A Sigmet issued for the accident area warned of occasional severe turbulence below 15,000 feet msl while Airmets forecasted moderate icing
conditions, with a freezing level between 6000 and 11,000 feet msl, plus instrument conditions and mountain obscuration.
had also been reported at 6,500 feet over North Wales.
And they still have an AIRMET for icing over the area, and that runs from Great Falls to Idaho Falls, and that says moderate icing
between the freezing level and 20,000.
In our study a value of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], [N.sub.1] will be used, where Ni corresponds to the value of moderate icing
determined as in (4).
We also encountered moderate icing
, with about a quarter of our windscreen getting rapidly covered with ice.
Pilots told us that in moderate icing
or freezing rain, the wings often carry slushy areas or spots of ice that may or may not pop off in flight.
An upper air sounding 18 miles from Greene County taken an hour after the accident "showed high icing potential within the cloud layers above the surface." Infrared satellite imagery from 1730 to 1815 showed cloud top temperatures of -12 C, corresponding to cloud tops around 12,000 feet and "indicating the likelihood of moderate or greater icing along the airplane's route of flight...." Airmets called for moderate icing
conditions below 12,000 feet msl and an Indianapolis Center statement warned of occasional moderate rime and mixed icing between 2000 and 5000 feet msl.
This made it plenty ripe for moderate icing
conditions in that altitude window.
The NTSB accident report stated the probable cause of the accident was "continued flight into known light-to-moderate icing conditions over mountainous terrain." Reporting stations along the flight path advertised widespread marginal VFR and Airmets had been issued for moderate icing
, mountain obscuration and moderate turbulence, and wind shear.
is indicated by an orange "U" with two vertical lines through it.
We had flown in known and published moderate icing
conditions for more than 10 minutes.
An Airmet for moderate turbulence below 8000 feet, and moderate icing
between the freezing level and 22,000 feet were valid.