Cessna 172N accident near Bolinas, California, September 7, 2017

The NTSB's narrative final · quoted from the NTSB record

During a personal cross-country flight, the non-instrument-rated private pilot reported to air traffic control that the airplane would be descending below the marine layer clouds to fly along the coastline to the intended destination. Before the pilot's descent, he asked the controller about the cloud base altitude, but the controller did not have that information. The pilot stated that he would descend to 1,500 ft. The controller asked the pilot if he was going to follow the shoreline, and the pilot replied, "yes, at 2,100 [ft]." Shortly afterward, the controller advised the pilot to contact the next sector. About 13 minutes later, the pilot attempted to contact the next sector, but that sector controller was not able to understand the transmission. The controller instructed the pilot to contact approach control in 15 miles. No further communications from the pilot were received. The wreckage evidence indicated that the airplane impacted rising terrain. Witnesses in the area did not see the accident airplane but heard a loud noise about the time of the accident. The witnesses reported a thick fog layer near the accident site. The airframe and engine examinations found no mechanical anomalies that would have precluded normal operation. The accident site was in an area that had a low- to mid-level pressure troughs, which induced an onshore flow toward the mountainous terrain along the coastline. The persistent onshore flow was favorable for the development of a marine layer and mountain obscuration. An AIRMET that was valid for the time of the accident indicated mountain obscuration and instrument flight rules (IFR) conditions along the coastal water; the accident site was less than 1-mile northeast of the coastline. The closest official weather observation site to the accident site, which was located 15 miles northeast of the accident site, reported an overcast ceiling at 2,600 above ground level (agl). An unofficial weather site that was 5 miles east-northeast of the accident site reported a temperature and dew point that were nearly equal and a relative humidity above 85%. These conditions increased the potential for instrument meteorological conditions (IMC) in the area. In addition, the cloud ceilings near the accident site were likely at 800 ft mean sea level according to the upper air sounding data. A review of archived ForeFlight data and DUATS and Leidos records revealed that the pilot received several weather briefings in preparation for the flight, and that the briefings identified marginal VFR and IMC conditions along the intended route of flight. Thus, the pilot had sufficient weather forecast information to understand that IFR and mountain obscuration conditions would have existed northward along the coastline.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.