5 ms·

exactly! to OP: For example: "If you get simultaneous overspeed and stall warnings and your VSI is deeply negative, you are in a stall and need to ignore the

by borism 15y ago

exactly! to OP: For example: "If you get simultaneous overspeed and stall warnings and your VSI is deeply negative, you are in a stall and need to ignore the overspeed warning and execute stall recovery procedures." Actually, when I word it that way, they shouldn't even have needed special training to figure that out: the VSI should have made it obvious. wow, and how were they supposed to know that VSI is reliable? but it is in the right direction: those guys screwed up big time, and a big contributing factor was almost certainly inadequate training in dealing with emergencies of this nature. I think you're too early to jump onto conclusions based solely on impartial and interim report. You're not alone though - seems that opinion of internet experts is divided between putting full blame on pilots or putting full blame on sensors. I think actual situation was a bit more complex than that.

epochwolf 15y ago
Because the VSI doesn't use the pitot tubes? IIRC, it uses a gyroscope. [Confirmed: http://news.ycombinator.com/item?id=2595230 http://news.ycombinator.com/item?id=2595230 ] The chance of having a failure of two separate, redundant systems is unlikely. Pitot tube failure in a thunderstorm is far more likely than gyroscope failure. Anyway, the computer had reported loss of reliable airspeed so they should have ignored all instruments that use airspeed and fly on what's left. There are procedures for this kind of failure. It looks like they didn't follow them. Nova did a program on this. It's on netflix. http://www.netflix.com/WiMovie/70148706 http://www.netflix.com/WiMovie/70148706 They showed the correct solution was to fly the plane based on angle of attack and throttle settings. You don't need airspeed to keep the plane in the air long enough to figure something else out.
- borism 15y ago
  Because the VSI doesn't use the pitot tubes? IIRC, it uses a gyroscope. [Confirmed: http://news.ycombinator.com/item?id=2595230 http://news.ycombinator.com/item?id=2595230 ] Not really confirmed. Even that post says (read carefully): The attitude, turn, and heading indicators rely on gyroscopes that are propelled by a vacuum pump and/or electric motors. Airspeed, altimeter, and _vertical speed indicators_ rely on a pitot-static system. The pitot tube must be exposed to the air that is uninterrupted by the plane's passage. The static port(s) must be positioned where the air is calm and undisturbed. http://en.wikipedia.org/wiki/Variometer http://en.wikipedia.org/wiki/Variometer
  - lucasjung 15y ago
    The VSI (and the altimeter) uses the pitot-static system, but not the pitot tube. The pitot-static system consists of a static pressure port, the pitot tube, and (on more sophisticated aircraft) a computer for interpreting the readings from the two sensors. The altimeter and VSI only rely on the static port (VSI is just the derivative of altitude with respect to time), while the airspeed indicator relies on the satic port and the pitot tube in combination. If the static port has a problem, none of those three instruments will work properly. Whenever I've seen this happen, altitude remains stuck at some fixed number (usually sea level, but not always), VSI is zero (becuase the derivative of a constant is zero), and airspeed is way off (but continues to change). If just the pitot tube has a problem (as was the case in this mishap), the airspeed indicator won't work (usually stuck at some fixed number, most often zero) but the alitimeter and VSI will be fine. The VSI and altimeter were both telling them that they were falling out of the sky like a rock, and they should have paid attention. If they were unsure as to whether it was a problem with the pitot tube, the static port, or both, they had a tie-breaker: the stall warning system uses a completely different set of sensors (they measure the airflow over the wings, looking for signs consistent with a stall). The stall warning system was telling them they were in a stall, the VSI and altimeter were giving them information consistent with a stall, and the airpseed was not changing as it should have in response to low-throttle and nose-high. Put those together and the most logical explanation is that the airspeed indicator is full of shit. That's what I was talking about when I said that human pilots have (or should, at least) the ability to figure out the truth when multiple sensors are in disagreement. An analogy: there's an old adage that you shouldn't go sailing with two compasses: three, or one, but not two. The reasoning is that if you have two compasses and one goes bad, you won't know which one to trust, so you might as well have none. If you have three, you can trust the two that agree with each other. A lot of avionics systems operate on this principle: three sensors, with the results combined by a voting system. Computers do this well. However, when you have a human in the loop, with access to information the computers don't have, this adage is actually a bunch of bullshit. If you have two compasses and they disagree, you're better off than if you only had one because at least you know immediately that something is wrong. Then you can pull in other clues to figure out which compass is right: look at the sun, or the stars, or rely on your knowledge of prevailing wind and current conditions in your general area. If the compasses disagree by more than a few degrees, you'll be able to quickly figure out which is wrong. If they only disagree by a few degrees, they'll probably get you safely close enough to land that you can go back to navigating by dead reckoning, and then figure out which compass is wrong.
lucasjung 15y ago
>You're not alone though - seems that opinion of internet experts is divided between putting full blame on pilots or putting full blame on sensors. I actually blame both: if the sensors hadn't broken, the mishap would not have occurred. However, even once the sensors broke, if the pilots had better SA, they still could have prevented the mishap. Even then, I don't blame the pilots so much as the training system that failed to prepare them for this situation, given what I already mentioned about how reliant these aircraft are on their pitot-static systems.

