Flight 615 was a transcontinental east-west service connecting Boston, Hartford, Cleveland, Chicago, Oakland, and San Francisco. The aircraft had departed Chicago at 10:59 p.m. Central time for the overnight leg to Oakland. At around 4:16 a.m. Pacific time, pilot Marion W. Hedden of Los Altos made contact with the Oakland control tower, preparing for his landing approach. He mentioned no trouble. The night was routine, the kind of early-morning arrival that airline crews completed thousands of times without incident. Nine minutes later, the aircraft was cleared for approach, and then silence.

The Civil Aeronautics Board investigation revealed that the crash was caused by the pilot's decision to abandon prescribed instrument landing procedures. Instead of following the established approach path, Hedden relied on visual reference and used the copilot's automatic direction finder. The ADF, a navigation instrument that points toward a radio beacon, guided the aircraft three miles off the correct course and below the prescribed minimum altitude of 3,500 feet. At that altitude and in that position, the mountainous terrain southeast of Oakland became a wall. Tolman Peak, rising from the East Bay hills, was directly in the path of the deviated approach.

The crash site on Tolman Peak, in the hills between Oakland and Union City, remains accessible to hikers who know where to look. Fragments of the aircraft are still scattered across the remote canyon, slowly being absorbed by the chaparral and live oaks of the East Bay hills. The disaster occurred in the early years of commercial aviation's rapid expansion, a period when accident investigation was still developing the systematic approach that would eventually make air travel the safest form of transportation. The lesson of Flight 615, that instrument procedures exist because mountains do not move, was learned at the cost of 50 lives on a hillside that most residents of the Bay Area have never visited.