Profits 1st, Safety 2nd? Pt 2

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Message 2033078 - Posted: 19 Feb 2020, 21:59:32 UTC - in response to Message 2033044.  

Looms are certainly one of the biggest problems on aircraft (and indeed on just about every modern transportation device).
Not only do they suffer from flexing, but they suffer from thermal issues, not related to overloading, but just the simple fact the cores are imperfect conductors, thus they have resistance, Ohms Law kicks in and there is heating due to simple cable losses. Bundle a hand full of cables together isn't an issue, but have a bundle of a hundred or more and those in the middle can get very warm, warm enough to soften the insulation, but not enough to melt it or set it on fire.
Add to that the fact that aircraft like the B737 operates over a very wide temperature range, from say +40C (or more) sitting on the ground in the tropics, to a few minutes later the air frame cooling to -40C (or lower), and you can get a dimensional change in the air frame of about 0.2% - plenty to stress a tight cable, or move a slack one over a chaffing edge, twice on every flight.
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Message 2033080 - Posted: 19 Feb 2020, 22:05:24 UTC - in response to Message 2033072.  

And just about every other civil airliner lacks the armour plating you are talking about.
The plating one sees on high-wing turbo-prop aircraft is not there to stop bits of engines entering the fuselage, but to stop ice flying off the propellers from damaging the fuselage.
They rely on segregation the engines are below the wing, the passengers are above it, and there is the wing in the way. Also the requirement that bits of the engine are retained within the engine casing, or ejected out of the back (and thus normally rapidly away from the aircraft).
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Message 2033084 - Posted: 19 Feb 2020, 22:33:31 UTC - in response to Message 2033042.  

Not all the regulations are limit requirements, some, like those for wiring, are very tight range requirements, stray from those bounds and you don't get certified.
It is totally beyond me why these regulations still date back, in part, to the 1930s and 1940s!!!! Its not that long ago that it was an absolute mandate to use a particular type of "waxed" cotton (or was it linen?) thread to lace all looms, in one continuous run from start to finish, no breaks permitted! Imagine what a pain that was if a modification or repair that involved adding a couple of wires and changing the destinations of another couple, when the loom concerned had been fed into the wing before the fuel tanks were installed.
And why do the required regulations not move with design date of a new sub-type (B737-200 to B737-300 etc)? Because nobody thought that an aircraft type would ever double in size, and it was never foreseen that we would see the proliferation of sub-types in the manner that we've seen in the last 20 or so years, so they are effectively locked for a type to the initial certification of that type - stupid or what.

There was a group set up a few years ago to investigate and advise on this issue, but it got mired in the detail, lost its way, re-found its path and did make some recommendations, but the FAA decided it was too difficult for them the write the framework regulations and it went no further. There is now pressure growing to go back to those reports and get the recommendations implemented, one group of which are to do with how to manage the requirements for sub-types that come along years after the first certified design. We've seen late sub-types with the B737 - How many years since first certification is it? And how many sub-types have we seen in that half century? And how many major technology changes have there been? But they still have to use the same basic set of wiring regulations as were in place back the late 1960s.

But the FAA has a means of getting round the regulation torpor - Airworthiness Directives can be used to over-rule the regulations, but they rarely use this route (as it is seen by some as cheating?). I believe this was used to mandate the use of alternative lacing materials and changes to the insulation materials to become Low Smoke Zero Halogen - I understand this followed the disastrous fire at Manchester a few years back.
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Message 2033091 - Posted: 19 Feb 2020, 23:03:13 UTC - in response to Message 2033080.  
Last modified: 19 Feb 2020, 23:05:51 UTC

And just about every other civil airliner lacks the armour plating you are talking about.
The plating one sees on high-wing turbo-prop aircraft is not there to stop bits of engines entering the fuselage, but to stop ice flying off the propellers from damaging the fuselage.
They rely on segregation the engines are below the wing, the passengers are above it, and there is the wing in the way. Also the requirement that bits of the engine are retained within the engine casing, or ejected out of the back (and thus normally rapidly away from the aircraft).

Thanks for some very good comment.

Yes, there is the ejected ice physical protection that I've seen on prop/turbo-prop aircraft.

