 The measures distinguished between Small Irregular Carriers, services operating light chartered aircraft weighing less than 10,000 pounds when fully loaded (changed in 1949 to 12,500), and Large Irregular Carriers, those flying equipment, the DC-3 for example, in excess of that weight.The small remained unrestricted, but the large carriers faced a host of new economic and safety regulations along with a warning to adhere to 'infrequent and irregular' service.The most important of the new rules was the need for a Letter of Registration in order to fly large aircraft; the CAB issued 142 of these in 1947 but stopped in August 1948 when the board began an investigation of the practices of large irregular carriers, which it accused of operating as scheduled services.