An aircraft seat assembly (12) is convertible between a first, narrower configuration of three seats (RA, RN, RW) and a second, widened seat configuration which adds an even increase in space between armrests. A seat assembly (12) includes a tube-in-tube support frame for the seats which allows the conversion to be accomplished by a simple sliding movement of the aisle and middle seat frames (RA, RM) toward and away from the window seat frame (RW) which is fixed in position. Movement of the aisle and middle seat frames (RA, RM) automatically centers the middle seat frame (RM) between the aisle and window seat frames (RA, RW) and automatically changes the position of the two inside armrests to provide even spacing therebetween. A lock mechanism (124, 130) locks the seat frame parts together, both in the first, narrower configuration and in the second, widened configuration. The lock mechanism is locked and unlocked by movement of a handle (H) located at the aisle end of the aisle seat (RA). A key lock is provided to keep the lock mechanism (124, 130) locked to prevent unauthorized or unintended movements of the seat frames to convert the seat assembly (12).