rohitvats wrote:Just to add: The static line jump happens from much lower altitude and the parachute deploys immediately as the paratrooper exits the aircraft (because the chord used to open the parachute is rigged to a 'static line'). The aim of static line jump is to ensure that maximum number of paratroopers fall on or as close to DZ as possible. Free fall is used by small body of troops. And also requires more training than vanilla para-jump. Not all army paratroopers are free-fall qualified.
You are right that static lines are very useful for short altitude jumps. But they don't help in grouping the paratroopers together. For example, most of the times, Russians don't use a static line. They use a stabilizing chute instead. Some believe that the Russian system groups the paratroopers better. Also, conical parachutes and static lines are not tied together. One can jump from a static line using a rectangular parachute.
Besides, there are other disadvantages of the static line + D-bag system. Because the lines and the D-bags are flaying behind the aircraft, they increase the drag significantly. As a result, the aircraft generally cannot drop the same number of paratroopers as the number that it can carry. Also, two or more personnel have to stay back and pull the lines and D-bags in. Till this is done, the aircraft can't close the doors which act like giant airbrakes. As a result, the aircraft can't accelerate till the D-bags are pulled in and the doors are closed. On the other hand, the Russians paratroopers leave nothing behind on the plane. Hence the plane can drop the same number of paratroopers as it can carry, require nobody to pull the lines in, close the doors as soon as the last paratrooper has exited, and be on their way as soon as possible.
Additionally, the drops can happen at speeds of up to 400 kmph. Paratroopers can exit from the front door (in front of the engine) allowing the equipment to be dropped from the rear ramp simultaneously. On the other hand, the Americans have a separate drop zones for personnel and and a different one for equipment.