During an approach, what is the most important indicator of possible wind shear?

The most important indicator of possible wind shear during an approach is the power and vertical velocity required to maintain the proper glidepath. Wind shear is characterized by rapid changes in wind velocity and direction, which can significantly affect an aircraft's performance during critical phases of flight such as approach and landing.

When experiencing wind shear, pilots may notice that they need to make substantial adjustments to the throttle (power) and may encounter sudden changes in vertical speed (velocity) to stay on the intended glidepath. If there’s an increase in required power to maintain altitude or a decrease in vertical speed, it could signal that the aircraft is entering a shearing wind environment. These indicators are essential for making timely adjustments to ensure a safe landing.

The other factors, while relevant to flying, do not provide direct insights into wind shear conditions in the same way. For example, trim adjustments relate to control inputs but do not inherently indicate changing wind profiles. Similarly, heading changes might occur due to various factors not necessarily linked to wind shear, such as course corrections or drift. Altimeter readings can provide useful altitude information but do not directly reveal wind conditions affecting approach stability. Thus, the power and vertical velocity indicators are critical for recognizing and responding to wind shear threats effectively.