What protects the flight controls in the event of a PCU runaway?

• A. Bungee Breakouts
• B. Hydraulic fuses
• C. Electrical isolation
• D. Trim interlock

Answer: (A)

When a PCU runaway happens, the flight-control surfaces must be protected from being driven beyond safe limits by the hydraulic actuator. The CRJ is equipped with bungee breakout devices at the PCU-to-surface connections. These devices are designed to “break away” under excessive actuator force, disconnecting the PCU from the control surface and preventing continued hydraulic drive from moving the surface. This mechanical disconnect acts as a fail-safe to stop the runaway from damaging the control surfaces and keeps the aircraft controllable.

Hydraulic fuses would only isolate a leaking hydraulic line, electrical isolation deals with electrical faults, and a trim interlock prevents unwanted trim motion. None of these specifically address preventing a PCU-driven surface from moving uncontrollably, which is why the breakout mechanism is the protective feature used in this scenario.