isolation valve and directional control valve solenoids mounted in the HCU. The transmission of these signals from the EEC is dependent upon the 28 V supply and the weight on wheels switch within the EEC. The supply of the signal to the directional control valve solenoid is also dependent upon the closure of the aircraft permission switch in that line. This switch is closed by the Throttle Lever Angle signal via the spoiler/elevator computer and the Engine Interface Unit energization of the isolation valve solenoid and the directional control valve solenoid allows hydraulic pressure into the system - this event being relayed to the EEC by the pressure switch mounted in the HCU. Pressure in the lower actuators releases the locks and these events are signaled to