# Aviation Accident Report: United Airlines Flight 21/Conclusion

CONCLUSION
FINDINGS
Upon all the evidence available to the Board at this time, we find the probable facts relating to the accident involving aircraft NC 25678 which occurred near Chicago, Illinois, on December 4, 1940, to be as follows:
* The accident occurred at approximately 5:48 p.m., and resulted in major damage to aircraft NC 25678 and the death of the crew of three and seven of the thirteen passengers.
* United Air Lines Transport Corporation, the operator of the aircraft, held a currently effective and appropriate certificate of public convenience and necessity and an air carrier operating certificate.
* The flight crew, Captain Philip Scott and First Officer George S. Young, were physically qualified and held proper certification of competency to operate as air carrier pilots over the route between New York and Chicago.
* Aircraft NC 25678 was currently certificated as airworthy at the time of the accident and had been maintained in accordance with company procedure and the applicable maintenance competency letters issued to United.
* United 21 was cleared in accordance with proper procedure from New York, New York, to Cleveland, Ohio, including stops at Philadelphia and Allentown, Pennsylvania, and Akron, Ohio, and from Cleveland, Ohio, to Chicago, Illinois.
* Before take-off from LaGuardia Field, New York, the right engine was cutting out on one magneto but this difficulty cleared up prior to take-off and did not recur during the flight between New York and Cleveland. ​
* At the time of departure from LaGuardia Field and at all intermediate stops between New York and Chicago, the gross weight of the airplane did not exceed the approved standard gross weight and its load was properly distributed.
* After arrival at Cleveland, the right engine was carefully inspected and checked by maintenance personnel of United and found to be in proper operating condition, which finding was concurred in Captain Scott.
* The difficulty with the right engine did not contribute in any way to the accident.
* At the time of departure from Cleveland for Chicago, the airplane carried sufficient fuel to permit flight at normal cruising speed to Chicago and thereafter for more than three hours.
* At 5:22 p.m., United 21 reported over the Chicago radio range station and cruised in that vicinity at the 5000 and 6000-foot levels until 5:33 p.m., when Captain Scott began his descent to the airport for a landing.
* The weather conditions existing at the airport were: Ceiling of 900 feet with low scattered clouds at 600 feet, visibility of one mile with light fog, occasional drizzle and light snow, wind from the north northwest of 8 to 10 miles per hour, and temperature of 32 degrees.
* Icing conditions existed to varying degrees in the overcast in the vicinity of the airport, including both rime and glaze type of ice, being especially pronounced at the 3000 and 4000-foot levels. These conditions were known to both the flight personnel of United 21 and United's dispatcher at Chicago. ​
* The weather conditions which existed were accurately forecast by both the United States Weather Bureau and by United's meteorologists.
* The runways at the Chicago Municipal Airport were in a slippery condition.
* Notwithstanding this condition, the east-west runway was sufficiently long and was adequate for a landing under the prevailing wind and weather conditions and had been used by a number of pilots landing at about this time.
* The northwest runway was limited in length by a railroad embankment and a landing on it under the existing wind and weather conditions would permit little, if any, margin of safety.
* Captain Scott cannot be adversely criticized for descending through the overcast for a landing, and United's dispatcher at Chicago cannot be adversely criticized for permitting him to do so.
* United 21 received a traffic clearance from the Air Traffic Control Tower operator permitting the use of either the northwest or the east-west runway.
* At 5:41 p.m., United 21 reported contact at an altitude of 900 feet above the ground near the Chicago range station.
* At the time Captain Scott broke out of the overcast he had accumulated a quantity of ice on his windshield which impaired his vision sufficiently to cause him to circle the airport for the purpose of clearing his windshield.
* After flying for a short period over, and in the vicinity of, the airport under the overcast, he reported to the Airport Control Tower that he ​would land on the east-west runway, but 40 seconds later changed his plan and stated that he would land on the northwest runway.
* No sudden emergency occurred which made it necessary for Captain Scott to land on the northwest runway.
* Under all the circumstances Captain Scott made an error of judgment in choosing the northwest runway for a landing.
* Upon reaching a point approximately 700 feet southeast of the southeast corner of the airport at an altitude of approximately 160 feet, the airplane stalled, fell off to the left, and crashed while in an incipient spin.
