In what sequence do the boots activate during the de-ice process?

The correct sequence for the activation of the boots during the de-ice process is indeed the horizontal stab, followed by lower in, upper in, lower out, and upper out. This order is crucial for effective ice removal, as each section of the aircraft must be treated in a specific manner to ensure that any ice accumulation is thoroughly cleared.

Starting with the horizontal stabilizer is essential because maintaining control and stability of the aircraft is a priority, particularly during flight. Ice accumulation on the horizontal stabilizer can severely affect the aircraft's pitch stability. Therefore, activating the de-ice process on this surface first helps to restore aerodynamic functions.

Next, the lower in and upper in boots are activated. This approach provides a thorough treatment of the leading edge surfaces, where ice tends to form. In particular, ensuring that the upper surfaces are cleared after the lower surfaces increases the effectiveness of the de-icing process.

The final steps involve activating the lower out and upper out boots. This sequence helps to ensure that any remaining ice that might have accumulated is efficiently expelled from these surfaces. This systematic progression is designed to maximize de-icing effectiveness while minimizing any potential aerodynamic disruptions during the process.