When must the tail stand on a B737-900ER be positioned?

The correct answer highlights the crucial safety procedure concerning the positioning of the tail stand on a B737-900ER. The tail stand must be positioned immediately after two sets of chocks are placed to ensure the aircraft is stable and secure on the ground. This is vital for preventing possible movement of the aircraft, which could lead to accidents or injuries during the unloading process or when personnel are working in proximity to the aircraft.

The placement of chocks serves as an initial safeguard against the aircraft rolling, while positioning the tail stand adds an additional layer of safety by supporting the rear of the aircraft. This practice significantly reduces the risk of any unintentional lift or movement of the tail during ground operations, making it a critical part of ramp safety protocols.