3 ms·
MCAS uses AoA (angle of attack) not Pitot Tube. There are two AoA on the 737 Max, MCAS only used a single one (but switched every time you started the aircraft
by Someone1234 7y ago
MCAS uses AoA (angle of attack) not Pitot Tube.
There are two AoA on the 737 Max, MCAS only used a single one (but switched every time you started the aircraft). Reporting in Seattle Times suggests that it was originally designed to use one AoA combined with a G-Sensor[0]. The two in combination would trigger MCAS to make a small adjustment.
The problem is that the aircraft's actual design was failing to be "common type" compatible (i.e. no re-training). So they kept on expanding MCAS's usage to compensate. For low speed the G-Sensor didn't work, so was removed, and flight authority was increased.
More from the article:
> This original version of MCAS, according to two people familiar with the details, was activated only if two distinct sensors indicated such an extreme maneuver: a high angle of attack and a high G-force. [...] About a third of the way through flight testing in 2016, as first reported by The Seattle Times in March, Boeing made substantial changes to MCAS.
> The flight-test pilots had found another problem: The same lack of smooth stick forces was also occurring in certain low-speed flight conditions. To cover that issue too, engineers decided to expand the scope and power of MCAS.
> Because at low speed a control surface must be deflected more to have the same effect, engineers increased the power of the system at low speed from 0.6 degrees of stabilizer nose-down deflection to 2.5 degrees each time it was activated.
> On the stabilizer, maximum nose down is about 4.7 degrees away from level flight. So with the new increased authority to move the stabilizer, just a couple of iterations of the system could push it to that maximum.
> Because there are no excessive G-forces at low speed, the engineers removed the G-force factor as a trigger. But that meant MCAS was now activated by a single angle-of-attack sensor.
> One of the people familiar with MCAS’s evolution said the system designers didn’t see any need to add an additional sensor or redundancy because the hazard assessment had determined that an MCAS failure in normal flight would only qualify in the “major” category for which the single sensor is the norm.
[0] https://www.seattletimes.com/seattle-news/times-watchdog/the-inside-story-of-mcas-how-boeings-737-max-system-gained-power-and-lost-safeguards/ https://www.seattletimes.com/seattle-news/times-watchdog/the...