FUNCTIONAL DESCRIPTION
The elevator trim subsystem consists of the following components:
FIN Component Panel Zone Access Door
52CC RH elevator trim actuator 342 342-AB
53CC LH elevator trim actuator 332 332-AB
33CC captain's (LH) and first officer's (RH) main elevator trim switch
210
63CC standby elevator trim switch 10CC 210
69CC elevator trim reset switch/light 10CC 210
50CC trim test switch 30VE (LH) 210
68CC main trim circuit lockout relay 29VE 310
55CC trim speed mode relay 29VE 310 311-AB
51CC elevator trim test relay 51CC 210
38CC, 58CC
nose up control relay 23VE 210
49CC, 59CC
nose down control relay 23VE 210
39CC nose down runaway override relay 23VE 210
48CC nose up runaway override relay 23VE 210
42CC, 45CC
trim master relays 23VE 210
54CC auto-trim relay 23VE 210
64CC stby elevator control relay (nose up) 23VE 210
65CC stby elevator control relay (nose down) 23VE 210
31CC, 61CC
circuit breaker 12VE 210
13CX, 14CX
circuit breaker 18VE 210
15CX fast/slow motor current sense relay 29VE 310 311-AB
LH (RH) Elevator Trim Actuator 53CC (52CC)
Flexible Shaft
A
A
RH Elevator Trim Actuator
Reduction Lever
LH and RH Trim Tab Connection
Reduction Lever
LH Elevator Trim Actuator DETAIL
Horizontal Stabilizer Attachment
Fig. 34 Elevator Trim Actuators
The LH elevator trim actuator is installed in the root trailing edge section of the horizontal stabilizer in line with the LH trim tab. A similar actuator is installed in line with the RH trim tab. The following description applies to both actuators.
The actuator has two separate (fast and slow) DC motors. Which motor control circuit is in operation depends on the airspeed. At airspeeds below 200 knots the fast motor circuit is operative and above 200 knots the slow motor circuit. The fast and slow motors of one actuator are electrically connected in parallel with the associated motors of the other actuator. The actuators are controlled by either:
- main elevator trim switches on the captain's and first officer's control wheels - a standby elevator trim switch on the trim control panel
- the automatic flight control system (AFCS) in the auto-trim mode
Internal limit switches stop actuator movement before the mechanical limits are reached.
The rotary output of the motors is converted in a reduction gearbox to a dual shaft linear output. Total shaft travel between the electrical limits is 25 mm (0.98 in.). The twin shafts of the actuator are connected to a lever in the leading edge of the elevator. Two push-pull rods connected to the output side of the lever transmit the actuator movement to the tab.
Two shunts are installed, one in series with each motor. The shunts are used for sending current sensing signals to the EICAS. If one actuator is not drawing current and the other actuator is, a caution message indicating a trim motor failure is displayed on the EICAS.
For synchronization purposes, the two actuators are mechanically interconnected by a flexible shaft. If one or both motors of an actuator fail, the other actuator will:
- drive its associated trim tab directly
- drive the other trim tab indirectly through the failed actuator.
The actuator has an integral synchro-type position transmitter. The transmitter sends an elevator trim position indicating signal to the EICAS.
Main Elevator Trim Switches 33CC
Dornier 328 Dornier
328
A
Elevator Trim Switch (Captain) Elevator Trim Switch (First Officer)
A B
Fig. 35 Elevator Trim - Controls
The captain's main elevator trim switch is installed on the outboard horn of the LH control wheel. An identical switch is installed for the first officer on the outboard horn of the RH control wheel. The following description applies to both switches.
Each elevator trim switch is labelled UP and DN and contains two three-position center biased rocker switches. Both rocker switches must be activated together to operate the elevator trim actuators. The two rocker switches have a total of eight sets of contacts (four per switch).
Standby Elevator Trim Switch 63CC
The standby elevator trim switch is installed in the aft RH section of the trim control panel in the center pedestal. The switch is labelled DN ELEV UP and is identical to the main trim switches 33CC. Its purpose is to control the elevator trim actuators should a fault occur in the main trim control circuit. A lockout circuit is activated when the standby trim switch is operated. This circuit disables the main trim (and auto-trim) circuits of the elevator trim actuators. These circuits then remain disabled until the reset switch/light 69CC is pressed.
Elevator Trim Reset Switch/Light 69CC
A lockout circuit disables the main trim (and auto-trim) circuits when the standby trim switch is operated.
