Date: 202107
Local time of day: 0601-1200
Location: OSH.Airport, WI
Flight conditions: Mixed
Light: Daylight
Aircraft: Small Aircraft; Low Wing; 1 Eng; Retractable Gear
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Personal
Flight phase: Initial Climb
Airspace: Class D OSH
ATC / advisory: Tower OSH
Reporter organization: Personal
Reporter function: Instructor; Pilot Not Flying
Reporter qualification: Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Instrument
Human factors: Workload
Anomaly: Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter VFR In IMC; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented
Contributing factors: Airspace Structure; Procedure; Weather
Primary problem: Weather

Narrative: The pilot flying was a XXX-hour Private pilot; without an instrument rating; in the left seat. I was providing some instruction from the right seat; but was mostly there to provide backup to a relatively new pilot on her first big cross-country trip. The Pilot Flying (PF) by this point had approximately 30 hours in type; and was becoming comfortable with the complexity of this high-performance single and the speed at which things happen. By prior agreement; I was serving as PIC on all flights. On this flight from OSH to ZZZ; the planned training topics included: - the OSH VFR departure procedure - managing the very busy traffic environment around OSH - enroute simulated attitude instrument flying - practice IFR approach We were departing OSH during EAA Airventure; shortly after the airport opened in the morning. It was busy; but by no means crazy. We had reserved an IFR departure slot; but opted not to confirm it the day before; as the forecast was CAVU. We wanted to leave the scarce IFR slot for someone who truly needed it; and I wanted to give the PF the opportunity to perform the Oshkosh VFR departure. We received a full ForeFlight briefing that morning; which showed CAVU at OSH and for the entire route of flight. From the ground at OSH; there wasn't a cloud to be seen. Aircraft were departing runway 27 and I believe also 36. We were assigned runway 27; which (per the Oshkosh NOTAM) required that we fly a heading of 270-360 at or below 1;300' MSL (500' AGL) until clear of the class D. As our route of flight was SE; we planned to depart runway 27 to the NW; fly around the north side of the class D; then proceed SE on course. All went as planned until we reached approx. 1-2nm west of the departure end of OSH runway 27. We were climbing through 1100' to 1300' MSL; and the weather had suddenly and severely deteriorated. I estimate that visibility was perhaps 3SM; and as we continued our climb; we began to enter the ragged underside of an overcast layer that I estimate was around 400' AGL (1;200' MSL). I instructed the PF to stop the climb; descend as necessary to remain below the cloud layer; and fly a heading of 360. She leveled off at 1;150' MSL. Knowing that this was only 300-350' AGL; we were both on extremely high alert for towers; aided by the VFR sectional. I could see Lake Butte des Morts approximately 2nm north; which (a) would relieve us from worrying about towers; and (b) had a sky that looked much brighter. Once over Lake Butte des Morts; we could clearly see Lake Winnebago to the east; to which we proceeded at 1;200' MSL. Once over Lake Winnebago; weather improved very rapidly; and the rest of the flight was uneventful. We were still listening to the OSH tower as we reached Lake Winnebago; and they continued to dispatch pilots to the NW from runway 27. Now that I had a small amount of mental capacity to spare; I called tower and informed them that conditions NW of the field were 'just barely VFR'; and to exercise extreme caution. They sent one more aircraft in that direction; who reported that conditions were 'definitely not VFR'. That pilot returned to land at OSH; which was then closed to VFR departures. In hindsight; I made several errors in judgment that morning: 1. The subsequent pilot was quite right: the weather conditions were definitely not VFR. When I made that report to the tower; I was fixated on visibility (which was; I believe; barely VFR -- perhaps 3SM). But when I had time to think more clearly; it was obvious that we were only just clear of cloud; not 500' below. Furthermore; even if clear-of-cloud had been legal (which it was not); 1;150-1;200' MSL was a very dangerous altitude; in this area; it was true scud running. We passed less than 1NM from a tower less than 100' below us (although we had it in sight). 2. The moment that we encountered weather that was worse than forecast; we should've done as the later pilot did: inform the tower that conditions were not VFR; and coordinate an eastbound turn (not with standing the requirements in the NOTAM). In hindsight; I gave far too much deference to the procedure in the NOTAM; and did not want to deviate from its mandate. I didn't want to be 'the guy who screwed up the OSH departure and snarled traffic for everyone' or worse; create collision risk by being where I shouldn't be. Yet it's obvious that the extremely professional controllers would've managed the situation; and it would've been far preferable to scud running. I failed to exercise my ultimate authority as PIC (Pilot In Command) that morning; and in my opinion this was the most critical error. 3. In hindsight; the one clue that we missed was a METAR at PCZ; 29nm NW of OSH; showing OVC005. This should've told us that; although weather at and east of OSH was extremely VFR; it was much worse only a short distance away. At the very least; this could've prepared us for the possibility that we would encounter poor weather nearby; and have a complete plan ready. Had we been able to perform a standard downwind departure to the east; as we would any other time; it's highly likely that we would've remained CAVU the entire time.




Synopsis: General aviation pilot departed airport VFR when when weather near the airport was marginal with low clouds in the area.