MD-90 Auxiliary Control System

The auxiliary control system (ACS) on the MD-90 automatically controls and monitors eight functions. It also provides fault-data retrieval and maintenance capabilities for the electronic engine controller and engine vibration monitor. Because the ACS is installed only on the MD-90, the flight and maintenance crews for this airplane require additional information about ACS functions and troubleshooting capabilities. Using this information can help them reduce the number and length of MD-90 dispatch delays.

An auxiliary control system (ACS) was installed exclusively on MD-90 commercial airplanes to assist with the operation of several improved features, including powered control surfaces and the elevator hydraulic pressure control system. The ACS performs eight different monitoring and controlling functions related to engine maintenance and control surfaces. It also offers maintenance personnel the capability for fault storage and retrieval. Proper use of the ACS troubleshooting tool is the first step in preventing dispatch delays or the need to replace the system.

The auxiliary control unit (ACU), located in the radio rack, is the primary component of the ACS and performs all functions of the system. It contains two independent computing channels, A and B, and each channel has a dedicated power supply and microprocessor. With two exceptions, either channel can provide the outputs required to perform all ACS functions in the event of a single channel failure. The elevator load feel (ELF) function is an exception because each channel is dedicated to the control of a single actuator only. The engine faults display system (EFDS) function is the other exception because it is included in channel A only.

Proper operation of the ACS requires an understanding of

  1. ACS functions.

  2. ACS troubleshooting and maintenance capabilities.

ACS Functions
Correctly using and maintaining the ACS requires understanding its baseline functions:

Elevator load feel (ELF).
The powered elevator surfaces on the MD-90 cause a loss of natural feedback to the control column. The ELF compensates for this loss with feedback created by the ACS. The ELF controls the position of two independent linear jackscrew actuators that adjust the artificial ELF system as a function of flight condition (fig. 1). These actuators are an integral part of the ELF system. The total feel at the column is the sum of the force established by the springs and ratio changer on one side of the elevator, with the similar force established by the other side. If a channel fails, it parks its actuator at a predetermined position. As a result, less effort than normal may be required to control the pitch of the airplane at high speeds, and greater effort may be required at low speeds.

Elevator hydraulic pressure control (EHPC).
The new powered elevator hydraulic control system on the MD-90 requires a new, automatic means of monitoring and controlling its pressures. The EHPC performs this function by monitoring hydraulic pressure and controlling two hydraulic-pressure bypass valves in the powered elevator hydraulic control system (fig. 2). An annunciation in the flight deck indicates that the flight crew has selected elevator hydraulic power to either the on or off state.

The EHPC uses two motor-operated shutoff valves (MOSV) to control pressure to the EHPC actuators. ACU control of the on/off state of the MOSVs is based on their positions and the pressure sensed at the P1, P2, P3, and P4 locations in the hydraulic system.

Because both regulators apply pressure to the same two tandem hydraulic cylinders, they reduce their nominal 3,000 lb per square inch (psi) to 1,500 psi so that both halves of the cylinders are operated at 1,500 psi. If system pressure on one side of a cylinder becomes too high or too low, the ACU shuts off the pressure on that side by closing the associated MOSV. The regulator on the other side senses the condition and provides full system pressure for operation of one-half of the tandem cylinders at full system pressure.

EHPC valve control is logically divided between the two channels of the ACU. In the event of a single-channel failure, the other channel controls both valves. In the case of a dual-channel failure or loss of power, the valves are left in their most recent positions.

Rudder hook monitor (RHM).
The RHM monitors the position of the Q-bellows-driven rudder limiter and warns the crew of an out-of-range condition (fig. 3). This function, which improves the reliability of the MD-90, communicates rudder-limit status to the digital flight guidance computer (DFGC).

The RHM provides dual-redundant monitoring of the Q-bellows rudder limiter mechanism, and it provides annunciation to the flight deck. The annunciation indicates that the hook is either at its correct position, within a predetermined tolerance band, or in a position beyond this tolerance. The latter causes the rudder to be either overrestricted or underrestricted for the existing equivalent airspeed. Either channel acting alone is capable of providing the RHM function.

Rudder stop limiter (RSL).
The RSL improves the reliability of the airplane by controlling and monitoring the deployment of a backup rudder limiter system that restricts rudder throw to a maximum of 10 deg in either direction from the faired position at airspeeds greater than 215 kn (fig. 4). It also provides flight-deck annunciation.

The ACU controls two solenoid valves that port pressure from the left and right hydraulic systems to two hydraulic linear actuators. Either actuator, when pressurized by its associated solenoid valve, is capable of deploying the rudder stops.

