Airline flight crew reported 'dirty sock odor' during preflight after transferring packs to APU.

Attributes
ACN	1691920
Time
Date	201909
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Aircraft 1
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	Parked
Flight Plan	IFR
Component
Aircraft Component	APU
Person 1
Function	Pilot Flying Captain
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Person 2
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument
Events
Anomaly	Aircraft Equipment Problem Less Severe Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor

Narrative:

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.

Original NASA ASRS Text

Title: Airline flight crew reported 'dirty sock odor' during preflight after transferring packs to APU.

Narrative: 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.

Data retrieved from NASA's ASRS site 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.