4 ms·

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

by 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.
- borism 15y ago
  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) AFAIK stall warning system uses AoA vanes, located on the nose close to static system pitots. Completely different system? Yes. Same working principle? Yes. http://msquair.files.wordpress.com/2010/03/aoa-rs-probes-a330.png http://msquair.files.wordpress.com/2010/03/aoa-rs-probes-a33...
  - lucasjung 15y ago
    Interesting. AoA probes are one way to provide stall warning, but I wasn't aware that's what Airbus used on the A-330. They also have other uses. There are also other ways to provide stall warning such as directly sampling flow over the wings (as I mentioned previously). Most light props actually use a very crude version of this. For example, the stall warning on the Cessna I flew my first few hours in was a horn on the lower part of the leading edge. In an impending stall scenario, the pressure over that part of the wing would go negative, sucking air out through the horn and causing it to sound. AoA vanes actually do work on different principles than pitot probes. The newer, fancy-schmancy ones use a whole bunch of tiny ports, each measuring pressure at different angles, and a computer interprets the results to give AoA. On older planes (including the one I have the most experience in), it's literally a vane, as in "weather vane:" it rotates into the direction of the relative airflow, so whatever angle it is at relative to horizontal is the AoA. Even if the AoA system did work on the same principle, the more important point is that it's a separate system, meaning that a failure in the pitot-static system would not cause a failure in the stall warning system. EDIT: from the picture you linked, the AoA vanes on an A-330 are of the type I'm more accustomed to: literal vanes that rotate into the airflow.

by borism 15y ago

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.

borism 15y ago
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) AFAIK stall warning system uses AoA vanes, located on the nose close to static system pitots. Completely different system? Yes. Same working principle? Yes. http://msquair.files.wordpress.com/2010/03/aoa-rs-probes-a330.png http://msquair.files.wordpress.com/2010/03/aoa-rs-probes-a33...
- lucasjung 15y ago
  Interesting. AoA probes are one way to provide stall warning, but I wasn't aware that's what Airbus used on the A-330. They also have other uses. There are also other ways to provide stall warning such as directly sampling flow over the wings (as I mentioned previously). Most light props actually use a very crude version of this. For example, the stall warning on the Cessna I flew my first few hours in was a horn on the lower part of the leading edge. In an impending stall scenario, the pressure over that part of the wing would go negative, sucking air out through the horn and causing it to sound. AoA vanes actually do work on different principles than pitot probes. The newer, fancy-schmancy ones use a whole bunch of tiny ports, each measuring pressure at different angles, and a computer interprets the results to give AoA. On older planes (including the one I have the most experience in), it's literally a vane, as in "weather vane:" it rotates into the direction of the relative airflow, so whatever angle it is at relative to horizontal is the AoA. Even if the AoA system did work on the same principle, the more important point is that it's a separate system, meaning that a failure in the pitot-static system would not cause a failure in the stall warning system. EDIT: from the picture you linked, the AoA vanes on an A-330 are of the type I'm more accustomed to: literal vanes that rotate into the airflow.