These twelve historical accident reports represent the spectrum of icing knowledge during the late 1930s through the early 1960s. The flight data recorders, if they even existed, recorded a minimum of data and often were not readable, so the development of factual information required painstaking investments in time and money.
United 21 and Northwest 5 represent the benchmarks of icing accident investigation during this period. The icing discussion for United 21 begins on page 47. The Northwest captain was the only survivor of flight 5, and his thought processes are described in that report in great detail. Both investigations involved flight tests of the DC-3 in icing; it is interesting to compare the information developed in these investigations with the later investigations of Zantop and Frontier, and particularly Mid Continent 16. The latter appears to be almost identical to United 21, yet the investigation seems almost reluctant to incorporate or refer to those earlier accidents.
The report for the Maersk Air Fokker F-27 accident in 1969 provides a perspective of icing from a European standpoint at that time. Note that the probable cause does not indicate a role for the approximately 1 inch of ice accretion during this stall-type accident.
The wide range of interpretations given to the influence of icing in these reports may suggest an inconsistent knowledge of icing within the investigative communities of the times. Although a number of fundamental lessons were captured very early on, the difficulties of obtaining and disseminating previously learned lessons may have made it difficult to sustain the very same lessons.