Final Answer:
The frequency to tune in for ground control after landing, as instructed by the tower controller on "point niner," is 121.9.
Step-by-step explanation:
When the tower controller instructs a pilot to contact ground on "point niner," it indicates the frequency for ground control. In aviation communication, "point niner" stands for the frequency 121.9. This frequency is specifically designated for ground control operations at the airport.
Once an aircraft lands and receives instructions from the tower controller to switch to ground frequency, the pilot would adjust the radio communication to 121.9 to establish contact with ground control. Ground control manages aircraft movement on the airport surface, including taxiing, parking, and other ground operations.
Tuning into this frequency allows the pilot to communicate with ground controllers to navigate the taxiways, receive further instructions, or request services upon landing.