51 ms·

Except you can estimate the Aoa pretty well just using airspeed and load factor.

• tobib 8y ago
  Can you elaborate please?
• esoterae 8y ago
  Estimates don't fly in aviation.
  • lutorm 8y ago
    That's a ridiculous statement. All sensor data are estimates. If the IMU and airspeed sensors are good enough to feed the artificial horizon, autopilot, etc, they're surely good enough to produce an AoA estimate that's good enough to serve as a cross-check on the physical sensor.
    • peteradio 8y ago
      > That's a ridiculous statement. All sensor data are estimates. Some more reliable than others. You have no fucking clue what direction the wind is coming down on the wings based on horizon. You have a skewed perception of what wind is. On the ground it is always horizontal because it can't go into the ground. Once you get up into the sky the dynamics are insane. Get out of your armchair.
      • lutorm 8y ago
        Are you a pilot? This is the basic way that a pilot can know whether he's close to stall without anything more than an airspeed indicator. It's basic flight physics. The lift developed by the wing is equal to CL.q.A. A is the wing area, the dynamic pressure q is basically indicated by the airspeed indicator, and CL is a known function of AoA. Lift is by definition equal to the load factor * mass. For a good, in-depth explanation, see http://www.av8n.com/how/htm/aoa.html#sec-ias-aoa http://www.av8n.com/how/htm/aoa.html#sec-ias-aoa
        • peteradio 8y ago
          I have my PhD in physics. And CL is determined how?
          • lutorm 8y ago
            Rearrange and get: CL(AoA) = Az.m/(q.A) (Az = vertical acceleration.) The relationship between coefficient of lift and AoA is a known function that depends on the airfoil. As long as you're below the critical angle of attack, it's an invertible function. (If you're not below the critical AoA, you've already departed controlled flight so it's too late for MCAS.)
            • peteradio 8y ago
              > In flight, the lift is nearly always equal to the weightlab times load factor. (From your link) This is not true at or near stall conditions.
              • lutorm 8y ago
                How so? As far as I understand, it will hold to a very high degree as long as the flight conditions are stable over the time scale needed to establish the airflow pattern over the wing, which is very short. (If the flight conditions are that unstable, a mechanical AoA vane will struggle too, since it has mechanical inertia.)
        • bbojan 8y ago
          The pilot can "know whether he's close to stall without anything more than an airspeed indicator" only if he's not in a turn (see accelerated stall). While you are right in theory - if you knew the load factor and indicated airspeed (IAS) you could determine AoA - how would you measure IAS in practice? Normally a pitot tube would give you incorrect readings at non-zero angles of attack, and that's why airplanes use an input from an AoA sensor for correction. Which brings you back to where you started - an AoA sensor. You are second-guessing tens (hundreds?) of thousands of engineers who thought about these problems for more than a hundred years.
      • Gibbon1 8y ago
        Yeah fly into a strong updraft while climbing.
        • russh 8y ago
          It was more fun flying into a downdraft in the middle of power on stall training.
• cmurf 8y ago
  I don't think that's accurate or consistent enough to meet the requirements of FAR 25.207. Load factor is constantly changing in flight. You only have a constant load factor with zero turbulence and ~~straight and level flight~~ unaccelerated flight. edited Obviously I can climb at 500fpm at 1.0g, but if the rate itself is changing I'm at something other than 1.0g. It also can't withstand failure of the selected pitot-static system, because it depends directly on it. So now you need a mechanism to detect such failures, and use a different pitot-static system, and do all of that switching while maintaining the requirements in FAR 25.207. I'm unconvinced that's feasible.
  • lutorm 8y ago
    Pitot-static failure is already a big deal for much more immediate reasons than an AoA estimate. And I'm puzzled why load factor changing would be a problem? The air data computer knows what the load factor is at any given time. To be clear: I didn't imply that you'd want to use this as the single source of AoA, only that you can use this in concert with the physical sensors to determine if the sensors are producing a valid value. You need 3 sources to be able to throw out a bad measurement, 2 AoA sensors and an airspeed-based measurement should be a perfectly valid, single-fault-tolerant, way to outvote a bad sensor. Remember that these accidents happened at high airspeed. You'd rip the wings off the airplane before you'd stall it at those speeds. It's not a subtle failure.
• peteradio 8y ago
  What is load factor? Air speed from GPS or something?
  • danaliv 8y ago
    Load factor is the number of Gs the aircraft structure is supporting. In unaccelerated flight it’s 1. Straight and level, an established climb, and an established descent are all unaccelerated. In a turn, an airplane experiences a higher load factor as a function of the bank angle. The lift from the wings is what makes the airplane turn. Since some of the lift is being diverted to the horizontal, the total lift must be increased to keep the airplane from sinking. The result is a higher load due to the acceleration in the turn. The beginning of a climb or descent also changes load, but once established, the load returns to 1.
• ddalex 8y ago
  How do you measure the load factor?
  • lutorm 8y ago
    These airplanes all have inertial measurement units feeding the flight instruments. On older aircraft, there would at the very least be a simple "G-meter". There are design limits on the load factor so it's important to track it to make sure it's not exceeded.
  • cmurf 8y ago
    It's a ratio, so you could use a vertical accelerometer. But this isn't enough information to derive angle of attack even with airspeed. Of course, a plane can be in straight and level flight with 1.0 g loading, at 200kts, and maintain that with many different angles of attack due to weight and center of gravity differences, both of which are changing throughout the flight as fuel is consumed from tanks in different locations.
    • lutorm 8y ago
      Indeed. But the system already knows what the mass is, this is needed to calculate takeoff performance, rotation speed, stall speed, maneuvering speed, etc. CG doesn't enter into it except to second order due to horizontal stabilizer lift.
      • cmurf 8y ago
        It's still not enough information to compute AOA. Let's say I'm configured for 300kts in level flight. Now I pull back on power changing nothing else. I'll end up in a decent, and eventually that will stabilize to e.g. 1000fpm at 300kts. I'm still at 1.0g, and 300kts, but I'm in a decent, obviously angle of attack is less than in level flight. So airspeed and load factor aren't enough, nor is including mass. Horizontal stabilizer lift is a downward force and must be countered with lift from the wing to maintain level flight. It's effectively the same as adding weight. Angle of attack absolutely changes as CG changes. This is a central purpose of weight and balance computation. W&B also changes as fuel is consumed. So you'd have to take that into account. Anyway, you can't infer angle of attack or the coefficient of lift from only airspeed, load factor, and mass. You need more information.
• toast0 8y ago
  The 737 calculates airspeed using the pitot tube measurements and the angle of attack sensor; if the AoA sensor is erroneous, so is your airspeed.