At which altitude does the airspeed indicator start reading "too low"?

The airspeed indicator begins to read "too low" at high altitudes due to the reduced atmospheric pressure and density that affect the performance of the aircraft. At high altitudes, the thinner air provides less lift, and the maximum threshold for safe airspeed becomes more critical. Flying at a high altitude can result in needing to maintain higher airspeeds to control the aircraft effectively because of reduced engine performance and increased stall risk linked to the airflow over the wings becoming less effective.

At lower altitudes, pilots typically have greater air density, which allows for more lift and generally safer flight conditions with a more forgiving airspeed margin. Therefore, understanding that airspeed indicators can be deceptive at high altitudes is crucial for safe flight operations. This makes high altitudes the key focus when considering when airspeed may be perceived as "too low."