because it is taller than the average ridge height used
in the model. When the wave forecast was adjusted
for the air downwind of Mount Rainier the wave
strength at 42,000 feet grew to an amplitude of 8,255
feet with a maximum vertical velocity of 4,551 fpm
with severe turbulence likely. The definition of
“severe” turbulence is that the airplane is out of control, at least briefly. But the chance of severe
turbulence appeared to be well above 11,000 feet in
the forecast model.
The forecasts did show a chance of rain showers
on the lee side of the mountains, and NEXRAD indicated the existence of very light to light rain
downwind of Mount Rainier. But the forecast for
light rain and only light to moderate turbulence
would not be enough to keep a reasonable IFR qualified pilot from making the flight.
The forecast turned out to be wrong in terms of
turbulence intensity and the existence of a mountain
wave, but the pilot still had fair warning. A camera
recovered from the wreckage shows a number of
images taken by the pilot or passenger as the airplane flew past Mount Rainier at 11,000 feet. The
Lancair was flying on top of a broken to overcast
layer and had a clear view of the volcanic mountain.
But in the photos investigators could see a band of
higher clouds immediately downwind of the mountain with tops as high as the mountain. The clouds
showed some signs of undulation motions or wave
action at the cloud top, with a higher band of cirriform cloud layer farther downwind.
We all learned in ground school that clouds
downwind of a mountain peak with clear air to
windward is often an indicator of mountain wave
turbulence. The classic mountain wave cloud is a
lenticular—shaped like a lens—cloud that forms over
a mountain peak or ridgeline. But any type of cloud
that forms only downwind of a mountain peak, particularly a cloud that shows apparent motion or
ragged structure, almost certainly contains turbulence, and possibly a vortex.
Until flying past Mount Rainier the Lancair pilot
had only routine IFR communications with
Anchorage center and then Seattle approach controllers. When he was handed off to Seattle center, the
pilot was cleared direct to Yakima at 11,000 feet. The
pilot’s confirmation of the altimeter setting was his last
communication with controllers. One minute later the
Seattle controller noticed the Lancair had descended
to 9,700 feet. The controller’s calls went unanswered.
Weather radar and satellite images showed the
Lancair had entered the clouds downwind of Mount
Rainier. The advanced flat glass avionics system was
recording date, time, latitude, longitude, altitude,
pitch, bank, heading, course, indicated airspeed, true
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