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> What they call 100% for this test is actually 150% of what the plane is expected to experience in a worst-case real-world situation. [..] The difference betwe

> What they call 100% for this test is actually 150% of what the plane is expected to experience in a worst-case real-world situation. [..] The difference between failing at 99% and failing at 101% So, what is it 100% of? Not that I even care enough about Boeing clickbait to skimread the article, but this discussion makes it seem like "percent" is a really poor fit for whatever this is actually measuring and trying to communicate.

deleted 7y ago
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bencoder 7y ago
100% of its expected actual failure point, based on its design. The plane is rated to 2/3rds of that (I think this is pretty standard in aviation) - the other "100%" - meaning it should never go above that in flight, and if it does it would require a very costly teardown/inspection.
mlyle 7y ago
So, first, airliners aren't really expected to experience more than 1.3G's of loading in normal or even unusual operation. They are expected to be able to survive 2.5G's (limit load) without structural damage. The 777X satisfied this. They are expected to be able to survive a few seconds of 1.5x this-- 3.75G-- without outright failure but possibly with (irreparable) structural damage. Instead, it catastrophically failed at a loading corresponding to 3.70G's. They are going to reinforce and fix the specific failure that happened at 3.7G's, and make an argument based on strain gauges and instrumentation that another, distant failure was not likely before 3.75G's.
- inferiorhuman 7y ago
  So, first, airliners aren't really expected to experience more than 1.3G's of loading in normal or even unusual operation. This is 100% true. But the unexpected happens. China Airlines 006 saw 5G due to pilot error. Everyone survived and the plane (Boeing 747-SP) saw another 25 years of service. Additionally planes age. What's your failure point after 10-15 years of service? Another part that's worrisome is the original angle. Originally the failure was reported as a cargo door blowing out. The pictures released show otherwise. So the question remains: who got reporters to go with a cargo door failure and why? They are going to reinforce and fix the specific failure that happened at 3.7G's, and make an argument based on strain gauges and instrumentation that another, distant failure was not likely before 3.75G's. That depends on how much you trust Boeing's modeling and assembly skills. IMO the pickle forks on the NG cast at least a little bit of shade on Boeing's reputation.
  - benmarks 7y ago
    > Additionally planes age. What's your failure point after 10-15 years of service? In practice there is/are none, because would-be points of failure are continuously replaced at a schedule well ahead of the number of cycles at which they are known or expected to fail. This is true for aircraft of all sizes, not just passenger jets.
    - inferiorhuman 7y ago
      In practice there is/are none, because would-be points of failure are continuously replaced at a schedule well ahead of the number of cycles at which they are known or expected to fail. This is true for aircraft of all sizes, not just passenger jets. Sure, but what does this mean for the 777X? With the NG, even if you reduce the inspection interval to a few thousand cycles you're only looking spending an hour on an inspection. How often will you have to inspect the skin (and/or whatever else failed or saw damage) on the 777X? How involved will the inspection be? The thing I can't quite get over is that something led Dominic Gates to run with "the door blew out" as the story. Why? If a torn fuselage skin on a destructive test is so minor, why go with misdirection? Think of it in terms of MCAS. Conceptually relying on multiple AoA inputs is simple, MAX specific training is (relatively) simple. Yet the MAX is still grounded and we're seeing leaks that indicate a suitable fix is dramatically more complex than Boeing has been suggesting.
      - jacquesm 7y ago
        > How often will you have to inspect the skin (and/or whatever else failed or saw damage) on the 777X? How involved will the inspection be? After every lightning strike, 1.5 strikes / aircraft / year on average.
  - mhandley 7y ago
    With regards to the cargo door story - a door did blow out, but it was a passenger door, not a cargo door. Passenger doors are plug-fit (they're bigger than the hole, and pressure holds them in place), so it's not really possible for a passenger door to blow out. Cargo doors are not plug fit, so a latching failure could cause a door to blow out. Everyone in the industry remembers the DC10 cargo door failures. I suspect the early rumours said a door had blown out, and as that's not really a possible failure mode for a passenger door, it was assumed it must have been a cargo door. Now, in this case a passenger door did blow out, but it wasn't a door failure. The fuselage some distance below a door failed, and split right up past the door. The partially unsupported door then blew out. But the door was peripheral to the story. Boeing could definitely had handled this much better, but it's not really surprising that in the absence of full facts the rumour mill got this one wrong.
danaliv 7y ago
100% of ultimate load, which is 150% of limit load. The limit load is what goes into the flight manuals and is what the aircraft is certified to withstand. The ultimate load is a safety factor on top of limit load and is what the engineers design for. The writer has some of the numbers mixed up. Transport category positive load limits are 2.5 to 3.8 (depending on max takeoff weight), not 1.3. I don’t know where the 1.3 number came from, but it might be from the loads required to be demonstrated during flight tests (1.3g pull-up at 1.3x the stalling speed in the landing configuration IIRC).

