A plane is outside of the normal human habitat, thus the analogy doesn't work.
Not a valid analogy.
There are two cases where it is done: people who are brain dead with no hope of recovery often have life support terminated, and people who choose it either through a living will or assisted suicide. In both of these cases, I think it is moral.
There is an inconsistent standard applied here by many religions: If a fetus is a full-human with a soul, then birth should not be a factor: why does entry into heaven require baptism after birth (why not have a ceremony where you baptise the woman's stomach?)? Why don't people have funerals for month-old fetuses? [legally] Why do we have "birth certificates" and not "conception certificates"?
It is my understanding that a significant fraction of fertilized eggs never implant into the uterus (can a biologist confirm this please?) or are otherwise lost (menstration doesn't stop?) within weeks of conception. Doesn't this pose a problem for the life-starts-at-conception view?