What major indication can signal a retreating blade stall?

The indication that can signal a retreating blade stall is characterized by abnormal vibrations and a nose pitch-up. This phenomenon occurs in rotorcraft when the blades on the retreating side of the rotor experience a stall due to inadequate angle of attack, often during high-speed flight. As the helicopter maneuvers, the airflow over the retreating blades decreases, which can lead to vibration, a loss of lift, and an unstable attitude, causing the nose of the aircraft to pitch upwards. This behavior is a critical warning sign for pilots, as it indicates a potential loss of control, requiring immediate corrective action to avoid an accident.

In contrast, options such as high fuel consumption, rapid altitude increase, or smooth flight characteristics do not directly relate to the mechanics of a retreating blade stall. High fuel consumption is more generic and can result from various operational conditions. Rapid altitude increase typically suggests positive lift and climbing performance, while smooth flight characteristics imply stable flight without indications of stall. Thus, recognizing abnormal vibrations and changes in pitch is essential for ensuring safe rotorcraft operation and preventing stall situations.