The Boeing 737 NG has stabilizer trim cutout switches marked as MAIN ELECT (left) and AUTOPILOT (right). Turning off the right switch disables automatic stabilizer trim in autopilot mode and automatic stabilizer trim by the STS in manual mode. The stabilizer can still be trimmed with electric trim switches on the control wheel or manually with the hand crank. Turning off the left switch disables the electric trim on the control wheel.
The Boeing 737 MAX has stabilizer cutout switches marked as PRI (left) and B/U (right).
With the MAX, Boeing added MCAS to the existing STS functionality. If Boeing kept the cutoff switches functionality intact, then turning off the right switch would disable both STS and MCAS, keeping the electric trim operable. Instead, in case of misbehaving MCAS Boeing requires to cut off BOTH switches without explaining what each of the switches does. With both switches turned off the only option to change stabilizer trim is to use a hand crank.
- On the NG, if only the left switch is cut out, does it disable electric trim on the control wheel only, or it completely disables electric assist no matter where the command comes from, so in effect when the left switch is turned off it does not matter what is the position of the right switch?
- What is the meaning of the new MAX labels?
- What is the exact functionality of these switches individually?
- Why the algorithm of turning off auto trim system has been changed?