SERVICE DIRECTIVE BULLETIN NO. 0110 

 Revision 4 

Page 1 of 9 

 DATE: January 23, 2012 

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

 Longitudinal Trim Actuator Assemblies 

2. MODEL: All F-28F, 280F, and 280FX models equipped with 24 volt D.C. system; 

 Any F-28F model equipped with a 12 volt D.C. system with trim relays 

 (S/N 815 only); 

 Any F-28C, F-28C-2, or 280C models that have been converted to a 24 volt 

D.C. system+ 

3. EFFECTIVITY: F-28F Series: Serial Numbers 828 and prior 

 280FX Series: Serial Numbers 2133 and prior 

 

4. REFERENCE: Enstrom F-28A/F-28C Series Maintenance Manual, Latest Revision 

 Enstrom 280/280C Series Maintenance Manual, Latest Revision 

 Enstrom F-28F/280FX 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 KUP14D55-24, KUP14D55-

472, M83536/10-015M, or M83536/10-024M trim relays (Figure 1). Aircraft equipped with 

solid state trim switch units P/N 4199059-1 or 4192102-( ) (Figure 2) are not at risk because 

those units do not have a failure mode that could cause a runaway. Aircraft equipped with a 

reversible trim motor, P/N 28-16621 (Ford Motor Co. C1AZ-14553A) or P/N AD1R-10 

(Signal Electric), are not at risk because the trim system does not use relays or solid state 

trim switch units. NOTE: If the electric motor on the trim actuator has three wires, it is not 

affected. All motors in systems with relays or solid state trim switch units have only two 

wires. 
 SERVICE DIRECTIVE BULLETIN NO. 0110 

 Revision 4 

January 23, 2012 Page 2 of 9 

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 28-01063, containing the upgraded bracket is available 

and to provide instructions to perform the modification. 

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 KUP14D55-

24, KUP14D55-472, M83536/10-015M, or M83536/10-024M trim relays. See Figure 1 for a 

pictorial example of a relay. Note: This includes aircraft modified per SIL 0164. 

If the aircraft is ever converted from the solid state trim switch units to relays, this bulletin 

must be accomplished at the same time. 

Compliance with this SDB is not required for aircraft equipped with a reversible trim motor, 

P/N 28-16621 (Ford Motor Co. C1AZ-14553A) or P/N AD1R-10 (Signal Electric). 

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 circuit breaker pulled, but 

this SDB should be complied with before further flight. 

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 F-28F/280F Series Maintenance 

Manual or in accordance with the F-28C/ 280C Maintenance Manual, as appropriate. 

6.1 MODIFICATION: 

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

B. Remove the seat deck and the copilot side floor panel for access to the trim actuator 

assemblies. 
 SERVICE DIRECTIVE BULLETIN NO. 0110 

 Revision 4 

January 23, 2012 Page 3 of 9 

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

accordance with the applicable maintenance manual (F-28F/280F: Paragraph 12-7.B 

and 12-7.C; F-28A/F-28C/280/280C: Section 11, page MM-11-9). 

1. Lateral trim actuator only: Remove the spring assembly to facilitate removal of 

the trim actuator assembly as shown on Sheet 2, Zone A-4. Mark the hole 

locations on the spring bracket used for the springs before disassembly. 

2. Remove the mounting hardware as shown on Sheet 2. 

NOTE 

When removing the actuator assemblies, note the orientation of hardware and 

location of spacers, etc. Retain the hardware and re-install in the original 

configuration. 

D. 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. 

2. Retain the washers. 

3. Discard the screws. 

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

assembly so they can be reinstalled correctly. 

E. Disassemble the trim switch assembly. 

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

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

lock washers, and AN340-4 nuts as shown on Sheet 1, Zone A-3. 

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. 

F. 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 D.4. 

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 E.1. NOTE: AN515-4R10 screws must 

be replaced by MS35206-219 screws provided in the kit. 
 SERVICE DIRECTIVE BULLETIN NO. 0110 

 Revision 4 

January 23, 2012 Page 4 of 9 

G. Unscrew P/N 28-16618 spring housing assembly (counter clockwise) from the trim 

motor assembly. 

H. Disassemble the P/N 28-16613-1 actuator and P/N 28-162043-1 bracket from the 

spring housing assembly. (P/N 28-162043-1 bracket is used for the lateral trim 

actuator only.) 

NOTE 

On aircraft manufactured prior to 1993, the actuator was installed to the spring 

housing assembly with NK5008 screws. The NK5008 screws do not have 

provisions for safety wire. Replace the NK5008 screws with kit-supplied AN503-

8-6 or AN503-8-8 screws, as applicable.

1. Lateral Assembly: Remove safety wire, AN503-8-6 screw, AN503-8-8 screws, 

and AN960-8L washers from the spring housing assembly as shown on Sheet 1, 

Zone B-3. 

a. Retain the washers and AN503-8-8 screws. 

b. Discard the AN503-8-6 screw, actuator, and bracket. 

2. Longitudinal Assembly: Remove NK5008-5 screw, AN503-8-6 screws, and 

AN960-8L washers from the spring housing assembly as shown on Sheet 1, Zone 

B-3. 

a. Retain the washers and AN503-8-6 screws. 

b. Discard the NK5008-5 screw and actuator. 

I. 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. Lateral Assembly: Install new P/N 28-162043-3 bracket (lateral trim actuator 

only). 

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

AN960-8L washers. Use retained hardware from step H.1.a. 

