Aircraft and spacecraft autopilots are now part of the electronic controller. The hydraulic circuits are similar except that mechanical servo valves are replaced with electrically controlled servo valves, operated by the electronic controller. This is the simplest and earliest configuration of an analog fly-by-wire flight control system. In this configuration, the flight control systems must simulate "feel". The electronic controller controls electrical feel devices that provide the appropriate "feel" forces on the manual controls. This was used in Concorde, the first production fly-by-wire airliner. In more sophisticated versions, analog computers replaced the electronic controller. The canceled 1950s Canadian supersonic interceptor, the Avro Canada CF-105 Arrow, employed this type of system. Analog computers also allowed some customization of flight control characteristics, including relaxed stability. This was exploited by the early versions of F-16, giving it impressive maneuverability. A digital fly-by-wire flight control system can be extended from its analog counterpart. Digital signal processing can receive and interpret input from multiple sensors simultaneously (such as the altimeters and the pitot tubes) and adjust the controls in real time. The computers sense position and force inputs from pilot controls and aircraft sensors. They then solve differential equations related to the aircraft's equations of motion to determine the appropriate command signals for the flight controls to execute the intentions of the pilot. The programming of the digital computers enable flight envelope protection. These protections are tailored to an aircraft's handling characteristics to stay within aerodynamic and structural limitations of the aircraft. For example, the computer in flight envelope protection mode can try to prevent the aircraft from being handled dangerously by preventing pilots from exceeding preset limits on the aircraft's flight-control envelope, such as those that prevent stalls and spins, and which limit airspeeds and g forces on the airplane. Software can also be included that stabilize the flight-control inputs to avoid pilot-induced oscillations. Since the flight-control computers continuously feedback the environment, pilot's workloads can be reduced. Also, it is now possible to fly military aircraft that have relaxed stability. The primary benefit for such aircraft is more maneuverability during combat and training flights, and the so-called "carefree handling" because stalling, spinning and other undesirable performances are prevented automatically by the computers. Digital flight control systems enable inherently unstable combat aircraft, such as the Lockheed F-117 Nighthawk and the Northrop Grumman B-2 Spirit flying wing to fly in usable and safe manners. The Federal Aviation Administration (FAA) of the United States has adopted the RTCA/DO-178C, titled "Software Considerations in Airborne Systems and Equipment Certification", as the certification standard for aviation software.