SERVICE DIRECTIVE BULLETIN NO. T-039 

 Revision 3 

Page 1 of 10 

 

DATE: July 6, 2011 

1. SUBJECT: Cyclic Trim System – Cyclic Trim Assembly Kit for the Lateral and 

 Longitudinal Trim Actuator Assemblies 

2. MODEL: TH-28, 480, 480B 

3. EFFECTIVITY: S/N 5138 and prior 

4. REFERENCE: Enstrom TH-28/480 Series Maintenance Manual, Latest Revision 

5. BACKGROUND: 

Enstrom has received two reports of failure in the cyclic trim system in the field. A failed 

trim relay has reportedly caused the trim actuator to runaway and bottom out against the 

motor housing. Such an event will cause the circuit breaker to trip, leaving the trim fixed. If 

the trim runaway occurs in the aft or right direction, the actuator will bottom out and no loss 

of control authority will occur, although the cyclic control forces will be higher than normal. 

On further analysis of the condition, it was determined that if the trim runaway occurs in the 

forward or left directions, the actuator will run until it fouls the limit switch bracket and the 

circuit breaker trips. In this case, there may be a significant loss of control authority in 

addition to the higher control forces which could result in an unsafe condition. 

This condition can only occur on aircraft equipped with P/N 4199059-3 Trim Switch 

Module Assembly containing a M83536/10-024M Trim Relay (Figure 1). Aircraft equipped 

with P/N 4199059-1, 4192102-4 or 4192102-903 Trim Switch Module Assembly containing 

a solid state relay (Figure 2) are not at risk because those switches do not have a failure 

mode that could cause a runaway. 

Enstrom has developed a replacement limit switch bracket to provide a positive stop for the 

trim actuator. In the event of a trim actuator runaway, the new bracket will stop the actuator, 

causing the circuit breaker to trip, before any significant loss of control authority occurs. 

The cyclic forces may be high, but the aircraft will be controllable. 

This Service Directive Bulletin (SDB) is intended to make operators aware that the cyclic 

trim assembly modification kit, P/N 4230045-1, containing the upgraded bracket is available 

and to provide instructions to perform the modification. 
 SERVICE DIRECTIVE BULLETIN NO. T-039 

 Revision 3 

July 6, 2011 Page 2 of 10 

6. COMPLIANCE: 

Before further flight, insert the attached page into the Emergency Section of the Flight 

Manual. 

Compliance with this SDB must be completed within the next 5 hours or at the next annual 

or 100 hour inspection, whichever occurs first, for aircraft equipped with P/N M83536/10-

024M trim relays. 

Until the new bracket is installed, operators are cautioned to be aware of anomalies in the 

trim system that might indicate an impending issue with the relay. This may include trim 

not operating when the switch is engaged, the trim not stopping when the switch is released, 

or the circuit breaker trips. If these or other unusual conditions are noted, the operator should 

pull the trim circuit breaker. The flight may be completed with the trim circuit breaker 

pulled, but this SDB should be complied with before further flight. 

If the aircraft is ever converted from the solid state switches to relays, this bulletin must be 

accomplished at the same time. 

Modify the lateral and longitudinal trim actuator assemblies in accordance with the 

procedure provided in paragraph 6.1. Upon completion, perform an operational check and 

flight test as specified in paragraphs 6.2 and 6.3. 

NOTE 

Perform all maintenance in accordance with the TH-28/480 Series Maintenance Manual. 

6.1 MODIFICATION: 

A. Determine if the trim actuator assembly has shims between the switch plate and the 

housing (Figure 3 and Figure 6). If the assembly contains shims complete the 

following: 

1. Using the cyclic control, fully extend the motor. 

2. Measure the distance from the motor housing to the bottom of the spring capsule 

(Figure 6). Record this distance (‘Max’) in Table 1. 

3. Repeat for the second trim actuator assembly, if necessary. 

B. Set the lateral and longitudinal trim actuator assemblies to a neutral position (midtravel). 

C. Disconnect the battery. 

NOTE 

Remove the battery, if required, to facilitate removal of the longitudinal trim 

actuator assembly. 