3. Longitudinal Assembly: Reinstall two AN503-8-6 screws and one kit-supplied 

AN503-8-6 screw and AN960-8L washers. Use retained hardware from step 

H.2.a. 
 SERVICE DIRECTIVE BULLETIN NO. 0110 

 Revision 4 

January 23, 2012 Page 5 of 9 

4. Safety wire with MS20995C25 as shown on Sheet 1, 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. 

5. Lubricate the threaded shaft with Aeroshell 22 grease. 

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

K. Re-assemble the trim actuator assembly. 

1. Install the new switch assembly onto the trim actuator assembly with the two kitsupplied AN502-10-10 screws and the retained washers from step D.1. 

2. Safety wire with MS20995C32 as shown on Sheet1, Zone B-1. 

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. 

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

spring housing. 

1. Install the electrical connectors. 

2. 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. 

M. 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 3). 

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 

The retracted travel or minimum stroke limit is 0.125” +0.075”/-0.00”. The 

extended travel or maximum stroke limit is 1.95” +0.00”/-0.05”. 
 SERVICE DIRECTIVE BULLETIN NO. 0110 

 Revision 4 

January 23, 2012 Page 6 of 9 

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 4). 

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. Verify there is no interference between the P/N 28-162043-3 bracket and the 

switch assembly hardware. 

N. Install the lateral and longitudinal trim actuator assemblies in reverse procedure as 

removed. 

1. Lateral trim actuator assembly only: Install the spring assembly. Install the spring 

ends in the holes marked on the bracket in Step C.1. 

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. 
 SERVICE DIRECTIVE BULLETIN NO. 0110 

 Revision 4 

January 23, 2012 Page 7 of 9 

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. 

6.4 PARTS: 

 A. Kit P/N 28-01063-1 includes the following parts unless otherwise noted. 

Part Number Description Quantity 

28-01063 Drawing Sheet 1 and Sheet 2 1 

28-162043-3 Bracket (Lateral trim actuator assembly only) 1 

28-16613-3 Actuator 2 

28-16614-5 Bracket 2 

28-16624-11 Shield 2 

AN502-10-10 Screw 4 

AN503-8-6 Screw 3 

AN503-8-8 Screw 3 

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 

VC-3 Vibra-Tite Thread Locker A/R 

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

8. MAN-HOURS: 8 hours (4 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. 0110 

 Revision 4 

January 23, 2012 Page 8 of 9 

Figure 1: Trim Relay (M83536/10-024M Relay Shown) 

Figure 2: Solid State Trim Switch Unit 

(NOTE: Color, size, location of the solid state trim switch unit may vary.) 
 SERVICE DIRECTIVE BULLETIN NO. 0110 

 Revision 4 

January 23, 2012 Page 9 of 9 

Figure 3: Maximum Stroke Limit at 1.95” 

 Figure 4: Trim Actuator Assembly (Lateral Trim Actuator Retracted) 

Micro Switch (Extend) 

Installation Screws 

Micro Switch (Retract) 

Installation Screws 
SPECIAL ADDENDUM TO FLIGHT MANUAL EMERGENCY PROCEDURES PER: 

SERVICE BULLETIN DIRECTIVE 0110 REV 4 

January 23, 2012 Page 1 of 2 

Trim Failure/Trim Runaway 

Operators must observe the following: 

They should locate the TRIM circuit breaker or circuit breaker switch so they can find it 

quickly in an emergency. 

Until the new bracket is available and 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 or the trim not stopping when 

the switch is released. If these or other unusual conditions are noted, the operator should: 

A. Immediately stop using the trim and pull the TRIM circuit breaker or select the trim 

switch to OFF to deactivate the circuit. Leave the circuit breaker out for the remainder of 

the flight. 

B. If the pilot determines the flight can be continued safely, without use of the trim, flight

may be continued to the next destination. 

C. If the pilot has any safety concerns, a landing should be made as soon as practical. 

a. If there is a significant reduction in the longitudinal control, the pilot should plan 

a landing where a shallow approach and a run on landing can be made. 

i. Avoid any maneuvers, such as rapid decelerations, which may require 

forward cyclic for recovery. 

ii. Perform a run-on landing as there may not be enough forward cyclic to 

level the aircraft and stop rearward motion if it is allowed to start drifting 

rearward from a hover. 

b. If there is a significant reduction in the lateral cyclic control, it may be difficult 

or impossible to make turns to the left. 

i. The pilot should plan a landing to an area where there is ample room to 

maneuver. The aircraft will fly in a right crab, and maintaining a straight 

course may be difficult. 

ii. Lateral course corrections can be made using pedal and longitudinal 

cyclic, but the aircraft may not be aligned with the ground course. 

iii. Perform an approach to a low hover; forward speed can be stopped, but 

there may be some sideward drift. 
SPECIAL ADDENDUM TO FLIGHT MANUAL EMERGENCY PROCEDURES PER: 

SERVICE BULLETIN DIRECTIVE 0110 REV 4 

January 23, 2012 Page 2 of 2 

iv. Once forward speed is reduced in a low hover, the pilot can roll off the 

throttle and align the aircraft with the direction of motion using the pedals 

prior to touching down. 

v. If the pilot is having difficulty maintaining the approach course, the pilot 

should consider making a 360° turn to the right to line up on the final 

approach. 

vi. Directional control is easier to maintain at airspeeds above 60 knots, but 

the pilot must plan to reduce forward speed prior to touchdown. 

D. Ground the aircraft at the end of the flight. The aircraft should be grounded until the 

relays are replaced.