Originally Posted by
Double07
Please excuse me if I am repeating something that has been discussed earlier in this thread, but from the schematic for the “Horizontal Stabilizer Trim Control System – Functional Description – Electric Trim” (see PPRUNE thread entitled
737MAX Stab Trim architecture, post #194), one can see that when MCAS is active, then STS in inactive.
And when MCAS is inactive, then STS is active.
Is it possible that after a pilot electrically trims the aircraft nose-up after an MCAS nose-down trim event, then the STS system will activate to trim the aircraft nose-down again before the next MCAS nose-down trim event? Recall that the STS system trims the aircraft in the direction opposite to the speed change. So if the pilot has just trimmed with a nose-up command, then wouldn’t the STS system counter with a nose-down trim command? This same operation would still apply if the autopilot thought the aircraft had a higher angle of attack as a result of a defective AoA sensor (the autopilot controls the STS trim even when the autopilot is off). Could this help to explain the failure of the ET302 pilot to trim back to a fully neutral trim after an MCAS trim event?
STS respects the control column cutout switches, so any time that virtually any amount of forward or backward pressure was being applied to the yoke, STS would not be able to make any input. Additionally, STS has its own Stall ID mode which, in the presence of a stall signal, would command the trim nose down and override any other STS input. This nose down trim input is also inhibited by the control column cutout switches.
One thing that confuses me here is that I’ve read that STS activates 5 seconds after release of the manual trim switches. MCAS has a similar 5 second delay. This may mean that any STS trim would be canceled by an MCAS trim event. But could there be a delay in MCAS activation relative to STS activation? By the way, STS trim and manual electric trim have the same trim rates, but differ in direction. Also, any STS trim should be canceled by the simultaneous activation of manual electric trim by the pilot. But if the pilot released the manual trim button when he believed the aircraft trim to be at neutral, then STS might give a short nose-down trim command before MCAS activates to give a larger nose-down command.
Both have a 5 second delay because MCAS is actually a sub-function of STS (though MCAS is not inhibited by the control column cutout switches). STS can trim in both directions. If near neutral trim, STS might make an input (nose up or nose down) but any further inputs would be inhibited if the control column was displaced from neutral.
Also, the same schematic shows that the two pedestal cutout switches on the 737MAX operate as a logical “AND” function (“&” function) as follows:
Manual electric trim = [PRI] & [B/U]
Autopilot trim = [PRI]
STS speed trim = [PRI]
MCAS trim = [PRI] & [B/U]
This means that there is no way to turn off all automatic trim functions while keeping the manual electric trim operative. However, it would be a trivial change to have the switches operate as follows:Manual electric trim = [PRI]
Autopilot trim = [PRI] & [B/U]
STS speed trim = [PRI]
MCAS trim = [PRI] & [B/U]
In this case the pilots would be able to turn off the autopilot trim and MCAS trim by turning off only the B/U cutout switch while keeping the manual electric trim and STS trim active by leaving the PRI cutout switch on. This would make the 737 MAX operate more like the 737NG aircraft, allowing full use of manual electric trim at all times. Why has this not been done?
Under current procedures, we never use the cutout switches individually on the 737NG or the MAX. We always use them together. There have been extensive discussions regarding the why's and wherefore's of the change in switch functionality, but procedurally it makes no difference at all.
Is it because it would have required re-certification of the aircraft by giving the pilots control over the MCAS function? Was certification of the 737NG with new lift-causing engines dependent upon the MCAS correction function operating only in the background without control from the pilots?
Pilots have never had direct control over the STS function, so I'll speculate that giving them direct control over MCAS never crossed anyone's mind. MCAS was required for certification, but I don't think those certification rules cared whether the system operated in the foreground or background.