My use of the word 'simultaneously' is perfectly legitimate. The end result of what happens abord airplane A must SIMULTANEOUSLY be true for both airplane A and airplane B. Otherwise, we have conflicing realities when only one can be correct. I'm not sure how you can come to the conclusion that the collapsing dominos meet in the middle of airplane A according to both airplane A and airplane B. From the reference frame of airplane B there is a difference of 0.75 seconds between each end of the domino chain starting, therefore the meeting point of the collapsing dominos will be closer to the end that starts later. This I don't understand.