Date: 201309
Local time of day: 0601-1200
Location: D21.TRACON, MI
Flight conditions: VMC
Light: Daylight
Aircraft: Beechcraft / Beech Aircraft Corp Undifferentiated or Other Model
Operator: Air Taxi
Operating under: Part 135
Flight phase: Initial Approach
ATC / advisory: TRACON D21
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Situational Awareness
Second reporter function: First Officer; Pilot Flying
Anomaly: Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert
Contributing factors: Chart Or Publication; Procedure; Weather; Airport
Primary problem: Ambiguous

Narrative: On descent to PTK I received the ATIS and the weather had improved to 2 miles from 1 1/4 miles. We also had ground contact and forward visibility of at least 3-4 miles. I asked Approach to vector us in close so as to maybe get the visual. The approach they would have vectored us for was past the airport. We called the airport in sight and ATC cleared us for a Visual approach. We landed without incident. After post-flight I received a call from the Tower instructing me that the field was actually IFR and that I was given an illegal clearance. He suggested I should have asked for a contact approach or executed the 27L B/C approach. I said I actually thought the field was VFR based on my forward visibility; forgetting about the last ATIS. I did not tell him that we were not authorized to conduct 'contact approaches'. He said the call was more for informational purposes; but the Approach Controller was going to file an ASAP.Due to the fact I had good VFR visibility; I accepted the visual approach when the ATIS/Tower was still reporting IFR conditions. I did not have the company's 3 mile restriction and should have conducted the approach procedure or get an updated visibility report.


Second narrative: I mentioned the idea of conducting a contact approach. We then discussed the uncertainty of this being an approved procedure per our company op specs and elected not to. In lieu of a contact approach I asked the pilot not flying to inquire with ATC about a vector more direct to the field as we may be able to get in on a visual approach. He complied and ATC directed as such. At 4 NM/5 SM I observed the field and asked the pilot not flying to call it 'in sight'. Thinking that; as a result of receiving visual contact greater than 3 miles out; we weren't in violation and could proceed visually. We landed on 27L without incident. Shortly after arrival; Tower contacted the FBO asking to speak with a pilot. The Captain took the phone call and was informed that we accepted an illegal clearance from ATC as the field was still reporting as IFR.Despite having good forward visibility; it is not a pilot's determination that makes the field IFR/VFR. Without Tower issuing a special observation or some declaration indicating the field is VFR it must be construed as IFR. I now understand fully that it is the Tower's responsibility to disseminate weather information pertaining to the corresponding airport and the local Approach Controller and pilots to react accordingly.


Synopsis: PTK Tower reported IMC conditions but a flight crew was able to see the airport at about 4 miles and reqested a visual approach but was then advised after landing that the clearance was not legal.