6 ms·

>344 single points of failure How can a system have more that one single point of failure?

by ducktective 5y ago

WalterBright 5y ago
Airliners are designed so that no single failure can bring it down.
- funklute 5y ago
  That's not entirely true. Any reasonably complex system will almost certainly have single points of failure, including airliners. Those single points of failures might be very unlikely, but they are still there. E.g. what if the front fell off?
  - WalterBright 5y ago
    It is entirely true. Source: I worked on 757 flight controls design for 3 years. > what if the front fell off? All structural components are redundant and have redundant attachments. The wing spars are doubled, for example.
    - deleted 5y ago
      [deleted]
    - histriosum 5y ago
      There are certainly single points of failure in airliner design. An example that springs to mind is the stabilizer trim jackscrew. There are supporting systems which try to mitigate the risk of failure or the likelihood of failure, but bottom line, if the jackscrew fails, there is no redundancy. You've lost stab trim, potentially making the situation unrecoverable. Of course Alaska 261 is one such example of a jackscrew failure.
      - WalterBright 5y ago
        I worked for 3 years on the design the 757 stab trim jackscrew. We considered every failure, and had redundancy for it. The AL flight cracked its nut, and the balls fell out. I don't know the details of that design, but on the 757 the ice scrapers on both sides of the nut will hold the nut in place if the balls fall out. If I recall correctly, if the AL pilot had simply left the trim system alone after it failed, and landed the airplane, it would have been fine. Instead, he kept fiddling with it, driving it up and down, until it tore the nut to pieces. The loud screech it made when running in a half broken configuration should have chilled anyone's blood to bone at 30,000 feet. As that incident, and the later MAX crashes clearly demonstrated, once something goes wrong with the stab trim system, you move it only enough to get the airplane into a flyable state. Then, you turn it off and leave it the frack alone. Land it, and let the mechanics figure it out with it safely in the hangar. One incident where the stab trim failed and the poor pilots were just passengers to their doom was that 747 that took off from the base, and some tank in the cargo hold broke loose, slid back, smashed through the bulkhead and broke off the whole stab trim system. That really wasn't the stab trim system's fault, you can't really design for a tank falling on it. It was the fault of no redundancy in the straps holding the tank in place. The video of the crash, shown endlessly on the news, makes my gut turn to water.
        histriosum 5y ago
        I fully agree that the risk tradeoffs made on these designs are highly unlikely to fail. The point is, though, that if they DO fail there IS NO redundancy. There aren't two jackscrew mechanisms. It's a single point of failure. Highly reliable single points of failure don't become redundant because of their high reliability -- they are still single points of failure. And sometimes those must be accepted -- even in safety critical systems.
- jibalt 5y ago
  Tell that to the passengers on the two 737 MAX planes that crashed.
binarymax 5y ago
The sequential deployment process had many possible individual points of failure along the way - if any step went wrong then the full deployment failed.
- ducktective 5y ago
  well yes, but wouldn't we refer to them simply by "points of failure" not "single"?
  - deleted 5y ago
    [deleted]
  - jbay808 5y ago
    I think it's single in the sense that only a single failure in any of those points would render the entire project a failure, not that there's only one point that can possibly fail. Contrast, say, a single-engine jet plane with a twin-engine jet plane that can still make it to the airport safely with the remaining engine, should one engine fail mid-flight.
  - furyofantares 5y ago
    "single point of failure" is a phrase that refers to any subsystem which the whole system is dependent on.
  - bryanlarsen 5y ago
    Most failure points on Webb have redundancies. So their failure doesn't brick the telescope, both the part and its redundancy have to fail before Webb is bricked. The single points brick Webb with only a single failure.
  - jacquesm 5y ago
    Because a point of failure could have multiple redundancies, and in this case they did not, every one of those points was implemented in a non-redundant way.
  - jibalt 5y ago
ISL 5y ago
If you're in a car being held above the ground by a chain, every single link is a potential single-point of failure.
- jacquesm 5y ago
  What a fantastic comment. In a visual and immediate way to explain such a complex concept in one sentence.
- systemvoltage 5y ago
  But the chain itself can be considered as one component and it becomes a single point of failure. Or if we go in the opposite direction, then we can breakdown links and say "Every single grain boundary is a single point of failure".
  - ISL 5y ago
    Partially-agreed on the former, but not the latter. So long as the chain is not loaded near its tensile limit, then the grain boundaries that support the load within each link do so in parallel and are therefore redundant. The selection of the allegory of the chain was intentional -- each link must be properly formed, or the entire chain will fail. If it breaks, it is surely correct to say, "the chain broke", but in truth, it was actually link-86. For JWST, the remarkable/audacious thing is that many links in the chain from launch to observation are potential single-point failures. Furthermore, many of them haven't ever been tested independently in space... ever. It is a hell of a triumph that JWST has gotten this far already. If even a small fraction of the instrumentation works at this point, we are going to learn a ton about the universe, simply due to JWST's position, collecting-area, and mirror-diameter.
    - systemvoltage 5y ago
      You're right. I think I was searching for a better analogy. Abstractly: Sub-system SPOF conditions can be bundled up as a single System SPOF condition by multiplying the probabilities. Also, I don't want to underplay JWST's success or its challenges. But, when saying 300+ SPOF conditions, one has to specify at what abstraction level. Otherwise, it can be misleading.
deleted 5y ago
[deleted]
mabbo 5y ago
I think linguistically the term can be hard to parse. If any one of 344 pieces were to fail during deployment, then all of the deployment has failed and the entire $10B was a loss. Consider the engine in your car- how many single pieces of it could fail before the entire engine can't work? The difference with Webb is that most of those points were single actions that had to work once. And we're now at a stage where most of them did not fail.
- jibalt 5y ago
  "A single point of failure (SPOF) is a part of a system that, if it fails, will stop the entire system from working." >I think linguistically the term can be hard to parse. Yeah, "solitary" might make more sense ... or "non-redundant". Or "point of single failure".
lordnacho 5y ago
I think SPF means a construction where there's no redundancy provided? Though it does raise the question of how this is counted: table with 4 legs, any of them breaks, table is still standing. Table with three legs, any of them breaks, table is broken.
_Microft 5y ago
Single point of failure is a term for non-redundant, critical parts of a system: https://en.m.wikipedia.org/wiki/Single_point_of_failure https://en.m.wikipedia.org/wiki/Single_point_of_failure
burtonator 5y ago
Are these 'points of failure' ? not single points of failure? I assume some of these have redundancy but could fail.
BurningFrog 5y ago
In your body, the brain, heart and lungs are all single points of failure. If either one fails, you die.
- stolsvik 5y ago
  You have two lungs, though. One can fail and you will still live. As with kidneys. Strange that there is only one heart.
  - digitalsushi 5y ago
    It's strange but we're still pretty bad at using more than a single computer core on average.
  - jibalt 5y ago
    Lungs and kidneys can work independently, but two pumps running at the same time on the same circuit will cause more failures ... it's like parallel vs. sequential circuits.
stjohnswarts 5y ago
There are 344 individual items that can make the $10 billion satellite be a paper weight in orbit. Those are the very definition of single points of failure.
jibalt 5y ago
BY the phrase not meaning what you think it means. https://en.wikipedia.org/wiki/Single_point_of_failure https://en.wikipedia.org/wiki/Single_point_of_failure "A single point of failure (SPOF) is a part of a system that, if it fails, will stop the entire system from working."

