Nightly News | April 16, 2010
>>> taking fewer pills.
>>> we are back continuing our coverage of this volcano . a lot of people have been wondering what would really happen if jets tried to fly through some of this dust, this volcanic ash ? some people have not understood this flight ban is not optional. they cannot fly through it. tonight we learn why from nbc 's tom costello .
>> reporter: for modern jet aircraft , volcanic ash can quickly turn a flight onto a harrowing, death-defying plunge. it happened in 1989 over alaska . klm flight 867 flew through a thick volcanic ash . the plane fell from 28,000 feet to 13,000 before the captain managed to regain power and land safely.
>> we dropped and were thrown up again.
>> reporter: this photograph shows what happens when volcanic ash cakes an aircraft's engines . as the grains of rock, sand and glass anti-er the engine they begin to melt coating the plane 's turbines and clogging the engine. when that happens, the airflow is choked off, the engine seizes and shuts down. restarting an ash-coated engine takes tremendous skill and a good deal of luck. in 1982 , a british airways 747 crew over the indian ocean at night, never saw a volcanic ash cloud and suddenly faced a real emergency.
>> all four engines stopped. we glided the airplane from 37,000 feet to 15,000 feet.
>> reporter: fortunately engines cleaned out the ash. it can have a sand-blasting effect, peeling off paint, clogging passenger ventilation systems.
>> we learned a great deal of what volcanic ash can do to jet engines .
>> reporter: a lethal threat keeping tens of thousands of passengers grounded. tom costello , nbc news, washington .
>>> when we come back here