The lockout circuit remains active even after the trim operation has been completed. The main trim circuits can be re-enabled by pressing an elevator trim reset switch/light. The switch is labelled PRESS TO RESET and is installed in the trim control panel to the left of the standby elevator trim switch. A white STBY ELEV annunciator in the front face of the switch comes on to indicate when the standby trim circuit - and thus the lockout circuit - is active. The switch/light is guarded by a transparent spring-loaded cover.
Trim Test Switch 50CC
The trim test push switch is installed in the LH engine/propeller test panel 30VE located in the forward LH section of the center pedestal. The switch is labelled TRIM TEST and is used to test the trim runaway override circuits of the following systems:
- elevator trim system - aileron trim system - rudder trim system
To test the elevator trim runaway override circuit, press and hold the TRIM TEST switch and then press either the LH or RH main elevator trim switch to UP and then DN. If the runaway override circuits are serviceable the actuator will operate in the reverse direction to that commanded. When the test switch is released the actuator will start again in the commanded direction.
OPERATION
Power Supplies
The elevator trim subsystem is supplied with 28 VDC and 26 VAC electrical power as follows:
Component Bus Circuit Breaker No.
trim actuator main control and power circuit
ESSENTIAL BUS 3PP ELEV TRIM 31CC
trim actuator standby control and power circuit
BUS 1 2PP STBY EL TRIM 61CC
LH trim actuator position indicating circuit
BUS 4XP ELEV TRIM IND LH 13CX
RH trim actuator position indicating circuit
BUS 6XP ELEV TRIM IND RH 14CX
System Power-Up
The elevator trim control and indicating circuits are energized when 28 VDC and 26 VAC
Trim Control
Note:
The captain's and first officer's main trim control circuits are similar. The operation of the captain's control circuit is described.
If both rockers of the captain's (first officer's) trim switch 33CC on the LH (RH) control wheel are held to UP, the following contacts operate:
- series contacts A1/A3 and B1/B3 close and supply 28 VDC to the nose up circuit
- parallel contacts A2/A4 and B2/B4 open and remove the ground from the nose up fault protection circuit.
Main Trim Control
The captain's (first officer's) elevator trim master relay 42CC (45CC) now energizes through diode 40CC (43CC), followed by the nose up control relays 38CC and 58CC. The diode 41CC (42CC) prevents cross coupling from the nose up to the nose down circuits. Note that initially, the ground circuit for the coils of 42CC (45CC) and 38CC is formed by the diode 79CC, contacts B2/B3 of the test relay 51CC and the nose down contacts of the first officer's and captain's trim switch. This ground circuit is replaced by contacts B1/B2 of relay 58 CC when this relay energizes. 28 VDC is now supplied to the nose up side of both actuators through the following series circuit:
- contacts D1/D2 of relay 38CC - contacts D1/D2 of relay 58CC - contacts B3/B2 of relay 48CC - contacts A3/A2 of relay 54CC - contacts B3/B2 of relay 68CC
- the contacts of the RH and LH actuator internal nose up limit switches
- the contacts of relay 55CC. Relay 55CC is the trim speed mode relay controlled by MADC 1. If the relay is de-energized (airspeed below 200 knots), electrical power is connected to the fast motor of each actuator. If the relay is energized (airspeed above 200 knots) the slow motors will run.
The ground circuit for the motors is provided by the following series circuit:
- the contacts of relay 55CC
- the contacts of the LH and RH actuator internal nose down limit switches (and then direct to ground if either actuator is at its maximum nose down travel)
or by the following series circuit:
- contacts C2/C3 of relay 68CC - contacts B2/B3 of relay 54CC - contacts B2/B3 of relay 39CC - contacts D2/D3 of relay 59CC - contacts B2/B3 of relay 49CC.
The actuators run in the commanded direction until either the trim switch is released or until the nose up internal limit switches operate. The operation sequence is similar when nose down is commanded, except that nose down control relays 49CC and 59CC energize instead of 38CC and 58CC.
The captain's and first officer's main trim switches form mutually exclusive circuits for the trim master relays. If, example, the captain's switch is set to UP and the first officer's switch is set to DN, neither relay 42CC nor 45CC will energize and the complete trim circuit is inhibited until one of the switches is released.
Standby Trim Control and Main Trim Lockout
The elevator trim standby circuit is controlled by the standby elevator trim switch 63CC. If both rockers of the switch are held to UP, the following contacts operate:
- series contacts A1/A3 and B1/B3 close and supply 28 VDC to the standby nose up relay 64CC - parallel contacts A2/A4 and B2/B4 open and remove the ground from the nose up fault protection
circuit.
