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.

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Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.