D. Remove the lateral and longitudinal trim actuator assemblies from the aircraft in 

accordance with Paragraph 12-69 of the maintenance manual. 

1. Remove the mounting hardware as shown on Sheet 3, Zones A-1 and B-1. 
 SERVICE DIRECTIVE BULLETIN NO. T-039 

 Revision 3 

July 6, 2011 Page 3 of 10 

E. Disassemble the trim actuator assembly. 

1. Remove P/N 28-16636-3 trim switch assembly from the actuator assembly by 

removing the safety wire and two AN502-10-8 and AN960-10 washers. Retain 

the washers. The screws may be discarded. 

2. Mark the wires at the connector locations of the P/N 28-16636 trim switch 

assembly so they can be reinstalled correctly. 

3. Retain shims P/N 4166031-11, if installed. 

F. Disassemble the trim switch assembly. 

1. Remove P/N 28-16624-3 shield from the trim switch assembly by removing four 

AN515-4R12 or AN515-4R10 screws, AN960-4L washers, AN935-4 lock 

washers, and AN340-4 nuts. 

2. Retain all removed hardware except the AN515-4R10 screws. 

3. Disconnect the two wires from the V3-1 micro-switches that pass through the 

shield. 

4. Remove and discard P/N 28-16614-3 switch bracket and P/N 28-16624-3 shield. 

G. Re-assemble the switch assembly. 

1. Route wires through new P/N 28-16624-11 shield. 

2. Slide new shrink tubing onto the wires, as required. 

3. Reconnect wires to the switches per the locations marked in step E.2. 

4. Position the shrink tubing over the connections, and shrink. 

5. Assemble new P/N 28-16614-5 bracket, new P/N 28-16624-11 shield, and micro 

switches using hardware retained in step F.1. NOTE: AN515-4R10 screws must 

be replaced by MS35206-219 screws provided in the kit. 

H. Disassemble the spring housing assembly from the actuator assembly. 

1. Unscrew P/N 4166031-( )1

 spring housing assembly (counter clockwise) from the 

actuator assembly. 

2. Remove NK500-8-5 screw and AN960-8L washer from P/N 4166031-( ) spring 

housing assembly as shown on Sheet 3, Zone B-4. 

3. Discard screw and washer. 

 

1

 Dash number of spring housing assembly varies based on configuration. 
 SERVICE DIRECTIVE BULLETIN NO. T-039 

 Revision 3 

July 6, 2011 Page 4 of 10 

I. Disassemble the P/N 28-16613-1 actuator from the spring housing assembly. 

1. Remove the safety wire and two AN503-8-6 screws as shown on Sheet 3, Zone 

B-4. 

2. Retain the screws. 

3. Discard the actuator. 

J. Install new P/N 28-16613-3 actuator onto the spring housing assembly. 

1. Heat the actuator, if necessary, to facilitate installation over the housing. 

NOTE 

It may be necessary to chamfer the I.D. of the actuator to get it to slide onto the 

housing. 

NOTE 

Ensure that the trim actuator, P/N 28-16613-3, is installed properly over the 

spring housing assembly. 

2. Reinstall two AN503-8-6 screws and one kit-supplied AN503-8-6 screw. 

3. Safety wire with MS20995C25 as shown on Sheet3, Zone B-4. 

NOTE 

Cotter pins, Safety wire, and VC-3 Vibra-Tite Thread Locker are not supplied 

with the kit and must be furnished by the installer. 

4. Lubricate the threaded shaft with Aeroshell 22 grease. 

K. Screw the spring housing assembly back onto the threaded shaft. 

L. Re-assemble the trim actuator assembly. 

1. Assemble the new switch assembly and the trim actuator assembly with the two 

kit-supplied AN502-10-10 screws and the retained washers from step E.1. 

NOTE: Re-assemble using the same shims, P/N 4166031-11, installed between 

the switch assembly and the motor housing, if used in the original installation. 

Substitute the kit-supplied AN502-10-12 screws for AN502-10-10, if necessary. 