However, I was thinking more of for example:

Southwest Airlines Flight 1380: an Uncontained Engine Failure?
wrote:
... The fuselage is protected by a Kevlar band wrapped around it. It's wrapped in the same position aft on the plane as the fan blades. That’s why there’s no windows there [row 11], according to John Baker, PE, in EngTips, our online engineers forum.

In light of damage occurring from debris blown back from the engine in flight, it would seem the Kevlar bands, in line with the fan blades would only offer protection while the aircraft is on the ground, which is not when the blades are spinning their fastest and would be encountering their highest forces...


... except in the case for the Boeing 737 flight control cables, my thoughts would be to have a kevlar band wrapped around a suitably strong protective pipe that immediately surrounds the cables for the vulnerable parts of their routing.

However, the experience from the Flight-1380 failure suggests that there can be a broad sweep of a debris cone to protect from...


Must make special note to, as far as possible, sit ahead of the engines or way back at the tail!

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Martin
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Message 2033094 - Posted: 19 Feb 2020, 23:10:27 UTC - in response to Message 2033060.  
Last modified: 19 Feb 2020, 23:15:54 UTC

... Defending against a fan blade into the fuselage...

That scenario is designed for.

See:
Uncontained Engine Failure
wrote:
Defences

Aircraft/Engine design features to mitigate against the risks associated with uncontained engine failure include:

  • Fan blade containment ring - to provide a measure of protection in the event of fan blade separation,
  • Aircraft systems "Routing Segregation" - covering both rotor burst and rapid depressurisation scenarios,
  • Fuel tank "Dry Bays" located in the most likely disk trajectories.




Again, how so not so for the Boeing 737 for the "Routing Segregation"?...

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Message 2033095 - Posted: 19 Feb 2020, 23:13:26 UTC - in response to Message 2033084.  
Last modified: 19 Feb 2020, 23:14:49 UTC

Thanks for that. Incredible!!!

Hopefully the increased funding that Trump has post-haste awarded the FAA can fund an update of those old outdated regs!

(And also undo the cheapo-work-arounds contrived for the multiple 'models' cheap certifications!!)


Which comes back to the question for how they were allowed to so badly stagnate and fall behind...

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Message 2033096 - Posted: 19 Feb 2020, 23:14:52 UTC - in response to Message 2033078.  

Bundle a hand full of cables together isn't an issue, but have a bundle of a hundred or more and those in the middle can get very warm, warm enough to soften the insulation, but not enough to melt it or set it on fire.

There are insulation materials for wires which are not subject to thermal softening, but I doubt the FAA has certified them for use on Part 25 aircraft. Aslo bundles of power cables in electric conduit are very much subject to heating and there are long ago made regulations on the required size of the conductors and the size of the conduit and the number of conductors to prevent the issue. Also a thermal concern in an aircraft is pressure altitude and convection. In unpressurized areas convection cooling will be greatly reduced.

However that isn't the end of the thermal issues, the strands of a copper wire itself will become brittle when subjected to repeated hot cold cycles. If enough break then the resistance shoots up there and it becomes a run away condition. Many a house fire starts this way in an overloaded extension cord.

This all very well known electric and mechanical engineering issues and is not unique to aircraft.
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Message 2033128 - Posted: 20 Feb 2020, 8:28:56 UTC

Also a thermal concern in an aircraft is pressure altitude and convection. In unpressurized areas convection cooling will be greatly reduced.

Its not so much the cabling as the air frame that changes size. But then the cabling suffers localised cooling at every point it passes through part of the structure, even if it is protected with thermal insulation and anti-chafe material. Many years ago we had an issue on a plant due to thermal cycling of the conduit carrying a number of cables, which were pulling out of connectors at one end - a lot of monitoring later it was found that the cables were walking along the conduit (they were not loomed as it hadn't been though necessary as they were in a conduit). The solution was to make sure that 50% of the cables were laid in one direction and 50% the other way round, plus a very loose loom was created that could be anchored to the ends and hold the whole bundle in one place within the conduit.
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Message 2033129 - Posted: 20 Feb 2020, 8:35:09 UTC - in response to Message 2033095.  

Well one would hope that the extra funding will release the blockage on the regulations that has existed for many years (probably more years the age of the B737)
But will it change the culture of reluctance to change in the face of developing technology, that's the real question.

