If both temperature and altitude increase, what happens to an aircraft's indicated airspeed?

When both temperature and altitude increase, the indicated airspeed of an aircraft decreases. This is primarily because indicated airspeed is a function of the air density, which decreases with increasing altitude and higher temperatures. As an aircraft climbs, the atmosphere thins, resulting in a drop in air density.

Under normal conditions, indicated airspeed is measured by the difference in pressure between the static air outside and the pressure within the aircraft. At higher altitudes, the lower air density leads to a lower pressure difference for a given true airspeed. While the true airspeed may remain constant, the indicated airspeed reflects the variations in air density due to the changes in altitude and temperature. Therefore, as density decreases, the indicated airspeed decreases as well.

This principle is critical in understanding how aircraft performance is affected during flight in various atmospheric conditions.