2. Safety wire with MS20995C32 as shown on Sheet3, Zone B-4. 

NOTE 

Cotter pins, Safety wire, and VC-3 Vibra-Tite Thread Locker are not supplied 

with the kit and must be furnished by the installer. 

M. Apply power to the trim actuator assembly to check and adjust the travel of the 

spring housing. 

1. Install the electrical connectors. 
 SERVICE DIRECTIVE BULLETIN NO. T-039 

 Revision 3 

July 6, 2011 Page 5 of 10 

2. Install and reconnect the battery. 

3. Both trim actuator assemblies can be electrically tested from either the left or 

right hand side of the aircraft, whichever location is more convenient. Set the 

trim actuator assembly on a portable bench or suitable stable surface to perform 

the check and adjustments. 

N. Check the spring housing travel and adjust the switch positions to ensure that the 

retracted and extended spring housing travel are within the limits specified on the 

drawing or as determined in the note below. Enlist a second person to assist with the 

cyclic operation while conducting measurements and adjustments (Figure 5). 

CAUTION 

To prevent damage to the switch assembly, immediately stop applying trim to 

the cyclic if the actuator will run out beyond the length allowed by the bracket 

slot. 

NOTE 

If no shims are installed, the retracted travel or minimum stroke limit is 0.125” 

+0.05”/-0.00”. The extended travel or maximum stroke limit is 1.95” +0.00”/-

0.05”. If shims are installed, the extended travel or maximum travel limit is the 

‘Max’ dimension measured in Step A.2 +0.00”/-0.05”. The minimum travel limit 

‘Min’ is the difference between this dimension and 1.825” +0.05”/-0.00” (Figure 

6). 

1. To adjust the spring housing travel, loosen the switch mounting hardware to set 

the switch position(s) as necessary to bring the spring housing travel within 

limits. For example, to shorten the spring housing extension, loosen the switch 

mounting hardware for the extending switch and adjust the switch position 

inward (Figure 3). 

2. Check spring housing travel to verify that proper adjustment was made. 

NOTE 

If is not possible to achieve the limits after adjusting the switch positions, either 

the bracket slot or the switch screw slots may be filed to achieve the travel 

limits. The maximum amount that the bracket slot may be filed is 0.040”. 

3. When the switch adjustments are complete, apply VC-3 Vibra-Tite to the 

exposed screw threads and nut face. 

4. Disconnect the battery. 

5. Remove the battery if it was re-installed in Step M. 

O. Install the lateral and longitudinal trim actuator assemblies in the aircraft in 

accordance with Paragraph 12-74 of the maintenance manual. 

P. Install and reconnect the battery. 

Q. Check the clearance of the spring housing with the cyclic bellcranks: 
 SERVICE DIRECTIVE BULLETIN NO. T-039 

 Revision 3 

July 6, 2011 Page 6 of 10 

1. Extend the actuator to full travel. (Trim full forward or full left). 

2. Move the cyclic full Aft/Right. 

3. Verify there is clearance between the end of the spring capsule and the bellcrank 

(Figure 4). 

4. If the spring capsule contacts the bellcrank, remove the P/N 4166031-11 shims 

from between the switch assembly and the actuator and re-install and re-safety 

the switch assembly. 

5. Repeat steps 1-4 as required. 

6. If the bellcrank contacts the spring capsule and there are no shims, re-check the 

travel in accordance with step N. 

7. If the travel is correct and the spring capsule still contacts the bellcrank, contact 

Enstrom Customer Service. 

6.2 OPERATIONAL CHECK: 

A. Conduct an operational check of the trim limits. 

1. Verify the trim operates in the correct direction. 

2. Cycle each time actuator full travel each direction to verify the limit switches 

work properly. 

3. Trim the cyclic full forward and verify the travel is not restricted in any direction. 

4. Repeat Step 2 for the other three directions. 

5. Return the trim to neutral (center the cyclic). 

6.3 FLIGHT TEST 

A. Perform a flight test to verify trim authority. 

1. Verify the aircraft can be trimmed in a hover. 

2. Verify the aircraft can be trimmed at VNE or VH, whichever is lower. 

NOTE 

VH is the speed in level flight at maximum continuous power. Do not exceed any 

engine, transmission or VNE limits. 

