Last week’s downing of Malaysian Airlines Flight 17 over disputed territory of Eastern Ukraine has set off worldwide outcries against the conflict. But is it even possible to defend civilian jets against military-grade weapons? We need to look just how broad the range of threats they face is, and why existing countermeasures are ultimately ineffective. There are several methods for doing so: active homing, passive homing and semi-active homing. Active homing allows for “fire and forget” missiles that don’t need to be monitored throughout their flight, but does come at the cost of requiring a much larger, heavier and more expensive missile.”]
Source: https://gizmodo.com/why-cant-we-protect-commercial-planes-from-attacks-1606634061