What criteria warrant the establishment of a partial control group?

The establishment of a partial control group is warranted primarily under the specific conditions of a single aircraft operating on a drop zone (DZ) that is less than 2100 meters long and has a green light duration of less than 20 seconds. This scenario indicates potential limitations in altitude, timing, and spacing, which can complicate the drop process.

In drop zone operations, the green light signals that parachutists are clear to exit the aircraft. A DZ shorter than 2100 meters can potentially affect the parachutists' landing accuracy and safety. Additionally, if the green light duration is less than 20 seconds, it signifies a narrow window for parachutists to exit, which can complicate the control of the drop. The establishment of a partial control group in this context allows for close monitoring and adjusted coordination, ensuring that the drop remains controlled and safe despite the constraints.

Other scenarios, such as multiple aircraft with extended green lights or emergency drop situations, do not typically merit a partial control group under normal operational protocols. These conditions often involve different considerations for safety and coordination that do not align with the specific need for controlled parameters highlighted in the correct answer.