Air carrier lead crew chief reported a flight crew discrepancy with the horizontal stabilizer trim inoperative both electrically and manually. The lead stated their concern was that even though the trim motor was replaced and resolved the issue; the fault isolation manual does not have a reference for a jammed manual operation.

Narrative

Aircraft X arrived at ZZZ with MIC XXXXXXX; TRIM SWITCHES ON BOTH CONTROL COLUMNS INOP. TRIM WHEEL WILL NOTMOVE MANUALLY VIA HAND CRANK. The AMT received the aircraft and spoke to the crew. It was explained that on approach all of the horizontal stabilizer switches were INOP and that when the Stabilizer control wheel was tried it was jammed as well. The horizontal stab was in the green range and was not an issue during landing because the elevations were able to compensate for the inability to use the horizontal stab to adjust pitch. The AMT went to the FIM (Fault Isolation Manual) for a reference. It was turned over and the following shift troubleshot the problem IAW (In Accordance With) FIM 27-XX-XXX-XXX-XXX Stabilizer Trim Does Not Move Freely - Fault Isolation. Following the steps it was determined that the trim actuator needed to be replaced. The MIC was signed off R&R STAB TRIM MOTOR IAW AMM (Aircraft Maintenance Manual) 27-XX-XX. OPS CHECK GOOD. Nowhere in the FIM reference used does it require you to troubleshoot any further if replacing the trim actuator resolved the issue and the replacement of the actuator repaired the electrical and mechanical operation of the horizontal stabilizer. However; the FIM does not have a reference for a jammed manual operation of the horizontal stabilizer. The mechanical clutch in the jackscrew gearbox could still be faulty. Cause: The FIM does not address the mechanical clutch stabilizer gearbox. . HORIZONTAL STABILIZER TRIM CONTROL SYSTEM - AFT COMPONENTS. Describes how the manual operation of the horizontal stabilizer is activated within the gearbox. Functional Description - Brakes.The gearbox has two internal brakes and a mechanical clutch. Both of the brakes are ratchet type brakes. The clutch lets the stabilizer manual trim wheel input override the stabilizer trim actuator input.So; even if the electrical trim motor was jammed the mechanical clutch should have allowed the manual inputs to control the horizontal stabilizer. I believe in normal operation of the manual control wheels the electrical trim motor shaft spin freely enough that the mechanical clutch is not utilized. Following IAW AMM Task 27-XX-XX-XXX-XXX.Stabilizer Ball Nut and Jackscrew Gearbox Installation.H.(10) subtask 730-XXX Task 27-XX-XX-XXX-XXX.Stabilizer Clutch Friction System Test. This test has you remove the trim actuator and install wrench SPL-1684 on the internal spline on the stabilizer gearbox and make sure the wrench does not move (simulating a jammed motor). Attach an adapter to the Capt's control trim wheel and using a torque wrench Make sure the torque necessary to operate the stabilizer trim wheel is not more than 163 in-lb (18.42 N·m).163 in-lb (18.42 N·m) is equal to a tangential force at the handle of the trim wheel of approximately 46 lbf (205 N). And….H.(19) subtask 730-XXX requires AMM Task 27-XX-XXX-XXX.Horizontal Stabilizer Trim Control System Friction Test.In this test the adapter is attached to the Capt's control trim wheel and the trim motor is installed while performing this test. Turn the captain's control trim wheel clockwise (full APL NOSE UP) until it touches the stops.1 Make sure the torque necessary to operate the control trim wheel is not more than 62 in-lb (7.0 N·m) or not less than 22 in-lb (2.5 N·m). This is simulating the free wheeling of the trim motor when using manual inputs. Suggestions: I believe this aircraft should have AMM 27-XX-XX-XXX-XXX Stabilizer Clutch Friction System Test accomplished so that in the event of another electrical trim actuator jamming the manual reversion is deemed safe to use. In addition; either 27-XX-XX-XXX-XXXStabilizer Trim Does Not Move Freely - Fault Isolation has the 2 AMM subtasks: subtask 730-XXX Task 27-XX-XX-XXX-XXX.Stabilizer Clutch Friction System Test.; and SUBTASK 730-XXX 27-XX-XX-XXX-XXX.Horizontal Stabilizer Trim Control System Friction Test; or; another FIM task created for a jammed manual trim control wheel scenario.