Southwest Airlines said Friday it will begin offering Internet service on all 540 planes in its fleet by 2012. The airline, the largest carrier at BWI Marshall Airport, has been testing service on some planes for several months. It hasn't decided how much it will charge for the service. The airline said it has signed an equipment purchase contract with Wi-Fi provider Row 44 and will begin installing it in the second quarter. It expects to install equipment on around 15 aircraft per month initially, with the goal of increasing that number to 25 aircraft until it is on every flight.