Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Aircraft Equipment Problem Dissipated; Flight Crew Overcame Equipment Problem
Aircraft
After APU start and transfer of power/air to APU; I noticed a mildewy and 'dirty sock' smell in the cockpit. It was intermittent for about 20 minutes and cleared up before we departed. At the time I did not realize it was anything other than dirty air smell. Approximately 9 days after the flight; I was reading the ALPA publication 'Positive Rate' and there was a link to articles referencing bleed air leaks and oil leaking into the bleed air system. I realized then that we had experienced this and how serious it is. Upon further research I see that this same airplane (Aircraft X) had been written up for the same issue on [date]. Cause: It sounds like the cause could possibly be oil leakage into the bleed/pack system. Suggestions: I suggest highlighting the cause of the dirty sock/mildew smell through communications to all fleets/pilots. Please stress how toxic of an event this can be and that it needs to be written up.
When the APU was started and the air was transferred to the packs from the APU; we noticed a dirty sock smell. The odor lasted for about 20 minutes and eventually cleared up prior to push back. Aircraft X has been written up before for same issue. After discussing with my Captain and reading a publication from ALPA; we concluded we may have had a possible fume event with the air system on this aircraft. Cause: Could be a possible oil leak from the APU into the Bleed Air System.
Airline flight crew reported 'dirty sock odor' during preflight after transferring packs to APU.