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would have to be maintained for several minutes and over a distance of several miles to force an airplane down from 5500 feet above the ground.
The roughening effect of rain due to fixed or splashing rain drops on the airfoil is unknown, but the percentage increase in drag from that cause is believed to be small in airplanes such as the DC-3 with overlapped skin construction, exposed rivet heads, and other departures from an absolutely smooth wing surface.
While the conclusions of the National Advisory Committee for Aeronautics indicate that heavy rainfall will not disturb the performance or behavior of an airplane such as a DC-3 to any marked degree, the Committee believes that heavy rainfall would have a substantial effect on the performance of the airspeed indicator. Beyond a certain critical combination of airspeed and rain density, the airspeed head will flood and the water will accumulate in the pressure line. If such be the case the airspeed would no longer serve as a guide to the true flight condition.
However, in the case of NC 21789, a hand pressure pump was incorporated in the airspeed indicator system so that any accumulation of water could be manually discharged. It was impossible to determine whether this pump had been used, but had the airspeed indicator been affected due to water in the pressure line, it should not have caused serious complications for one of Captain Scroggins' experience because he had undoubtedly encountered a similar condition resulting from the pitot head freezing under icing conditions. Moreover, any inaccuracy of the airspeed indicator would be revealed by the attitude of the airplane, as shown by the artifical horizon, the altimeter, the power output of the engines, and the feel of the controls.
Based upon all the available evidence, it appears that the effect of the rainfall could not alone have caused the accident.
