|CASB Majority Report|
General | Forecast Weather | Surface Observations | Precipitation and Surface Temperature Record | Pilot Reports
On 12 December 1985, the Gander weather was dominated by a deep, almost stationary, low pressure system situated about 250 miles south of Greenland. This low produced a moist northwesterly flow, giving overcast conditions with occasional light snow, very light snow grams, and very light freezing drizzle.
The 12 December area forecast for the Fortune, St. Georges, Exploits, and Bonne Bay Regions issued at 0530Z, valid for the period 0600 to 1800Z, forecast ceilings of 1,500 to 2,500 feet above ground level (agl), with cloud tops 6,000 feet agl. Visibility was forecast to be two to six miles in light snow showers.
For the eastern regions, where an onshore or upslope flow was present, forecast ceilings were 500 to 1,500 feet agl with occasional light freezing drizzle.
Light to moderate rime icing in cloud was forecast except for moderate clear icing in the light freezing drizzle. The freezing level was forecast to be at the surface.
The terminal forecast for Gander, issued at 0430Z 12 December, indicated an overcast ceiling at 1,500 feet agl, accompanied by light snow showers, variable to an overcast ceiling at 500 feet agl, with visibility reduced to two miles in light snow showers and occasional light freezing drizzle.
The Gander weather was generally as forecast. Throughout the evening of 11 December and the early morning hours of 12 December, ceilings varied between 500 and 1,400 feet agl, with visibilities between 2 1/2 and 12 miles. Precipitation was present in the form of light to very light freezing drizzle, snow grains or snow.
Surface observations from Gander taken between 0600Z and 1030Z were as follows:
||measured ceiling 1,400 ft broken, 2,800 ft overcast, visibility 10 mi in light snow, barometric pressure 1011.6 mb, temperature 4 C, dew point -5 C, wind 330 T at 4 kt, altimeter setting 29.84 in. Hg, strato cumulus 6 tenths, strato cumulus 4 tenths.
|measured ceiling 1,200 ft overcast, visibility 2 1/2 mi in light snow grains, wind 330 T at 5 kt, strato cumulus 10 tenths.
||measured ceiling 1,200 ft broken, 2,200 ft overcast, visibility 2 mi in light snow grains, barometric pressure 1011.6 mb, temperature 4 C, dew point -5.C, wind 330 T at 4 kt, altimeter setting 29.85 in. Hg, strato cumulus 8 tenths, strato cumulus 2 tenths.
|600 ft scattered, measured ceiling 1,200 ft broken, 2,200 ft overcast, visibility 5 mi in very light freezing drizzle and light snow grains, wind 300 T at 5 kt, stratus fractus 3 tenths, strato cumulus 5 tenths, strato cumulus 2 tenths.
||600 ft scattered, measured ceiling 1,200 ft broken, 2,000 ft overcast, visibility 8 mi in very light freezing drizzle and light snow grains, barometric pressure 1011.6 mb, temperature -4°C, dew point -5°C, wind 300°T at 5 kt, stratus fractus 3 tenths, strato cumulus 5 tenths, strato cumulus 2 tenths.
||600 ft scattered, measured ceiling 1,200 ft broken, 2,200 ft overcast, visibility 10 mi in very light freezing drizzle and light snow grains, barometric pressure 1011.6 mb, temperature -4°C, dew point -5°C, wind 300°T at 4 kt, altimeter setting 29.85 in. Hg, stratus fractus 5 tenths, strato cumulus 4 tenths, strato cumulus 1 tenth.
|700 ft scattered, measured ceiling 1,200 ft overcast, visibility 12 mi in very light snow grains, wind 290°T at 4 kt, stratus fractus 5 tenths, strato cumulus 5 tenths.
||700 ft scattered, measured ceiling 1,200 ft overcast, visibility 12 mi in very light snow grains, barometric pressure 1011.8 mb, temperature -°C, dew point -5°C, wind 290°T at 4 kt, altimeter setting 29.86 in. Hg, stratus fractus 5 tenths, strato cumulus 5 tenths.
|700 feet scattered, measured ceiling 1,200 ft broken, 2,500 ft overcast, visibility 12 mi in very light snow grains, barometric pressure 1011.8 mb, temperature -4°C, dew point -5°C, wind 290°T at 2 kt, altimeter setting 29.85 in. Hg.
(For definitions of precipitation types and rates see Appendix B.)
Precipitation and Surface Temperature Record
Between 0600 and the time of the accident, the surface temperature recorded at Gander ranged between -3.8 and 4.2 degrees Celsius. The dew point ranged between -4.5 and -5.1 degrees Celsius.
Precipitation in the form of light snow fell from 0600 to 0645. Light snow grains commenced at 0645 and continued until 0945. Very light freezing drizzle was reported between 0740 and 0945. Between 0945 and the time of the accident, recorded precipitation consisted only of very light snow grains.