The solenoids are continuously powered by the ACU when the equivalent airspeed exceeds a nominal value of 215 kn. Otherwise, the solenoids are de-energized, hydraulic power is shut off, and the limiter stops are retracted. Position switches allow the ACU to determine when the limiter stops are in the fully deployed or fully retracted position.

Horizontal stabilizer in motion detector (HSMD).
The HSMD eliminates the need for the STABILIZER IN MOTION warning sensors installed on the MD-80. As such, it monitors the position of the horizontal stabilizer, and it causes the central aural warning system (CAWS) to sound a tone for each 1/2 deg of continuous movement. If the autopilot is engaged, the CAWS issues the STABILIZER IN MOTION voice warning after approximately 2 deg of travel.

Clapper spring monitor (CSM).
The CSM monitors the integrity of the ELF clapper spring mechanism and informs the flight crew of failures (fig. 5). A proximity switch in each load feel clapper spring assembly senses the clapper arm position caused by motion of the control column. At these stop positions, the elevators are also at their limit positions if they are in the powered mode. When the clapper arms move to a position corresponding to the elevators at either limit, the clapper proximity switches are actuated. When the column is at neutral, the clapper arms are pulled back to the zero-force feel position, and the switches are not actuated.

Split elevator annunciation (SEA).
The SEA is required on the MD-90 because the DFGC does not interface with the master warning and caution controller to alert the flight crew to a split elevator condition. The DFGC performs a continuous comparison test between the left and right elevator positions. If the test fails, the DFGC will send a split elevator signal to the ACU.

Engine faults display system (EFDS).
Installation of the V2500 engines on the MD-90 led to the requirement for an EFDS. The EFDS function includes starter switch solenoid hold-in, engine no. 4 bearing test, and maintenance capability.

ACS Troubleshooting and Maintenance Capabilities
The ACS MAINTENANCE menu on the MCDU offers five selections to permit various fault-finding and maintenance operations:

Access to the RETURN-TO-SERVICE TEST and the ERASE MAINT MEMORY selections is permitted only when the airplane is on the ground.

This menu can be used as a quick system check of real-time operational status (fig. 6). It displays an alphabetical list of all line replaceable units (LRU) that the ACU considers faulty. If an additional LRU becomes faulty, its name is placed at the end of the list. If a faulty LRU regains operational status while the CURRENT FAULTS menu is displayed, its name is removed from the list, and its position on the list displays is blanked out. Blank lines and names that appear out of alphabetical order are reformatted when the NEXT PAGE function key is selected on the last CURRENT FAULTS page, or when the CURRENT FAULTS menu is exited and reselected. No detail fault information is displayed in the CURRENT FAULTS menu.

On the ground, detail fault information is stored into volatile memory and is lost when the airplane powers down. Selecting RECORD FAULTS on the CURRENT FAULTS menu stores the detail fault information into nonvolatile memory. Selecting LED 01 on the FAULT REVIEW menu retrieves stored fault information. In flight, detail fault information is automatically stored into nonvolatile memory.

These menus provide detail fault information that the ACU has stored during the current and previous flights (fig. 7). A list of flight legs with stored faults is displayed when FAULT REVIEW is selected from the ACS/EFDS LRU MAINTENANCE menu. Flight legs without stored faults are not displayed. The most recent leg is displayed first, and the remaining legs appear in reverse chronological order.

When a flight leg is selected, the following detail fault information is available (fig. 8): LRU associated with the failure, flight mode or flight-deck effect message, flight number, altitude, airspeed, date and time, additional data prompt, and specific fault message. The MD-90 Aircraft Maintenance Manual describes proper corrective action for each fault message.

The RTS TEST examines the ACU and all its external connections. It is used following a maintenance action to verify proper operation of an LRU, and it performs diagnostic testing of the system to identify and isolate failures that may have been detected by the maintenance monitor. An RTS screen appears on the monitor to provide notification if the system contains one or more latched failures stored in nonvolatile memory. The screen indicates the need to perform a complete RTS TEST.

When the ACS is in ground mode, the ERASE MAINT MEMORY function permits the operator to clear maintenance memory of all faults that were stored in fault review history.

SENSOR READOUTS provide a real-time display of the following ACU input/ output sensor values:

The MD-90 ACS is a highly efficient tool intended to serve as a control, monitoring, fault-storage, and fault-retrieval system. In addition to these functions, it provides flight-deck annunciations regarding status of the elevator, rudder, stabilizer, and engines. When used properly by both flight and maintenance crews, the ACS can reduce the number and length of MD-90 dispatch delays.









Tim Pang
Avionic Systems
Boeing Long Beach

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