power boost. The brake valves are controlled via the rudder pedal toe brakes through mechanical linkages. Two shuttle valves in the pressure lines prevent fluid feedback between the pilot's and copilot's pedals. Four additional shuttle valves connect the pneumatic system to the brake system for emergency braking. An integral anti-skid system in installed to affect maximum braking efficiency. In order to minimize heat build-up in the brakes and reduce brake wear, pilots are advised to deploy the spoilers upon touchdown. Wheel speed transducers in each main wheel axle induce an AC frequency on the DC voltage input proportionate to the wheel speed as they are driven by the wheels. This frequency is compared to a normal deceleration curve and if it deviates it activates a small torque motor in the affected wheel control valve which shunts braking pressure to the return line by means of a spool valve. As the wheel rotation speed accelerates to normal tolerance limits, normal braking pressure is restored. Fuel The Learjet 25 generally has five fuel tanks. Two wing tanks, a fuselage tank and two wing tip tanks. Each wing tank extends from the centre bulkhead outboard to the wing tip and provides separate fuel for each engine. A tank cross-flow valve is installed to prevent fuel transfer between wing tanks. Flapper-type check valves, located in the various wing ribs, allow free fuel flow inboard but restrict outboard flow. A jet pump and electrical boost pump are mounted in each wing tank near the centre bulkhead to supply fuel under pressure to the respective engine fuel system. The tip tanks provide additional fuel capacity to enable longer times aloft. A jet pump installed in each tip tank transfers fuel into the wing tanks. Fuel can also flow via the flapper check valves into the wing tanks, but the lower half of the fuel in the tip tanks must be transferred with the jet pump. Most Learjet 25 aircraft were fitted with a fuselage tank. The fuselage tank can be filled by the wing boost pumps through the transfer line and the fuel transfer valve. When the tank is full, a float switch de-energizes the wing boost pumps and closes the valve. During fuel transfer, the fuselage tank transfer pump pumps fuel into both the wing tanks. Electrical systems The aircraft is equipped with AC (Alternating Current) and DC (Direct Current) electrical systems. The DC system