Date: 201312
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Weather elements / visibility: Snow
Light: Night
Aircraft: Cessna 404 Titan
Operator: Air Carrier
Operating under: Part 135
Mission: Cargo / Freight / Delivery
Flight phase: Parked
Aircraft component: Pneumatic System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Powerplant; Maintenance Airframe
Human factors: Communication Breakdown; Distraction; Workload
Second reporter function: Technician
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Inflight Shutdown; General Flight Cancelled / Delayed; General Declared Emergency; General Maintenance Action
Contributing factors: Aircraft; Human Factors; Weather
Primary problem: Human Factors

Narrative: Pilot reported his left pneumatic pump was inoperative (Inop). Myself and another mechanic went to replace it. Used the Cessna Manual to get the part number and review the maintenance procedure for any special considerations. There was nothing unusual about the procedure. We were being called away to perform other Maintenance and 'tag-teamed' the operation. We had problems seating the nuts. We finally got it installed. The discrepancy was cleared and the plane departed. About 10 minutes out of ZZZ; the aircraft declared an emergency; shut down the Left engine due to low oil pressure; and landed at ZZZ1. Maintenance there found one of the nuts was loose on the pneumatic pump; causing an oil leak. They reseated the pump and the plane was released to service. As I looked back on it; I only remember tightening three of the nuts. I must have [forgotten] to tighten the fourth. It was dark; outside; temperatures were in the low teens and it was snowing. We were called away to help with other Maintenance issues. All these contributed to the incident. Since two of us were working together; we both thought the other tightened the fourth nut.


Second narrative: I was working the night shift on the Ramp at ZZZ. At about XA: 00am; a pilot reported that the pneumatic pump on left engine was inoperative. Another mechanic and I began to change the pump. This is a busy time; so we worked on it together; but got called off this job a couple of times to perform other tasks. We had problems seating the pump. It has four nuts; and they were a problem to get to. We eventually got the pump changed and cleared the discrepancy. The plane left ZZZ. About 20 minutes into its flight; an engine oil problem developed and the pilot had to shut the engine down and divert to ZZZ1. Our Maintenance personnel at ZZZ1 found one loose nut. I thought the other Mechanic X had secured the nuts. Obviously; he had not. There were several contributing factors: It was very cold and dark. We were working outside in the elements. We were interrupted during the repair. You have to be MORE careful when working with a second person! Communication is key.


Synopsis: A Lead Technician and Line Technician are informed that a Cessna C-404 aircraft they had replaced a left pneumatic pump on had declared an emergency; shut down the Left engine due to low oil pressure and diverted. Working outside in snow; cold; dark with very little lighting; heavy workload and lack of communication contributed to one bolt and nut not be properly tightened.