What protection is provided in the event of a stab trim runaway?

What protection is provided in the event of a stab trim runaway?

• A. Stab trim brakes controlled by the SSCU
• B. Hydraulic lockout of stabilizers
• C. Automatic rudder trim reversal
• D. Mechanical lock on trim actuator

Answer: (A)

When stab trim runaway happens, the system protects the airplane by quickly stopping the stabilizer movement with a brake. The SSCU (Stabilizer/Trim Control Unit) monitors the stabilizer trim and, if it detects abnormal, uncontrolled motion, commands the stab trim brakes to engage. This rapid braking halts further trim movement and holds the stabilizer in place, preventing the elevator from moving toward dangerous angles and giving the pilot a chance to regain control. This approach is preferred because it provides an immediate, controlled stop of trim motion without relying on fully locking out hydraulic power or attempting a surface reversal. Mechanical or hydraulic lockouts would either interfere with normal trim function or could fail to respond quickly enough; reversing trim via rudder trim isn’t how this protection is designed to work. So, brakes controlled by the SSCU deliver the reliable protection needed during a stab trim runaway.

When stab trim runaway happens, the system protects the airplane by quickly stopping the stabilizer movement with a brake. The SSCU (Stabilizer/Trim Control Unit) monitors the stabilizer trim and, if it detects abnormal, uncontrolled motion, commands the stab trim brakes to engage. This rapid braking halts further trim movement and holds the stabilizer in place, preventing the elevator from moving toward dangerous angles and giving the pilot a chance to regain control.

This approach is preferred because it provides an immediate, controlled stop of trim motion without relying on fully locking out hydraulic power or attempting a surface reversal. Mechanical or hydraulic lockouts would either interfere with normal trim function or could fail to respond quickly enough; reversing trim via rudder trim isn’t how this protection is designed to work. So, brakes controlled by the SSCU deliver the reliable protection needed during a stab trim runaway.