Thunderstorm Avoidance Lessons
Since the advent of commercial aviation, thunderstorms caused delays, diversions, and accidents. Airlines attempted to find ways to avoid enroute weather while maintaining scheduled operations. In 1949, American Airlines and the Unites States Navy conducted tests of an onboard weather radar system installed in a Convair airliner (“Flying Lab,” 1949, p. 28). In 1956, American Airlines equipped its fleet of DC-7s with airborne weather radar (“AA to Equip,” 1955, p. 20). Airlines quickly embraced the innovative technology, and Aviation Week (Christian, 1955) quoted a Northwest Airlines pilot: “We all wonder how we ever got along without it. Pretty soon the public won’t fly in anything but radar-equipped airplanes” (Christian, 1955, p. 40).
Radar was life-changing for the crews who formerly navigated through areas of embedded thunderstorms by guesswork and prayers or significant routing changes. This new technology quickly became trusted by flight crews and dispatchers, and Job (1994) states:
…since the advent of airborne weather radar, more and more aircraft were being dispatched in marginal weather, with the captain having the primary responsibility for avoiding severe conditions…too much reliance was probably being placed on an instrument which could not ‘see’ the turbulence itself. (pp. 58-59)
Several aircraft accidents in the late 1950s and early 1960s demonstrated this faulty reliance by dispatchers expecting pilots to avoid enroute thunderstorms and weather but failing to provide vital information on enroute weather conditions. These accidents included Capital Airlines flight 75 in 1959 (CAB, 1959), Mohawk Airlines flight 112 in 1963 (CAB, 1964), two different Braniff International Airways flights, flight 250 in 1966 (NTSB, 1968), and flight 352 in 1968 (NTSB, 1969).
One of these accidents that emphasized this misguided reliance on crews for inflight weather avoidance was highlighted by the NTSB (1968) in its report on Braniff International Airways flight 250 (p. 15). On August 6, 1966, a BAC 1-11 designated as flight 250 departed from Kansas City bound for Omaha. Prior to departure from Kansas City, the crew discussed the expected enroute weather with another Braniff flight crew that had just arrived from Chicago. The incoming crew described “a solid line of very intense thunderstorms with continuous lightning and no apparent breaks” (Job, 1994, p. 53).
Dispatchers were aware that another Braniff flight had delayed its takeoff from Sioux City because of weather at Omaha, and yet another Braniff flight between St. Louis and Omaha had diverted to Kansas City after its pilot elected to completely avoid the intense squall line of thunderstorms (NTSB, 1968, p. 14). Dispatchers failed to inform the crew of flight 250 of the other crews’ decisions to avoid the weather. The dispatcher testified to the NTSB (1968): "If he received a severe weather warning for an area through which company aircraft were operating, it was doubtful that he would forward this information to en route aircraft. In his opinion the crews in the area would be better able to evaluate the weather than he." (p. 15)
Flight 250 continued ahead toward the line of thunderstorms, requesting a deviation to the left to avoid weather (Job, 1994, p. 55). It suddenly encountered a severe gust of wind in the turbulent shear zone near the line of thunderstorms. The gust broke the elevator and rudder from the tail, and seconds later the right wing failed (Job, 1994, p. 56). Witnesses who had been outside watching the approaching storm saw the aircraft plummet to the ground, killing everyone on board (Job, 1994, p. 54).
The NTSB (1968) determined the probable cause to be “inflight structural failure caused by extreme turbulence during operation of the aircraft in an area of avoidable hazardous weather” (p. 59). The word “avoidable” in the Board’s probable cause is tied to directly to the dispatcher’s error of omission. The dispatcher possessed weather knowledge but failed to provide it to the crew of flight 250. As a result, 42 people perished (NTSB, 1968, p. 1).