Helicopter
Maintenance Powerplant; Maintenance Airframe
Maintenance Lead Technician 1.5; Maintenance Technician 19
Situational Awareness; Time Pressure; Distraction
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Maintenance
General Maintenance Action
Aircraft; Procedure; Human Factors
Aircraft X was in a full 300 hour inspection for two weeks in ZZZ at Base XXX. I was leading the project working the hours of XA:00 to XM:00 everyday. We were short on man-power and the ones that arrived to help were only able to help a couple days at a time and then another hand or hands would show up to help. The base mechanic out of ZZZ1 refused to come help. I started the inspection at the tail and was lubing the splines and got pulled to the front due to a crack found in the broom closet which turned out to be structural. We had to remove all the controls and the servo rack up top in order for the repair to get done. A vendor was called in to repair it which took a full two days and I was up there removing anything else he needed. We finished all the inspections and the aircraft went back to the base and after a flight the pilot noticed a nut sitting in the drain hole on the thrust reverser gearbox fairing. Base mechanic was called in and found the nut on the furthest aft hangar bearing bolt (there are one on each side); came off. The other bolt was still in and cotter pinned and the bolt was still in on the left side with no nut. I was having to do a bunch of jumping around due to different hands coming in on several different days and lost focus. I thoroughly believe I put the nut on but didn't cotter pin it.
Technician reported not installing a cotter pin in the nut for a bolt on the aft bearing hangar for the Thrust Reverser gearbox fairing.