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of altitude. Aside from the improbability in the light of existing meteorological knowledge of so strong a descending current, and aside from the fact that vertical currents lose their violence near the ground so that the pilot would have in any case been able to check a descent induced in that fashion before the ground had been reached, the time element seems to eliminate a steady vertical current as a possible primary cause of the accident. The earliest point at which the pilot could by any possibility have entered a strong descending current to the point of contact with the ground was only about two miles. It could not possibly have taken over one minute to cover that distance, and to have descended from cruising altitude to ground level within that time would have required a mean rate of descent of over 5000 feet per minute, and a descending current of even more velocity than that. It scene impossible to credit the existence of any such condition.
The Board has also given consideration to the possibility that in the event of severe turbulence, the occupant of the jump seat in the pilots' compartment might have inadvertently disturbed the pilots' control of the airplane. The jump seat installation in NC 21789 consisted of a 12 x 18 inch flat board about ½ inch thick. It is supported by two steel pins about ½ inch in diameter on one side, one of which is spring loaded to hold it in place. These pins are placed in holes in the secondary structure provided for that purpose. The other side is supported, when in a down position, by resting on a channel which supports a part of the mail pit. There were no back or side arms attached. The seat was designed so that it might be raised upward from one side to permit pilots to pass to or from the cockpit.
