5 ms·

I'd like to share AvE's video[0] here of a tear down of an AoA sensor. It's really quite interesting. More info on the AoA sensor's role in the Wiki entry[1].

I'd like to share AvE's video[0] here of a tear down of an AoA sensor. It's really quite interesting. More info on the AoA sensor's role in the Wiki entry[1]. [0] https://www.youtube.com/watch?v=NhZ0D-JRtz0 https://www.youtube.com/watch?v=NhZ0D-JRtz0 [1] https://en.wikipedia.org/wiki/Boeing_737_MAX#Maneuvering_Characteristics_Augmentation_System_(MCAS) https://en.wikipedia.org/wiki/Boeing_737_MAX#Maneuvering_Cha... TLDR/TLDW: AoA sensor measures "angle of attack." AoA sensors are physically over-engineered, but the electrical components may be questionable. If an AoA sensor fails, it may cause a stall. The 737 Max is equipped with two sensors, however, effectively utilized only one at a time because it cost extra for the "AOA-disagree" feature.

raverbashing 7y ago
If I'm not mistaken, the Ethiopian case had a 'Anti-Ice warning' from the AOA sensor, why did MCAS consider the data of a sensor that potentially had an issue is anybody's guess. > effectively utilized only one at a time because it cost extra for the "AOA-disagree" feature. Yeah, props to Boeing who put a basic security feature as an "extra cost" option. But hey, who needs to know their AoA sensors are failing, especially when the plane is commanded by that, right?
deleted 7y ago
[deleted]
tzs 7y ago
Has anyone tried building aircraft AoA sensors other than vane type sensors? For example, when I was designing a home weather station for myself, I wondered if the anemometer and wind vane could be replaced with ultrasonic sensors and emitters. If you take four ultrasonic emitter/ultrasonic sensor pairs and arrange them in a square, and measure the time of flight of four pulses, one from each corner emitter to the diagonally opposite sensor, you can determine the component of the wind vector along that diagonal. Combine the components from the diagonals, and you get the wind vector. A bit of research turned up that this was not an original idea, and it does work. There are commercial anemometers and wind vanes (are they still called wind vanes when they don't have a vane?) for terrestrial use that work this way. There have also been many home made ones, such as this one [1]. Offhand, I don't see why such a thing would not work for an airplane. [1] http://embedded-lab.com/blog/making-an-ultrasonic-anemometer/ http://embedded-lab.com/blog/making-an-ultrasonic-anemometer...
- ncmncm 7y ago
  It works at low wind speeds and in the absence of turbulence. Would it work the same at 400 knots? Its possible failure modes, too, seem a lot more complicated than for a physical vane.
- macoovacany 7y ago
  Off the top of my head, there are at least two more potential ways of measuring AoA: 1. Similar to your ultrasonic lattice, but using a cone of hot wire anemometers, so that each wire is set at an offset angle to the 'zero' direction. As the AoA changes relative to the cone, the resistance on the different hot wires changes (some wires become more perpendicular to the freestream, thus cooler; others will align closer to the freestream with less opportunity to cool). By measuring the resistance differences with a bridge, it's possible to derive, angle of attack, sideslip and airspeed. 2. An elliptical body with various pressure ports around the circumference, plus a pitot port. The angle of the flow can be measured by the difference in pressure between the circumference ports (and dividing by the pitot pressure).
  - macoovacany 7y ago
    Hot wire anemometers are very, very fragile. Installation on aircraft is not practical - a bug strike would render it inoperative.
- mannykannot 7y ago
  [1] is a description of an available device using two pitots on different axes, together with a static-pressure reference: "The AOA tube is constructed with two tiny machined holes to create differential pressure sources. One hole is bore-sighted at the front of the AOA tube along the longitudinal axis, while the second, located at the bottom of the AOA probe, is pointed downward at a 45-degree angle to act as a reference source. "Garmin’s normalized AOA includes an additional static-air-pressure source to allow the air-data computer to calculate airspeed and air density, important in calculating precise AOA. The addition of the static-pressure reference input makes the AOA indication accurate regardless of the airplane’s weight, speed or configuration." [1] https://www.flyingmag.com/how-it-works-angle-attack-indicator https://www.flyingmag.com/how-it-works-angle-attack-indicato...

raverbashing 7y ago

If I'm not mistaken, the Ethiopian case had a 'Anti-Ice warning' from the AOA sensor, why did MCAS consider the data of a sensor that potentially had an issue is anybody's guess. > effectively utilized only one at a time because it cost extra for the "AOA-disagree" feature. Yeah, props to Boeing who put a basic security feature as an "extra cost" option. But hey, who needs to know their AoA sensors are failing, especially when the plane is commanded by that, right?

deleted 7y ago

[deleted]

tzs 7y ago

Has anyone tried building aircraft AoA sensors other than vane type sensors? For example, when I was designing a home weather station for myself, I wondered if the anemometer and wind vane could be replaced with ultrasonic sensors and emitters. If you take four ultrasonic emitter/ultrasonic sensor pairs and arrange them in a square, and measure the time of flight of four pulses, one from each corner emitter to the diagonally opposite sensor, you can determine the component of the wind vector along that diagonal. Combine the components from the diagonals, and you get the wind vector. A bit of research turned up that this was not an original idea, and it does work. There are commercial anemometers and wind vanes (are they still called wind vanes when they don't have a vane?) for terrestrial use that work this way. There have also been many home made ones, such as this one [1]. Offhand, I don't see why such a thing would not work for an airplane. [1] http://embedded-lab.com/blog/making-an-ultrasonic-anemometer/ http://embedded-lab.com/blog/making-an-ultrasonic-anemometer...

ncmncm 7y ago
It works at low wind speeds and in the absence of turbulence. Would it work the same at 400 knots? Its possible failure modes, too, seem a lot more complicated than for a physical vane.
macoovacany 7y ago
Off the top of my head, there are at least two more potential ways of measuring AoA: 1. Similar to your ultrasonic lattice, but using a cone of hot wire anemometers, so that each wire is set at an offset angle to the 'zero' direction. As the AoA changes relative to the cone, the resistance on the different hot wires changes (some wires become more perpendicular to the freestream, thus cooler; others will align closer to the freestream with less opportunity to cool). By measuring the resistance differences with a bridge, it's possible to derive, angle of attack, sideslip and airspeed. 2. An elliptical body with various pressure ports around the circumference, plus a pitot port. The angle of the flow can be measured by the difference in pressure between the circumference ports (and dividing by the pitot pressure).
- macoovacany 7y ago
  Hot wire anemometers are very, very fragile. Installation on aircraft is not practical - a bug strike would render it inoperative.
mannykannot 7y ago
[1] is a description of an available device using two pitots on different axes, together with a static-pressure reference: "The AOA tube is constructed with two tiny machined holes to create differential pressure sources. One hole is bore-sighted at the front of the AOA tube along the longitudinal axis, while the second, located at the bottom of the AOA probe, is pointed downward at a 45-degree angle to act as a reference source. "Garmin’s normalized AOA includes an additional static-air-pressure source to allow the air-data computer to calculate airspeed and air density, important in calculating precise AOA. The addition of the static-pressure reference input makes the AOA indication accurate regardless of the airplane’s weight, speed or configuration." [1] https://www.flyingmag.com/how-it-works-angle-attack-indicator https://www.flyingmag.com/how-it-works-angle-attack-indicato...