On the MD80, you have to arm the altitude after you select it on the MCP. I would like to know why that is, since it seems pretty dangerous to me, the plane can fly higher or lower than what you requested if you forget to arm the altitude (even if there is an alarm).
Is is because it was technically impossible when it was designed (I do not really believe in this one).
Is it a pilot request ? If so, why ?
I also would like to know if pilots find this irritating or it does not matter to them ?