How does the ASI react when flying at higher altitudes due to the thinning air?

When flying at higher altitudes, the true airspeed of the aircraft increases due to the thinning air, which leads to a corresponding increase in indicated airspeed as shown by the airspeed indicator (ASI). This phenomenon occurs because, at higher elevations, the air density decreases, allowing the aircraft to move through the air more efficiently. As the aircraft maintains a constant indicated airspeed, the pressure difference at the pitot tube is greater due to reduced air density, resulting in a higher reading on the ASI.

Among the other options, the suggestion that the airspeed decreases steadily does not accurately describe the relationship between altitude and indicated airspeed; as altitude increases, the true airspeed – and consequently the indicated airspeed if the aircraft is not adjusting its speed – tends to increase. The notion that the ASI remains constant fails to recognize the impact of altitude changes on the aircraft's performance. Lastly, the idea that the ASI becomes erratic does not apply here under normal flying conditions; it would only behave erratically if there were a malfunction or obstruction affecting the pitot static system.

Thus, the correct understanding involves recognizing that the thinning air at higher altitudes generally causes the indicated airspeed to increase relative to sea level conditions.