Date: 201708
Local time of day: 0601-1200
Location: OAK.Tower, CA
Flight conditions: VMC
Light: Daylight
Aircraft: Medium Transport; Low Wing; 2 Turbojet Eng
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Flight phase: Climb
Airspace: Class C OAK
ATC / advisory: Tower OAK
Reporter organization: Air Taxi
Reporter function: Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Flying
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Ground Personnel
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Environment - Non Weather Related
Primary problem: Environment - Non Weather Related

Narrative: We executed a standard RNAV arrival into OAK. Had no altitude/speed deviations according to those published on the STAR. We then started receiving vectors and altitude assignments from ATC. Once we called the field in sight; and the traffic we were following in sight (a B737); we executed a visual approach in VMC conditions to runway 30. Three days later we were notified by our company that during our approach; we were a few hundred feet low according to noise abatement procedures. At the time of our supposed breach of procedure; we were on assigned altitudes from ATC; then on a visual approach; and at no time dangerously or unacceptably low. I'd be interested to know whether or not the ATC instructions we were following lead to non-compliance.Make compulsory noise abatement procedures more easily accessible to the flight crew. We were completely unaware of the procedure we were apparently violating.


Second narrative: On our descent into Oakland we were assigned the WNDSR 2 RNAV arrival; runway 30 transition.  Rather than flying the 112 track after CRSEN; we were given a heading after HOPTA to join the right downwind for a visual approach to runway 30. Oakland Approach was stepping us down from 5000 feet and eventually gave us an altitude of 3000 feet for a downwind vector. We were also told by OAK approach control to follow a B737 on final for runway 30.  The final approach fix; MITOE for the ILS Rwy 30 is 8.3 nm from the end of the runway; which is a little south of Hayward airport. Since the B737 was on a 3-4 nm final; we called the aircraft and airport in sight. We were cleared for the visual approach to runway 30; we began our right base entry and turn just abeam/outside of Hayward airport. We held our last assigned altitude of 3000 feet before beginning our descent for the visual approach. At no time during our visual approach; did Oakland Approach control say we were violating a noise abatement procedure or flying too low on the base leg. According to their report; we were around 2431 feet AGL on the base leg. After landing; nobody told us to call a specific number. We learned about this when [the company] sent us an email with the attachment from the Noise Abatement and Environmental Affairs office.First; we were cleared for the visual approach. If this noise abatement procedure is applicable below 3000 feet; OAK approach control should have extended our downwind leg before turning base for the visual.  Second; we were never told to hold our last assigned altitude prior to turning final. That would have prevented us from descending into the noise abatement sector. In the future; I plan on turning my base leg at or above the last assigned altitude to prevent penetrating the noise sector.


Synopsis: Air taxi flight crew reported being informed by their company after the event that they violated noise standards during arrival to OAK. The crew was not aware of the procedures and was following published arrival procedures and ATC vectors until cleared for the visual approach.