What condition causes THR LK?

THR LK, or Thrust Lock, is a specific condition in the A320 aircraft's autothrust system. This situation arises primarily when the autothrust system encounters a failure, leading the thrust to be locked to a pre-determined value. This ensures that the aircraft maintains a certain level of power and thrust, typically to prevent the loss of the flight's climb capability or to stabilize the flight in specific scenarios.

When THR LK is activated, the autothrust does not automatically adjust thrust according to the flight conditions or inputs from the thrust levers. Instead, it remains fixed, which can be crucial for maintaining control during abnormal situations or system failures. Thus, understanding that A reflects this mechanical safeguard against autothrust failure clarifies why that option is the correct condition causing THR LK.

The remaining options, such as an ALPHA FLOOR condition, typically signify a different situation, focusing more on stall prevention rather than a direct failure of the autothrust system. Similarly, PWS activation and windshear are distinct events related to weather conditions and do not result specifically in thrust lock behavior.