Unless you are the type of traveler who prefers to shut down all possibility of contact with fellow passengers by lowering it into place immediately upon boarding an airplane, you may have noticed that flight attendants will often ask you to lower your upright armrest prior to takeoff. Nervous fliers might wonder why such a procedure is routine when it would surely make it more difficult to escape from seats during an emergency — after all, that is precisely the reason why tray tables must be stowed and seats in the upright position.
One explanation we've heard from a flight attendant is that it prevents passengers from moving around — and potentially slamming into their neighbors — in the event of turbulence. At Quora, however, one poster has a more thorough explanation, for which he invokes Newton's first law of motion: "An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force."
In the airplane scenario, the armrest is that object in motion. "It is attached to the aircraft," he explains, "which is typically traveling at 135 to 155 knots for take off and landing. If the aircraft was to come to a sudden halt, the armrest would swivel forward at the same speed as the aircraft was at prior to halting — i.e, the armrest could slam into your side with a force as if it was traveling at more than 135 knots," which could immobilize the passenger, and, therefore hinder any necessary evacuation.
So it comes down to safety, after all — as these things often do.
But, if you decide to keep the armrest down, be sure to follow airplane etiquette to determine who gets to rest on it.
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