A Mechanic and Inspector report about an incorrectly installed horizontal stab jackscrew gimbal/ACME nut on an MD-80 type aircraft. The gimbal assembly is symmetrical and had been installed 180 degrees out of proper orientation; preventing proper lubrication of the ACME nut for the jackscrew. Maintenance diagrams were reported to be unclear.
Synopsis
A Mechanic and Inspector report about an incorrectly installed horizontal stab jackscrew gimbal/ACME nut on an MD-80 type aircraft. The gimbal assembly is symmetrical and had been installed 180 degrees out of proper orientation; preventing proper lubrication of the ACME nut for the jackscrew. Maintenance diagrams were reported to be unclear.
Narrative
While performing [horizontal] jackscrew installation on an MD-88 aircraft 'X'; June 2010; orientation of the jackscrew gimbal nut came into question. I performed the same installation procedure on a [different] MD-88; aircraft 'Y' two weeks earlier. The MD-88 Maintenance Manual (M/M) 27-40-01-401 step 9-B states: 'Position the ACME screw and nut; support; and gimbal ring in vertical stabilizer with upper stop on nut facing forward'. My interpretation was to have the face of the upper stop facing forward; not have the stop assembly on the forward side of the gimbal/ACME nut when installed in the aircraft. A consensus was taken of fellow AMT's; Lead AMT's; and an Inspector; the findings confirmed that I did install the jackscrew gimbal nut assembly 180 degrees out from proper orientation.Maintenance Control and Operations were contacted and informed of the situation. Aircraft 'Y' was immediately grounded in ZZZ. Company Line Maintenance in ZZZ1 sent an AMT/Crew to investigate the issue. After confirmation of improper installation a crew from company hangar in ZZZ2 was dispatched to correct the problem. To my knowledge no damage occurred to the aircraft or components in question. To prevent further installation confusion I believe a clarification of that M/M step should be performed to alleviate any gray areas.
Second reporter narrative
During the installation of a horizontal stabilizer jackscrew on an MD-80 aircraft; I was involved in the 'OK to install' [portion] of the actuator jackscrew assembly. I was also involved in part of the installation process per MD-88 AMM 27-40-01-401 Steps 8.B.(X) through 8.B.(Y); 9.B.(X) through 9.B.(4) and 9.B.(Y) through 9.B.(Z). It has come to my attention that the orientation of the jackscrew gimbal nut was 180 degrees out from proper orientation. AMM 27-40-01-401 Step 9.B.(4) states; 'position ACME screw and nut; support; and gimbal ring in vertical stabilizer with upper stop on nut facing forward.' My interpretation was that the face of the stop needed to face forward. Instead; the stop assembly needed to be on the forward side of the screw [jackscrew] with the gimbal in place in the vertical stabilizer. The actual face of the stop would now be facing aft. To ensure proper installation of the gimbal ring and nut in the future; the AMM 27-40-01-401 Step 9.B.(4) should be revised to clarify that the Stop block should be positioned on the forward side of the screw when seated in the vertical stabilizer fitting.
NASA callback
Reporter stated the jackscrew gimbal/ACME nut is symmetrical and can be installed 180 degrees out from proper orientation. There are two stops on the gimbal assembly; one upper stop and one stop on the lower side of the gimbal; they are diagonally opposite from each other. There is also a grease fitting on the same side of the gimbal that has the upper stop. Reporter stated when the first gimbal nut was installed in aircraft 'X' both he and another Mechanic; including an Inspector; all of them with experience believed they had interpreted the Maintenance Manual (MM) diagrams correctly. But when he started the second gimbal replacement; other mechanics and a different Inspector who had also performed the same type of installation told him he mis-read the manual.Reporter stated He was told the grease fitting was also used as a guide for correct installation and orientation of the gimbal/Acme nut upper stop; because if the grease fitting is not positioned facing forward the gimbal/ACME nut for the jackscrew cannot be lubed. But the MM does not even reference the grease fitting in the installation procedures. That's why he believes a change to the maintenance diagrams that better identify the correct orientation of the gimbal/ACME nut position is necessary.Reporter stated the effects of an improper orientation of the gimbal/ACME nut stops would not be noticed during 'normal' flight because the horizontal stab jackscrew does not travel to the extreme limits of aircraft nose full-up or full-down position; which is when the gimbal stops would come into play. But the issue of not being able to lube the gimbal/ACME nut is also a concern.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.