Portal:Civil Aeronautics Board

“ The stall characteristics of the DC-3 with power off are considered to be quite satisfactory. When a stall occurs it develops at the center section before progressing to the wing tip and good lateral control is maintained almost to the stalling speed. An incipient stall is accompanied by vibration and tail buffeting which develops at a speed of about five miles per hour above the actual stalling speed. There is no marked tendency for the airplane to roll off in either direction, and it may be held in the stall without rolling unless the stall has been induced by pulling the nose up sharply. ”

“ It is reasonable to believe, therefore, that Captain Scott, after choosing the northwest runway under the unfavorable conditions existing, would attempt to make his approach at as low an altitude as possible and at a minimum possible flying speed in order to avoid overshooting the runway. According to the testimony of the pilots, it appears that in landing on the northwest runway a drag approach is made with gear down, three-quarter flaps, and about 25 percent horsepower. We have previously discussed the stall characteristics of a DC-3 airplane. ”

“ Although pilots are not in agreement as to the degree of effect of ice upon the performance of the aircraft, they all agree that ice does raise the stalling speed. Wind tunnel tests under simulated icing conditions confirm this conclusion, which also follows from a consideration of the basic theory of wing action. The stall is not accompanied by the usual visual or mechanical warnings and may occur at an attitude which is not expected by the pilot. Tail buffeting is often decreased or entirely eliminated. ”

“ It is improbable that a momentary failure of one engine to take the throttle could have resulted from cooling of the motors, because, according to pilot testimony, one of the principal advantages of the drag approach under partial power is that the motors remain warm and available for an emergency.
The evidence indicates that at the time of impact both motors were under little, if any, power, and the similarity of their condition indicates that both engines were doing exactly the same thing at that time. ”

“ 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. ”

“ A fire hose was played upon the engines following the accident and since the carburetor adapter of the right engine was cracked, it is probable that the water found in the right engine came from that source.
It was the opinion of engine experts [14] at the hearing that the water found in the engines did not indicate ice accumulation in the carburetors. They stated that if there had been ice in the carburetors sufficient to interfere with the operation of the engines, the inspection would have disclosed a substantial amount of water in the blower sections whereas none was found there. ”

“ In recovering, the plane usually loses a minimum of 500 feet of altitude. The speed at which the stall develops is variable depending upon many factors such as loading, flap position, and degree of bank. Pilots employed by air carriers using DC-3 equipment are given periodic flight checks which include both power on and power off stalls, so that all airline pilots using this equipment are familiar with its stall characteristics. United's procedure requires such checks of its captains every six months. ”

“ The only evidence in the record which suggests that one motor might have failed at that time is the statement of one witness, an airline pilot, who thought that the maneuver performed by the airplane resembled a stall aggravated by the use of the opposing motor. ”

“ 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 normal landing procedure for the northwest runway at Chicago is to make an approach for landing as low and with as little excess speed as possible. Mr. Addems, Superintendent of Flight Operations for United, testified that under normal conditions the approach to the northwest runway must be made with extreme accuracy. The pilot must come over the boundary with a minimum of speed and at a height of not more than 25 to 30 feet above the high tension wires bounding the field. ”

“ Rime ice is usually milk in color, of a rough uneven globular form, described as tending to build up very quickly but brittle in nature and easy to break off with de-icing equipment. Glaze ice is described as clear and smooth, tending to build up very rapidly but clinging to and preserving the form of the object upon which it accumulates. Glaze ice is harder to remove and when broken off by the action of the de-icer boot, tends to stick in patches and build up over the cap strip behind the boot. Glaze ice will not blow off as easily as does the more brittle and rough rime ice. ”

“ An overcast was expected in the Chicago area with a base at 1000 feet and a top at 4000 to 5000 feet. Light and moderate snow and possible light freezing drizzle was expected below 5000 feet with an inversion of temperature above 32 degrees existing over 5000 feet. According to United's procedure, the captain of the flight is required to prepare his own forecast upon the same form as the meteorologist. Captain Scott's forecast indicates he expected to find a mild to moderate icing condition with ice of both rime and glaze types in the region between Toledo and Chicago. ”

“ After a preliminary examination of the wreckage at the scene of the crash, the Safety Bureau authorized the removal of the aircraft to the United Air Lines hangar at the Chicago Municipal Airport for disassembly and inspection. Until the close of the public hearing, when the aircraft was released by the Board to United Air Lines, the wreckage was under continuous guard by the Chicago Police Department and all inspections, including the dismantling and tear-down of the engines and accessories, were completed under careful observation of staff experts of the Board. ”

