快猫短视频

SEVERE turbulence can kill aircraft passengers. Now, in test flights over the
Rocky Mountains, NASA engineers have successfully detected clear-air turbulence
up to 10 seconds before an aircraft hits it.

Clear-air turbulence often catches pilots by surprise. Invisible to radar, it
is difficult to forecast and can hurl passengers about the cabin. In December
1997, one passenger died and a hundred others were injured when unexpected rough
air caused a United Airlines flight over the Pacific to drop 300 metres in a few
seconds.

However, passengers can avoid serious injury by fastening their seatbelts.
鈥淏uckling in is the only antidote for this sort of thing,鈥� says Rod Bogue,
project manager at NASA鈥檚 Dryden Flight Research Center in Edwards,
California.

The centre鈥檚 new turbulence detector is based on lidar, or laser radar. Laser
pulses are sent ahead of the plane and these are then reflected back by
particles in the air. The technique depends on the Doppler effect. The
wavelength of the light shifts according to the speed at which the particles are
approaching. In calm air, the speed equals the plane鈥檚 airspeed. But as the
particles swirl in choppy air, their speed of approach increases or decreases
rapidly. The rate of change in speed corresponds to the severity of the
turbulence.

In a series of tests that began last month, a research jet flew repeatedly
into disturbed air over the mountain ridges near Pueblo, Colorado. The lidar
detector spotted turbulence between 3 and 8 kilometres ahead, and its forecasts
of strength and duration corresponded closely with the turbulence that the plane
encountered.

Bogue says that he had 鈥渁 comfortable amount of time鈥� to fasten his seatbelt.
The researchers are planning to improve the lidar鈥檚 range with a more powerful
beam. The system could be installed on commercial aircraft in the next few
years.