>344 single points of failure How can a system have more that one single point of failure?

WalterBright 5y ago

Airliners are designed so that no single failure can bring it down.

funklute 5y ago
That's not entirely true. Any reasonably complex system will almost certainly have single points of failure, including airliners. Those single points of failures might be very unlikely, but they are still there. E.g. what if the front fell off?
- WalterBright 5y ago
  It is entirely true. Source: I worked on 757 flight controls design for 3 years. > what if the front fell off? All structural components are redundant and have redundant attachments. The wing spars are doubled, for example.
  - deleted 5y ago
    [deleted]
  - histriosum 5y ago
    There are certainly single points of failure in airliner design. An example that springs to mind is the stabilizer trim jackscrew. There are supporting systems which try to mitigate the risk of failure or the likelihood of failure, but bottom line, if the jackscrew fails, there is no redundancy. You've lost stab trim, potentially making the situation unrecoverable. Of course Alaska 261 is one such example of a jackscrew failure.
    - WalterBright 5y ago
      I worked for 3 years on the design the 757 stab trim jackscrew. We considered every failure, and had redundancy for it. The AL flight cracked its nut, and the balls fell out. I don't know the details of that design, but on the 757 the ice scrapers on both sides of the nut will hold the nut in place if the balls fall out. If I recall correctly, if the AL pilot had simply left the trim system alone after it failed, and landed the airplane, it would have been fine. Instead, he kept fiddling with it, driving it up and down, until it tore the nut to pieces. The loud screech it made when running in a half broken configuration should have chilled anyone's blood to bone at 30,000 feet. As that incident, and the later MAX crashes clearly demonstrated, once something goes wrong with the stab trim system, you move it only enough to get the airplane into a flyable state. Then, you turn it off and leave it the frack alone. Land it, and let the mechanics figure it out with it safely in the hangar. One incident where the stab trim failed and the poor pilots were just passengers to their doom was that 747 that took off from the base, and some tank in the cargo hold broke loose, slid back, smashed through the bulkhead and broke off the whole stab trim system. That really wasn't the stab trim system's fault, you can't really design for a tank falling on it. It was the fault of no redundancy in the straps holding the tank in place. The video of the crash, shown endlessly on the news, makes my gut turn to water.
      - histriosum 5y ago
        I fully agree that the risk tradeoffs made on these designs are highly unlikely to fail. The point is, though, that if they DO fail there IS NO redundancy. There aren't two jackscrew mechanisms. It's a single point of failure. Highly reliable single points of failure don't become redundant because of their high reliability -- they are still single points of failure. And sometimes those must be accepted -- even in safety critical systems.
jibalt 5y ago
Tell that to the passengers on the two 737 MAX planes that crashed.