328 DORNIER LUFTFAHR T
70- FOR TRAINING PURPOSES ONLY -Effectivity:328-100 Mod.10
TM-Flight Controls e auto-trim circuit
elay 64CC
T = change over(transition)
Electrical Schematic for:
Main Trim Switch 33CC/63CC
Fig. 36 Elevator Trim - Electrical Schematic (Sheet 1 of 3)
Dornier 328 TR AINING MANUAL
LINE /BASE MAINTENANCE- FOR TRAINING PURPOSES ONLY -Page: 27-Issue: 08/95 ETM/ETX
TM-Flight Controls
SIGN 646 SIGN 6
11TTDAU2 refer to ATA 22
autotrim
1
Fig. 37 Elevator Trim - Electrical Schematic (Sheet 2 of 3)
53CC
refer to ATA 22
refer to ATA 22 C1
Fig. 38 Elevator Trim - Electrical Schematic (Sheet 3 of 3) Actuator Speed Control
The fast and slow modes of the elevator trim actuators are controlled automatically by micro air data computer 1 (MADC 1) as a function of the airspeed. Refer to (Chapter 34 - NAVIGATION) for details on the MADC. The MADC controls two relays (55CC and 15CX) in the elevator trim circuit. At airspeeds below 200 knots the ground circuit for the coil of each relay is open and the relays are de-energized. At airspeeds above 200 knots the relays are energized.
When the relays are de-energized (below 200 knots):
- contacts A and B of relay 55CC connect the trim control circuit to the fast motor of the LH and RH trim actuator respectively
- contact C of relay 55CC sends a fast speed mode indicating signal to the EICAS
- contacts A and B of relay 15CX send a fast motor sensing signal from the LH actuator to the EICAS - contacts C and D of relay 15CX send a fast motor sensing signal from the RH actuator to the EICAS.
When the relays are energized (above 200 knots):
- contacts A and B of relay 55CC connect the trim motor control circuit to the slow motor of the LH and RH trim actuator respectively
- contact C of relay 55CC sends a slow speed mode indicating signal to the EICAS - contacts A and B of relay 15CX send a slow motor sensing signal to the EICAS
- contacts C and D of relay 15CX send a slow motor sensing signal from the RH actuator to the EICAS.
Main Trim Runaway Override
An electrical fault in the actuator control circuitry, such as sticking contacts in one of the main trim switches, could initiate a trim runaway. Override circuits ensure that a runaway of the trim actuators can be stopped and reversed by instinctive operation of the same main trim switch in the opposite direction to the runaway. Note that an attempt to override a runaway with the opposite trim switch will cause the actuators to stop but not to reverse.
The following example describes the operation of the circuit if a runaway towards nose down occurs due to sticking contacts in the LH main trim switch 33CC.
Relay 42CC is the captain's elevator trim master relay. Relays 49CC and 59CC are the nose down control relays. All three relays energize when the LH trim switch 33CC is held to the DN position. 49CC and 59CC connect 28 VDC to the actuator nose down circuit. If contacts A5/A7 and B5/B7 of the captain's trim switch stick remain closed when the switch is released the three relays will remain energized. This will cause the trim actuators to run uncommanded towards nose down.
The trim runaway will be indicated to the flight crew by the continuing pitch down movement and by the elevator trim position indicator on the EICAS. The runaway can be stopped and reversed by holding the LH trim switch to the UP position. This energizes the nose down runaway override relay 39CC through:
- contacts B1/B2 of relay 42CC - contacts A2/A3 of relay 58CC - contacts C2/C1 of relay 59CC.
When UP is selected the nose up control relays 38CC and 58CC energize. Note, however, that relay 39CC must be allowed to energize before 58CC. This is achieved by connecting the coils of 38CC and 39CC in parallel so that they energize together. 58CC cannot energize until 38CC (and thus 39CC) has energized. Once 39CC is energized it is held closed by its own contacts A1/A2. Contact B of relay 39CC interrupts the 28 VDC supply to the actuator nose down circuit and completes the ground circuit for the nose up circuit.
Relays 38CC and 58CC connect 28 VDC to the nose up circuit of the actuators. The actuators now reverse direction and override the trim runaway. If the fault still exists on the trim switch when the overriding switch is released, the actuators will again run uncommanded towards nose down. The actuator main trim circuit can be disabled by momentarily operating the standby trim switch 63CC to UP or DN.
The operation sequence is similar in reverse if the trim runaway is in the nose up direction. The operation sequence is similar if the runaway is caused by the RH main trim switch 33CC.
