Originally Posted by
Ian W
From earlier in the thread the ADIRUs disagreed because of the AoA disagree. I was pointing out that the aircraft systems had already sensed an AoA disagree otherwise they would not alert to a UAS.
The UAS on Lion Air resulted from the left and right airspeed signals not tracking each other. This was most likely caused by the the left /right sensed AOA difference, but declaring UAS did not involve a direct comparison of the two AOA signals. On 737 the difference between measured static pressure and actual static pressure is a function of AOA. In order to account for this, measured static pressure is compensated based on measured AOA to minimize the resulting static pressure error that would result from a change in AOA. This AOA compensation for static pressure is done independently by the two air data systems that use the sensors on the two sides of the airplane. As a result, if the left and right static pressure ports measure identical pressures but AOA as sensed by the vanes on the two sided of the airplane differ, the compensated static pressure signals will between left and right. This difference causes the left and right barometric altitudes to differ slightly and the computed left and right airspeeds to differ slightly. It is the difference in airspeeds left to right that leads to the system issuing its UAS indication.