deleted 7y ago

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bencoder 7y ago

100% of its expected actual failure point, based on its design. The plane is rated to 2/3rds of that (I think this is pretty standard in aviation) - the other "100%" - meaning it should never go above that in flight, and if it does it would require a very costly teardown/inspection.

mlyle 7y ago

So, first, airliners aren't really expected to experience more than 1.3G's of loading in normal or even unusual operation. They are expected to be able to survive 2.5G's (limit load) without structural damage. The 777X satisfied this. They are expected to be able to survive a few seconds of 1.5x this-- 3.75G-- without outright failure but possibly with (irreparable) structural damage. Instead, it catastrophically failed at a loading corresponding to 3.70G's. They are going to reinforce and fix the specific failure that happened at 3.7G's, and make an argument based on strain gauges and instrumentation that another, distant failure was not likely before 3.75G's.

inferiorhuman 7y ago
So, first, airliners aren't really expected to experience more than 1.3G's of loading in normal or even unusual operation. This is 100% true. But the unexpected happens. China Airlines 006 saw 5G due to pilot error. Everyone survived and the plane (Boeing 747-SP) saw another 25 years of service. Additionally planes age. What's your failure point after 10-15 years of service? Another part that's worrisome is the original angle. Originally the failure was reported as a cargo door blowing out. The pictures released show otherwise. So the question remains: who got reporters to go with a cargo door failure and why? They are going to reinforce and fix the specific failure that happened at 3.7G's, and make an argument based on strain gauges and instrumentation that another, distant failure was not likely before 3.75G's. That depends on how much you trust Boeing's modeling and assembly skills. IMO the pickle forks on the NG cast at least a little bit of shade on Boeing's reputation.
- benmarks 7y ago
  > Additionally planes age. What's your failure point after 10-15 years of service? In practice there is/are none, because would-be points of failure are continuously replaced at a schedule well ahead of the number of cycles at which they are known or expected to fail. This is true for aircraft of all sizes, not just passenger jets.
  - inferiorhuman 7y ago
    In practice there is/are none, because would-be points of failure are continuously replaced at a schedule well ahead of the number of cycles at which they are known or expected to fail. This is true for aircraft of all sizes, not just passenger jets. Sure, but what does this mean for the 777X? With the NG, even if you reduce the inspection interval to a few thousand cycles you're only looking spending an hour on an inspection. How often will you have to inspect the skin (and/or whatever else failed or saw damage) on the 777X? How involved will the inspection be? The thing I can't quite get over is that something led Dominic Gates to run with "the door blew out" as the story. Why? If a torn fuselage skin on a destructive test is so minor, why go with misdirection? Think of it in terms of MCAS. Conceptually relying on multiple AoA inputs is simple, MAX specific training is (relatively) simple. Yet the MAX is still grounded and we're seeing leaks that indicate a suitable fix is dramatically more complex than Boeing has been suggesting.
    - jacquesm 7y ago
      > How often will you have to inspect the skin (and/or whatever else failed or saw damage) on the 777X? How involved will the inspection be? After every lightning strike, 1.5 strikes / aircraft / year on average.
- mhandley 7y ago
  With regards to the cargo door story - a door did blow out, but it was a passenger door, not a cargo door. Passenger doors are plug-fit (they're bigger than the hole, and pressure holds them in place), so it's not really possible for a passenger door to blow out. Cargo doors are not plug fit, so a latching failure could cause a door to blow out. Everyone in the industry remembers the DC10 cargo door failures. I suspect the early rumours said a door had blown out, and as that's not really a possible failure mode for a passenger door, it was assumed it must have been a cargo door. Now, in this case a passenger door did blow out, but it wasn't a door failure. The fuselage some distance below a door failed, and split right up past the door. The partially unsupported door then blew out. But the door was peripheral to the story. Boeing could definitely had handled this much better, but it's not really surprising that in the absence of full facts the rumour mill got this one wrong.

danaliv 7y ago

100% of ultimate load, which is 150% of limit load. The limit load is what goes into the flight manuals and is what the aircraft is certified to withstand. The ultimate load is a safety factor on top of limit load and is what the engineers design for. The writer has some of the numbers mixed up. Transport category positive load limits are 2.5 to 3.8 (depending on max takeoff weight), not 1.3. I don’t know where the 1.3 number came from, but it might be from the loads required to be demonstrated during flight tests (1.3g pull-up at 1.3x the stalling speed in the landing configuration IIRC).