Douglas DC-6B accident at San Francisco Bay, California, 20 Apr 1953

Accident narrative

On April 20, 1953, Western Air Lines Flight 636, a Douglas DC-6B, crashed into San Francisco Bay approximately midway between San Francisco International Airport and Oakland Municipal Airport at about 2308. The aircraft was demolished and sank. Two of the ten occupants survived.

**The flight**

Flight 636 originated in Los Angeles for Oakland, with an intermediate scheduled stop at San Francisco. The crew comprised Captain Robert E. Clark, First Officer Robert C. Jacobson, Flight Engineer Robert R. League, and Stewardesses Barbara Brew and Beverlee Nelson. Captain Clark had 11,500 total flying hours, including 79 hours in the DC-6B, and had recently made 12 trips between San Francisco and Oakland.

The flight proceeded routinely to San Francisco, arriving at 22h0. Thirty passengers deplaned, leaving five aboard for Oakland. A walk-around inspection by the flight engineer and a company mechanic revealed no discrepancies. No fuel was added to the 1,700 gallons aboard, and the gross weight was well within allowable limits.

**Trans-Bay clearance and departure**

Because the ceiling and visibility at both airports were below standard visual flight minimums, the flight operated under a "Visual Trans-Bay" clearance. This procedure expedited traffic between the two airports, requiring aircraft to remain clear of clouds and fly no lower than 500 feet. If a flight could not maintain visual contact with the land or water, the procedure required the crew to immediately advise approach control, climb to 2,000 feet, intercept the northwest course of the Oakland range, and hold.

At 2300, San Francisco reported a measured ceiling of 800 feet with broken clouds and 10 miles visibility. Oakland reported a 700-foot measured ceiling, overcast, with 10 miles visibility.

Flight 636 took off from San Francisco's Runway 28R at 2305. Two minutes later, the flight called the Oakland tower, advised it was on a trans-bay clearance, and requested landing instructions. Oakland cleared the flight to enter the traffic pattern for Runway 27R, giving the wind as west at 10 miles per hour. The crew acknowledged the clearance.

Oakland surveillance radar detected the flight as it completed a right turn toward the airport. At about 2308, tower operators at both airports saw a large orange flash in the direction of the aircraft's track. The target disappeared from the radar scope, and the operator marked its last position 5.5 miles from the Oakland radar on a bearing of 217 degrees.

**Rescue and survivor accounts**

Coast Guard and Navy aircraft and vessels were dispatched. Rescuing helicopters found scattered to broken clouds at 400 to 500 feet over the crash scene, requiring them to descend to stay clear of the overcast. Below 300 feet, visibility improved to 12 miles or better, and lights on both sides of the bay were clearly visible. A Coast Guard boat rescued Stewardess Nelson and passenger Vilas F. Adams. Six bodies were recovered; the captain and flight engineer were not found.

Stewardess Nelson stated the takeoff appeared normal. About five minutes out, she sensed a gradual descent, heard a decrease in power similar to a normal landing, and felt what she presumed was the nose-wheel striking the runway. She was in the water for about one hour before rescue.

Passenger Adams stated he could see the lights on both sides of the bay after takeoff. He judged the aircraft was about 500 feet high after turning toward Oakland. He then noticed they were about 20 feet off the water and appeared to be below the shore lights. In about 15 seconds, the aircraft was down to 10 feet. He unfastened his seat belt and stood up just as the crash occurred, accompanied by a blinding flash. He reported the flight was well below the clouds, the water was smooth, the wings were level, and the engines were running smoothly at impact. He estimated he was in the water 50 minutes.

**What the investigation found**

The aircraft crashed in water averaging 30 feet deep. The largest recovered piece was the wing center section with the two main landing gears attached, which were fully retracted. One propeller hub was recovered, but damage precluded determining its pitch or RPM. The four engines were not recovered due to deep, soft mud on the bay floor. The Board found no evidence of mechanical malfunction in the recovered components.

Other trans-bay flights operating within an hour of the accident reported ceilings varying from 400 to 1,000 feet. The Board noted that the flight likely encountered a cloud condition lower than indicated by preflight reports. In endeavoring to stay clear of clouds as required by the clearance, the pilot descended below the minimum altitude of 500 feet. The Board believed that in doing so, the pilot may have lost visual reference to the surface. The smooth water made judging distance extremely difficult.

The investigation considered sensory illusions that can occur when attempting to maintain orientation by visual reference to distant lights. A nosed-up attitude can cause distant lights to appear lower, creating a false illusion that the aircraft is higher than it actually is. The Board noted that refraction through the windshields and the unlighted water surface could have been contributory factors.

The Board ruled out an erroneous altimeter setting. Prior to landing at San Francisco, the flight acknowledged a barometric pressure of 29.90 inches, and at departure, both airports reported 29.89 inches. It remained unknown why the pilot did not refer to the altimeter or execute the prescribed missed approach procedure. The Board concluded it was reasonable to assume the pilot permitted the aircraft to descend into the bay under a low and spotty overcast while maintaining visual reference to the distant shore, misled by an optical illusion relative to altitude.

**Probable cause**

The Board determined that the probable cause of this accident was "the pilot's action in continuing descent below the 500-foot prescribed minimum altitude until the aircraft struck the water." The Board found that "a probable contributing factor to the aircraft striking the water was the sensory illusion experienced by the pilots."