Originally Posted by
737 Driver
The flap position monitoring is a function of the Flap Slate Electronic Unit, so my guess is that the FSEU would send a signal to MCAS (which is a subroutine of the FCC) regarding flap position. There are one or more digital data busses which allow the various electronic components to exchange information, so I figure this would be the easier option than running a dedicated sensor to the flap handle. Reference the DFDR data from ET302 mentioned in the post immediately prior, there could be some threshold value beyond which the flaps are presumed up (i.e. < 0.2 degrees, slats retracted, etc.).
But FDR data is quite clear that it starts automatic long trim down after flap handle is up but before the flap is seated. This is also clear from the ET302 description in the history of flight:
At 05:39:45, Captain requested flaps up and First-Officer acknowledged. One second later, flap handle moved from 5 to 0 degrees and flaps retraction began.
At 05:40:00 shortly after the autopilot disengaged, the FDR recorded an automatic aircraft nose down (AND) activated for 9.0 seconds and pitch trim moved from 4.60 to 2.1 units. The climb was arrested and the aircraft descended slightly.
At 05:40:06, left and right flap position reached a recorded value of 0.019 degrees which remained until the end of the recording.