I guess my question remains. fadecfault or someone else:
Is the Stall ID information in the SYMD and FCC different? As is now clear from the traces we are not facing an MCAS range AoA situation for JT610 (or previous flight), the SYMD must have full Stall ID to start the stick shaker. It means the aircraft is flying in the stall AoA range the whole previous flight and for all the fatal flight except a short moment. With this AoA, does the FCC have Stall ID?
A) If not the Stall trim loop in the FCC has other trigger criteria than the pre-Stall MCAS loop. Why in such a case is MCAS allowed to have laxer rules than Stall? The trim authority seems to be the same, other than it's not clear if the Stall trim is stopped after 10s, waits and then continues. It might continue until the AoA goes below stall value.
B) If the Stall criteria in the FCC is the same for MCAS and Stall trim the Lion Air scenario is not a concern for the 737 MAX but for both 737 NG and MAX.
The absence of FAA being active on the NG points to A. But is this the case really? Or is there something wrong in my logic?
I could not find a full logic diagram for Speed Trim Stall recovery. I agree with you this accident could happened to a NG crew not paying attention.
Here is the Text I found for the NG:
"The FCC trims the stabilizer nose down as speed decreases and
to allow for speed trim above the stickshaker AOA and idle thrust.
When the stabilizer position reaches its limit, the trim stops. When
the trim starts to exceed the aft column cutout position, the trim
stops. The FCC stops the speed trim function if the roll angle is
more than 40 degrees."
Aircraft with Elevator Feel Shift Module:
"During a stall, the FCCs command the stabilizer to trim the
airplane nose down. The EFSM and column cut-out switch
modules operate to make sure the pilot cannot easily stop this
automatic stabilizer movement with the elevator control column
nose up input."
Here is a summary for the smyd stall warning:
Normal Stall Warning
Normal stall warning uses this data:
- Angle of attack (AOA)
- TE flap position
- LE flap and slat asymmetry information
- LE uncommanded motion (UCM) information
- Engine thrust settings (N1 and N2)
- Computed airspeed (VCAS)
- Wing and engine thermal anti-ice switch (TAI) status
A normal stall warning schedule sets the AOA trip point for each
TE flap position. The AOA trip point values change from 13.0 to
23.5 degrees depending on flap position. If the AOA is more than
the trip point for a TE flap position, the SMYDs send a stall
warning signal to the column stick shakers.
The SMYDs use the other data for bias calculations. If a bias is
active, the SMYDs send a stall warning signal at a lower AOA.
The SMYDs calculate all biases simultaneously and use the
highest bias. The SMYDs subtract the highest active bias from the
normal trip point schedule to decrease the trip point. These are the
biases that the SMYD calculates:
- LE flap and slat asymmetry bias (10.2 - 15.0)
- High thrust bias (0 - 13.6)
- LE UCM bias (2.3 - 6.7)
- TAI bias (0.8 - 5.3)
Last edited by fadecfault
on Sun Dec 02, 2018 11:11 am, edited 1 time in total.
The views and opinions written here are my own and do not reflect those of my employer.