CAUTION 

VNE and VH vary depending on aircraft gross weight, center of gravity, pressure 

altitude, and temperature. Do not exceed VNE for the flight conditions. See the 

appropriate Rotorcraft Flight Manual. 
 SERVICE DIRECTIVE BULLETIN NO. T-039 

 Revision 3 

July 6, 2011 Page 7 of 10 

6.4 PARTS: 

 A. Kit P/N 4230045-1 includes the following parts unless otherwise noted. 

Part Number Description Quantity 

4230045 Drawing Cyclic Trim Assembly Kit Drawing, 

Sheet 2 and Sheet 3 

1 

28-16614-5 Bracket 2 

28-16624-11 Shield 2 

28-16613-3 Actuator 2 

AN502-10-10 Screw 4 

AN502-10-12 Screw 2 

AN503-8-6 Screw 2 

MS35206-219 Screw 8 

FIT-221V-1/4 Shrink Tubing 2 inches 

NOTE 

The following parts are not supplied with the kit 

and must be furnished by the installer. 

Aeroshell 22 Grease A/R 

MS20995C25 Safety Wire (See Note below) A/R 

MS20995C32 Safety Wire (See Note below) A/R 

MS24665-151 Cotter Pin 2 

MS24665-153 Cotter Pin 2 

VC-3 Vibra-Tite Thread Locker A/R 

7. SPECIAL TOOLS: Heat gun and precision ruler, or equivalent measuring device 

8. MAN-HOURS: 4 hours (2 hours per trim actuator assembly) 

9. WARRANTY: Per Enstrom Warranty Policy 

10. WEIGHT CHANGE: None 

11. LOG BOOK ENTRY: 

Record this modification in detail as required for maintenance actions. 

12. REPETITIVE ACTION: None 
 SERVICE DIRECTIVE BULLETIN NO. T-039 

 Revision 3 

July 6, 2011 Page 8 of 10 

Figure 1: Lateral Cyclic Trim Switch Module and Trim Actuator Assembly 

Figure 2: Solid State Trim Switch Module (Lateral Trim Actuator) 

LATERAL TRIM ACTUATOR 

ASSEMBLY

Actuator Retracted 

TRIM SWITCH MODULE

P/N 4199059-3 Assembly 

P/N M83536/10-024M Relays 

Cyclic Bellcrank (Ref) 

TRIM SWITCH MODULE 

P/N 4199059-1 Assembly 

P/N 4192102-4 Assembly 

P/N 4192102-903 Assembly 
 SERVICE DIRECTIVE BULLETIN NO. T-039 

 Revision 3 

July 6, 2011 Page 9 of 10 

Figure 3: Trim Actuator Assembly (Lateral Trim Actuator Retracted) 

Figure 4: Spring Housing and Cyclic Bellcrank Clearance 

Clearance 

Cyclic Bellcrank 

Spring Housing 

Fully Extended 

Spring Housing 

(Retracted) 

Micro Switch (Extend) 

Installation Screws 

Micro Switch (Retract) 

Installation Screws 

Motor Housing (Ref) 

Location of P/N 4166031-11 

Shim, If Installed 
 SERVICE DIRECTIVE BULLETIN NO. T-039 

 Revision 3 

July 6, 2011 Page 10 of 10 

Figure 5: Maximum Travel Limit at 1.95” 

Figure 6: Actuator Travel Limits with a Shim 

Table 1. Actuator Travel Limits for Trim Actuator Assemblies with a Shim

S/N 

Extended Travel Limit 

‘Max’(+ 0.00”/-0.05”) 

(inch) 

Retracted Travel Limit 

‘Min’ (+ 0.05”/-0.00”) 

(inch) (Note 1) 

 

Note 1: Subtract 1.825 from ‘Max’ to calculate the ‘Min’ travel limit.