5 ms·

I was wondering the same. What is the logic behind this setup? That basically means that it is no better than having one sensor. Because if any one of them brea

I was wondering the same. What is the logic behind this setup? That basically means that it is no better than having one sensor. Because if any one of them breaks, you have broken sensor active on next or following flight. Also I recall there was something said about Lion Air crash that the crew on the day before experienced some flight control problems which was not communicated to the crew involved in crash? Did they use other sensor on the day before and that was faulty too, or isn't that is how it actually worked?

Scarblac 8y ago
It's worse than having one sensor, the chance that one of two breaks is larger than that just a single one breaks. And by alternating you ensure that the broken sensor will be used.
- phkahler 8y ago
  Was talking to my flight instructor about twin engine planes. He was adamant that they were LESS safe than single engines because 1) you are twice as likely to have an engine failure and 2) the statistics indicate (non-intuitively) that an engine-out in a twin is more likely to lead to fatalities. He said the insurance companies used to think twins were safer but they have adjusted their rates after looking at actual data. I was fully on-board with point #1, but #2 was a surprise to me. I suppose that's what Burt Rutan tried to address with the Boomerang design.
  - ebg13 8y ago
    To your second point, my assumption as a non-pilot is that a symmetrically balanced glide is more controllable than lopsided thrust that could put you into a spin?
    - mcguire 8y ago
      Loss of one engine at low speed will leave a twin uncontrollable. See Vmc (https://en.wikipedia.org/wiki/V_speeds?wprov=sfla1 https://en.wikipedia.org/wiki/V_speeds?wprov=sfla1).
      - cheerlessbog 8y ago
        If you promptly shut off the other engine, does it become equivalent to single engine failure?
        VBprogrammer 8y ago
        Not really. Twin engine aircraft don't have to meet the same crash worthiness standards and usually have higher stall speeds (in this case minimum crash speeds). To be fair the statistics are swayed slightly by a number of factors, training accidents and different risk profiles of the flights.
        mcguire 7y ago
        In general, a twin will weigh considerably more than a single as well as having greater drag, making for shorter glides.
  - cjbprime 8y ago
    Yeah, there's a pilot proverb about how having a second engine "helps you get to the crash site faster".
    - FabHK 8y ago
      Or "the plane is certified such that the remaining engine has enough power to take you to the crash site". Indeed. As outlined above, you double the probability that a mechanical problem occurs. (For p small, 1-(1-p)^2 is around 2p). Now, if the pilots are very much on top of things (well trained and current), then they can control things and safely land at the nearest suitable airport. However, if they make a mistake (and that's surprisingly easy [1]), it could easily be fatal. A single-engine plane with an engine failure, in contrast, becomes a bad glider, with a decent chance of surviving the ensuing forced landing. [1] For example, the TransAsia ATR 72 twin turbo prop clipping a bridge in Taiwan after the crew erroneously shut down the "good" engine. https://en.wikipedia.org/wiki/TransAsia_Airways_Flight_235 https://en.wikipedia.org/wiki/TransAsia_Airways_Flight_235 https://www.youtube.com/watch?v=RSoZOsIC25M https://www.youtube.com/watch?v=RSoZOsIC25M
      - pas 7y ago
        Would it be possible to make single-engine planes usable for commercial passenger flights with similar economics?
      - DuskStar 8y ago
        Just want to point out that "shutting down the 'good' engine" isn't some isolated incident - that was a contributing factor to the British Midland Flight 92 crash [0] as well. 0: https://en.wikipedia.org/wiki/Kegworth_air_disaster https://en.wikipedia.org/wiki/Kegworth_air_disaster
