When starting ATM/PTU, why is it important to check the position of the flaps/slats with a ground observer?

When starting the Auxiliary Power Unit (APU) or Power Transfer Unit (PTU), checking the position of the flaps and slats with a ground observer is crucial primarily because the application of hydraulic power will cause these surfaces to move to the commanded position. This movement is automatic and depending on the aircraft's configuration, can lead to unexpected or unwanted aircraft surface movement that may pose safety risks.

Having a ground observer allows the crew to visually confirm the current position of these surfaces before any hydraulic systems are engaged. This check helps ensure that no obstructions are in the way and prevents any potential harm to personnel or equipment. It is a vital safety measure to confirm that when hydraulic power is applied, the aircraft surfaces will not encounter anything that could lead to damage or safety hazards, such as colliding with ground equipment or personnel.

Overall, situational awareness regarding the position of flaps and slats enhances operational safety during APU or PTU operations.