What can cause the PTU to activate?

The activation of the PTU (Power Transfer Unit) is primarily linked to situations where there is a need to maintain hydraulic system pressure, particularly when there is a discrepancy in hydraulic pressure between the left and right hydraulic systems.

In the case of "airborne gear disagree," this indicates that there is a conflict concerning the position of the landing gear, typically where the gear is indicated to be down in one system but up in another, leading to a loss or imbalance in hydraulic pressure across the systems. The PTU is designed to automatically activate under these conditions to assist in equalizing the pressure and allowing the aircraft to maintain control during critical phases of flight such as landing.

Other scenarios such as engine start, emergency landing, or system shutdown do not typically lead to PTU activation under normal operating procedures. An engine start mainly contributes to the aircraft's hydraulic systems activation but does not directly invoke PTU action. Emergency landings might require various other systems but are not a direct cause for PTU activation. Likewise, a system shutdown reflects a loss of hydraulic function rather than a call for PTU operation. Thus, "airborne gear disagree" is the correct scenario that can lead to the PTU activating.