If you aren't willing to redesign then you can put something LIKE this in the after update event of the Aircraft_Reg combo box/textbox
Select Case Me.Aircraft_Reg
Case A5-EAA, A5-EAB, A5-EAC, A5-EAD
Me.Aircraft_Type = "A320"
Case A5-RAA, A5-RAB, A5-RAC, A5-RAD
Me.Aircraft_Type = "B737"...