binarymax 5y ago

The sequential deployment process had many possible individual points of failure along the way - if any step went wrong then the full deployment failed.

ducktective 5y ago
well yes, but wouldn't we refer to them simply by "points of failure" not "single"?
- deleted 5y ago
  [deleted]
- jbay808 5y ago
  I think it's single in the sense that only a single failure in any of those points would render the entire project a failure, not that there's only one point that can possibly fail. Contrast, say, a single-engine jet plane with a twin-engine jet plane that can still make it to the airport safely with the remaining engine, should one engine fail mid-flight.
- furyofantares 5y ago
  "single point of failure" is a phrase that refers to any subsystem which the whole system is dependent on.
- bryanlarsen 5y ago
  Most failure points on Webb have redundancies. So their failure doesn't brick the telescope, both the part and its redundancy have to fail before Webb is bricked. The single points brick Webb with only a single failure.
- jacquesm 5y ago
  Because a point of failure could have multiple redundancies, and in this case they did not, every one of those points was implemented in a non-redundant way.
- jibalt 5y ago

ISL 5y ago

If you're in a car being held above the ground by a chain, every single link is a potential single-point of failure.

jacquesm 5y ago
What a fantastic comment. In a visual and immediate way to explain such a complex concept in one sentence.
systemvoltage 5y ago
But the chain itself can be considered as one component and it becomes a single point of failure. Or if we go in the opposite direction, then we can breakdown links and say "Every single grain boundary is a single point of failure".
- ISL 5y ago
  Partially-agreed on the former, but not the latter. So long as the chain is not loaded near its tensile limit, then the grain boundaries that support the load within each link do so in parallel and are therefore redundant. The selection of the allegory of the chain was intentional -- each link must be properly formed, or the entire chain will fail. If it breaks, it is surely correct to say, "the chain broke", but in truth, it was actually link-86. For JWST, the remarkable/audacious thing is that many links in the chain from launch to observation are potential single-point failures. Furthermore, many of them haven't ever been tested independently in space... ever. It is a hell of a triumph that JWST has gotten this far already. If even a small fraction of the instrumentation works at this point, we are going to learn a ton about the universe, simply due to JWST's position, collecting-area, and mirror-diameter.
  - systemvoltage 5y ago
    You're right. I think I was searching for a better analogy. Abstractly: Sub-system SPOF conditions can be bundled up as a single System SPOF condition by multiplying the probabilities. Also, I don't want to underplay JWST's success or its challenges. But, when saying 300+ SPOF conditions, one has to specify at what abstraction level. Otherwise, it can be misleading.

deleted 5y ago

[deleted]

mabbo 5y ago

I think linguistically the term can be hard to parse. If any one of 344 pieces were to fail during deployment, then all of the deployment has failed and the entire $10B was a loss. Consider the engine in your car- how many single pieces of it could fail before the entire engine can't work? The difference with Webb is that most of those points were single actions that had to work once. And we're now at a stage where most of them did not fail.

jibalt 5y ago
"A single point of failure (SPOF) is a part of a system that, if it fails, will stop the entire system from working." >I think linguistically the term can be hard to parse. Yeah, "solitary" might make more sense ... or "non-redundant". Or "point of single failure".

lordnacho 5y ago

I think SPF means a construction where there's no redundancy provided? Though it does raise the question of how this is counted: table with 4 legs, any of them breaks, table is still standing. Table with three legs, any of them breaks, table is broken.

_Microft 5y ago

Single point of failure is a term for non-redundant, critical parts of a system: https://en.m.wikipedia.org/wiki/Single_point_of_failure https://en.m.wikipedia.org/wiki/Single_point_of_failure

burtonator 5y ago

Are these 'points of failure' ? not single points of failure? I assume some of these have redundancy but could fail.

BurningFrog 5y ago

In your body, the brain, heart and lungs are all single points of failure. If either one fails, you die.

stolsvik 5y ago
You have two lungs, though. One can fail and you will still live. As with kidneys. Strange that there is only one heart.
- digitalsushi 5y ago
  It's strange but we're still pretty bad at using more than a single computer core on average.
- jibalt 5y ago
  Lungs and kidneys can work independently, but two pumps running at the same time on the same circuit will cause more failures ... it's like parallel vs. sequential circuits.

stjohnswarts 5y ago

There are 344 individual items that can make the $10 billion satellite be a paper weight in orbit. Those are the very definition of single points of failure.

jibalt 5y ago

BY the phrase not meaning what you think it means. https://en.wikipedia.org/wiki/Single_point_of_failure https://en.wikipedia.org/wiki/Single_point_of_failure "A single point of failure (SPOF) is a part of a system that, if it fails, will stop the entire system from working."