The FAA's thinking changed in the 1980s with the creation of the new commuter category of certification. A number of turboprops used in commuter flying at the time were bumping up against the 12,500-pound certification limit for small airplanes. It was impractical to modify these airplanes to the more stringent transport category rules so that they could operate at higher takeoff weights, so the commuter category was created. To certify in commuter category an airplane has to meet many, but not nearly all, of the transport rules, but, in general, two pilots are required.
An early airplane approved in the commuter category was the Beech King Air 300, which very closely resembles the Super King Air 200, except the 300 has more powerful engines and a maximum takeoff weight above 12,500 pounds. Pilots flying the 300 need a type rating because it is in the commuter category, and they must observe balanced field takeoff requirements. But the cockpit and pilot workload of the 300 is virtually identical to the 200 that requires neither a type rating or second pilot. In an admirable flash of absolute logic the FAA recognized that it is the pilot, not the airplane, that makes the difference when deciding on how many pilots are needed. Clearly the King Air 200 had a long and successful record being flown by single pilots before the 300 was created, so the airplane and its workload was not the issue. It was obvious that the 300 could be flown safely by a single pilot, but the FAA didn't want to give up the safety standards it sets for large airplanes. The answer was the single-pilot type rating. The way it works is that the airplanes - the King Air 300, 350, CJs, Beech Premier, Mustang and many more - are approved for single-pilot operation, but the pilot must have a type rating that qualifies him to fly solo. To earn that rating the pilot must be trained and checked under an approved program. These airplanes are eligible to be flown by a single pilot, but not just any single pilot.
The FAA still begins with the assumption that a jet or airplane weighing more than 12,500 pounds will require a crew of two and it's up to the manufacturer to demonstrate otherwise. Approval for single-pilot operation is one of the final steps in certification of a new business jet, or an airplane in the commuter category.
Many of the requirements for single-pilot operation are obvious. For example, the landing gear handle needs to be easily reachable from the left seat, which is not always the case in jets. The design of the basic controls is also crucial. For example, it would be almost impossible to get single-pilot approval for a jet with tiller steering for the nosewheel because at crucial times during takeoff or landing the left seat pilot needs one hand for the tiller and the other for the throttles, with nothing left for the flight controls. All other controls, switches and essential items must also be located so a single pilot can easily see and reach them. And the autopilot must be fully and seamlessly integrated into the airplane and its navigation systems, and the complete autopilot system must be functional for every flight with only one pilot.
After the FAA determines that a pilot can physically manage the cockpit from the left seat, actual flight testing determines if the airplane will be approved for single-pilot flight. What typically happens is that FAA certification pilots fly the new type from the left seat through various maneuvers, but most importantly, the pilots fly typical flights to determine the workload. The FAA pilots fly the airplane into busy airspace on standard IFR clearances with no special ATC handling and with no help from a copilot. And the FAA pilots do not have a great deal of time in the airplane because it is, of course, new, so they are taking a fresh look at the tasks a pilot must perform.