Runaway Override Test
The TRIM TEST push switch 50CC is used to test the trim runaway override circuits of the following systems:
- elevator trim system - aileron trim system - rudder trim system
To test the elevator trim runaway override circuit, press and hold the captain's or first officer's main elevator trim switch to UP or DN. The trim actuators will operate in the commanded direction. Now press and hold the TRIM TEST switch. If the runaway override circuits are serviceable, the actuators will reserve direction. When the test switch is released the actuators will start again in the commanded direction. The following example describes the operation of the circuit when the LH (captains) elevator trim switch 33CC is held to UP and the test switch pressed.
When the LH main trim switch 33CC is held to the UP position, relays 42CC, 38CC and 58CC energize as previously described. If the TRIM TEST switch is now pressed and held, the trim test relay 51CC energizes. The nose up runaway override relay 48CC now energizes through:
- contacts B1/B2 of relay 42CC - contacts A2/A1 of relay 58CC - diode 75CC
- contacts B1/B2 of relay 51CC - contacts A2/A3 of relay 59CC - contacts C2/C1 of relay 58CC.
Once 48CC is energized it is held closed by its own contacts A1/A2. The nose down relays 49CC and 59CC also energize through the B contacts of relay 51CC and connect 28 VDC to the nose down circuit of the trim actuators. The actuator ground circuit is completed through contacts B2/B1 of relay 48CC.
The actuators now reverse direction.
The test sequence is similar in reverse when the LH trim switch is held to DN. The operation sequence is similar if the RH main trim switch 33CC is tested.
Main Trim Circuit Fault Protection
Protection circuits disable the main trim control circuit should an unwanted connection with 28 VDC occur. This is achieved by connecting critical wiring to ground through a series/parallel contact arrangement of the main trim switches.
The protected circuits are:
- the main nose up and nose down control circuits when no main trim switch is operated - the main nose up control circuit when a main trim switch is operated to nose down - the main nose down control circuit when a main trim switch is operated to nose up.
The main nose up control circuit is connected to ground through the following series connections:
- contacts A2/A3 of relay 51CC
- contacts B2/B4 and/or A2/A4 of the RH main trim switch 33CC - contacts B2/B4 and/or A2/A4 of the LH main trim switch 33CC.
The main nose down circuit is connected to ground through the series circuit:
- contacts B2/B3 of relay 51CC
- contacts B8/B6 and/or A8/A6 of the RH main trim switch 33CC - contacts B8/B6 and or A8/A6 of the LH main trim switch 33CC.
Any unwanted 28 VDC connection will cause the ELEV TRIM circuit breaker 31CC to trip and remove all power from the main trim control circuit.
Diodes in the main trim control circuits prevent cross-coupling between:
- the captain's nose up and nose down control circuits (diodes 40CC and 41CC)
- the first officer's main nose up and nose down control circuits (diodes 43CC and 44CC).
Standby Trim Circuit Fault Protection
Protection circuits disable the standby trim control circuit should an unwanted connection with 28 VDC occur. This is achieved in a similar manner to the main trim control circuit.
The protected circuits are:
- the standby nose up and nose down control circuits when the standby trim switch 63CC is not operated
- the standby nose up control circuit when the standby trim switch is operated to nose down - the standby nose down control circuit when the standby trim switch is operated to nose up.
The standby nose up circuit is connected to ground through contacts B2/B4 and/or A2/A4 of the standby trim switch 63CC. The standby nose down circuit is connected to ground through contacts B8/B6 and/or A8/A6 of the standby trim switch 63CC. Any unwanted 28 VDC connection will cause the STBY EL TRIM circuit breaker 61CC to trip and remove all power from the standby trim control circuit.
Diodes in the standby trim control circuits prevent cross-coupling between the standby nose up and nose down control circuits (diodes 67CC and 66CC).
Auto-Trim
Signals from the automatic flight control system (AFCS) are used to operate the trim actuators for auto-trim. Auto-trim is enabled automatically by a discrete 28 VDC signal from an integrated avionics computer (IAC 1). The signal controls a relay 54CC in the elevator trim circuit. If the auto-trim signal is not available from the AFCS, relay 54CC is de-energized and:
- its contacts A and B put the trim actuators under the control of the main trim control circuit
- its contacts C and D ensure that the feedback lines from the actuator control circuit to the IAC are open.
When the auto-trim signal is available relay 54CC energizes and:
- its contacts A and B switch control of the trim actuators from the main trim control circuit to the IAC - its contacts C and D close the feedback circuit from the actuator control circuit to the IAC.
- its contacts A and B switch control of the trim actuators from the main trim control circuit to the IAC - its contacts C and D close the feedback circuit from the actuator control circuit to the IAC.