Page:CAB Accident Report, TWA Flight 891.pdf/17

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There was no evidence of strong shears in the high—altitude winds. It is to be assumed that the storm cells moved about in conformty with the winds described above.

In View of the small rise in the surface temperature during the day because of the dense clouds, the scattered storms hitting the Po Valley in the afternoon seem to have been due to phenomena of forced updraft, caused by infiltration of cold air from the Alps, rather than to thermo- convective phenomena.

The calculation of the available energy for the phenomena of forced updraft, as the maxim estimate of the rising vertical currents in the storm cell at the presumed height of the crash, namely between 10,000 and 11,000 ft., comes out to about 12 ﬁlls, a figure which may go up to about 19 life if we consider the thermic instability also as being active.

Because of the aforesaid posmble speeds of the rismg currents, in contrast with the downward currents which, although weaker, were always present about the storm cell, we can admit outright the existence of the turbulence with strong accelerations.

Within the same cell there is also a manrmnn probability for the produc~ tion of very strong and localized electric charges, with a wide difference of potential eVen between adjacent pants and, therefore, a high probability of discharges.

Weather conditions at the place where TWA Flight 891/26 crashed, about 12.5 km. from the Malpensa airport, may be presumed to have been Slmllal“ to those reported by the weather stations of Malpensa and Linate. In fact, dense cloud formations covered the Milan zone at altitudes of from 2,000-h,000 to 1h,000-20,000 ft. At higher altitudes, towering cumulo- nimbus formations, in large cells, existed at up to 35,000 ft.

Below the 2 ,000-h,000 ft. the cloud ceiling may for short periods have dropped to 600-1,000 ft. during the showers. More—ornless steady rain and shower activities existed over the Alps and in the Po Valley until the system moved on toward the Adriatic Sea, permitting the entry of northerly Winds, after which the skies became clear.

An examination of the weather bulletins shows that at the time the TM Flight 891/26 to Paris took off (M20 p.111.) the weather over the airport was not good but neither was it prohibitive, even though the rumble of thunder, already audible at h:00 p.m., announced the approach of a storm. It was not until h:50 p.111. that storm actiwity of moderate intensity reached the airport. According to statements made by reasonaler reliable witnesses, at the time of the crash, 14:35 p.m., it was raining slightly in the vicinity of Olgiate Olona and the ceiling was estimated at 600-700 meters (SABENA'S flight 6148 entered from the clouds at about 2,600 ft. QWH, more or less in the same area and at approximately the same time, with a variation of plus or minus 30 seconds). ViSibili'bY was described as good enough: about 3-14 km. There had been a very heavy shower 5—10 minutes before the crash and it rained quite hard again, briefly, some time later. Some mtnesses heard thunder and saw flashes of lightning Just before and after the crash.