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Under these conditions, the bypass solenoid valve blocks hydraulic pressure to the mode selector valve. However, pressure trapped between the bypass check valves and the thermal relief valve moves the mode selector valve to the bypass position.
The mode selector valve blocks control pressure from the EHSV to the actuator cylinder. It also connects both sides of the actuator through a restriction.
During rapid backdrive of the failed PCU, the compensator/relief valve supplies fluid to the actuator through the anti-cavitation check valves.
Blocking Mode
The PCU is in the blocking mode when the ACE de-energizes the bypass solenoid and both coils of the blocking solenoid. The ACE also de-energizes the pressure reducer solenoid. This happens when the PCU has no electrical and hydraulic power. Also, the PCU is in the blocking mode when there is a failure of both adjacent PCUs or their ACEs and the elevator is near the faired position.
In the blocking mode, there is no hydraulic pressure on the mode selector valve. Its internal spring moves the valve to the blocking position. The mode selector valve blocks control pressure from the EHSV to the actuator cylinder. It also blocks hydraulic flow between both sides of the actuator piston.
Under these conditions, the actuator cannot move. When both PCUs on an elevator are in blocking mode, they lock the elevator in its position.
Indications
The status message FLIGHT CONTROL SYS shows when one or more elevator actuator has failed.

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ELEVATOR CONTROL -ELEVATOR PCU -FUNCTIONAL DESCRIPTION

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ELEVATOR CONTROL -ELEVATOR PCU -FUNCTIONAL DESCRIPTION -PCU MODES

General
The ACEs control the mode of operation of the elevator PCU.
Normal Mode

An elevator PCU operates in the normal mode when there is no failure on the PCU and there is no failure of its on-side ACE.
If the adjacent PCU is also in the normal mode, the ACEs energize both coils of the blocking solenoid and the pressure reducer solenoid. When these conditions are true, both PCUs operate with reduced hydraulic pressure.
Bypass Mode

If the PCU has an electrical or hydraulic failure or there is a failure of its ACE, the ACE de-energizes the bypass solenoid. The PCU is then in the bypass mode. The ACE also de-energizes the pressure reducer solenoid of the adjacent PCU. The adjacent PCU then operates in normal mode with full hydraulic system pressure.
When the elevator position is less than two degrees from neutral with flaps retracted, or five degrees with flaps extended, it is near to being faired. When the elevator is near to being faired, the ACE also de-energizes one coil of the blocking solenoid on the failed and the adjacent PCU. This prepares the two PCUs to go into the blocking mode should there be a failure on the adjacent PCU or its ACE.
Blocking Mode
If there is a failure with the second PCU or its ACE when the elevator is near to being faired, all PCU solenoids de-energize. When it is not near to faired, the energized blocking solenoids on each PCU stay energized.
As soon as the airloads move the elevator panel near to faired, both blocking solenoids de-energize. At this time, a hydraulic lock in the PCUs keeps the elevator in the faired position.

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ELEVATOR CONTROL -ELEVATOR PCU -FUNCTIONAL DESCRIPTION -PCU MODES

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ELEVATOR CONTROL -ELEVATOR ASSEMBLY

Purpose
The elevators supply short-term pitch control of the airplane.
Physical Description

The elevators are light weight composite material structures. The skin is reinforced graphite/epoxy bonded to a core of Nomex honeycomb. The overall dimensions are approximately 70 inches by 403 inches (1.7 m by 10.2 m). Each elevator panel weighs approximately 327 lb (148 kg).
The elevator attaches to the stabilizer rear spar with seven hinges. There are four PCU attach fittings on each elevator. Jumpers and static dischargers supply electrostatic protection.
Functional Description

The elevator surface deflection limits are at the PCU internal stops. The elevators have a maximum movement of 33 degrees trailing edge up (TEU) and 27 degrees trailing edge down (TED) under no load conditions.
Under load conditions, the maximum PFC and ACE command signals move the elevators to a nominal position between 30 degrees TEU and 25 degrees TED. You adjust the elevator PCUs on the ground with the stabilizer at the neutral position. You adjust the PCUs so the elevator control surfaces are at neutral in reference to an index plate on the tail section of the fuselage.
　
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