Page:CAB Accident Report, Western Air Lines Flight 636.pdf/6

 resulted from the pilot's failure to maintain sufficient altitude to avoid contact of the aircraft with the water. The precise reason or reasons for the pilot's action or lack of action in allowing the aircraft to descend into the water is a matter of conjecture. However, there were several pertinent conditions and circumstances that can be considered as contributory factors. These were the type of operation being conducted, the weather conditions that existed over the Bay and the sensory illusions that can occur under certain conditions.

The type of operation being conducted was somewhat of a special nature wherein flights between the Oakland and San Francisco Airports are permitted to fly at altitudes below the minimums normally prescribed for scheduled airline operations and also below the normal Visual Flight Rule weather minimums. This has been authorized to expedite traffic between these two airports in view of the short distance involved and the fact that such flights are made entirely within controlled airspace. Special procedures have been established in the form of sliding scale minimums for various combinations of visibility and ceiling values. Also, aircraft must remain clear of clouds and fly not less than 500 feet above the surface. If unable to remain clear of clouds at 500 feet or if unable to maintain visual contact with the surface, such flights are required to climb to 2,000 feet, intercept the northwest course of the Oakland range and hold for clearance to make a standard instrument approach. It is evident that, at the time of the subject flight, the cloud base was lower than 500 feet over portions of the Bay area. Reports indicate that the ceiling in the area of the accident was approximately 400 feet. It was also found that the visibility was at least 12 miles at an altitude of 300 feet.

It therefore appears that in proceeding over the Bay, the subject flight encountered a cloud condition lower than indicated from preflight reports and that the pilot, endeavoring to stay clear of clouds as required for this operation, descended below the minimum altitude of 500 feet. In doing so, the pilot may have lost visual reference to the surface both with respect to the lights on shore and to the surface of the water. As the waters of the Bay were reported as smooth, a condition existed that made it extremely difficult if not impossible to judge distance above the water especially as it was at night and when no other means of reference were available for visual orientations.

In this connection, the third condition enters the then existing situation. This is a condition wherein an erroneous belief of an aircraft's altitude can occur when attempting to maintain orientation by means of visual reference to distant lights. In this case the aircraft was approaching the shore some five miles distant where there were numerous lights. But the concentration of the much stronger lights at the airport proper could well cause that cluster of lights to appear as a single foci, and thus bring into being the condition so aptly described by P. P. Cocquyt's "The Sensory Illusion of Pilots." Therein, the author explains the condition necessary to cause a pilot to believe that he is higher than he really is, and so invite quick disaster if at extremely low altitude, as was the case in the subject flight. Briefly, the error in estimate of altitude stems from the fact that a nosed-up