“ Before flying on the civil airways under instrument weather conditions, approval must be secured from Airway Traffic Control for the flight, including the altitude at which it is to be flown. The Cleveland and the Chicago Airway Traffic Control centers were concerned with the flight of United Trip 21 between Cleveland and Chicago. The jurisdiction of the Cleveland center includes the civil airway between Cleveland and a point 25 miles east of Goshen, Indiana, and the jurisdiction of the Chicago center extends from that point to the Chicago Airport. ”

“ The 4:10 weather observation reported the ceiling at 1000 feet, an overcast sky with scattered clouds at 600 feet, a variable visibility of 11⁄4 miles, with a light freezing drizzle and light smoke, a temperature of 33 degrees, a dewpoint temperature of 31 degrees, with a wind from the west northwest of 11 miles per hour, and included a pilot report locating the top of the overcast at 5000 feet and light to moderate icing existing between 1000 and 5000 feet. ”

“ It is definitely established that Captain Scott's windshield was covered with ice when he first broke out under the overcast, since he reported to the Control Tower that it would be necessary for him to circle the field before landing in order to get the ice off of his windshield. The record indicates that the hot air blower windshield de-icing equipment is not always entirely effective during heavy icing conditions in keeping the windshield clear of ice. ”

“ 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. ”

“ In addition, while it is the usual Airway Traffic Control procedure to bring aircraft into an airport by stepping them down through 1000-foot levels, Airway Traffic Control varied this procedure in the case of United 21 in order to avoid holding the trip in icing conditions while awaiting a clearance to land. Captain Scott was told to hold at 6000 feet until Airway Traffic Control could give the trip an approach clearance so that it might descend through the overcast as rapidly as possible. ”

“ If the aircraft was being flown very close to its stalling speed and power applied suddenly, the failure of one engine to respond would be sufficient to precipitate a stall. If the aircraft had already stalled and one engine failed upon the application of power in an attempt to recover, the stall would be aggravated. ”

“ Although there are reports of interference with the pitot operation due to a build-up of ice on the mast supporting the pitot tubes, the effect of this ice would be, ordinarily, to make the air speed indicator read low. A low reading of the air speed indicator, according to pilots, may ordinarily be expected from the effect of ice on the air speed pitot tubes themselves with the effect of causing the pilot to maintain a higher speed than is actually indicated, thus operating as a safety factor. ”

“ The unpredictability of icing conditions and the fact that such conditions may be hazardous under some circumstances and not under others, and the effect to be given to such conditions in conjunction with other circumstances such as the length and state of the runways and the conditions of wind direction and velocity, all make necessary a reliance upon human judgment acting upon all of the facts and circumstances available at the time. ”

“ From the testimony of several airline pilots who have been using the Chicago airport for many years, it appears that the southeast-northwest runway is too short for anything but normal operation under favorable conditions with the airline equipment now in use. ”

“ The ship landed normally, showing no characteristics of icing, and on inspection on the ground just a few spots of rough ice remained clinging to his de-icer boots. He used his de-icing equipment while in the overcast, not shutting his boots off until he passed over the range station. The windshield de-icer worked effectively and on landing his windshield was free of ice. Captain Holsinger described the windshield de-icer used on United planes as a hot air blower equipped with a booster pump having two fixed outlets, one in front of each pilot. ”

“ They are then permitted to land as quickly as is consistent with safety and, unless an emergency exists, no landing priority is given to any aircraft. ↑ The aircraft was equipped with a flight recorder which was installed in the tail and which was undamaged in the crash. The flight recorder trace substantiates the progress of the flight as reported in the radio communications but provides no dependable evidence as to the last few minutes of flight, the trace ending in a blot covering the period for the last 15 minutes of flight. ”

“ 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 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. ”

“ Captain Scott then elected to fly on top of the overcast, reporting as follows:
"United 21 OK. Will cross on top no matter what."
At 4:21 a change is clearance was requested and obtained from the Chicago Airway Traffic Control for Trip 21 to fly 500 feet on top, crossing McCool at 5500 feet. At 4:34 Trip 21 reported over Goshen 500 feet on top of the overcast, reporting on outside temperature of 33 degrees at this altitude. ”