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attitude of the aircraft causes a distant light or concentration of lights to appear lower (and the aircraft thus higher), and vice versa. This simple false illusion has demonstrably caused a number of accidents, and many near-accidents, under conditions of light and weather similar to those being encountered by Flight 636. Refraction, and apparent displacement, of lights through windshields, with many conflicting and confusing reflections, is another element that may have been involved. Another contributory factor could have been the unlighted water surface offering little or no visual stimuli for estimating altitude.
Notwithstanding the points mentioned above, there remains the fact that the pilot had two altimeters in the cockpit. It was disclosed that prior to landing at San Francisco the flight received and acknowledged the San Francisco barometric pressure of 29.90 inches. There was no appreciable change in pressure between this time and the time of departure from San Francisco when both airports reported the pressure at 29.89 inches. Therefore, it can be concluded that there was no possibility of erroneous altimeter setting existing as a factor in the accident. Why the pilot did not refer to the altimeter is unknown. Pursuing this trend of thought, there arises the question as to why the pilot did not follow the prescribed procedure of climbing to 2,000 feet and intercepting the northwest leg of Oakland range when he found it impossible to maintain visual contact at 500 feet.
In reviewing this accident, the Board concludes that the crew was definitely qualified to operate the aircraft. The evidence is conclusive that the aircraft was in an airworthy condition. It is, therefore, reasonable to assume that in the conduct of the flight the pilot permitted the aircraft to descend into the Bay under a low and spotty overcast while maintaining visual reference to the distant shore, in the belief that he still was safely above the water. Obviously the pilot must have been misled by some form of optical illusion relative to altitude.
With regard to Trans-Bay Operations, the Board, subsequent to this accident, inquired of the Administrator regarding the adequacy of the procedures prescribed for Visual—contact flight, particularly with respect to 4-engine aircraft. The Administrator has advised that this matter has been reviewed and re-evaluated both by the CAA and by a joint industry and CAA group since the accident, and they have concluded that the procedures in effect insure a reasonable degree of safety consistent with normal standards.[1]
- ↑ Section 61.261 of the Civil Air Regulations governing minimum flight altitudes specifies a minimum of 1,000 feet for VFR (Visual Flight Rules) operations "Provided, that other altitudes may be established by the Administrator for any route or portion thereof where he finds, after considering the character of the terrain being traversed, the quality and quantity of meteorological service, the navigational facilities available, and other flight conditions, that the safe conduct of the flight permits or requires such other altitudes."
This deviation authority has been exercised in four cases by the Administrator in authorizing lower VFR flight altitudes for the following routes:
- Fort Worth and Dallas, Texas (day and night)
- Spartanburg and Greenville, South Carolina (day and night)
- Winston-Salem and Greensboro, North Carolina (day and night)
- San Francisco and Oakland, California (day and night)
