At the end when helicopter is arriving they that must board fast because the helicopters cannot stay under the radar for long. This makes no sense. Planes/helicopters can fly under the radar because of the curvature of the earth, the position of the radar or certain terrain features blocking the beam. Unless the radar is moving there is no time limit of how long a plane/helicopter can stay under the radar. Staying under the radar depends only on the location and flying height.