 In this case, aircraft sent on minelaying sorties almost always returned while those on other intruder missions were almost always intercepted.These aircraft had to drop their payloads from very low altitudes, so they generally followed routes over water, including rivers, flying at low altitudes for most of the mission.At first it was not obvious why they were surviving, as there could be many reasons; fighters might not be able to see them against the ground, AA guns positioned inland might not be able to aim at them, etc.