Originally Posted by
Takwis
That question is near the top of my list, as well. Why change from individual cutouts for manual and autoplot trim? The higher ups don't seem to be asking it, and Boeing certainly is not giving any meaningful answers. That, and understanding the 'blips' would go a long way to solving some of the mystery.
While looking at the schematics for the electric trim system on the MAX posted by yoko1, and comparing them to the NG schematics, I noticed something interesting:
- on the NG the cutout switches were positioned between the thumb switches and the motor. So the thumb switches were powered directly from the circuit breaker panel.
- on the MAX the cutout switches are positioned between the circuit breaker and the thumb switches, so the cutout switches can disconnect the thumb switches from the circuit breaker.
This has some implications about what is recorded on the FDR when the cutout switches are used and you try to use the thumb switches.
Previously I had questions about how the thumb switch commands are recorded by the FDR, and if for some reason it can fail to record them. Reading the schematics cleared that out somewhat for me:
- on both the NG and the MAX the captain and F/O switches are connected together in parallel, so there is no possibility to distinguish between the captain switches and the F/O switches on the FDR.
- there are 3 outputs from the switches: "up" commands, "down" commands, and an "arm" command. The "up" / "down" commands are controlled by one half of the switch, the "arm" command is controlled by the other.
- to be able to control the motor the switches need to send one of the "up" / "down" commands and the "arm" command as well.
- the "up" and "down" and "arm" commands go through various relays and switches to the motor.
- the "up" and "down" commands go from the switches directly to the Flight Data Acquisition Unit so, as long as the thumb switches are powered, any use of the "up" / "down" half of the switches can be recorded by the FDR.
- I didn't find such a connection to the FDAU for the "arm" command, it seems that it is either not recorded on the FDR or, if it is recorded, it goes through a less direct path, possibly through the FCC, as the "arm" command seems to disable the autopilot interlock signal to the FCC.
- all this means that on NG using the cutout switches and then later trying to use the column trim switches would be communicated to the FDR as "up" / "down" main trim commands, without a corresponding stabilizer movement. However on the MAX any attempted "up" / "down" commands from the switches wouldn't show after the cutout switches are used, since the thumb switches are disconnected from the circuit breaker by the cutout.
Originally Posted by
sadtraveller
I'm short on time to find the post, but there was compelling circumstantial evidence presented in this thread a couple of days ago that pilot trim inputs are sent to the FDR from the stab trim motor (whereas mcas/speed trim inputs are sent from the trim computer). In such a case, it is in fact very probable that failed pilot trim inputs will not be recorded by the FDR.
I was the one that assumed that was the case, but I was wrong, after looking at yoko1's schematics and other schematics in detail they are actually not sent from the trim motor.
For my analysis I used information from these sources:
MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
(Yoko1's post containing schematics of the stabilizer trim control system on the MAX)
https://www.satcom.guru/2018/11/stabilizer-trim.html
(contains a high level schematic for the NG version of the stabilizer trim)
https://aviaforum.ru/threads/katastr...2/post-2392734
(contains a more detailed schematic for the NG version).