a problem.
Parts get delivered out of spec sometimes, and they get bounced to engineering to evaluate if the design as a whole will still work.
I.e. it shouldn't actually be a problem unless the other joins are near the limits of their tolerances.
So, what's missing from CNN's reporting is why they are demanding a rework. The local newspaper, which should have a better handle on the situation, doesn't do any better at reporting this.
Suppose you've got a gearbox, with a shaft in it, and a stack of parts on the shaft. The shaft with its parts must fit in the gearbox. The gearbox has a max and min size, and each part has a max and min size. The engineer divides up the tolerances so that if they are all at max size, and the gearbox housing is at min size, the parts will fit.
Now, if a gear comes back from the manufacturer larger than its max size, but enough of the other parts are not near their max size, the stack will still fit and it's ok. (But engineering still has to approve the fit.)
This is not in any way a crisis. It simply costs extra money because the out-of-spec part gets bounced to engineering for review.
Yes, I did these exact calculations on the 757 stab trim gearbox.
I had always assumed that (as a compiler guru) you had a CS background. :)
Here is Walter's wikipedia: https://en.m.wikipedia.org/wiki/Walter_Bright
He went to Caltech, studied Aerospace Engineering, wrote many books, has given lots of presentations
For my own notoriety, I am a large clam that has slaved away at data for 10 years in Financial Services. I'm about to quit my job for a TOTALLY AWESOME job somewhere else
Because time and money are not infinite and +/-0.005 is a hell of a lot cheaper than +/-0.0005.
This is an assembly of stamped and sheared aluminum and composites. Machine shop level accuracy gets real expensive real quick with those construction methods and this isn't intentional defense industry pork so the cost does need to be kept under control.
For a rough part, you can measure how much material you need to take off, do it in a few passes without stopping to remeasure, and you're within 5 thousandths of an inch or so.
For a shaft that is going to slide or rotate in a bore without too much force, you need to be within a thousandth or two, so for the last pass you will stop, measure carefully, and only take a little bit of material off at a time so that the cutting forces are low and there is less deflection in the tool to throw off the size of the part.
For a press fit (two parts are sized precisely enough that they can be pressed together with a hydraulic press and then never come apart), you need tolerances in the tenths of a thousandth of an inch. For this, you'll dust off your special expensive micrometer, and it's important to let the part cool before measuring, because the heat from machining can cause the part to expand and cause you to take off too much material, which would ruin the part.
So even for the same person on the same machine with the same material, the effort (and therefore cost, if you're doing this commercially) can vary quite a bit based on the tolerance needed.
I don't know aerospace, but I imagine the required tolerances require vary from "looks good from here" (seats in the cabin) to "must ride on a film of oil this many microns thick when spinning at 10,000 rpm." And the manufacturing processes can be anything from "intern with a saw" to "specialized metrology lab with most expensive machines in the world" depending on what engineering specifies.
If the outer diameter of a gear tooth is too large, it can bind with the root of the matching gear, and this can cause rapid wear, and an unplanned disassembly at some point very early in the service life.
Last year's issue related to joints around the rear pressure bulkhead. This article indicates the new issue is a similar problem related to the forward pressure bulkhead.
As this seems to be something that's given quite a bit of thought, do you have an idea on what the maximum diameter hole through the fuselage to safely maintain sufficient cabin pressure during cruise flight at, say, PL390? More specifically, what's the maximum rate of pressurization that the cabin pressurization system is designed to support?
But I do know that there can be a hole the size of a quarter and nobody will notice a pressure drop. You're not going to knock an airliner down by punching holes in the fuselage, either (some people think a bullet hole will cause it to pop like a balloon, nothing of the sort will happen).
The phrase "Boeing said 787 planes already in service do not need to be grounded" kind-of implies that there are aircraft with this defect in service, but it need not necessarily do so.
That would would explain why the FAA remains involved on an issue it does not consider to be a safety concern, especially as it implies there may be even more quality control issues than previously realized.
Are these problems specific to the problem-plagued 787-10 or 787s in general (all models of which have had issues)?
Is this specific to models built in South Carolina? I have read some Asian airlines are specifying Washington construction only.
Any business reporter (not specific to aviation) who has covered Boeing in the last few years would know that these two questions would explain if this is bad or terrible for Boeing.
Though personally I liked Saginaw Gear the best, as they did the forgings for the stab trim jackscrew, and made the best of the best.
Generally, a forged part is going to be free of the voids and surface defects of a casting, and waste less material than cutting from an rolled or extruded piece of stock.
"GM in 1999 spun off its parts group as Delphi Automotive including Saginaw Steering. Saginaw Division was renamed Delphi Steering"
"With Delphi entering a lengthy bankruptcy proceeding, GM purchased Delphi Steering through their subsidiary, GM Global Steering Holdings LLC, in 2009 and renamed the company, Nexteer Automotive"
"Nexteer was acquired by Pacific Century Motors on November 29, 2010"
"Originally, Pacific Century was a subsidiary of E-town, the Beijing municipal government's investment company, and Tempo Group, a Chinese car component manufacturer."
"In March 2011, AVIC Automobile Industry Holding Co., a Chinese state-owned parts manufacturer, purchased a 51 percent interest in Pacific Century Motors."
When I amped up the horsepower of my Dodge, I replaced the cast bell housing with a forged steel one, because I like my feet. (I've seen pictures of the flywheel coming through the side of a car.) All the spinning parts in the engine were replaced with forged ones, too.