What conditions will cause the true landing speed of an aircraft to be the highest?

High temperature with high humidity results in the highest true landing speed of an aircraft primarily due to the effects of density on air. When the temperature is high, the air becomes less dense because warmer air expands. Similarly, high humidity adds water vapor to the air, which also alters its density, as water vapor is less dense than the dry air it displaces. Both factors contribute to a reduction in air density.

As air density decreases, the lift generated by the wings is reduced, necessitating a higher landing speed to achieve the required lift for safe landing. This means that when an aircraft approaches landing in these conditions, it must maintain a speed that compensates for the lack of lift caused by the lower-density air.

In contrast, low temperature and low humidity conditions maintain a higher air density, which means that the aircraft can achieve necessary lift at a lower speed. Normal temperature with low humidity provides a more balanced environment, while high altitude with low pressure typically results in lower air density, also influencing landing speeds but not necessarily to the extent of high temperature and humidity combined.