by borism 15y ago

epochwolf 15y ago

Because the VSI doesn't use the pitot tubes? IIRC, it uses a gyroscope. [Confirmed: http://news.ycombinator.com/item?id=2595230 http://news.ycombinator.com/item?id=2595230 ] The chance of having a failure of two separate, redundant systems is unlikely. Pitot tube failure in a thunderstorm is far more likely than gyroscope failure. Anyway, the computer had reported loss of reliable airspeed so they should have ignored all instruments that use airspeed and fly on what's left. There are procedures for this kind of failure. It looks like they didn't follow them. Nova did a program on this. It's on netflix. http://www.netflix.com/WiMovie/70148706 http://www.netflix.com/WiMovie/70148706 They showed the correct solution was to fly the plane based on angle of attack and throttle settings. You don't need airspeed to keep the plane in the air long enough to figure something else out.

borism 15y ago
Because the VSI doesn't use the pitot tubes? IIRC, it uses a gyroscope. [Confirmed: http://news.ycombinator.com/item?id=2595230 http://news.ycombinator.com/item?id=2595230 ] Not really confirmed. Even that post says (read carefully): The attitude, turn, and heading indicators rely on gyroscopes that are propelled by a vacuum pump and/or electric motors. Airspeed, altimeter, and _vertical speed indicators_ rely on a pitot-static system. The pitot tube must be exposed to the air that is uninterrupted by the plane's passage. The static port(s) must be positioned where the air is calm and undisturbed. http://en.wikipedia.org/wiki/Variometer http://en.wikipedia.org/wiki/Variometer
- lucasjung 15y ago
  The VSI (and the altimeter) uses the pitot-static system, but not the pitot tube. The pitot-static system consists of a static pressure port, the pitot tube, and (on more sophisticated aircraft) a computer for interpreting the readings from the two sensors. The altimeter and VSI only rely on the static port (VSI is just the derivative of altitude with respect to time), while the airspeed indicator relies on the satic port and the pitot tube in combination. If the static port has a problem, none of those three instruments will work properly. Whenever I've seen this happen, altitude remains stuck at some fixed number (usually sea level, but not always), VSI is zero (becuase the derivative of a constant is zero), and airspeed is way off (but continues to change). If just the pitot tube has a problem (as was the case in this mishap), the airspeed indicator won't work (usually stuck at some fixed number, most often zero) but the alitimeter and VSI will be fine. The VSI and altimeter were both telling them that they were falling out of the sky like a rock, and they should have paid attention. If they were unsure as to whether it was a problem with the pitot tube, the static port, or both, they had a tie-breaker: the stall warning system uses a completely different set of sensors (they measure the airflow over the wings, looking for signs consistent with a stall). The stall warning system was telling them they were in a stall, the VSI and altimeter were giving them information consistent with a stall, and the airpseed was not changing as it should have in response to low-throttle and nose-high. Put those together and the most logical explanation is that the airspeed indicator is full of shit. That's what I was talking about when I said that human pilots have (or should, at least) the ability to figure out the truth when multiple sensors are in disagreement. An analogy: there's an old adage that you shouldn't go sailing with two compasses: three, or one, but not two. The reasoning is that if you have two compasses and one goes bad, you won't know which one to trust, so you might as well have none. If you have three, you can trust the two that agree with each other. A lot of avionics systems operate on this principle: three sensors, with the results combined by a voting system. Computers do this well. However, when you have a human in the loop, with access to information the computers don't have, this adage is actually a bunch of bullshit. If you have two compasses and they disagree, you're better off than if you only had one because at least you know immediately that something is wrong. Then you can pull in other clues to figure out which compass is right: look at the sun, or the stars, or rely on your knowledge of prevailing wind and current conditions in your general area. If the compasses disagree by more than a few degrees, you'll be able to quickly figure out which is wrong. If they only disagree by a few degrees, they'll probably get you safely close enough to land that you can go back to navigating by dead reckoning, and then figure out which compass is wrong.

lucasjung 15y ago

>You're not alone though - seems that opinion of internet experts is divided between putting full blame on pilots or putting full blame on sensors. I actually blame both: if the sensors hadn't broken, the mishap would not have occurred. However, even once the sensors broke, if the pilots had better SA, they still could have prevented the mishap. Even then, I don't blame the pilots so much as the training system that failed to prepare them for this situation, given what I already mentioned about how reliant these aircraft are on their pitot-static systems.