  - eitland 8y ago
    FWIW: 5 years ago or so I was on a plane that lost[0] an engine in rough weather shortly after take off. The only reason I know is because the captain informed us about it and added that since it was technically an emergency landing we would be greeted by the fire department. He then turned the plane around and landed as if nothing had happened. The bloke across the corridor, an old pilot, complained about how there never happened anything remotely exiting on any of his flights ;-) [0]: to be as accurate as possible I think what he said was they'd lost oil pressure on one engine and decided to shut it down voluntarily.
  - cmurf 8y ago
    For my commercial MEL training, most of the time was spent on assymetric thrust handling due to a single powerplant failure. When both are working, or neither are working, the plane handles about the same as a single engine. When one engine fails, while you've lost 50% horsepower, you've lost more like 90% of excess horsepower which is what you need to accelerate and to climb. And assymmetric thrust is dangerous at slow airspeed. You must react exactly correctly and quickly. Failure on takeoff roll means an immediate abort. Failure at takeoff with runway remaining means cutting power and landing on what remains. Failure once airborne and you must quickly identify the failed engine, apply full opposite rudder (toward the good engine), if you don't do this while in a full power low airspeed climb the airplane will absolutely roll over. Many accidents are the result of continuing when they should have aborted, and being anemic or slow with corrective rudder. Turbine powered airplanes are way different, they have a lot more excess thrust available so powerplant failure is not dire. They have so much more power that they will still easily accelerate to a safe single engine takeoff speed and climb away for a circle to land, which is safer than aborting after certain speeds that smaller twins won't get to on takeoff. That's all part of the computation for "balanced field length".
- mannykannot 8y ago
  A broken AofA sensor has other consequences, such as the differing pressure altitude and airspeed on the left and right panels, so I would guess that it is expected that the problem would be found and fixed after the flight it occurred on (though this did not happen in the Lion Air case.)
  - pas 7y ago
    They replaced "something", but probably haven't confirmed that, and haven't coordinated with the incoming crew to watch out for that, etc.
mannykannot 8y ago
This came up in an earlier discussion. Someone suggested that, in attempting to fix the problem reported from the previous flight, the relevant systems were shut down, and this reset the AofA sensor selection mechanism.
michaelt 8y ago
I've heard it suggested [1, 2] the original design for MCAS could only make a small adjustment a single time, which the pilots could easily overcome with the stick, classifying it a less safety-critical component as it couldn't possibly crash the plane. Later, the MCAS ended up adjusting the trim multiple times, by an amount the pilots couldn't override with the stick. Thus it should have been safety level DAL-A (loss of plane) [3] - but the safety analysis and MCAS design weren't updated to reflect that. And the FAA, in what with hindsight seems like a ridiculously naive move, allowed Boeing to self-certify their own faulty safety analysis. [1] https://www.seattletimes.com/business/boeing-aerospace/failed-certification-faa-missed-safety-issues-in-the-737-max-system-implicated-in-the-lion-air-crash/ https://www.seattletimes.com/business/boeing-aerospace/faile... [2] https://news.ycombinator.com/item?id=19419454 https://news.ycombinator.com/item?id=19419454 [3] https://en.wikipedia.org/wiki/DO-178B#Software_level https://en.wikipedia.org/wiki/DO-178B#Software_level

