MD-80 Series (DC-9-80) Undifferentiated or Other Model
Maintenance Powerplant; Maintenance Airframe
Maintenance Technician 5
Communication Breakdown; Confusion; Situational Awareness
Aircraft Equipment Problem Critical; Ground Event / Encounter Other / Unknown
General Flight Cancelled / Delayed; General Maintenance Action
Aircraft; Human Factors; Procedure
Aircraft had been placed AOG in ZZZ1 after landing from ZZZ. Pilot stated that horizontal stabilizer was in a runaway nose down position and felt heavy during landing. Myself and another mechanic were sent from ZZZ2 to repair the Aircraft X and make serviceable. When we landed we got a turnover from the pilot and reviewed the logbook write up. Once we reviewed everything we attempted to operate the Stabilizer Primary Trim System and try and duplicate the fault; during the operation of the stabilizer primary trim we discovered the trim was no longer on a runaway nose down and in fact would operate normally in the nose down position; however; it would not operate in a nose up position. Contract Maintenance personnel that had been on the aircraft when it landed confirmed the pilots original defect of a runaway nose down situation. Me; [Mechanic X]; and my Partner; [Mechanic Y]; agreed we would replace the Primary Trim Motor as our first step; when we opened the packaging with the new trim motor we realized it had been removed as a serviceable unit from Aircraft Y and was missing half the gasket needed between the motor and the gearbox assembly. We notified Maintenance Control of the missing gasket and all agreed we would install the motor with the used gasket for troubleshooting purposes and verify if that would be a positive fix; Maintenance Control agreed to order a gasket aog and concurred with the installation of the motor; which was carried out per the Aircraft Maintenance Manual (AMM) using the old gasket and documented into the aircraft logbook. The access panel that was removed in order to access the trim motor; referred to as the 'doghouse'; remained off the aircraft during the troubleshooting for the remainder of the day. Once the motor was installed; we ops checked the system and found the fault was not fixed; all that occurred was that now we could actuate the stabilizer trim approx 1/8' [of an inch] and it would return approx 1/16' [of an inch]. We contacted maintenance control and stated our find and we agreed we would leave the motor installed and wait for the gasket to arrive the following morning. We continued to troubleshoot the system into the night. Prior to our duty time reaching 16 hours; we contacted maintenance control and gave a pass down of what had been accomplished for the day and agreed we would install the 'doghouse' panel to protect the motor and metal structure from the elements and temporarily secure it so as to allow quick access in the morning for the replacement of the gasket. The following morning when we arrived at the aircraft we contacted maintenance control as requested to notify them we were on the aircraft; and were asked to continue troubleshooting the system; at which point we followed the AMM Troubleshooting tree; and performed Power Checks at the Primary Nose up contactor; where power was confirmed to be found. In order to find power this required us to move the stabilizer nose up and nose down and no damage occurred during this time. Once we confirmed the presence of power at the contactor; we replaced the primary nose up contactor and performed an ops check of the stabilizer and found the fault to remain. We contacted maintenance control to report our progress; it was at this time we were asked by the maintenance controller to inspect the Jackscrew Assembly. Due to the fact the stabilizer was in a nose up position; we needed to move the stabilizer into a nose down position. During the movement of the stabilizer nose down position past the '0' detent we heard a noise and stopped the movement of the stabilizer. We looked around for the culprit of the noise and found nothing so continued to slowly move the stabilizer nose down; once it reached a '1' detent; we heard and felt the aircraft jump; at this point I realized the culprit was the stabilizer and believed the jackscrew had been damaged. Once I exited the aircraft and looked towards the tail section I immediately realizedthe 'doghouse' panel had been forcefully dislodged from the aircraft. I put on a safety harness and went to assess the damage and see what had caused the damage. Once i reached the horizontal stabilizer i realized the right elevator fairing had managed to lodge itself into the 'doghouse' panel and then be crushed by the panel while forcing it to come off the structure and ripping the screws securing it out. Once I assessed the damage we notified maintenance control of the incident along with my supervisor on duty in ZZZ2; and was told to take detailed pictures of the damage and submit them to maintenance control along with a written statement of the incident.Only preventative measure I can think of is that we should have secured the doghouse panel with all the required screws; gone to the hotel and gotten some rest and the next morning first thing should have been remove the doghouse panel and installed the gasket before continuing the troubleshooting of the system.
Reporter stated the flight crew had to continuously move the Horizontal Stab Trim 'Pickle Switches' on the Aileron Control wheel to a Nose-Up position because the MD-80 would immediately start moving Nose-Down as soon as they released the Trim switches. Captain had to use thrust levers and spoilers to augment lift. Trouble shooting indicated the Stab Primary Trim Motor braking was not holding the Stab jackscrew in position when the 'pickle switches' were released. The air loads on the MD-80's horizontal stab's leading edge would tend to lift (push) the horizontal stab up into a Nose-Down configuration. Replacement of the Primary Trim Motor did not correct the problem.  Reporter stated they were both removed from performing any further maintenance on the MD-80 and returned to their Maintenance Base after the 'Doghouse' Vertical Stab access panel was damaged. Another Maintenance crew was sent and they replaced the Stab Alternate Trim Motor. Why or how the Alternate Trim Motor contributed to the runaway Nose-Down conditions was unclear to him at this time.
When we arrived into the location; we decided upon changing the [Horizontal Stabilizer] Trim motor after consulting with the Capt. When we unboxed the Serviceable unit; which had been removed from another MD-80 aircraft; we discovered it had a partial gasket installed; instead of a serviceable one. We decided to install the unit without the gasket in order to ascertain if it was a positive fix; then have a new gasket sent. Maintenance control concurred.  We then installed the 'doghouse' panel with 4 fasteners; since we would be removing it the next day in order to install the gasket. We then continued troubleshooting the system per the AMM for the rest of our duty time until around late afternoon. After finding out our reservations at Hotel X had been modified; I was [later] sent to Hotel Y.  The next day; late morning; after continuing troubleshooting; and moving the stab up and down numerous times; we were directed by Maintenance Control to inspect the jackscrew for wear/damage. We moved the stab in order to access the panel on the side of the Vertical stab; and heard a thump; we thought possibly something had fallen next to the plane; and after waiting a few seconds; moved the stab again; when again a thump was heard. We went out to look at the plane; and upon inspection observed the doghouse panel leading edge sticking up from its faired position. The plane was powered down and after a closer inspection was made; found the inboard edge of the RH Stab had caught the lower edge of the doghouse panel. Since we had been cycling the stab all day I believe the wind caught the edge of the panel and dislocated it to the point where it was caught by the stab when it was moved. Completely securing the panel would have prevented this from happening; and I will ensure any panel I remove is secured prior to ops checking systems.
Two Aircraft Maintenance Technicians (AMTs) report how a 'Doghouse' access panel temporarily re-installed on the Vertical Stab of an MD-80 aircraft was damaged during Maintenance trouble shooting for an inflight runaway Nose-Down condition. Aircraft would not hold a Nose-up configuration when the Horizontal Stab's Primary Trim Motor 'pickle' switches on the Aileron Control wheel were released. Pilots used thrust and spoilers to augment lift.