Weather observations at Gander are taken from two locations. The primary observation site is located on the roof of the terminal building; a second observation site is located at ground level about 200 feet from the terminal building. When freezing precipitation is present or suspected, weather observers use an ice accretion indicator composed of a small piece of aluminum alloy similar to that found in aircraft structure. It is placed outside at the observation site and inspected for the presence of freezing precipitation at each observation. Mandatory weather observations are taken every 30 minutes, at the hour and half hour.
When freezing precipitation is present, the indicator is removed and a new one installed at each mandatory observation. Prior to installation, the indicator is pre-cooled to ambient temperature. Thus, when observed at the next observation following installation, the indicator shows the type and quantity of freezing precipitation which occurred in the previous 30 minutes.
The weather observer on duty at the time of the occurrence testified at the Board's public inquiry that, in the several hours preceding the accident, he had made regular visits to both observation sites to check on the icing indicators. He stated that, at his 0900 observation, he observed a small amount of freezing drizzle on the indicator at the roof observation site. He described it as small areas comprising 10 to 15 per cent of the surface area of the indicator. Also present, mixed in with the freezing drizzle, were snow grains which had adhered to the surface of the indicator. Together, the freezing drizzle and snow grains covered approximately 30 per cent of the indicator's surface. The result was a thin, rough layer resembling medium grit sandpaper which could be removed with a finger-nail. The indicator at the ground level observation site was substantially the same. As a result of this observation, the precipitation on the 0900 surface observation report was indicated as very light freezing drizzle and light snow grains.
Following the 0900 observation, the indicators at both observation sites were changed in accordance with standard procedure. The freezing precipitation observed on this indicator at 0930 again consisted of freezing drizzle mixed with snow grains, but the quantity was less than that observed at 0900. However, the texture of the surface was the same, and the decrease in quantity was not sufficient to result in a change to the precipitation indicated on the 0900 surface observation report.
Following the 0930 report, the indicators were again changed. They were inspected at 0943, at which time, only snow grains were observed in a small quantity. There was no freezing drizzle present. As a result of this observation, the observer determined that the freezing drizzle had ended and that the intensity of the snow grains had reduced. Accordingly, a special weather observation report was issued at 0945 which indicated precipitation as very light snow grains.
Similar observations of small amounts of snow grams were made at 1000 and 1030. During his several visits to the observation site after 0945, the observer did not observe any evidence of freezing drizzle.
Throughout the period, no appreciable difference was noted between the indicators at the two observation sites.
Precipitation accumulation is measured over a six-hour period. The measurements pertinent to the accident were for the period 0601 to 1200Z, 12 December 1985. In that time, the measured precipitation was freezing drizzle - trace (less than 0.2 millimetres); snow grains 0.2 centimetres, water equivalent 0.2 millimetres.
A British Aerospace VC-10 aircraft landed at Gander at 0626, approximately four hours before the accident, and departed Gander at 0716, three hours before the accident. The flight crew of this aircraft reported that no significant icing was encountered on either approach or departure. Precipitation described as light grainy snow was reported during the approach, station stop, and departure. No significant or unusual weather conditions were encountered.
A Boeing 737 aircraft departed Gander approximately 45 minutes after the arrival of MF1285R and 30 minutes before the accident. The pilot of that aircraft testified that the cloud base was about 700 feet agl, with cloud tops about 4,000 feet asl. During climb-out, he encountered moderate icing in cloud. He estimated that it took approximately one minute to climb through the cloud layer. During this time, he observed about one-quarter inch of ice accrete on the windscreen centre post. Below the cloud, he did not perceive any icing, either in flight or during the taxi out on the take-off roll. It was his belief that the ice which did accumulate on the aircraft was mainly clear ice. The ice did not present any difficulties because of the very short duration of time spent in cloud. It dissipated quickly after climbing clear of cloud. He funkier testified that the flight conditions after take-off were smooth. No turbulence or wind shear was experienced.
A Piper PA-31 landed on runway 31 at Gander at 1016, approximately 30 seconds after the accident. The crew of this aircraft reported icing conditions in the cloud layer between 4,000 feet asl and 700 feet agl. The icing was sufficient to obscure the view from the cockpit. Only a small area of the windshield was reported to be clear. Precipitation described as drizzle was reported to be falling. No other significant or unusual weather conditions were reported.
About 20 minutes after the accident, another Boeing 737 aircraft landed at Gander. The captain of that aircraft testified that, during his approach to land, cloud tops were encountered at about 4,000 feet asl, and the cloud base was about 700 feet agl. He could not recall encountering any icing. He did encounter very light precipitation below cloud. He described it as a very light drizzle but could not determine if it was freezing drizzle. Flight conditions on approach were further described as smooth with light wind and no turbulence.