Under what condition will true airspeed and groundspeed increase proportionally?

True airspeed (TAS) and groundspeed (GS) can be influenced by various factors, but they tend to increase proportionally under the condition of higher density altitude. As the density altitude rises, the air becomes less dense, allowing an aircraft to achieve a higher true airspeed for the same indicated airspeed, due to reduced aerodynamic drag.

In addition, when flying at a higher density altitude, the engine performs better because it operates in thinner air, leading to improved performance overall. Since groundspeed is the resulting speed of the aircraft over the ground, it also increases as true airspeed rises, given that wind conditions remain constant.

This proportionality is significant in understanding how aircraft perform in various atmospheric conditions, particularly at higher elevations where the effects of altitude on performance are critical for flight planning and safety.