Page:CAB Accident Report, United Airlines Flight 21.pdf/31



The sky conditions which prevailed in the vicinity of the Chicago Municipal Airport at the time of the accident consisted of a stratus cloud deck the ragged base of which afforded a variable ceiling of from 600 feet to 1200 feet above the field level. The evidence indicates that this ceiling was modified by a secondary layer of lower scud at 600 feet which persisted when the primary cloud layer lifted to the maximum within this range of variability. Intermittent snow flurries of light intensity were falling which, in conjunction with a light fog and occasional freezing drizzle, resulted in a changeable horizontal visibility, the average being officially reported as about one mile. Surface temperature and dewpoint observations were indicative of icing conditions. The top of the cloud layer was reported at 5000 feet above sea level approximately one hour prior to the accident, but evidence adduced during the investigation indicated that this changed in the interim to an ill-defined upper limit to the cloud mass with a still higher layer above. Partial merging of these cloud systems made positive between-layer flight erratic.

These conditions were produced as attendant phenomena associated with the passage of a low pressure system southeastward across the Great Lakes from the Alberta region. The weak cold front of the this disturbance moved past Chicago at about 4:00 p.m., the air mass to the rear of this discontinuity being of a conditionally unstable type. Convergence effects within this air mass were sufficient to bring about saturation, and, since the temperature was within the critical range, conditions were favorable for ice accretion upon the exposed surfaces of an aircraft flown within or slightly below the main cloud layer. Icing conditions of this type ordinarily would produce a rough opaque ice which might be considered an intermediate phenomenon between glaze and rime.