* Up to the instant before the crash the approach path was normal for a landing on the northwest runway, no structural or mechanical failure or malfunctioning of any part of the aircraft had occurred, and the aircraft was trimmed for a normal approach with gear down, flaps in the three-quarter position, and operating under approximately 25 percent horsepower.
* Power was applied almost simultaneously with the stall, and no engine failure caused the stall or contributed to the crash.
* Visibility from the cockpit was impaired during the approach due either to an accumulation of ice or mist on the windshield.
* The aircraft had accumulated ice of a rough granular nature on the de-icer boots on the leading edges of the wings about ⅜ of an inch thick extending back for a distance of 2 inches on the top and bottom of the boots, and ice of a clear gaze type from 1/32 to 1/16 of an inch in thickness extending back on the boots almost to the attachment strip. Ice was also present on the de-icer boots on the tail surfaces. ​
* The stalling speed of the aircraft was raised an undetermined amount by the ice accumulation on the leading edges of the wings.
* Captain Scott failed under the conditions set forth above to maintain adequate flying speed.
PROBABLE CAUSE
Upon the basis of the foregoing findings of fact and upon all the evidence available at this time, we find that the probable cause of the accident to aircraft NC 25678 (United 21) on December 4, 1940, was the failure of the pilot to exercise that degree of caution and skill required to avert a stall while approaching for a landing on the short northwest runway. A substantial contributing factor to the accident was the error in judgment of Captain Scott in choosing that short runway for his landing.
RECOMMENDATIONS
* An adequate stall-warning device has been recognized as much needed in connection with air carrier services and other aircraft operations for a long time. Some research has been conducted along this line, but to date nothing has been developed which appears to be reliable under all conditions, particularly under icing conditions. The Board recommends that the Administrator of Civil Aeronautics give this matter the utmost encouragement through appropriate channels, including the aviation industry, in an effort to accelerate the further development of an adequate stall-warning device. The Board also recommends the continuance of research into the more functional problem of developing wing surfaces which are not subject to critical behavior at or near the stalling speed.
* The need for adequate windshield wipers or other devices for improving vision from the cockpit under adverse atmosphere conditions is self-evident and was especially brought out at the Chicago hearing. The need for such a device has been long recognized by the industry and research has been conducted by the National Advisory Committee for Aeronautics and by some operators in an effort to develop a satisfactory windshield wiper. In order to further this project, and to encourage the most rapid development of an adequate windshield wiper, the Board recommends that the Administrator of Civil Aeronautics take steps through appropriate channels, including air carriers, to expedite the development and adoption of an adequate device.
* The Board recommends that the Administrator of Civil Aeronautics stimulate the development of a more accurate means of measuring the rate of ice accretion and the adoption of an appropriate device for this purpose. The Board recommends further that the appropriate governmental agencies be requested to determine the relation of ice accretion to the terms generally used when reporting ice, namely, "light, medium and heavy".
* The testimony of several pilots indicates that the formation of ice in and on the pitot tubes and on the pitot tube mast may lead to erroneous readings. It is understood that this problem is the subject of study and research by the air carriers and various governmental agencies. The Board recommends accelerated research in connection with the development and adoption of an air speed indicator which is not susceptible to incorrect readings due to formation of ice.
* The Board recommends that the National Advisory Committee for Aeronautics continue and intensify its research looking toward the development ​of an improved means to prevent the accumulation of ice on wings and other parts of aircraft without thereby interfering with aerodynamic characteristics.
* The Board recommends that the Administrator undertake a study of measures that should be adopted at all air carrier airports to keep Airport Control Tower operators, or others responsible for the issuance of information to pilots, informed by reports from the field personnel, pilots or other sources, of hazardous field conditions in order that such information may be promptly relayed to incoming or outgoing pilots.
* When the accident happened, the airplane severed power lines leading into the airport, causing discontinuance of the lighting and communication facilities needed to furnish information for the operation of aircraft. The Board recommends that an auxiliary power supply be installed at all airports used by air carriers for instrument or night approaches. The auxiliary power supply should be independent of any source outside the boundaries of the airport and should be applicable to the operation of radio transmitters, field lights and obstruction lights. The Board is considering a requirement to this effect.
BY THE CIVIL AERONAUTICS BOARD:
(Edward P. Warner, Vice Chairman, participated in the preparation of this report but was absent from the United States on official duty at the time of its adoption.)