Cessna 172 Roll Servo Cable Guard, Rev B

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TECHNICAL SERVICE BULLETIN: 104798-000 Cessna 172 Roll Servo Cable Guard, Rev B Original Bulletin: January 17, 2025 PLEASE READ THIS BULLETIN IN ITS ENTIRETY BEFORE CONTACTING DYNON AVIONICS Description In Cessna 172 aircraft, during some phases of flight with autopilot engaged, low tension on the aileron control cable may cause the aileron crossover cable to sag and rest on the autopilot servo. This sagging increases the potential for binding in the flight control system. Dynon has received no reports of such binding occurring in the field. This technical service bulletin provides a means to prevent such binding in the event of the control cable sagging as described. TECHNICAL SERVICE BULLETIN: Cessna 172 Roll Servo Cable Guard, DOC# 104798-000, REV B Page | 2 Applicability and Affected Equipment This bulletin affects the following certified aircraft models. • Cessna 172 Series, Models 172F–172S, with a Dynon autopilot system installed according to the instructions in the document: 103562-000 AP Servo Install and Maintenance Manual – Cessna 172 F-S, Revision E or earlier. Required Actions Within 12 months after the effective date of this service bulletin or 100 flight hours, whichever comes first, install the new Dynon-provided parts. To request parts, fill out and submit the following form: Cessna 172 Roll Servo Cable Guard Parts Request Form. Aircraft with serial numbers 17266333, 172S9081, 17270117, 17269958, and 17265614 also require installation of new roll servo brackets. Contact Dynon Certified Technical Support at dynoncertified.com/contact and request part number 503107- 001 (upper bracket) and part number 503108-001 (lower bracket). This action must be performed by an appropriately-rated certified mechanic and must be entered into the aircraft records showing compliance with this service bulletin in accordance with 14 CFR 43.9(a) and the instructions in this service bulletin. The record must be maintained as required by 14 CFR 91.417. The following non-typical tools are required to complete this service bulletin: • Swage compression tool appropriate for ST2-2 stop swages, • Cutters for 1/16" diameter wire rope, • Calibrated tensiometer. Reference the following document for procedure below: Autopilot Servo Installation and Maintenance Manual, Cessna 172, Revision G at dynoncertified.com/docs. 1. Secure ailerons in center of travel. 2. Access the area between WS100 and WS118 in accordance with the manufacturer's service manual. 3. Remove roll servo bracket assembly per instructions in Section 11.8: Roll Servo Removal and Replacement. Retain hardware for reinstallation. TECHNICAL SERVICE BULLETIN: Cessna 172 Roll Servo Cable Guard, DOC# 104798-000, REV B Page | 3 4. If replacing roll servo brackets, unfasten hardware that secures roll servo to upper and lower brackets, and then use same hardware to attach roll servo to new upper and lower brackets as shown. Do not fully tighten upper bracket fasteners. 5. Assemble new bridle cable as shown. TECHNICAL SERVICE BULLETIN: Cessna 172 Roll Servo Cable Guard, DOC# 104798-000, REV B Page | 4 6. Wrap new bridle cable around capstan and attach capstan to roll servo as shown. Do not fully tighten capstan's castle nut. TECHNICAL SERVICE BULLETIN: Cessna 172 Roll Servo Cable Guard, DOC# 104798-000, REV B Page | 5 7. Insert and position roll servo/bracket assembly into aircraft as shown. 8. Slide roll servo assembly inboard until it contacts the flange of WS100 rib. 9. Raise upper bracket until it contacts upper rear spar flange. 10. Ensure there is a setback for the bend radii on rear spar flanges, fully tighten upper bracket fasteners. 11. If replacing brackets, use existing holes in wing skins and rear spar to match drill #18 holes through upper and lower brackets as shown. Insert/remove temporary fasteners as needed to secure roll servo assembly while drilling. TECHNICAL SERVICE BULLETIN: Cessna 172 Roll Servo Cable Guard, DOC# 104798-000, REV B Page | 6 12. Use retained hardware to reattach roll servo/bracket assembly to wing skins and rear spar as shown. TECHNICAL SERVICE BULLETIN: Cessna 172 Roll Servo Cable Guard, DOC# 104798-000, REV B Page | 7 13. Connect bridle cable to aileron crossover cable as shown. 14. Position bridle cable clamps so that: • Capstan's swage engagement hole remains in the 6 'o' clock position. • Clamps are located as shown above. • Clamps are positioned to avoid contacting any structure. 15. Make sure bridle cable does not contact servo's cable guard. The bridle cable will prematurely wear if bridle cable and cable guard make contact when control is in a neutral position. 16. If cable guard interferes with bridle cable, adjust cable guard position (see Section 11.6: Cable Guard Adjustment), and then repeat Step 15. 17. Tighten all bridle cable clamp nuts to 35-40 in-lb. 18. Use a feeler gauge to measure gaps between bridle cable clamp halves (top and bottom). Make sure gaps meet the following criteria: • Gap measurements on both sides of clamp are not less than 0.003" and not more than 0.050". If clamp gap measurements do not meet criteria above, measure diameter of control cable, and then contact Dynon Technical Support for a resolution. Out-of-specification cable clamps can slip under load. 19. Using turnbuckle, tension bridle cable to 15-20 lbs. Make sure capstan's swage engagement hole remains in the 6 'o' clock position. The tension on bridle cable should never exceed manufacturer's specified tension for control cable. TECHNICAL SERVICE BULLETIN: Cessna 172 Roll Servo Cable Guard, DOC# 104798-000, REV B Page | 8 20. Use provided clips to safety wire bridle cable turnbuckle halves and barrel. 21. Tighten capstan's castle nut until slot in it lines up with hole for cotter pin in servo shaft. DO NOT overtighten the castle nut! Tightening the castle nut beyond 4 in-lbs may prevent the capstan from rotating freely in the event of shear screw failure.