SEC STAB TRIM FAIL
(Caution)

Displayed when the Secondary Channel is selected and inoperative.

The pilot action is to select the other stabilizer trim channel using the Pitch Trim Select switch.

The secondary pitch trim channel is selected and the DCU is not receiving the secondary valid 1 and secondary valid 2 discrete signals.

The fault is detected if the MCU is not sending Valid discrete signal to the HST ECU or if the pilot's and the copilot's trim switch are defective and The HSTA brake 2 and brake 4 drivers are OFF (i.e. brakes are engaged) or the trim is commanded to operate.

NOTE: When no power is applied, the HSTA brakes are engaged. During a trim command, power is sent to the applicable solenoids to release the brakes.

• Horizontal Stab Trim Actuator (HSTA) Secondary Motor
• Horizontal Stab Trim Electronic Control Unit (HST ECU) (A130)
• Motor Control Unit (MCU) (A131)
• Horizontal Stab Trim Actuator (HSTA) (MPE44)
• Associated Wiring

Advisory Wire/Service Bulletin: None

Full Throttle Blog/Forum Articles/Infoservice/Newsletter: None

Flight Operation Notifications Manual (FONM): None

1. Interrogate MDC CURRENT FAULTS.
2. Perform Operational test of the Pitch Trim System.

   1. Turn the TRIM STAB switch to SEC.
   2. Aircraft on Ground: Pull the circuit breakers that follow and reset after a few seconds:

      • PRI STAB TRIM/RUD LIM, CB1-B15
      • SEC STAB TRIM/RUD LIM, CB2-B13
      • PRI STAB MOTOR, LCDPC CB5-A4
      • SEC STAB MOTOR, RDCPC CB6-A5

   3. If the fault cannot be cleared, verify the Pilot and Copilot Trim Switch:
      The SEC STAB TRIM FAIL message will be triggered if both trim switches are defective. Through the MDC, identify if the Trim Switch are defective (Bit Definitions: 1 = Failed).

      • HST ECU Label 352 Bit 27 (Copilot)
      • HST ECU Label 352 Bit 26 (Pilot)

      If required, do the continuity check of the Trim Switches as per Wiring Manual.

3. Perform Operational test of the Pitch/Roll Trim Switch.
4. Perform Reset procedure for HST ECU:
   Reset Procedure For Horizontal Stabilizer Trim Electronic Control Unit
   This procedure provides instructions to the flight crew on how to clear Pitch Trim related fault messages when the aircraft is on ground. The HST ECU RESET PROCEDURE can be used whenever one of the following messages are posted:

   • PRI STAB TRIM FAIL (Caution)
   • SEC STAB TRIM FAIL (Caution)
   • STAB TRIM FAULT (Advisory)

   HST ECU RESET PROCEDURE

   1. PTSS.................................................................................................PRI
   2. PULL PRI STAB TRIM / RUD LIM CCB (CB1-B15)......................OPEN
   3. PULL SEC STAB TRIM / RUD LIM CCB (CB2-B13)......................OPEN
   4. INSERT PRI STAB TRIM / RUD LIM CCB (CB1-B15)....................CLOSED
   5. INSERT SEC STAB TRIM / RUD LIM CCB (CB2-B13)....................CLOSED
      Wait 45 seconds for HST ECU SPOST to complete. Ensure no pitch trim related faults are posted.
   6. PTSS..................................................................................................SEC
      Verify SEC STAB TRIM FAIL (amber) or STAB TRIM FAULT (cyan) are not posted.
   7. PTSS..................................................................................................PRI

5. Examine HST ECU:

   1. The HST ECU has a number of LEDs that are visible when the unit is powered up. There are basically three sets. Right, Middle and Left. The main ones of concern are the Right and Left lights.
   2. If there are no lights present on the right or left side of the unit this means either the command or monitor side is not getting power.
   3. When this happens reapply power and monitor unit. There should be a Fault present on the EICAS.
   4. If this does not clear the fault, try to re-rig the unit.
   5. If this still does not clear the fault return the unit as loss of either Command or Monitor Power.

6. Perform removal and Installation of HST ECU:

   1. Beware of HST ECUs that are hard to install and remove.
   2. This is an indication of the ARINC 600 connector not aligning up properly.
   3. This can cause damage of the rack connector and the connector on the unit.
   4. This can also cause a variety of intermittent faults.
   5. To correct loosen the four rack screws that hold the connector in place. Then install and remove the unit until the unit can be seated properly.

7. Perform Electrical rigging of the Pitch trim Control System.

   1. Some faults can be cleared by re-rigging the unit.
   2. The HST ECU stores the specific rig values for the HSTA and RTLA. Any changes to these units CAN cause faults. Rig the HST ECU.

8. If the fault persists continue as follow until the fault is found: (ref WM 27-41-23).

   1. On the Data Concentrator Unit (DCU) connector A14BP1, verify that you have 28 VDC coming in from the HST ECU for the following Pin No.

      • PIN 1H - SECONDARY VALID number 1 IN.
      • PIN 5K - SECONDARY VALID number 2 IN.

   2. The DCU will not receive the 28 VDC from the HST ECU if the HST ECU does not receive a valid signal from the Motor Control Unit (MCU).
      The MCU Channel 2 shall compute the signal MCU Valid. The MCU channels each contains monitoring for detecting the following types of failures:

      • Motor powered when motor is not enabled.
      • Motor enable disagree with motor brake mode.
      • Motor turning in the wrong direction.

   3. Verify the MCU power with the MDC (Bit Definitions: 1 = Failed):

      • The MDC Label 352 Bit 14 (MCU Secondary Power On Valid).
      • The MDC Label 272 Bit 27 (MCU Secondary Power On (Secondary Channel Relay engage)).

      If failed do the MCU power check as per Wiring Manual.

   4. On the HSTA, do the continuity check of the Motor 1 Brake coil 2 and the Motor 2 Brake coil 4. Ref. WM 27-41-05 Connector MPE44P3 and MPE44P2.
   5. If the fault cannot be cleared, replace the following Line Replaceable Unit (LRU):

      • MCU
      • HST ECU
      • HSTA Motor Brake 1 or 2
      • HSTA

9. Do close out.