How exactly does MCAS communicate with the flaps setting to know if the flaps have been retracted?

I just noticed on the preliminary report for ET302, the flaps were set at 5 degrees. It doesn’t say when or if the pilots retracted them completely. So I’ll assume the flaps were at 5 degrees the entire flight. So it makes me curious about these systems communicate, if I understand correctly, MCAS and everything was perfectly fine, the only faulty part was the AoA, so there shouldn’t have been a problem with the systems determining whether or not flaps were set.

  • $\begingroup$ Probably it doesn't. In the real world assumptions do exist. $\endgroup$ Feb 7, 2020 at 2:58
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    $\begingroup$ I don't know how the 737 does things. In other airplanes, there is a position sensor at different degrees that is sent to a flap control computer. The position sensors may be at 3° and 7°. If the 3° sensor is active but not the 7° sensor, it would be assumed the flaps were selected to 5°. The flap control computer makes that determination and then communicates that position to any other system that requires it. $\endgroup$
    – wbeard52
    Feb 7, 2020 at 4:04
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    $\begingroup$ Airliners built since the 70s us the ARINC 429 databus standard for intercommunication of black boxes en.wikipedia.org/wiki/ARINC_429. So when flaps are at a given position, it will set a bit on a particular data word on he bus, and MCAS can read that bit off the bus and know where the flaps are. Not quite sure of the MAX is using that standard or something newer, so not posted as an answer. $\endgroup$
    – John K
    Feb 8, 2020 at 0:28

1 Answer 1


MCAS is implemented on the Flight Control Computers (FCCs). The FCCs communicate over an aviation data bus, ARINC format. The B737NG used ARINC 700 series standard for communication - sensor information is linked to the FCCs via the avionics data bus. The B737MAX may or may not use a later standard ARINC bus.

There are two FCCs on the B737MAX. From this link:

Computer redundancy. As of 2019, the two flight control computers of Boeing 737 never cross-checked each other's operations; i.e., each was a single non-redundant channel. This lack of robustness existed since the early implementation and persisted for decades. The updated flight control system will use both flight control computers and compare their outputs. This switch to a fail-safe two-channel redundant system, with each computer using an independent set of sensors, is a radical change from the architecture used on 737s since the introduction on the older model 737-300 in the 1980s. Up to the MAX in its prior to groundings version, the system alternates between computers after each flight.[191] The two computers architecture allowed switching in flight if the operating computer failed, thus increasing availability. In the revised architecture, Boeing required the two computers to monitor each other so that each one can vet the other.

So not only do the FCCs know if the flaps are retracted, they now know if their computations correlate.


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