A321
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Physiological - Other; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Flight Crew Overcame Equipment Problem; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft
During preflight immediately after switching from ground air to APU air FO (First Officer) and Captain both noted musty; dirty sock smell on flight deck. After calling the #1 FA (Flight Attendant); she also could smell the odor in the forward cabin. Other FA's could not smell odor in other parts of cabin. APU air was discontinued with a dissipation of odor. All passengers and crew were deplaned at the gate. FO complained of a head ache and was removed from further flight duty. Due to the length of time required for replacing FO and required maintenance trouble shooting Captain was released by [crew software] due to FAR part 117 crew duty limitations. APU was written up on Dates for requiring oil service.[I] suspect APU oil loss as a contributing factor.Without knowing status of maintenance action on APU; this cannot be answered. Must follow up with ZZZ Maintenance personnel.
Turned APU bleed on; smelt the dirty socks odor. Reached up and turned off bleed. Asked Captain if he smelt that; he said 'what the dirty socks smell'. Wrote it up and got everybody off.When reviewing the AML I saw the APU oil had been serviced twice in the last 3 days. Should have known something was wrong.APU oil isn't serviced that often. When it has to be serviced twice in 3 days it needs to be looked at and not just serviced.
Air Carrier flight crew reported a fume event during preflight boarding. Maintenance was requested and the aircraft was eventually swapped.