I really feel for the inspectors who see something that is very out-dated and potentially not safe and know that there is a solution that would improve safety because it is certified and has worked very well on newer aircraft, but can't be applied to the one being looked at because it isn't certified for it because the regulation set under which the one being inspected won't allow it. Galling or what.
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Message 2033172 - Posted: 20 Feb 2020, 14:48:41 UTC - in response to Message 2033128.  

Also a thermal concern in an aircraft is pressure altitude and convection. In unpressurized areas convection cooling will be greatly reduced.
Its not so much the cabling as the air frame that changes size.
Wasn't thinking of the physical length of the cable as much as the current carrying capacity of the cable. In free space at sea level a 14AWG can carry X amps. As you reduce pressure altitude the cable no longer has convective cooling and can only radiate heat, so it becomes X minus Z amps. I'm sure the NASA guys have a table for use in a vacuum.
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Message 2033174 - Posted: 20 Feb 2020, 15:16:13 UTC

Another move to cut costs & increase profits?
Well, it failed...for now
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Message 2033177 - Posted: 20 Feb 2020, 15:46:29 UTC - in response to Message 2033174.  

Another move to cut costs & increase profits?
Well, it failed...for now

Nasty unhealthy stuff...

There is already that type of abusive nastiness in such as call centres that 'facilitates' psychopathic bonus chasing line managers to burn out their operatives in just a few weeks, ready for HR to draft in the next batch of slaves, and to repeat the cycle. Abusively easy in the 'commoditized' unskilled world...

All a game of greedy modern day slavery? There should be laws against that...

That ain't any way that I work...


All in our overly greedy world,
Martin
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Message 2033178 - Posted: 20 Feb 2020, 15:50:52 UTC - in response to Message 2033172.  
Last modified: 20 Feb 2020, 15:51:03 UTC

Wasn't thinking of the physical length of the cable as much as the current carrying capacity of the cable. In free space at sea level a 14AWG can carry X amps. As you reduce pressure altitude the cable no longer has convective cooling and can only radiate heat, so it becomes X minus Z amps. I'm sure the NASA guys have a table for use in a vacuum.

Hopefully...

The aviation world work to much better/wider safety margins than to need to start relying on convective cooling for their electrical cables?!...

But then, to get back on topic to the later models of the Boeing 737, how far have the original design margins been eroded due to the ever increasing loads of the successive 'upgrades' additions?...


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Message 2033179 - Posted: 20 Feb 2020, 15:55:39 UTC - in response to Message 2032974.  
Last modified: 20 Feb 2020, 16:02:16 UTC

How many more problems are there to find?
737 Max: Debris found in planes' fuel tanks
Boeing's crisis-hit 737 Max jetliner faces a new potential safety issue as debris has been found in the fuel tanks of several of the planes.
The head of Boeing's 737 programme has told employees that the discovery was "absolutely unacceptable".

"Prof Simon" gives a brief very direct succinct demonstration of that:

YouTube: F.O.D. - Prof Simon
wrote:
Prof Simon does a test. Not on a Boeing aircraft.



Graphically fatal stuff.

Some of the comments make for interesting reading as to how that might have happened... Especially so if the 'debris' includes parts that should be elsewhere?...


All in our deadly overly greedy world,
Martin


F.O.D.: Foreign object damage/debris
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Message 2033193 - Posted: 20 Feb 2020, 17:37:39 UTC

Someone I worked with years ago spent some time working as a final cleaner on the Harrier assembly line. His job was to crawl through as much of the aircraft as possible once all the assembly work had been done and remove all the left-over bits using his hands, then a vacuum cleaner, he said there always bins full ranging from small bits of swarf to fairly large tools and fittings. After that was done the aircraft was placed on what might be described as a spit and rotated slowly, at the dead of night (when it was very quiet) and then more bits were found... Then the fuel system was flushed through from the engine back to the tanks with empty filters in place, and more stuff was found. Even after this the first few flights always produced some more bit. The Harrier was a much smaller aircraft than a B737, so it doesn't surprise me that there is stuff lurking in the dark corners. The question is How does the amount found compare with say an Airbus a220? Also, Has the amount increased recently (in the months preceding the halt on production)?
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Message 2033207 - Posted: 20 Feb 2020, 19:53:38 UTC - in response to Message 2033178.  