Scarblac 8y ago

It's worse than having one sensor, the chance that one of two breaks is larger than that just a single one breaks. And by alternating you ensure that the broken sensor will be used.

phkahler 8y ago
Was talking to my flight instructor about twin engine planes. He was adamant that they were LESS safe than single engines because 1) you are twice as likely to have an engine failure and 2) the statistics indicate (non-intuitively) that an engine-out in a twin is more likely to lead to fatalities. He said the insurance companies used to think twins were safer but they have adjusted their rates after looking at actual data. I was fully on-board with point #1, but #2 was a surprise to me. I suppose that's what Burt Rutan tried to address with the Boomerang design.
- ebg13 8y ago
  To your second point, my assumption as a non-pilot is that a symmetrically balanced glide is more controllable than lopsided thrust that could put you into a spin?
  - mcguire 8y ago
    Loss of one engine at low speed will leave a twin uncontrollable. See Vmc (https://en.wikipedia.org/wiki/V_speeds?wprov=sfla1 https://en.wikipedia.org/wiki/V_speeds?wprov=sfla1).
    - cheerlessbog 8y ago
      If you promptly shut off the other engine, does it become equivalent to single engine failure?
      - VBprogrammer 8y ago
        Not really. Twin engine aircraft don't have to meet the same crash worthiness standards and usually have higher stall speeds (in this case minimum crash speeds). To be fair the statistics are swayed slightly by a number of factors, training accidents and different risk profiles of the flights.
      - mcguire 7y ago
        In general, a twin will weigh considerably more than a single as well as having greater drag, making for shorter glides.
- cjbprime 8y ago
  Yeah, there's a pilot proverb about how having a second engine "helps you get to the crash site faster".
  - FabHK 8y ago
    Or "the plane is certified such that the remaining engine has enough power to take you to the crash site". Indeed. As outlined above, you double the probability that a mechanical problem occurs. (For p small, 1-(1-p)^2 is around 2p). Now, if the pilots are very much on top of things (well trained and current), then they can control things and safely land at the nearest suitable airport. However, if they make a mistake (and that's surprisingly easy [1]), it could easily be fatal. A single-engine plane with an engine failure, in contrast, becomes a bad glider, with a decent chance of surviving the ensuing forced landing. [1] For example, the TransAsia ATR 72 twin turbo prop clipping a bridge in Taiwan after the crew erroneously shut down the "good" engine. https://en.wikipedia.org/wiki/TransAsia_Airways_Flight_235 https://en.wikipedia.org/wiki/TransAsia_Airways_Flight_235 https://www.youtube.com/watch?v=RSoZOsIC25M https://www.youtube.com/watch?v=RSoZOsIC25M
    - pas 7y ago
      Would it be possible to make single-engine planes usable for commercial passenger flights with similar economics?
    - DuskStar 8y ago
      Just want to point out that "shutting down the 'good' engine" isn't some isolated incident - that was a contributing factor to the British Midland Flight 92 crash [0] as well. 0: https://en.wikipedia.org/wiki/Kegworth_air_disaster https://en.wikipedia.org/wiki/Kegworth_air_disaster
- eitland 8y ago
  FWIW: 5 years ago or so I was on a plane that lost[0] an engine in rough weather shortly after take off. The only reason I know is because the captain informed us about it and added that since it was technically an emergency landing we would be greeted by the fire department. He then turned the plane around and landed as if nothing had happened. The bloke across the corridor, an old pilot, complained about how there never happened anything remotely exiting on any of his flights ;-) [0]: to be as accurate as possible I think what he said was they'd lost oil pressure on one engine and decided to shut it down voluntarily.
- cmurf 8y ago
  For my commercial MEL training, most of the time was spent on assymetric thrust handling due to a single powerplant failure. When both are working, or neither are working, the plane handles about the same as a single engine. When one engine fails, while you've lost 50% horsepower, you've lost more like 90% of excess horsepower which is what you need to accelerate and to climb. And assymmetric thrust is dangerous at slow airspeed. You must react exactly correctly and quickly. Failure on takeoff roll means an immediate abort. Failure at takeoff with runway remaining means cutting power and landing on what remains. Failure once airborne and you must quickly identify the failed engine, apply full opposite rudder (toward the good engine), if you don't do this while in a full power low airspeed climb the airplane will absolutely roll over. Many accidents are the result of continuing when they should have aborted, and being anemic or slow with corrective rudder. Turbine powered airplanes are way different, they have a lot more excess thrust available so powerplant failure is not dire. They have so much more power that they will still easily accelerate to a safe single engine takeoff speed and climb away for a circle to land, which is safer than aborting after certain speeds that smaller twins won't get to on takeoff. That's all part of the computation for "balanced field length".
mannykannot 8y ago
A broken AofA sensor has other consequences, such as the differing pressure altitude and airspeed on the left and right panels, so I would guess that it is expected that the problem would be found and fixed after the flight it occurred on (though this did not happen in the Lion Air case.)
- pas 7y ago
  They replaced "something", but probably haven't confirmed that, and haven't coordinated with the incoming crew to watch out for that, etc.

mannykannot 8y ago

This came up in an earlier discussion. Someone suggested that, in attempting to fix the problem reported from the previous flight, the relevant systems were shut down, and this reset the AofA sensor selection mechanism.

michaelt 8y ago

I've heard it suggested [1, 2] the original design for MCAS could only make a small adjustment a single time, which the pilots could easily overcome with the stick, classifying it a less safety-critical component as it couldn't possibly crash the plane. Later, the MCAS ended up adjusting the trim multiple times, by an amount the pilots couldn't override with the stick. Thus it should have been safety level DAL-A (loss of plane) [3] - but the safety analysis and MCAS design weren't updated to reflect that. And the FAA, in what with hindsight seems like a ridiculously naive move, allowed Boeing to self-certify their own faulty safety analysis. [1] https://www.seattletimes.com/business/boeing-aerospace/failed-certification-faa-missed-safety-issues-in-the-737-max-system-implicated-in-the-lion-air-crash/ https://www.seattletimes.com/business/boeing-aerospace/faile... [2] https://news.ycombinator.com/item?id=19419454 https://news.ycombinator.com/item?id=19419454 [3] https://en.wikipedia.org/wiki/DO-178B#Software_level https://en.wikipedia.org/wiki/DO-178B#Software_level