Final answer:
Pilots must be prepared to control the aircraft's speed manually if the flashing green SPEED message does not appear during descent through 100-ft radio altitude. Verification of autothrottle system status and readiness to manage speed manually, in accordance with approach and landing procedures, are required.
Step-by-step explanation:
If a pilot does not observe the flashing green SPEED message during descent through 100-ft radio altitude, they must be prepared to manually control the aircraft's speed. The SPEED message is a part of the flight deck's alerting system, which signals to the pilot when the autothrottle (if equipped) is no longer controlling airspeed during an approach, typically because the aircraft has descended below a predefined altitude. In most cases, receiving this message requires the pilot to assume manual control over the speed of the aircraft by adjusting the throttle as necessary. However, if this message does not appear as expected, the pilot should verify the status of the autothrottle system and be ready to manually manage speed while also ensuring alignment with all applicable approach and landing procedures.