The aviation world work to much better/wider safety margins than to need to start relying on convective cooling for their electrical cables?!...
Convective cooling happens, so why not exploit it?
You do understand the three methods of heat (energy) transfer? Conduction, Convection and Radiation don't you? You do understand that the connectors on the end of the wire is where the heat would have to conduct out. Well guess what. Those connection points are high resistance points and that is a heat source, not a heat sink. So now you have radiation, since your wire is in a loom is isn't really in free space so it doesn't radiate heat very well until it gets the whole loom red hot. So what's left? Convection. Convection can more the greatest amount of heat of all the methods. As we are talking pressure, the issue is there is less air at altitude so convection is reduced in effectiveness. To keep the wire cool the only answer is to use a bigger and heavier conductor, more weight less useful load.
Oh, in orbit, convection isn't available, even inside at pressure. If you can figure out why perhaps you will understand a bit of mechanical engineering.
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Message 2033214 - Posted: 20 Feb 2020, 21:07:39 UTC - in response to Message 2033207.  
Last modified: 20 Feb 2020, 21:19:12 UTC

The aviation world work to much better/wider safety margins than to need to start relying on convective cooling for their electrical cables?!...
Convective cooling happens, so why not exploit it?...

Dangerously not reliable for safety critical systems unless very thoroughly designed into the design from the outset (which is not usually the case for small-scale electrical cable bundles!).

Not to be confused with (reliably controlled) forced air cooling...

And still, the Boeing 737 electrical cable bundles can be seen to be dangerous for a catastrophic failure for which the pilots have no recovery possible. The fix is physically easy. Why not fix it to be safe?



All a good question of good design with good safety margins.

Is that the case for our deadly greedy world?
Martin


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Message 2033216 - Posted: 20 Feb 2020, 21:25:57 UTC - in response to Message 2033193.  
Last modified: 20 Feb 2020, 21:26:44 UTC

... Even after this the first few flights always produced some more [FOD] bit[s]. ... a much smaller aircraft than a B737, so it doesn't surprise me that there is stuff lurking in the dark corners. The question is How does the amount found compare with say an Airbus a220? Also, Has the amount increased recently (in the months preceding the halt on production)?

A very big question is whether such FOD is looked for in early inspections during the first few flights?...

An even bigger question is indeed what were the effects of Boeing sacking/redeploying the large number of their QA staff that checked the 737 production??...


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Message 2033217 - Posted: 20 Feb 2020, 21:30:27 UTC

The fix is not "simple" or "trivial" - it is a TOTAL re-wire of just about every system on the aircraft, a task that would take MONTHS on each plane as the aircraft would have to be stripped of just about every fixture and fitting to get to some of the wiring - think about it. Also there may not be the physical routes available without structural changes. Then there's all the re-certification of the design before the re-wiring can start, a pile of testing to make sure that no new faults have been introduced.

The big thing is that the regulators have to get to grips with keeping the regulations up with technology, and not lagging half a century behind it as we have seen with aircraft wiring.
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Message 2033220 - Posted: 20 Feb 2020, 21:40:14 UTC - in response to Message 2033217.  
Last modified: 20 Feb 2020, 21:44:54 UTC

The fix is not "simple" or "trivial" - it is a TOTAL re-wire of just about every system on the aircraft...

My understanding is that the wiring of catastrophic concern is one (16 foot?) section for the (two?) control wires to the tail jackscrew actuator.

Hence my personal uneducated suggestion of rerouting just that section for those (two?) control wires with new wiring for just the jackscrew control. Leave the existing bundle as-is to minimize the risk of disturbance. Positively test the whole lot afterwards...

(All the rest of the wiring should be reviewed for any catastrophic dangers...)


The big thing is that the regulators have to get to grips with keeping the regulations up with technology, and not lagging half a century behind it as we have seen with aircraft wiring.

Agreed for that one!

My own personal wish would be for the new regulations to be applied in real time to all current aircraft production (during production) as we all progressively learn more about preventing whatever failures...


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Message boards : Politics : Profits 1st, Safety 2nd? Pt 2


 
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