The Stall Warning System comprises of
*** Updated 23 Nov 2020 ***
The NG and MAX Stall Management System includes two Stall Management Yaw Damper (SMYD) computers. SMYD 1 uses data from AoA 1 and send the outputs the the Captains DU and stick shaker and SMYD 2 uses data from AoA 2 and send the outputs the the F/Os DU and stick shaker. The SMYDs also take data from the ADIRS, Flap Position Transmitter, FSEU, CDS, MCP, Anti-ice configuration and PSEU air/ground switch.
The 737 Originals and Classics have a Stall Warning Computer (SWC) rather than a SMYD. Its function is very similar.
The SMYDs are located in the E&E bay on the E3-2 shelf. The BITE modules can be seen on the front of the boxes.
The SMYD/SWC calculates the AoA at which the aircraft will stall and hence the minimum and maximum safe operating speed. These speeds are displayed on the speed tape and the AoA on the Pitch Limit Indicators (known as the eyebrows!).
When the SMYD calculates that the aircraft is approaching a stall several things happen:
The stick shaker is simply a motor with an eccentric (off-centre) weight that when energised causes a vibration on the control column. Each stick shaker is operated by its onside SMYD/SWC but if one side activates the shaker vibrates both columns through interconnects.
The system is armed in flight at all times
In both MAX MCAS accidents a stick shaker operated continuously throughout the flights until impact because of the eroneous AoA data. This undoubtedly added to the confusion, distraction and stress levels. It is the view of EASA, Allied Pilots Association, ALPA, BALPA, Ethiopian Airlines Group, and the UAE GCAA that the means to suppress an erroneous stick shaker, as is available on most other warnings, should be available to crew.
The FAA View of the Stick Shaker Issue
The issue of disabling the stick shaker in-flight was addressed in FAA AD 2020-24-02 as follows:
Comments Regarding Crew Procedure To Disable Stick Shaker
Comment summary: Several commenters, including the Allied Pilots Association, ALPA, BALPA, Ethiopian Airlines Group, and the UAE GCAA, expressed concerns regarding the attention getting nature of the stick shaker and requested a change to the procedures to include a means to suppress an erroneous stick shaker, including procedures to pull the associated stick shaker circuit breaker. In contrast, a commenter expressed a concern with the possible safety risks of including a procedure to pull the stick shaker circuit breaker in order to silence the warning.
FAA response: The FAA infers that the commenters are suggesting there is an unacceptably high flightcrew workload when stick shaker is activated erroneously. The 737 stall warning/stick shaker is, by design, attention getting and can be a distraction during an erroneously high-AOA event. However, after careful evaluation, the FAA has not changed the AFM non-normal procedure to include pulling the stick shaker circuit breakers in this final rule, for the following reasons.
The FAA evaluated all failure conditions of the new FCC software as part of certification of the proposed system changes. The new FCC software removes the potential for repeated, uncommanded MCAS inputs in the presence of an erroneous high AOA sensor input. This new design therefore removes the most significant contributor to unacceptably high flightcrew workload. With the new FCC software on the 737 MAX, the FAA tested and assessed all remaining flight deck effects, including erroneous stick shaker, during all foreseeable failure conditions, including high-AOA sensor failures during the most critical phases of flight (such as during takeoff or go-around). With the remaining flight deck effects and associated crew workload, these failures and effects were found compliant and safe.
The FAA considered the commenters' concerns that an erroneous stick shaker may pose a distraction for the crew, and evaluated that scenario with procedures that include steps to silence an erroneous stick shaker stall warning via a circuit breaker pull. The FAA finds that an erroneous stick shaker, while it may pose a distraction to the flightcrew, does not affect controllability of the airplane. The stick shaker circuit breaker locations also do not meet FAA requirements for convenient operation for emergency controls for the complete range of pilots from their normal seated position in the flight deck, leading to possible distraction from their primary duties to safely control and monitor the aircraft. Furthermore, inclusion of these additional steps would add cognitive and physical workload to an already substantial Airspeed Unreliable non-normal procedure, and errors in locating and pulling the correct circuit breaker may lead to other airplane hazards. Balancing the concerns associated with adding a procedure to pull circuit breakers against the distraction of an erroneous stick shaker, the FAA has concluded that the design is compliant and safe, and therefore no change to the proposed non-normal procedures related to silencing the 737 MAX stall warning is required for this AD.
The Southwest Airlines Pilots Association said in response to the FAA’s final AD that “Most MAX operators, with the exception of those in the United States, have developed a procedure for pulling a circuit breaker in just such an occurrence, thereby reducing distractions and flight deck confusion, No U.S. pilot should have to use captain’s emergency authority to accomplish this procedure when communication is near impossible due to noise and creating a significant distraction on the flight deck.”
The EASA View of the Stick Shaker Issue
EASA issued a proposed AD 20-184 on 24 Nov 2020 cancelling the ‘suspension of flight operations‘. In it was the following requirement:
EASA are unlikely to drop this requirement and will require the addition of steps in associated QRH procedures adding the pulling of the c/b's as required.
The EASA Airspeed Unreliable QRH procedure has some steps and notes which refer to deactivating the stick shaker:
As the aircraft appraoches the stall, the Elevator Feel Shift Module (EFSM) increases hydraulic system A pressure to the Elevator Feel and Centering Unit to double control column forces to counteract control column elevator up movement. The EFSM is inhibited when an AP is engaged.
Stall warning test requires AC power. Also, with no hydraulic pressure, the leading edge flaps may droop enough to cause an asymmetry signal, resulting in a failure of the stall warning system test. If this happens, switch the "B" system electric pump ON to fully retract all flaps and then repeat the test.
737-CL/NG/MAX System test switches on the aft overhead panel
The 737-1/200's had a different stall warning panel as shown below:
The OFF light may indicate either a failure of the heater of the angle of attack sensor a system signal failure or a power failure.
The test disc should rotate, indicating electrical continuity, when the switch is held to the test position.
737-1/200 Stall Warning Panel